/Users/deen/code/yugabyte-db/src/postgres/src/backend/access/transam/xlog.c

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/*-------------------------------------------------------------------------
 *
 * xlog.c
 *    PostgreSQL write-ahead log manager
 *
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/xlog.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <ctype.h>
#include <math.h>
#include <time.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <unistd.h>

#include "access/clog.h"
#include "access/commit_ts.h"
#include "access/multixact.h"
#include "access/rewriteheap.h"
#include "access/subtrans.h"
#include "access/timeline.h"
#include "access/transam.h"
#include "access/tuptoaster.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xloginsert.h"
#include "access/xlogreader.h"
#include "access/xlogutils.h"
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "catalog/pg_database.h"
#include "commands/tablespace.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "postmaster/bgwriter.h"
#include "postmaster/walwriter.h"
#include "postmaster/startup.h"
#include "replication/basebackup.h"
#include "replication/logical.h"
#include "replication/slot.h"
#include "replication/origin.h"
#include "replication/snapbuild.h"
#include "replication/walreceiver.h"
#include "replication/walsender.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/large_object.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/reinit.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/backend_random.h"
#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/pg_lsn.h"
#include "utils/ps_status.h"
#include "utils/relmapper.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
#include "pg_trace.h"

extern uint32 bootstrap_data_checksum_version;

/* File path names (all relative to $PGDATA) */
#define RECOVERY_COMMAND_FILE "recovery.conf"
#define RECOVERY_COMMAND_DONE "recovery.done"
#define PROMOTE_SIGNAL_FILE   "promote"
#define FALLBACK_PROMOTE_SIGNAL_FILE "fallback_promote"


/* User-settable parameters */
int     max_wal_size_mb = 1024; /* 1 GB */
int     min_wal_size_mb = 80; /* 80 MB */
int     wal_keep_segments = 0;
int     XLOGbuffers = -1;
int     XLogArchiveTimeout = 0;
int     XLogArchiveMode = ARCHIVE_MODE_OFF;
char     *XLogArchiveCommand = NULL;
bool    EnableHotStandby = false;
bool    fullPageWrites = true;
bool    wal_log_hints = false;
bool    wal_compression = false;
char     *wal_consistency_checking_string = NULL;
bool     *wal_consistency_checking = NULL;
bool    log_checkpoints = false;
int     sync_method = DEFAULT_SYNC_METHOD;
int     wal_level = WAL_LEVEL_MINIMAL;
int     CommitDelay = 0;  /* precommit delay in microseconds */
int     CommitSiblings = 5; /* # concurrent xacts needed to sleep */
int     wal_retrieve_retry_interval = 5000;

#ifdef WAL_DEBUG
bool    XLOG_DEBUG = false;
#endif

int     wal_segment_size = DEFAULT_XLOG_SEG_SIZE;

/*
 * Number of WAL insertion locks to use. A higher value allows more insertions
 * to happen concurrently, but adds some CPU overhead to flushing the WAL,
 * which needs to iterate all the locks.
 */
#define NUM_XLOGINSERT_LOCKS  8

/*
 * Max distance from last checkpoint, before triggering a new xlog-based
 * checkpoint.
 */
int     CheckPointSegments;

/* Estimated distance between checkpoints, in bytes */
static double CheckPointDistanceEstimate = 0;
static double PrevCheckPointDistance = 0;

/*
 * GUC support
 */
const struct config_enum_entry sync_method_options[] = {
  {"fsync", SYNC_METHOD_FSYNC, false},
#ifdef HAVE_FSYNC_WRITETHROUGH
  {"fsync_writethrough", SYNC_METHOD_FSYNC_WRITETHROUGH, false},
#endif
#ifdef HAVE_FDATASYNC
  {"fdatasync", SYNC_METHOD_FDATASYNC, false},
#endif
#ifdef OPEN_SYNC_FLAG
  {"open_sync", SYNC_METHOD_OPEN, false},
#endif
#ifdef OPEN_DATASYNC_FLAG
  {"open_datasync", SYNC_METHOD_OPEN_DSYNC, false},
#endif
  {NULL, 0, false}
};


/*
 * Although only "on", "off", and "always" are documented,
 * we accept all the likely variants of "on" and "off".
 */
const struct config_enum_entry archive_mode_options[] = {
  {"always", ARCHIVE_MODE_ALWAYS, false},
  {"on", ARCHIVE_MODE_ON, false},
  {"off", ARCHIVE_MODE_OFF, false},
  {"true", ARCHIVE_MODE_ON, true},
  {"false", ARCHIVE_MODE_OFF, true},
  {"yes", ARCHIVE_MODE_ON, true},
  {"no", ARCHIVE_MODE_OFF, true},
  {"1", ARCHIVE_MODE_ON, true},
  {"0", ARCHIVE_MODE_OFF, true},
  {NULL, 0, false}
};

/*
 * Statistics for current checkpoint are collected in this global struct.
 * Because only the checkpointer or a stand-alone backend can perform
 * checkpoints, this will be unused in normal backends.
 */
CheckpointStatsData CheckpointStats;

/*
 * ThisTimeLineID will be same in all backends --- it identifies current
 * WAL timeline for the database system.
 */
TimeLineID  ThisTimeLineID = 0;

/*
 * Are we doing recovery from XLOG?
 *
 * This is only ever true in the startup process; it should be read as meaning
 * "this process is replaying WAL records", rather than "the system is in
 * recovery mode".  It should be examined primarily by functions that need
 * to act differently when called from a WAL redo function (e.g., to skip WAL
 * logging).  To check whether the system is in recovery regardless of which
 * process you're running in, use RecoveryInProgress() but only after shared
 * memory startup and lock initialization.
 */
bool    InRecovery = false;

/* Are we in Hot Standby mode? Only valid in startup process, see xlog.h */
HotStandbyState standbyState = STANDBY_DISABLED;

static XLogRecPtr LastRec;

/* Local copy of WalRcv->receivedUpto */
static XLogRecPtr receivedUpto = 0;
static TimeLineID receiveTLI = 0;

/*
 * During recovery, lastFullPageWrites keeps track of full_page_writes that
 * the replayed WAL records indicate. It's initialized with full_page_writes
 * that the recovery starting checkpoint record indicates, and then updated
 * each time XLOG_FPW_CHANGE record is replayed.
 */
static bool lastFullPageWrites;

/*
 * Local copy of SharedRecoveryInProgress variable. True actually means "not
 * known, need to check the shared state".
 */
static bool LocalRecoveryInProgress = true;

/*
 * Local copy of SharedHotStandbyActive variable. False actually means "not
 * known, need to check the shared state".
 */
static bool LocalHotStandbyActive = false;

/*
 * Local state for XLogInsertAllowed():
 *    1: unconditionally allowed to insert XLOG
 *    0: unconditionally not allowed to insert XLOG
 *    -1: must check RecoveryInProgress(); disallow until it is false
 * Most processes start with -1 and transition to 1 after seeing that recovery
 * is not in progress.  But we can also force the value for special cases.
 * The coding in XLogInsertAllowed() depends on the first two of these states
 * being numerically the same as bool true and false.
 */
static int  LocalXLogInsertAllowed = -1;

/*
 * When ArchiveRecoveryRequested is set, archive recovery was requested,
 * ie. recovery.conf file was present. When InArchiveRecovery is set, we are
 * currently recovering using offline XLOG archives. These variables are only
 * valid in the startup process.
 *
 * When ArchiveRecoveryRequested is true, but InArchiveRecovery is false, we're
 * currently performing crash recovery using only XLOG files in pg_wal, but
 * will switch to using offline XLOG archives as soon as we reach the end of
 * WAL in pg_wal.
*/
bool    ArchiveRecoveryRequested = false;
bool    InArchiveRecovery = false;

/* Was the last xlog file restored from archive, or local? */
static bool restoredFromArchive = false;

/* Buffers dedicated to consistency checks of size BLCKSZ */
static char *replay_image_masked = NULL;
static char *master_image_masked = NULL;

/* options taken from recovery.conf for archive recovery */
char     *recoveryRestoreCommand = NULL;
static char *recoveryEndCommand = NULL;
static char *archiveCleanupCommand = NULL;
static RecoveryTargetType recoveryTarget = RECOVERY_TARGET_UNSET;
static bool recoveryTargetInclusive = true;
static RecoveryTargetAction recoveryTargetAction = RECOVERY_TARGET_ACTION_PAUSE;
static TransactionId recoveryTargetXid;
static TimestampTz recoveryTargetTime;
static char *recoveryTargetName;
static XLogRecPtr recoveryTargetLSN;
static int  recovery_min_apply_delay = 0;
static TimestampTz recoveryDelayUntilTime;

/* options taken from recovery.conf for XLOG streaming */
static bool StandbyModeRequested = false;
static char *PrimaryConnInfo = NULL;
static char *PrimarySlotName = NULL;
static char *TriggerFile = NULL;

/* are we currently in standby mode? */
bool    StandbyMode = false;

/* whether request for fast promotion has been made yet */
static bool fast_promote = false;

/*
 * if recoveryStopsBefore/After returns true, it saves information of the stop
 * point here
 */
static TransactionId recoveryStopXid;
static TimestampTz recoveryStopTime;
static XLogRecPtr recoveryStopLSN;
static char recoveryStopName[MAXFNAMELEN];
static bool recoveryStopAfter;

/*
 * During normal operation, the only timeline we care about is ThisTimeLineID.
 * During recovery, however, things are more complicated.  To simplify life
 * for rmgr code, we keep ThisTimeLineID set to the "current" timeline as we
 * scan through the WAL history (that is, it is the line that was active when
 * the currently-scanned WAL record was generated).  We also need these
 * timeline values:
 *
 * recoveryTargetTLI: the desired timeline that we want to end in.
 *
 * recoveryTargetIsLatest: was the requested target timeline 'latest'?
 *
 * expectedTLEs: a list of TimeLineHistoryEntries for recoveryTargetTLI and the timelines of
 * its known parents, newest first (so recoveryTargetTLI is always the
 * first list member).  Only these TLIs are expected to be seen in the WAL
 * segments we read, and indeed only these TLIs will be considered as
 * candidate WAL files to open at all.
 *
 * curFileTLI: the TLI appearing in the name of the current input WAL file.
 * (This is not necessarily the same as ThisTimeLineID, because we could
 * be scanning data that was copied from an ancestor timeline when the current
 * file was created.)  During a sequential scan we do not allow this value
 * to decrease.
 */
static TimeLineID recoveryTargetTLI;
static bool recoveryTargetIsLatest = false;
static List *expectedTLEs;
static TimeLineID curFileTLI;

/*
 * ProcLastRecPtr points to the start of the last XLOG record inserted by the
 * current backend.  It is updated for all inserts.  XactLastRecEnd points to
 * end+1 of the last record, and is reset when we end a top-level transaction,
 * or start a new one; so it can be used to tell if the current transaction has
 * created any XLOG records.
 *
 * While in parallel mode, this may not be fully up to date.  When committing,
 * a transaction can assume this covers all xlog records written either by the
 * user backend or by any parallel worker which was present at any point during
 * the transaction.  But when aborting, or when still in parallel mode, other
 * parallel backends may have written WAL records at later LSNs than the value
 * stored here.  The parallel leader advances its own copy, when necessary,
 * in WaitForParallelWorkersToFinish.
 */
XLogRecPtr  ProcLastRecPtr = InvalidXLogRecPtr;
XLogRecPtr  XactLastRecEnd = InvalidXLogRecPtr;
XLogRecPtr  XactLastCommitEnd = InvalidXLogRecPtr;

/*
 * RedoRecPtr is this backend's local copy of the REDO record pointer
 * (which is almost but not quite the same as a pointer to the most recent
 * CHECKPOINT record).  We update this from the shared-memory copy,
 * XLogCtl->Insert.RedoRecPtr, whenever we can safely do so (ie, when we
 * hold an insertion lock).  See XLogInsertRecord for details.  We are also
 * allowed to update from XLogCtl->RedoRecPtr if we hold the info_lck;
 * see GetRedoRecPtr.  A freshly spawned backend obtains the value during
 * InitXLOGAccess.
 */
static XLogRecPtr RedoRecPtr;

/*
 * doPageWrites is this backend's local copy of (forcePageWrites ||
 * fullPageWrites).  It is used together with RedoRecPtr to decide whether
 * a full-page image of a page need to be taken.
 */
static bool doPageWrites;

/* Has the recovery code requested a walreceiver wakeup? */
static bool doRequestWalReceiverReply;

/*
 * RedoStartLSN points to the checkpoint's REDO location which is specified
 * in a backup label file, backup history file or control file. In standby
 * mode, XLOG streaming usually starts from the position where an invalid
 * record was found. But if we fail to read even the initial checkpoint
 * record, we use the REDO location instead of the checkpoint location as
 * the start position of XLOG streaming. Otherwise we would have to jump
 * backwards to the REDO location after reading the checkpoint record,
 * because the REDO record can precede the checkpoint record.
 */
static XLogRecPtr RedoStartLSN = InvalidXLogRecPtr;

/*----------
 * Shared-memory data structures for XLOG control
 *
 * LogwrtRqst indicates a byte position that we need to write and/or fsync
 * the log up to (all records before that point must be written or fsynced).
 * LogwrtResult indicates the byte positions we have already written/fsynced.
 * These structs are identical but are declared separately to indicate their
 * slightly different functions.
 *
 * To read XLogCtl->LogwrtResult, you must hold either info_lck or
 * WALWriteLock.  To update it, you need to hold both locks.  The point of
 * this arrangement is that the value can be examined by code that already
 * holds WALWriteLock without needing to grab info_lck as well.  In addition
 * to the shared variable, each backend has a private copy of LogwrtResult,
 * which is updated when convenient.
 *
 * The request bookkeeping is simpler: there is a shared XLogCtl->LogwrtRqst
 * (protected by info_lck), but we don't need to cache any copies of it.
 *
 * info_lck is only held long enough to read/update the protected variables,
 * so it's a plain spinlock.  The other locks are held longer (potentially
 * over I/O operations), so we use LWLocks for them.  These locks are:
 *
 * WALBufMappingLock: must be held to replace a page in the WAL buffer cache.
 * It is only held while initializing and changing the mapping.  If the
 * contents of the buffer being replaced haven't been written yet, the mapping
 * lock is released while the write is done, and reacquired afterwards.
 *
 * WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
 * XLogFlush).
 *
 * ControlFileLock: must be held to read/update control file or create
 * new log file.
 *
 * CheckpointLock: must be held to do a checkpoint or restartpoint (ensures
 * only one checkpointer at a time; currently, with all checkpoints done by
 * the checkpointer, this is just pro forma).
 *
 *----------
 */

typedef struct XLogwrtRqst
{
  XLogRecPtr  Write;      /* last byte + 1 to write out */
  XLogRecPtr  Flush;      /* last byte + 1 to flush */
} XLogwrtRqst;

typedef struct XLogwrtResult
{
  XLogRecPtr  Write;      /* last byte + 1 written out */
  XLogRecPtr  Flush;      /* last byte + 1 flushed */
} XLogwrtResult;

/*
 * Inserting to WAL is protected by a small fixed number of WAL insertion
 * locks. To insert to the WAL, you must hold one of the locks - it doesn't
 * matter which one. To lock out other concurrent insertions, you must hold
 * of them. Each WAL insertion lock consists of a lightweight lock, plus an
 * indicator of how far the insertion has progressed (insertingAt).
 *
 * The insertingAt values are read when a process wants to flush WAL from
 * the in-memory buffers to disk, to check that all the insertions to the
 * region the process is about to write out have finished. You could simply
 * wait for all currently in-progress insertions to finish, but the
 * insertingAt indicator allows you to ignore insertions to later in the WAL,
 * so that you only wait for the insertions that are modifying the buffers
 * you're about to write out.
 *
 * This isn't just an optimization. If all the WAL buffers are dirty, an
 * inserter that's holding a WAL insert lock might need to evict an old WAL
 * buffer, which requires flushing the WAL. If it's possible for an inserter
 * to block on another inserter unnecessarily, deadlock can arise when two
 * inserters holding a WAL insert lock wait for each other to finish their
 * insertion.
 *
 * Small WAL records that don't cross a page boundary never update the value,
 * the WAL record is just copied to the page and the lock is released. But
 * to avoid the deadlock-scenario explained above, the indicator is always
 * updated before sleeping while holding an insertion lock.
 *
 * lastImportantAt contains the LSN of the last important WAL record inserted
 * using a given lock. This value is used to detect if there has been
 * important WAL activity since the last time some action, like a checkpoint,
 * was performed - allowing to not repeat the action if not. The LSN is
 * updated for all insertions, unless the XLOG_MARK_UNIMPORTANT flag was
 * set. lastImportantAt is never cleared, only overwritten by the LSN of newer
 * records.  Tracking the WAL activity directly in WALInsertLock has the
 * advantage of not needing any additional locks to update the value.
 */
typedef struct
{
  LWLock    lock;
  XLogRecPtr  insertingAt;
  XLogRecPtr  lastImportantAt;
} WALInsertLock;

/*
 * All the WAL insertion locks are allocated as an array in shared memory. We
 * force the array stride to be a power of 2, which saves a few cycles in
 * indexing, but more importantly also ensures that individual slots don't
 * cross cache line boundaries. (Of course, we have to also ensure that the
 * array start address is suitably aligned.)
 */
typedef union WALInsertLockPadded
{
  WALInsertLock l;
  char    pad[PG_CACHE_LINE_SIZE];
} WALInsertLockPadded;

/*
 * State of an exclusive backup, necessary to control concurrent activities
 * across sessions when working on exclusive backups.
 *
 * EXCLUSIVE_BACKUP_NONE means that there is no exclusive backup actually
 * running, to be more precise pg_start_backup() is not being executed for
 * an exclusive backup and there is no exclusive backup in progress.
 * EXCLUSIVE_BACKUP_STARTING means that pg_start_backup() is starting an
 * exclusive backup.
 * EXCLUSIVE_BACKUP_IN_PROGRESS means that pg_start_backup() has finished
 * running and an exclusive backup is in progress. pg_stop_backup() is
 * needed to finish it.
 * EXCLUSIVE_BACKUP_STOPPING means that pg_stop_backup() is stopping an
 * exclusive backup.
 */
typedef enum ExclusiveBackupState
{
  EXCLUSIVE_BACKUP_NONE = 0,
  EXCLUSIVE_BACKUP_STARTING,
  EXCLUSIVE_BACKUP_IN_PROGRESS,
  EXCLUSIVE_BACKUP_STOPPING
} ExclusiveBackupState;

/*
 * Session status of running backup, used for sanity checks in SQL-callable
 * functions to start and stop backups.
 */
static SessionBackupState sessionBackupState = SESSION_BACKUP_NONE;

/*
 * Shared state data for WAL insertion.
 */
typedef struct XLogCtlInsert
{
  slock_t   insertpos_lck;  /* protects CurrBytePos and PrevBytePos */

  /*
   * CurrBytePos is the end of reserved WAL. The next record will be
   * inserted at that position. PrevBytePos is the start position of the
   * previously inserted (or rather, reserved) record - it is copied to the
   * prev-link of the next record. These are stored as "usable byte
   * positions" rather than XLogRecPtrs (see XLogBytePosToRecPtr()).
   */
  uint64    CurrBytePos;
  uint64    PrevBytePos;

  /*
   * Make sure the above heavily-contended spinlock and byte positions are
   * on their own cache line. In particular, the RedoRecPtr and full page
   * write variables below should be on a different cache line. They are
   * read on every WAL insertion, but updated rarely, and we don't want
   * those reads to steal the cache line containing Curr/PrevBytePos.
   */
  char    pad[PG_CACHE_LINE_SIZE];

  /*
   * fullPageWrites is the master copy used by all backends to determine
   * whether to write full-page to WAL, instead of using process-local one.
   * This is required because, when full_page_writes is changed by SIGHUP,
   * we must WAL-log it before it actually affects WAL-logging by backends.
   * Checkpointer sets at startup or after SIGHUP.
   *
   * To read these fields, you must hold an insertion lock. To modify them,
   * you must hold ALL the locks.
   */
  XLogRecPtr  RedoRecPtr;   /* current redo point for insertions */
  bool    forcePageWrites;  /* forcing full-page writes for PITR? */
  bool    fullPageWrites;

  /*
   * exclusiveBackupState indicates the state of an exclusive backup (see
   * comments of ExclusiveBackupState for more details). nonExclusiveBackups
   * is a counter indicating the number of streaming base backups currently
   * in progress. forcePageWrites is set to true when either of these is
   * non-zero. lastBackupStart is the latest checkpoint redo location used
   * as a starting point for an online backup.
   */
  ExclusiveBackupState exclusiveBackupState;
  int     nonExclusiveBackups;
  XLogRecPtr  lastBackupStart;

  /*
   * WAL insertion locks.
   */
  WALInsertLockPadded *WALInsertLocks;
} XLogCtlInsert;

/*
 * Total shared-memory state for XLOG.
 */
typedef struct XLogCtlData
{
  XLogCtlInsert Insert;

  /* Protected by info_lck: */
  XLogwrtRqst LogwrtRqst;
  XLogRecPtr  RedoRecPtr;   /* a recent copy of Insert->RedoRecPtr */
  uint32    ckptXidEpoch; /* nextXID & epoch of latest checkpoint */
  TransactionId ckptXid;
  XLogRecPtr  asyncXactLSN; /* LSN of newest async commit/abort */
  XLogRecPtr  replicationSlotMinLSN;  /* oldest LSN needed by any slot */

  XLogSegNo lastRemovedSegNo; /* latest removed/recycled XLOG segment */

  /* Fake LSN counter, for unlogged relations. Protected by ulsn_lck. */
  XLogRecPtr  unloggedLSN;
  slock_t   ulsn_lck;

  /* Time and LSN of last xlog segment switch. Protected by WALWriteLock. */
  pg_time_t lastSegSwitchTime;
  XLogRecPtr  lastSegSwitchLSN;

  /*
   * Protected by info_lck and WALWriteLock (you must hold either lock to
   * read it, but both to update)
   */
  XLogwrtResult LogwrtResult;

  /*
   * Latest initialized page in the cache (last byte position + 1).
   *
   * To change the identity of a buffer (and InitializedUpTo), you need to
   * hold WALBufMappingLock.  To change the identity of a buffer that's
   * still dirty, the old page needs to be written out first, and for that
   * you need WALWriteLock, and you need to ensure that there are no
   * in-progress insertions to the page by calling
   * WaitXLogInsertionsToFinish().
   */
  XLogRecPtr  InitializedUpTo;

  /*
   * These values do not change after startup, although the pointed-to pages
   * and xlblocks values certainly do.  xlblock values are protected by
   * WALBufMappingLock.
   */
  char     *pages;      /* buffers for unwritten XLOG pages */
  XLogRecPtr *xlblocks;   /* 1st byte ptr-s + XLOG_BLCKSZ */
  int     XLogCacheBlck;  /* highest allocated xlog buffer index */

  /*
   * Shared copy of ThisTimeLineID. Does not change after end-of-recovery.
   * If we created a new timeline when the system was started up,
   * PrevTimeLineID is the old timeline's ID that we forked off from.
   * Otherwise it's equal to ThisTimeLineID.
   */
  TimeLineID  ThisTimeLineID;
  TimeLineID  PrevTimeLineID;

  /*
   * archiveCleanupCommand is read from recovery.conf but needs to be in
   * shared memory so that the checkpointer process can access it.
   */
  char    archiveCleanupCommand[MAXPGPATH];

  /*
   * SharedRecoveryInProgress indicates if we're still in crash or archive
   * recovery.  Protected by info_lck.
   */
  bool    SharedRecoveryInProgress;

  /*
   * SharedHotStandbyActive indicates if we're still in crash or archive
   * recovery.  Protected by info_lck.
   */
  bool    SharedHotStandbyActive;

  /*
   * WalWriterSleeping indicates whether the WAL writer is currently in
   * low-power mode (and hence should be nudged if an async commit occurs).
   * Protected by info_lck.
   */
  bool    WalWriterSleeping;

  /*
   * recoveryWakeupLatch is used to wake up the startup process to continue
   * WAL replay, if it is waiting for WAL to arrive or failover trigger file
   * to appear.
   */
  Latch   recoveryWakeupLatch;

  /*
   * During recovery, we keep a copy of the latest checkpoint record here.
   * lastCheckPointRecPtr points to start of checkpoint record and
   * lastCheckPointEndPtr points to end+1 of checkpoint record.  Used by the
   * checkpointer when it wants to create a restartpoint.
   *
   * Protected by info_lck.
   */
  XLogRecPtr  lastCheckPointRecPtr;
  XLogRecPtr  lastCheckPointEndPtr;
  CheckPoint  lastCheckPoint;

  /*
   * lastReplayedEndRecPtr points to end+1 of the last record successfully
   * replayed. When we're currently replaying a record, ie. in a redo
   * function, replayEndRecPtr points to the end+1 of the record being
   * replayed, otherwise it's equal to lastReplayedEndRecPtr.
   */
  XLogRecPtr  lastReplayedEndRecPtr;
  TimeLineID  lastReplayedTLI;
  XLogRecPtr  replayEndRecPtr;
  TimeLineID  replayEndTLI;
  /* timestamp of last COMMIT/ABORT record replayed (or being replayed) */
  TimestampTz recoveryLastXTime;

  /*
   * timestamp of when we started replaying the current chunk of WAL data,
   * only relevant for replication or archive recovery
   */
  TimestampTz currentChunkStartTime;
  /* Are we requested to pause recovery? */
  bool    recoveryPause;

  /*
   * lastFpwDisableRecPtr points to the start of the last replayed
   * XLOG_FPW_CHANGE record that instructs full_page_writes is disabled.
   */
  XLogRecPtr  lastFpwDisableRecPtr;

  slock_t   info_lck;   /* locks shared variables shown above */
} XLogCtlData;

static XLogCtlData *XLogCtl = NULL;

/* a private copy of XLogCtl->Insert.WALInsertLocks, for convenience */
static WALInsertLockPadded *WALInsertLocks = NULL;

/*
 * We maintain an image of pg_control in shared memory.
 */
static ControlFileData *ControlFile = NULL;

/*
 * Calculate the amount of space left on the page after 'endptr'. Beware
 * multiple evaluation!
 */
#define INSERT_FREESPACE(endptr)  \
  (((endptr) % XLOG_BLCKSZ == 0) ? 00 : (XLOG_BLCKSZ - (endptr) % XLOG_BLCKSZ))

/* Macro to advance to next buffer index. */
#define NextBufIdx(idx)   \
    (((idx) == XLogCtl->XLogCacheBlck) ? 00 : ((idx) + 1))

/*
 * XLogRecPtrToBufIdx returns the index of the WAL buffer that holds, or
 * would hold if it was in cache, the page containing 'recptr'.
 */
#define XLogRecPtrToBufIdx(recptr)  \
  (((recptr) / XLOG_BLCKSZ) % (XLogCtl->XLogCacheBlck + 1))

/*
 * These are the number of bytes in a WAL page usable for WAL data.
 */
#define UsableBytesInPage (XLOG_BLCKSZ - SizeOfXLogShortPHD)

/* Convert min_wal_size_mb and max wal_size_mb to equivalent segment count */
#define ConvertToXSegs(x, segsize)  \
  (x / ((segsize) / (1024 * 1024)))

/* The number of bytes in a WAL segment usable for WAL data. */
static int  UsableBytesInSegment;

/*
 * Private, possibly out-of-date copy of shared LogwrtResult.
 * See discussion above.
 */
static XLogwrtResult LogwrtResult = {0, 0};

/*
 * Codes indicating where we got a WAL file from during recovery, or where
 * to attempt to get one.
 */
typedef enum
{
  XLOG_FROM_ANY = 0,      /* request to read WAL from any source */
  XLOG_FROM_ARCHIVE,      /* restored using restore_command */
  XLOG_FROM_PG_WAL,     /* existing file in pg_wal */
  XLOG_FROM_STREAM      /* streamed from master */
} XLogSource;

/* human-readable names for XLogSources, for debugging output */
static const char *xlogSourceNames[] = {"any", "archive", "pg_wal", "stream"};

/*
 * openLogFile is -1 or a kernel FD for an open log file segment.
 * When it's open, openLogOff is the current seek offset in the file.
 * openLogSegNo identifies the segment.  These variables are only
 * used to write the XLOG, and so will normally refer to the active segment.
 */
static int  openLogFile = -1;
static XLogSegNo openLogSegNo = 0;
static uint32 openLogOff = 0;

/*
 * These variables are used similarly to the ones above, but for reading
 * the XLOG.  Note, however, that readOff generally represents the offset
 * of the page just read, not the seek position of the FD itself, which
 * will be just past that page. readLen indicates how much of the current
 * page has been read into readBuf, and readSource indicates where we got
 * the currently open file from.
 */
static int  readFile = -1;
static XLogSegNo readSegNo = 0;
static uint32 readOff = 0;
static uint32 readLen = 0;
static XLogSource readSource = 0; /* XLOG_FROM_* code */

/*
 * Keeps track of which source we're currently reading from. This is
 * different from readSource in that this is always set, even when we don't
 * currently have a WAL file open. If lastSourceFailed is set, our last
 * attempt to read from currentSource failed, and we should try another source
 * next.
 */
static XLogSource currentSource = 0;  /* XLOG_FROM_* code */
static bool lastSourceFailed = false;

typedef struct XLogPageReadPrivate
{
  int     emode;
  bool    fetching_ckpt;  /* are we fetching a checkpoint record? */
  bool    randAccess;
} XLogPageReadPrivate;

/*
 * These variables track when we last obtained some WAL data to process,
 * and where we got it from.  (XLogReceiptSource is initially the same as
 * readSource, but readSource gets reset to zero when we don't have data
 * to process right now.  It is also different from currentSource, which
 * also changes when we try to read from a source and fail, while
 * XLogReceiptSource tracks where we last successfully read some WAL.)
 */
static TimestampTz XLogReceiptTime = 0;
static XLogSource XLogReceiptSource = 0;  /* XLOG_FROM_* code */

/* State information for XLOG reading */
static XLogRecPtr ReadRecPtr; /* start of last record read */
static XLogRecPtr EndRecPtr;  /* end+1 of last record read */

/*
 * Local copies of equivalent fields in the control file.  When running
 * crash recovery, minRecoveryPoint is set to InvalidXLogRecPtr as we
 * expect to replay all the WAL available, and updateMinRecoveryPoint is
 * switched to false to prevent any updates while replaying records.
 * Those values are kept consistent as long as crash recovery runs.
 */
static XLogRecPtr minRecoveryPoint;
static TimeLineID minRecoveryPointTLI;
static bool updateMinRecoveryPoint = true;

/*
 * Have we reached a consistent database state? In crash recovery, we have
 * to replay all the WAL, so reachedConsistency is never set. During archive
 * recovery, the database is consistent once minRecoveryPoint is reached.
 */
bool    reachedConsistency = false;

static bool InRedo = false;

/* Have we launched bgwriter during recovery? */
static bool bgwriterLaunched = false;

/* For WALInsertLockAcquire/Release functions */
static int  MyLockNo = 0;
static bool holdingAllLocks = false;

#ifdef WAL_DEBUG
static MemoryContext walDebugCxt = NULL;
#endif

static void readRecoveryCommandFile(void);
static void exitArchiveRecovery(TimeLineID endTLI, XLogRecPtr endOfLog);
static bool recoveryStopsBefore(XLogReaderState *record);
static bool recoveryStopsAfter(XLogReaderState *record);
static void recoveryPausesHere(void);
static bool recoveryApplyDelay(XLogReaderState *record);
static void SetLatestXTime(TimestampTz xtime);
static void SetCurrentChunkStartTime(TimestampTz xtime);
static void CheckRequiredParameterValues(void);
static void XLogReportParameters(void);
static void checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI,
          TimeLineID prevTLI);
static void LocalSetXLogInsertAllowed(void);
static void CreateEndOfRecoveryRecord(void);
static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags);
static void KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo);
static XLogRecPtr XLogGetReplicationSlotMinimumLSN(void);

static void AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic);
static bool XLogCheckpointNeeded(XLogSegNo new_segno);
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible);
static bool InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
             bool find_free, XLogSegNo max_segno,
             bool use_lock);
static int XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
       int source, bool notfoundOk);
static int  XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source);
static int XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
       int reqLen, XLogRecPtr targetRecPtr, char *readBuf,
       TimeLineID *readTLI);
static bool WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
              bool fetching_ckpt, XLogRecPtr tliRecPtr);
static int  emode_for_corrupt_record(int emode, XLogRecPtr RecPtr);
static void XLogFileClose(void);
static void PreallocXlogFiles(XLogRecPtr endptr);
static void RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr RedoRecPtr, XLogRecPtr endptr);
static void RemoveXlogFile(const char *segname, XLogRecPtr RedoRecPtr, XLogRecPtr endptr);
static void UpdateLastRemovedPtr(char *filename);
static void ValidateXLOGDirectoryStructure(void);
static void CleanupBackupHistory(void);
static void UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force);
static XLogRecord *ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
       int emode, bool fetching_ckpt);
static void CheckRecoveryConsistency(void);
static XLogRecord *ReadCheckpointRecord(XLogReaderState *xlogreader,
           XLogRecPtr RecPtr, int whichChkpti, bool report);
static bool rescanLatestTimeLine(void);
static void WriteControlFile(void);
static void ReadControlFile(void);
static char *str_time(pg_time_t tnow);
static bool CheckForStandbyTrigger(void);

#ifdef WAL_DEBUG
static void xlog_outrec(StringInfo buf, XLogReaderState *record);
#endif
static void xlog_outdesc(StringInfo buf, XLogReaderState *record);
static void pg_start_backup_callback(int code, Datum arg);
static void pg_stop_backup_callback(int code, Datum arg);
static bool read_backup_label(XLogRecPtr *checkPointLoc,
          bool *backupEndRequired, bool *backupFromStandby);
static bool read_tablespace_map(List **tablespaces);

static void rm_redo_error_callback(void *arg);
static int  get_sync_bit(int method);

static void CopyXLogRecordToWAL(int write_len, bool isLogSwitch,
          XLogRecData *rdata,
          XLogRecPtr StartPos, XLogRecPtr EndPos);
static void ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos,
              XLogRecPtr *EndPos, XLogRecPtr *PrevPtr);
static bool ReserveXLogSwitch(XLogRecPtr *StartPos, XLogRecPtr *EndPos,
          XLogRecPtr *PrevPtr);
static XLogRecPtr WaitXLogInsertionsToFinish(XLogRecPtr upto);
static char *GetXLogBuffer(XLogRecPtr ptr);
static XLogRecPtr XLogBytePosToRecPtr(uint64 bytepos);
static XLogRecPtr XLogBytePosToEndRecPtr(uint64 bytepos);
static uint64 XLogRecPtrToBytePos(XLogRecPtr ptr);
static void checkXLogConsistency(XLogReaderState *record);

static void WALInsertLockAcquire(void);
static void WALInsertLockAcquireExclusive(void);
static void WALInsertLockRelease(void);
static void WALInsertLockUpdateInsertingAt(XLogRecPtr insertingAt);

/*
 * Insert an XLOG record represented by an already-constructed chain of data
 * chunks.  This is a low-level routine; to construct the WAL record header
 * and data, use the higher-level routines in xloginsert.c.
 *
 * If 'fpw_lsn' is valid, it is the oldest LSN among the pages that this
 * WAL record applies to, that were not included in the record as full page
 * images.  If fpw_lsn <= RedoRecPtr, the function does not perform the
 * insertion and returns InvalidXLogRecPtr.  The caller can then recalculate
 * which pages need a full-page image, and retry.  If fpw_lsn is invalid, the
 * record is always inserted.
 *
 * 'flags' gives more in-depth control on the record being inserted. See
 * XLogSetRecordFlags() for details.
 *
 * The first XLogRecData in the chain must be for the record header, and its
 * data must be MAXALIGNed.  XLogInsertRecord fills in the xl_prev and
 * xl_crc fields in the header, the rest of the header must already be filled
 * by the caller.
 *
 * Returns XLOG pointer to end of record (beginning of next record).
 * This can be used as LSN for data pages affected by the logged action.
 * (LSN is the XLOG point up to which the XLOG must be flushed to disk
 * before the data page can be written out.  This implements the basic
 * WAL rule "write the log before the data".)
 */
XLogRecPtr
XLogInsertRecord(XLogRecData *rdata,
         XLogRecPtr fpw_lsn,
         uint8 flags)
{
  XLogCtlInsert *Insert = &XLogCtl->Insert;
  pg_crc32c rdata_crc;
  bool    inserted;
  XLogRecord *rechdr = (XLogRecord *) rdata->data;
  uint8   info = rechdr->xl_info & ~XLR_INFO_MASK;
  bool    isLogSwitch = (rechdr->xl_rmid == RM_XLOG_ID &&
                 info == 2.36kXLOG_SWITCH2.36k);
  XLogRecPtr  StartPos;
  XLogRecPtr  EndPos;
  bool    prevDoPageWrites = doPageWrites;

  /* we assume that all of the record header is in the first chunk */
  Assert(rdata->len >= SizeOfXLogRecord);

  /* cross-check on whether we should be here or not */
  if (!XLogInsertAllowed())
    elog(ERROR, "cannot make new WAL entries during recovery");

  /*----------
   *
   * We have now done all the preparatory work we can without holding a
   * lock or modifying shared state. From here on, inserting the new WAL
   * record to the shared WAL buffer cache is a two-step process:
   *
   * 1. Reserve the right amount of space from the WAL. The current head of
   *    reserved space is kept in Insert->CurrBytePos, and is protected by
   *    insertpos_lck.
   *
   * 2. Copy the record to the reserved WAL space. This involves finding the
   *    correct WAL buffer containing the reserved space, and copying the
   *    record in place. This can be done concurrently in multiple processes.
   *
   * To keep track of which insertions are still in-progress, each concurrent
   * inserter acquires an insertion lock. In addition to just indicating that
   * an insertion is in progress, the lock tells others how far the inserter
   * has progressed. There is a small fixed number of insertion locks,
   * determined by NUM_XLOGINSERT_LOCKS. When an inserter crosses a page
   * boundary, it updates the value stored in the lock to the how far it has
   * inserted, to allow the previous buffer to be flushed.
   *
   * Holding onto an insertion lock also protects RedoRecPtr and
   * fullPageWrites from changing until the insertion is finished.
   *
   * Step 2 can usually be done completely in parallel. If the required WAL
   * page is not initialized yet, you have to grab WALBufMappingLock to
   * initialize it, but the WAL writer tries to do that ahead of insertions
   * to avoid that from happening in the critical path.
   *
   *----------
   */
  START_CRIT_SECTION();
  if (isLogSwitch)
    WALInsertLockAcquireExclusive();
  else
    WALInsertLockAcquire();

  /*
   * Check to see if my copy of RedoRecPtr is out of date. If so, may have
   * to go back and have the caller recompute everything. This can only
   * happen just after a checkpoint, so it's better to be slow in this case
   * and fast otherwise.
   *
   * Also check to see if fullPageWrites or forcePageWrites was just turned
   * on; if we weren't already doing full-page writes then go back and
   * recompute.
   *
   * If we aren't doing full-page writes then RedoRecPtr doesn't actually
   * affect the contents of the XLOG record, so we'll update our local copy
   * but not force a recomputation.  (If doPageWrites was just turned off,
   * we could recompute the record without full pages, but we choose not to
   * bother.)
   */
  if (RedoRecPtr != Insert->RedoRecPtr)
  {
    Assert(RedoRecPtr < Insert->RedoRecPtr);
    RedoRecPtr = Insert->RedoRecPtr;
  }
  doPageWrites = (Insert->fullPageWrites || Insert->forcePageWrites0);

  if (doPageWrites &&
    (!prevDoPageWrites ||
     (fpw_lsn != InvalidXLogRecPtr && fpw_lsn <= RedoRecPtr0)))
  {
    /*
     * Oops, some buffer now needs to be backed up that the caller didn't
     * back up.  Start over.
     */
    WALInsertLockRelease();
    END_CRIT_SECTION();
    return InvalidXLogRecPtr;
  }

  /*
   * Reserve space for the record in the WAL. This also sets the xl_prev
   * pointer.
   */
  if (isLogSwitch)
    inserted = ReserveXLogSwitch(&StartPos, &EndPos, &rechdr->xl_prev);
  else
  {
    ReserveXLogInsertLocation(rechdr->xl_tot_len, &StartPos, &EndPos,
                  &rechdr->xl_prev);
    inserted = true;
  }

  if (inserted)
  {
    /*
     * Now that xl_prev has been filled in, calculate CRC of the record
     * header.
     */
    rdata_crc = rechdr->xl_crc;
    COMP_CRC32C(rdata_crc, rechdr, offsetof(XLogRecord, xl_crc));
    FIN_CRC32C(rdata_crc);
    rechdr->xl_crc = rdata_crc;

    /*
     * All the record data, including the header, is now ready to be
     * inserted. Copy the record in the space reserved.
     */
    CopyXLogRecordToWAL(rechdr->xl_tot_len, isLogSwitch, rdata,
              StartPos, EndPos);

    /*
     * Unless record is flagged as not important, update LSN of last
     * important record in the current slot. When holding all locks, just
     * update the first one.
     */
    if ((flags & XLOG_MARK_UNIMPORTANT) == 0)
    {
      int     lockno = holdingAllLocks ? 00 : MyLockNo;

      WALInsertLocks[lockno].l.lastImportantAt = StartPos;
    }
  }
  else
  {
    /*
     * This was an xlog-switch record, but the current insert location was
     * already exactly at the beginning of a segment, so there was no need
     * to do anything.
     */
  }

  /*
   * Done! Let others know that we're finished.
   */
  WALInsertLockRelease();

  MarkCurrentTransactionIdLoggedIfAny();

  END_CRIT_SECTION();

  /*
   * Update shared LogwrtRqst.Write, if we crossed page boundary.
   */
  if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
  {
    SpinLockAcquire(&XLogCtl->info_lck);
    /* advance global request to include new block(s) */
    if (XLogCtl->LogwrtRqst.Write < EndPos)
      XLogCtl->LogwrtRqst.Write = EndPos;
    /* update local result copy while I have the chance */
    LogwrtResult = XLogCtl->LogwrtResult;
    SpinLockRelease(&XLogCtl->info_lck);
  }

  /*
   * If this was an XLOG_SWITCH record, flush the record and the empty
   * padding space that fills the rest of the segment, and perform
   * end-of-segment actions (eg, notifying archiver).
   */
  if (isLogSwitch)
  {
    TRACE_POSTGRESQL_WAL_SWITCH();
    XLogFlush(EndPos);

    /*
     * Even though we reserved the rest of the segment for us, which is
     * reflected in EndPos, we return a pointer to just the end of the
     * xlog-switch record.
     */
    if (inserted)
    {
      EndPos = StartPos + SizeOfXLogRecord;
      if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
      {
        uint64    offset = XLogSegmentOffset(EndPos, wal_segment_size);

        if (offset == EndPos % XLOG_BLCKSZ)
          EndPos += SizeOfXLogLongPHD;
        else
          EndPos += SizeOfXLogShortPHD;
      }
    }
  }

#ifdef WAL_DEBUG
  if (XLOG_DEBUG)
  {
    static XLogReaderState *debug_reader = NULL;
    StringInfoData buf;
    StringInfoData recordBuf;
    char     *errormsg = NULL;
    MemoryContext oldCxt;

    oldCxt = MemoryContextSwitchTo(walDebugCxt);

    initStringInfo(&buf);
    appendStringInfo(&buf, "INSERT @ %X/%X: ",
             (uint32) (EndPos >> 32), (uint32) EndPos);

    /*
     * We have to piece together the WAL record data from the XLogRecData
     * entries, so that we can pass it to the rm_desc function as one
     * contiguous chunk.
     */
    initStringInfo(&recordBuf);
    for (; rdata != NULL; rdata = rdata->next)
      appendBinaryStringInfo(&recordBuf, rdata->data, rdata->len);

    if (!debug_reader)
      debug_reader = XLogReaderAllocate(wal_segment_size, NULL, NULL);

    if (!debug_reader)
    {
      appendStringInfoString(&buf, "error decoding record: out of memory");
    }
    else if (!DecodeXLogRecord(debug_reader, (XLogRecord *) recordBuf.data,
                   &errormsg))
    {
      appendStringInfo(&buf, "error decoding record: %s",
               errormsg ? errormsg : "no error message");
    }
    else
    {
      appendStringInfoString(&buf, " - ");
      xlog_outdesc(&buf, debug_reader);
    }
    elog(LOG, "%s", buf.data);

    pfree(buf.data);
    pfree(recordBuf.data);
    MemoryContextSwitchTo(oldCxt);
  }
#endif

  /*
   * Update our global variables
   */
  ProcLastRecPtr = StartPos;
  XactLastRecEnd = EndPos;

  return EndPos;
}

/*
 * Reserves the right amount of space for a record of given size from the WAL.
 * *StartPos is set to the beginning of the reserved section, *EndPos to
 * its end+1. *PrevPtr is set to the beginning of the previous record; it is
 * used to set the xl_prev of this record.
 *
 * This is the performance critical part of XLogInsert that must be serialized
 * across backends. The rest can happen mostly in parallel. Try to keep this
 * section as short as possible, insertpos_lck can be heavily contended on a
 * busy system.
 *
 * NB: The space calculation here must match the code in CopyXLogRecordToWAL,
 * where we actually copy the record to the reserved space.
 */
static void
ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos, XLogRecPtr *EndPos,
              XLogRecPtr *PrevPtr)
{
  XLogCtlInsert *Insert = &XLogCtl->Insert;
  uint64    startbytepos;
  uint64    endbytepos;
  uint64    prevbytepos;

  size = MAXALIGN(size);

  /* All (non xlog-switch) records should contain data. */
  Assert(size > SizeOfXLogRecord);

  /*
   * The duration the spinlock needs to be held is minimized by minimizing
   * the calculations that have to be done while holding the lock. The
   * current tip of reserved WAL is kept in CurrBytePos, as a byte position
   * that only counts "usable" bytes in WAL, that is, it excludes all WAL
   * page headers. The mapping between "usable" byte positions and physical
   * positions (XLogRecPtrs) can be done outside the locked region, and
   * because the usable byte position doesn't include any headers, reserving
   * X bytes from WAL is almost as simple as "CurrBytePos += X".
   */
  SpinLockAcquire(&Insert->insertpos_lck);

  startbytepos = Insert->CurrBytePos;
  endbytepos = startbytepos + size;
  prevbytepos = Insert->PrevBytePos;
  Insert->CurrBytePos = endbytepos;
  Insert->PrevBytePos = startbytepos;

  SpinLockRelease(&Insert->insertpos_lck);

  *StartPos = XLogBytePosToRecPtr(startbytepos);
  *EndPos = XLogBytePosToEndRecPtr(endbytepos);
  *PrevPtr = XLogBytePosToRecPtr(prevbytepos);

  /*
   * Check that the conversions between "usable byte positions" and
   * XLogRecPtrs work consistently in both directions.
   */
  Assert(XLogRecPtrToBytePos(*StartPos) == startbytepos);
  Assert(XLogRecPtrToBytePos(*EndPos) == endbytepos);
  Assert(XLogRecPtrToBytePos(*PrevPtr) == prevbytepos);
}

/*
 * Like ReserveXLogInsertLocation(), but for an xlog-switch record.
 *
 * A log-switch record is handled slightly differently. The rest of the
 * segment will be reserved for this insertion, as indicated by the returned
 * *EndPos value. However, if we are already at the beginning of the current
 * segment, *StartPos and *EndPos are set to the current location without
 * reserving any space, and the function returns false.
*/
static bool
ReserveXLogSwitch(XLogRecPtr *StartPos, XLogRecPtr *EndPos, XLogRecPtr *PrevPtr)
{
  XLogCtlInsert *Insert = &XLogCtl->Insert;
  uint64    startbytepos;
  uint64    endbytepos;
  uint64    prevbytepos;
  uint32    size = MAXALIGN(SizeOfXLogRecord);
  XLogRecPtr  ptr;
  uint32    segleft;

  /*
   * These calculations are a bit heavy-weight to be done while holding a
   * spinlock, but since we're holding all the WAL insertion locks, there
   * are no other inserters competing for it. GetXLogInsertRecPtr() does
   * compete for it, but that's not called very frequently.
   */
  SpinLockAcquire(&Insert->insertpos_lck);

  startbytepos = Insert->CurrBytePos;

  ptr = XLogBytePosToEndRecPtr(startbytepos);
  if (XLogSegmentOffset(ptr, wal_segment_size) == 0)
  {
    SpinLockRelease(&Insert->insertpos_lck);
    *EndPos = *StartPos = ptr;
    return false;
  }

  endbytepos = startbytepos + size;
  prevbytepos = Insert->PrevBytePos;

  *StartPos = XLogBytePosToRecPtr(startbytepos);
  *EndPos = XLogBytePosToEndRecPtr(endbytepos);

  segleft = wal_segment_size - XLogSegmentOffset(*EndPos, wal_segment_size);
  if (segleft != wal_segment_size)
  {
    /* consume the rest of the segment */
    *EndPos += segleft;
    endbytepos = XLogRecPtrToBytePos(*EndPos);
  }
  Insert->CurrBytePos = endbytepos;
  Insert->PrevBytePos = startbytepos;

  SpinLockRelease(&Insert->insertpos_lck);

  *PrevPtr = XLogBytePosToRecPtr(prevbytepos);

  Assert(XLogSegmentOffset(*EndPos, wal_segment_size) == 0);
  Assert(XLogRecPtrToBytePos(*EndPos) == endbytepos);
  Assert(XLogRecPtrToBytePos(*StartPos) == startbytepos);
  Assert(XLogRecPtrToBytePos(*PrevPtr) == prevbytepos);

  return true;
}

/*
 * Checks whether the current buffer page and backup page stored in the
 * WAL record are consistent or not. Before comparing the two pages, a
 * masking can be applied to the pages to ignore certain areas like hint bits,
 * unused space between pd_lower and pd_upper among other things. This
 * function should be called once WAL replay has been completed for a
 * given record.
 */
static void
checkXLogConsistency(XLogReaderState *record)
{
  RmgrId    rmid = XLogRecGetRmid(record);
  RelFileNode rnode;
  ForkNumber  forknum;
  BlockNumber blkno;
  int     block_id;

  /* Records with no backup blocks have no need for consistency checks. */
  if (!XLogRecHasAnyBlockRefs(record))
    return;

  Assert((XLogRecGetInfo(record) & XLR_CHECK_CONSISTENCY) != 0);

  for (block_id = 0; block_id <= record->max_block_id; block_id++)
  {
    Buffer    buf;
    Page    page;

    if (!XLogRecGetBlockTag(record, block_id, &rnode, &forknum, &blkno))
    {
      /*
       * WAL record doesn't contain a block reference with the given id.
       * Do nothing.
       */
      continue;
    }

    Assert(XLogRecHasBlockImage(record, block_id));

    if (XLogRecBlockImageApply(record, block_id))
    {
      /*
       * WAL record has already applied the page, so bypass the
       * consistency check as that would result in comparing the full
       * page stored in the record with itself.
       */
      continue;
    }

    /*
     * Read the contents from the current buffer and store it in a
     * temporary page.
     */
    buf = XLogReadBufferExtended(rnode, forknum, blkno,
                   RBM_NORMAL_NO_LOG);
    if (!BufferIsValid(buf))
      continue;

    LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
    page = BufferGetPage(buf);

    /*
     * Take a copy of the local page where WAL has been applied to have a
     * comparison base before masking it...
     */
    memcpy(replay_image_masked, page, BLCKSZ);

    /* No need for this page anymore now that a copy is in. */
    UnlockReleaseBuffer(buf);

    /*
     * If the block LSN is already ahead of this WAL record, we can't
     * expect contents to match.  This can happen if recovery is
     * restarted.
     */
    if (PageGetLSN(replay_image_masked) > record->EndRecPtr)
      continue;

    /*
     * Read the contents from the backup copy, stored in WAL record and
     * store it in a temporary page. There is no need to allocate a new
     * page here, a local buffer is fine to hold its contents and a mask
     * can be directly applied on it.
     */
    if (!RestoreBlockImage(record, block_id, master_image_masked))
      elog(ERROR, "failed to restore block image");

    /*
     * If masking function is defined, mask both the master and replay
     * images
     */
    if (RmgrTable[rmid].rm_mask != NULL)
    {
      RmgrTable[rmid].rm_mask(replay_image_masked, blkno);
      RmgrTable[rmid].rm_mask(master_image_masked, blkno);
    }

    /* Time to compare the master and replay images. */
    if (memcmp(replay_image_masked, master_image_masked, BLCKSZ) != 0)
    {
      elog(FATAL,
         "inconsistent page found, rel %u/%u/%u, forknum %u, blkno %u",
         rnode.spcNode, rnode.dbNode, rnode.relNode,
         forknum, blkno);
    }
  }
}

/*
 * Subroutine of XLogInsertRecord.  Copies a WAL record to an already-reserved
 * area in the WAL.
 */
static void
CopyXLogRecordToWAL(int write_len, bool isLogSwitch, XLogRecData *rdata,
          XLogRecPtr StartPos, XLogRecPtr EndPos)
{
  char     *currpos;
  int     freespace;
  int     written;
  XLogRecPtr  CurrPos;
  XLogPageHeader pagehdr;

  /*
   * Get a pointer to the right place in the right WAL buffer to start
   * inserting to.
   */
  CurrPos = StartPos;
  currpos = GetXLogBuffer(CurrPos);
  freespace = INSERT_FREESPACE(CurrPos);

  /*
   * there should be enough space for at least the first field (xl_tot_len)
   * on this page.
   */
  Assert(freespace >= sizeof(uint32));

  /* Copy record data */
  written = 0;
  while (rdata != NULL)
  {
    char     *rdata_data = rdata->data;
    int     rdata_len = rdata->len;

    while (rdata_len > freespace)
    {
      /*
       * Write what fits on this page, and continue on the next page.
       */
      Assert(CurrPos % XLOG_BLCKSZ >= SizeOfXLogShortPHD || freespace == 0);
      memcpy(currpos, rdata_data, freespace);
      rdata_data += freespace;
      rdata_len -= freespace;
      written += freespace;
      CurrPos += freespace;

      /*
       * Get pointer to beginning of next page, and set the xlp_rem_len
       * in the page header. Set XLP_FIRST_IS_CONTRECORD.
       *
       * It's safe to set the contrecord flag and xlp_rem_len without a
       * lock on the page. All the other flags were already set when the
       * page was initialized, in AdvanceXLInsertBuffer, and we're the
       * only backend that needs to set the contrecord flag.
       */
      currpos = GetXLogBuffer(CurrPos);
      pagehdr = (XLogPageHeader) currpos;
      pagehdr->xlp_rem_len = write_len - written;
      pagehdr->xlp_info |= XLP_FIRST_IS_CONTRECORD;

      /* skip over the page header */
      if (XLogSegmentOffset(CurrPos, wal_segment_size) == 0)
      {
        CurrPos += SizeOfXLogLongPHD;
        currpos += SizeOfXLogLongPHD;
      }
      else
      {
        CurrPos += SizeOfXLogShortPHD;
        currpos += SizeOfXLogShortPHD;
      }
      freespace = INSERT_FREESPACE(CurrPos);
    }

    Assert(CurrPos % XLOG_BLCKSZ >= SizeOfXLogShortPHD || rdata_len == 0);
    memcpy(currpos, rdata_data, rdata_len);
    currpos += rdata_len;
    CurrPos += rdata_len;
    freespace -= rdata_len;
    written += rdata_len;

    rdata = rdata->next;
  }
  Assert(written == write_len);

  /*
   * If this was an xlog-switch, it's not enough to write the switch record,
   * we also have to consume all the remaining space in the WAL segment.  We
   * have already reserved that space, but we need to actually fill it.
   */
  if (isLogSwitch && XLogSegmentOffset0(CurrPos, wal_segment_size) != 00)
  {
    /* An xlog-switch record doesn't contain any data besides the header */
    Assert(write_len == SizeOfXLogRecord);

    /* Assert that we did reserve the right amount of space */
    Assert(XLogSegmentOffset(EndPos, wal_segment_size) == 0);

    /* Use up all the remaining space on the current page */
    CurrPos += freespace;

    /*
     * Cause all remaining pages in the segment to be flushed, leaving the
     * XLog position where it should be, at the start of the next segment.
     * We do this one page at a time, to make sure we don't deadlock
     * against ourselves if wal_buffers < wal_segment_size.
     */
    while (CurrPos < EndPos)
    {
      /*
       * The minimal action to flush the page would be to call
       * WALInsertLockUpdateInsertingAt(CurrPos) followed by
       * AdvanceXLInsertBuffer(...).  The page would be left initialized
       * mostly to zeros, except for the page header (always the short
       * variant, as this is never a segment's first page).
       *
       * The large vistas of zeros are good for compressibility, but the
       * headers interrupting them every XLOG_BLCKSZ (with values that
       * differ from page to page) are not.  The effect varies with
       * compression tool, but bzip2 for instance compresses about an
       * order of magnitude worse if those headers are left in place.
       *
       * Rather than complicating AdvanceXLInsertBuffer itself (which is
       * called in heavily-loaded circumstances as well as this lightly-
       * loaded one) with variant behavior, we just use GetXLogBuffer
       * (which itself calls the two methods we need) to get the pointer
       * and zero most of the page.  Then we just zero the page header.
       */
      currpos = GetXLogBuffer(CurrPos);
      MemSet(currpos, 0, SizeOfXLogShortPHD);

      CurrPos += XLOG_BLCKSZ;
    }
  }
  else
  {
    /* Align the end position, so that the next record starts aligned */
    CurrPos = MAXALIGN64(CurrPos);
  }

  if (CurrPos != EndPos)
    elog(PANIC, "space reserved for WAL record does not match what was written");
}

/*
 * Acquire a WAL insertion lock, for inserting to WAL.
 */
static void
WALInsertLockAcquire(void)
{
  bool    immed;

  /*
   * It doesn't matter which of the WAL insertion locks we acquire, so try
   * the one we used last time.  If the system isn't particularly busy, it's
   * a good bet that it's still available, and it's good to have some
   * affinity to a particular lock so that you don't unnecessarily bounce
   * cache lines between processes when there's no contention.
   *
   * If this is the first time through in this backend, pick a lock
   * (semi-)randomly.  This allows the locks to be used evenly if you have a
   * lot of very short connections.
   */
  static int  lockToTry = -1;

  if (lockToTry == -1)
    lockToTry = MyProc->pgprocno % NUM_XLOGINSERT_LOCKS;
  MyLockNo = lockToTry;

  /*
   * The insertingAt value is initially set to 0, as we don't know our
   * insert location yet.
   */
  immed = LWLockAcquire(&WALInsertLocks[MyLockNo].l.lock, LW_EXCLUSIVE);
  if (!immed)
  {
    /*
     * If we couldn't get the lock immediately, try another lock next
     * time.  On a system with more insertion locks than concurrent
     * inserters, this causes all the inserters to eventually migrate to a
     * lock that no-one else is using.  On a system with more inserters
     * than locks, it still helps to distribute the inserters evenly
     * across the locks.
     */
    lockToTry = (lockToTry + 1) % NUM_XLOGINSERT_LOCKS;
  }
}

/*
 * Acquire all WAL insertion locks, to prevent other backends from inserting
 * to WAL.
 */
static void
WALInsertLockAcquireExclusive(void)
{
  int     i;

  /*
   * When holding all the locks, all but the last lock's insertingAt
   * indicator is set to 0xFFFFFFFFFFFFFFFF, which is higher than any real
   * XLogRecPtr value, to make sure that no-one blocks waiting on those.
   */
  for (i = 0; i < NUM_XLOGINSERT_LOCKS - 1; i++18.0k)
  {
    LWLockAcquire(&WALInsertLocks[i].l.lock, LW_EXCLUSIVE);
    LWLockUpdateVar(&WALInsertLocks[i].l.lock,
            &WALInsertLocks[i].l.insertingAt,
            PG_UINT64_MAX);
  }
  /* Variable value reset to 0 at release */
  LWLockAcquire(&WALInsertLocks[i].l.lock, LW_EXCLUSIVE);

  holdingAllLocks = true;
}

/*
 * Release our insertion lock (or locks, if we're holding them all).
 *
 * NB: Reset all variables to 0, so they cause LWLockWaitForVar to block the
 * next time the lock is acquired.
 */
static void
WALInsertLockRelease(void)
{
  if (holdingAllLocks)
  {
    int     i;

    for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++20.5k)
      LWLockReleaseClearVar(&WALInsertLocks[i].l.lock,
                  &WALInsertLocks[i].l.insertingAt,
                  0);

    holdingAllLocks = false;
  }
  else
  {
    LWLockReleaseClearVar(&WALInsertLocks[MyLockNo].l.lock,
                &WALInsertLocks[MyLockNo].l.insertingAt,
                0);
  }
}

/*
 * Update our insertingAt value, to let others know that we've finished
 * inserting up to that point.
 */
static void
WALInsertLockUpdateInsertingAt(XLogRecPtr insertingAt)
{
  if (holdingAllLocks)
  {
    /*
     * We use the last lock to mark our actual position, see comments in
     * WALInsertLockAcquireExclusive.
     */
    LWLockUpdateVar(&WALInsertLocks[NUM_XLOGINSERT_LOCKS - 1].l.lock,
            &WALInsertLocks[NUM_XLOGINSERT_LOCKS - 1].l.insertingAt,
            insertingAt);
  }
  else
    LWLockUpdateVar(&WALInsertLocks[MyLockNo].l.lock,
            &WALInsertLocks[MyLockNo].l.insertingAt,
            insertingAt);
}

/*
 * Wait for any WAL insertions < upto to finish.
 *
 * Returns the location of the oldest insertion that is still in-progress.
 * Any WAL prior to that point has been fully copied into WAL buffers, and
 * can be flushed out to disk. Because this waits for any insertions older
 * than 'upto' to finish, the return value is always >= 'upto'.
 *
 * Note: When you are about to write out WAL, you must call this function
 * *before* acquiring WALWriteLock, to avoid deadlocks. This function might
 * need to wait for an insertion to finish (or at least advance to next
 * uninitialized page), and the inserter might need to evict an old WAL buffer
 * to make room for a new one, which in turn requires WALWriteLock.
 */
static XLogRecPtr
WaitXLogInsertionsToFinish(XLogRecPtr upto)
{
  uint64    bytepos;
  XLogRecPtr  reservedUpto;
  XLogRecPtr  finishedUpto;
  XLogCtlInsert *Insert = &XLogCtl->Insert;
  int     i;

  if (MyProc == NULL)
    elog(PANIC, "cannot wait without a PGPROC structure");

  /* Read the current insert position */
  SpinLockAcquire(&Insert->insertpos_lck);
  bytepos = Insert->CurrBytePos;
  SpinLockRelease(&Insert->insertpos_lck);
  reservedUpto = XLogBytePosToEndRecPtr(bytepos);

  /*
   * No-one should request to flush a piece of WAL that hasn't even been
   * reserved yet. However, it can happen if there is a block with a bogus
   * LSN on disk, for example. XLogFlush checks for that situation and
   * complains, but only after the flush. Here we just assume that to mean
   * that all WAL that has been reserved needs to be finished. In this
   * corner-case, the return value can be smaller than 'upto' argument.
   */
  if (upto > reservedUpto)
  {
    elog(LOG, "request to flush past end of generated WAL; request %X/%X, currpos %X/%X",
       (uint32) (upto >> 32), (uint32) upto,
       (uint32) (reservedUpto >> 32), (uint32) reservedUpto);
    upto = reservedUpto;
  }

  /*
   * Loop through all the locks, sleeping on any in-progress insert older
   * than 'upto'.
   *
   * finishedUpto is our return value, indicating the point upto which all
   * the WAL insertions have been finished. Initialize it to the head of
   * reserved WAL, and as we iterate through the insertion locks, back it
   * out for any insertion that's still in progress.
   */
  finishedUpto = reservedUpto;
  for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++19.1k)
  {
    XLogRecPtr  insertingat = InvalidXLogRecPtr;

    do
    {
      /*
       * See if this insertion is in progress. LWLockWait will wait for
       * the lock to be released, or for the 'value' to be set by a
       * LWLockUpdateVar call.  When a lock is initially acquired, its
       * value is 0 (InvalidXLogRecPtr), which means that we don't know
       * where it's inserting yet.  We will have to wait for it.  If
       * it's a small insertion, the record will most likely fit on the
       * same page and the inserter will release the lock without ever
       * calling LWLockUpdateVar.  But if it has to sleep, it will
       * advertise the insertion point with LWLockUpdateVar before
       * sleeping.
       */
      if (LWLockWaitForVar(&WALInsertLocks[i].l.lock,
                 &WALInsertLocks[i].l.insertingAt,
                 insertingat, &insertingat))
      {
        /* the lock was free, so no insertion in progress */
        insertingat = InvalidXLogRecPtr;
        break;
      }

      /*
       * This insertion is still in progress. Have to wait, unless the
       * inserter has proceeded past 'upto'.
       */
    } while (insertingat < upto0);

    if (insertingat != InvalidXLogRecPtr && insertingat < finishedUpto0)
      finishedUpto = insertingat;
  }
  return finishedUpto;
}

/*
 * Get a pointer to the right location in the WAL buffer containing the
 * given XLogRecPtr.
 *
 * If the page is not initialized yet, it is initialized. That might require
 * evicting an old dirty buffer from the buffer cache, which means I/O.
 *
 * The caller must ensure that the page containing the requested location
 * isn't evicted yet, and won't be evicted. The way to ensure that is to
 * hold onto a WAL insertion lock with the insertingAt position set to
 * something <= ptr. GetXLogBuffer() will update insertingAt if it needs
 * to evict an old page from the buffer. (This means that once you call
 * GetXLogBuffer() with a given 'ptr', you must not access anything before
 * that point anymore, and must not call GetXLogBuffer() with an older 'ptr'
 * later, because older buffers might be recycled already)
 */
static char *
GetXLogBuffer(XLogRecPtr ptr)
{
  int     idx;
  XLogRecPtr  endptr;
  static uint64 cachedPage = 0;
  static char *cachedPos = NULL;
  XLogRecPtr  expectedEndPtr;

  /*
   * Fast path for the common case that we need to access again the same
   * page as last time.
   */
  if (ptr / XLOG_BLCKSZ == cachedPage)
  {
    Assert(((XLogPageHeader) cachedPos)->xlp_magic == XLOG_PAGE_MAGIC);
    Assert(((XLogPageHeader) cachedPos)->xlp_pageaddr == ptr - (ptr % XLOG_BLCKSZ));
    return cachedPos + ptr % XLOG_BLCKSZ;
  }

  /*
   * The XLog buffer cache is organized so that a page is always loaded to a
   * particular buffer.  That way we can easily calculate the buffer a given
   * page must be loaded into, from the XLogRecPtr alone.
   */
  idx = XLogRecPtrToBufIdx(ptr);

  /*
   * See what page is loaded in the buffer at the moment. It could be the
   * page we're looking for, or something older. It can't be anything newer
   * - that would imply the page we're looking for has already been written
   * out to disk and evicted, and the caller is responsible for making sure
   * that doesn't happen.
   *
   * However, we don't hold a lock while we read the value. If someone has
   * just initialized the page, it's possible that we get a "torn read" of
   * the XLogRecPtr if 64-bit fetches are not atomic on this platform. In
   * that case we will see a bogus value. That's ok, we'll grab the mapping
   * lock (in AdvanceXLInsertBuffer) and retry if we see anything else than
   * the page we're looking for. But it means that when we do this unlocked
   * read, we might see a value that appears to be ahead of the page we're
   * looking for. Don't PANIC on that, until we've verified the value while
   * holding the lock.
   */
  expectedEndPtr = ptr;
  expectedEndPtr += XLOG_BLCKSZ - ptr % XLOG_BLCKSZ;

  endptr = XLogCtl->xlblocks[idx];
  if (expectedEndPtr != endptr)
  {
    XLogRecPtr  initializedUpto;

    /*
     * Before calling AdvanceXLInsertBuffer(), which can block, let others
     * know how far we're finished with inserting the record.
     *
     * NB: If 'ptr' points to just after the page header, advertise a
     * position at the beginning of the page rather than 'ptr' itself. If
     * there are no other insertions running, someone might try to flush
     * up to our advertised location. If we advertised a position after
     * the page header, someone might try to flush the page header, even
     * though page might actually not be initialized yet. As the first
     * inserter on the page, we are effectively responsible for making
     * sure that it's initialized, before we let insertingAt to move past
     * the page header.
     */
    if (ptr % XLOG_BLCKSZ == SizeOfXLogShortPHD &&
      XLogSegmentOffset0(ptr, wal_segment_size) > 0XLOG_BLCKSZ0)
      initializedUpto = ptr - SizeOfXLogShortPHD;
    else if (ptr % XLOG_BLCKSZ == SizeOfXLogLongPHD &&
         XLogSegmentOffset0(ptr, wal_segment_size) < 0XLOG_BLCKSZ0)
      initializedUpto = ptr - SizeOfXLogLongPHD;
    else
      initializedUpto = ptr;

    WALInsertLockUpdateInsertingAt(initializedUpto);

    AdvanceXLInsertBuffer(ptr, false);
    endptr = XLogCtl->xlblocks[idx];

    if (expectedEndPtr != endptr)
      elog(PANIC, "could not find WAL buffer for %X/%X",
         (uint32) (ptr >> 32), (uint32) ptr);
  }
  else
  {
    /*
     * Make sure the initialization of the page is visible to us, and
     * won't arrive later to overwrite the WAL data we write on the page.
     */
    pg_memory_barrier();
  }

  /*
   * Found the buffer holding this page. Return a pointer to the right
   * offset within the page.
   */
  cachedPage = ptr / XLOG_BLCKSZ;
  cachedPos = XLogCtl->pages + idx * (Size) XLOG_BLCKSZ;

  Assert(((XLogPageHeader) cachedPos)->xlp_magic == XLOG_PAGE_MAGIC);
  Assert(((XLogPageHeader) cachedPos)->xlp_pageaddr == ptr - (ptr % XLOG_BLCKSZ));

  return cachedPos + ptr % XLOG_BLCKSZ;
}

/*
 * Converts a "usable byte position" to XLogRecPtr. A usable byte position
 * is the position starting from the beginning of WAL, excluding all WAL
 * page headers.
 */
static XLogRecPtr
XLogBytePosToRecPtr(uint64 bytepos)
{
  uint64    fullsegs;
  uint64    fullpages;
  uint64    bytesleft;
  uint32    seg_offset;
  XLogRecPtr  result;

  fullsegs = bytepos / UsableBytesInSegment;
  bytesleft = bytepos % UsableBytesInSegment;

  if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
  {
    /* fits on first page of segment */
    seg_offset = bytesleft + SizeOfXLogLongPHD;
  }
  else
  {
    /* account for the first page on segment with long header */
    seg_offset = XLOG_BLCKSZ;
    bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

    fullpages = bytesleft / UsableBytesInPage;
    bytesleft = bytesleft % UsableBytesInPage;

    seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
  }

  XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, wal_segment_size, result);

  return result;
}

/*
 * Like XLogBytePosToRecPtr, but if the position is at a page boundary,
 * returns a pointer to the beginning of the page (ie. before page header),
 * not to where the first xlog record on that page would go to. This is used
 * when converting a pointer to the end of a record.
 */
static XLogRecPtr
XLogBytePosToEndRecPtr(uint64 bytepos)
{
  uint64    fullsegs;
  uint64    fullpages;
  uint64    bytesleft;
  uint32    seg_offset;
  XLogRecPtr  result;

  fullsegs = bytepos / UsableBytesInSegment;
  bytesleft = bytepos % UsableBytesInSegment;

  if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
  {
    /* fits on first page of segment */
    if (bytesleft == 0)
      seg_offset = 0;
    else
      seg_offset = bytesleft + SizeOfXLogLongPHD;
  }
  else
  {
    /* account for the first page on segment with long header */
    seg_offset = XLOG_BLCKSZ;
    bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

    fullpages = bytesleft / UsableBytesInPage;
    bytesleft = bytesleft % UsableBytesInPage;

    if (bytesleft == 0)
      seg_offset += fullpages * XLOG_BLCKSZ + bytesleft;
    else
      seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
  }

  XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, wal_segment_size, result);

  return result;
}

/*
 * Convert an XLogRecPtr to a "usable byte position".
 */
static uint64
XLogRecPtrToBytePos(XLogRecPtr ptr)
{
  uint64    fullsegs;
  uint32    fullpages;
  uint32    offset;
  uint64    result;

  XLByteToSeg(ptr, fullsegs, wal_segment_size);

  fullpages = (XLogSegmentOffset(ptr, wal_segment_size)) / XLOG_BLCKSZ;
  offset = ptr % XLOG_BLCKSZ;

  if (fullpages == 0)
  {
    result = fullsegs * UsableBytesInSegment;
    if (offset > 0)
    {
      Assert(offset >= SizeOfXLogLongPHD);
      result += offset - SizeOfXLogLongPHD;
    }
  }
  else
  {
    result = fullsegs * UsableBytesInSegment +
      (XLOG_BLCKSZ - SizeOfXLogLongPHD) + /* account for first page */
      (fullpages - 1) * UsableBytesInPage; /* full pages */
    if (offset > 0)
    {
      Assert(offset >= SizeOfXLogShortPHD);
      result += offset - SizeOfXLogShortPHD;
    }
  }

  return result;
}

/*
 * Initialize XLOG buffers, writing out old buffers if they still contain
 * unwritten data, upto the page containing 'upto'. Or if 'opportunistic' is
 * true, initialize as many pages as we can without having to write out
 * unwritten data. Any new pages are initialized to zeros, with pages headers
 * initialized properly.
 */
static void
AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
{
  XLogCtlInsert *Insert = &XLogCtl->Insert;
  int     nextidx;
  XLogRecPtr  OldPageRqstPtr;
  XLogwrtRqst WriteRqst;
  XLogRecPtr  NewPageEndPtr = InvalidXLogRecPtr;
  XLogRecPtr  NewPageBeginPtr;
  XLogPageHeader NewPage;
  int     npages = 0;

  LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);

  /*
   * Now that we have the lock, check if someone initialized the page
   * already.
   */
  while (upto >= XLogCtl->InitializedUpTo || opportunistic1)
  {
    nextidx = XLogRecPtrToBufIdx(XLogCtl->InitializedUpTo);

    /*
     * Get ending-offset of the buffer page we need to replace (this may
     * be zero if the buffer hasn't been used yet).  Fall through if it's
     * already written out.
     */
    OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
    if (LogwrtResult.Write < OldPageRqstPtr)
    {
      /*
       * Nope, got work to do. If we just want to pre-initialize as much
       * as we can without flushing, give up now.
       */
      if (opportunistic)
        break;

      /* Before waiting, get info_lck and update LogwrtResult */
      SpinLockAcquire(&XLogCtl->info_lck);
      if (XLogCtl->LogwrtRqst.Write < OldPageRqstPtr)
        XLogCtl->LogwrtRqst.Write = OldPageRqstPtr;
      LogwrtResult = XLogCtl->LogwrtResult;
      SpinLockRelease(&XLogCtl->info_lck);

      /*
       * Now that we have an up-to-date LogwrtResult value, see if we
       * still need to write it or if someone else already did.
       */
      if (LogwrtResult.Write < OldPageRqstPtr)
      {
        /*
         * Must acquire write lock. Release WALBufMappingLock first,
         * to make sure that all insertions that we need to wait for
         * can finish (up to this same position). Otherwise we risk
         * deadlock.
         */
        LWLockRelease(WALBufMappingLock);

        WaitXLogInsertionsToFinish(OldPageRqstPtr);

        LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);

        LogwrtResult = XLogCtl->LogwrtResult;
        if (LogwrtResult.Write >= OldPageRqstPtr)
        {
          /* OK, someone wrote it already */
          LWLockRelease(WALWriteLock);
        }
        else
        {
          /* Have to write it ourselves */
          TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_START();
          WriteRqst.Write = OldPageRqstPtr;
          WriteRqst.Flush = 0;
          XLogWrite(WriteRqst, false);
          LWLockRelease(WALWriteLock);
          TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
        }
        /* Re-acquire WALBufMappingLock and retry */
        LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
        continue;
      }
    }

    /*
     * Now the next buffer slot is free and we can set it up to be the
     * next output page.
     */
    NewPageBeginPtr = XLogCtl->InitializedUpTo;
    NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;

    Assert(XLogRecPtrToBufIdx(NewPageBeginPtr) == nextidx);

    NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);

    /*
     * Be sure to re-zero the buffer so that bytes beyond what we've
     * written will look like zeroes and not valid XLOG records...
     */
    MemSet((char *) NewPage, 0, XLOG_BLCKSZ);

    /*
     * Fill the new page's header
     */
    NewPage->xlp_magic = XLOG_PAGE_MAGIC;

    /* NewPage->xlp_info = 0; */  /* done by memset */
    NewPage->xlp_tli = ThisTimeLineID;
    NewPage->xlp_pageaddr = NewPageBeginPtr;

    /* NewPage->xlp_rem_len = 0; */ /* done by memset */

    /*
     * If online backup is not in progress, mark the header to indicate
     * that WAL records beginning in this page have removable backup
     * blocks.  This allows the WAL archiver to know whether it is safe to
     * compress archived WAL data by transforming full-block records into
     * the non-full-block format.  It is sufficient to record this at the
     * page level because we force a page switch (in fact a segment
     * switch) when starting a backup, so the flag will be off before any
     * records can be written during the backup.  At the end of a backup,
     * the last page will be marked as all unsafe when perhaps only part
     * is unsafe, but at worst the archiver would miss the opportunity to
     * compress a few records.
     */
    if (!Insert->forcePageWrites)
      NewPage->xlp_info |= XLP_BKP_REMOVABLE;

    /*
     * If first page of an XLOG segment file, make it a long header.
     */
    if ((XLogSegmentOffset(NewPage->xlp_pageaddr, wal_segment_size)) == 0)
    {
      XLogLongPageHeader NewLongPage = (XLogLongPageHeader) NewPage;

      NewLongPage->xlp_sysid = ControlFile->system_identifier;
      NewLongPage->xlp_seg_size = wal_segment_size;
      NewLongPage->xlp_xlog_blcksz = XLOG_BLCKSZ;
      NewPage->xlp_info |= XLP_LONG_HEADER;
    }

    /*
     * Make sure the initialization of the page becomes visible to others
     * before the xlblocks update. GetXLogBuffer() reads xlblocks without
     * holding a lock.
     */
    pg_write_barrier();

    *((volatile XLogRecPtr *) &XLogCtl->xlblocks[nextidx]) = NewPageEndPtr;

    XLogCtl->InitializedUpTo = NewPageEndPtr;

    npages++;
  }
  LWLockRelease(WALBufMappingLock);

#ifdef WAL_DEBUG
  if (XLOG_DEBUG && npages > 0)
  {
    elog(DEBUG1, "initialized %d pages, up to %X/%X",
       npages, (uint32) (NewPageEndPtr >> 32), (uint32) NewPageEndPtr);
  }
#endif
}

/*
 * Calculate CheckPointSegments based on max_wal_size_mb and
 * checkpoint_completion_target.
 */
static void
CalculateCheckpointSegments(void)
{
  double    target;

  /*-------
   * Calculate the distance at which to trigger a checkpoint, to avoid
   * exceeding max_wal_size_mb. This is based on two assumptions:
   *
   * a) we keep WAL for only one checkpoint cycle (prior to PG11 we kept
   *    WAL for two checkpoint cycles to allow us to recover from the
   *    secondary checkpoint if the first checkpoint failed, though we
   *    only did this on the master anyway, not on standby. Keeping just
   *    one checkpoint simplifies processing and reduces disk space in
   *    many smaller databases.)
   * b) during checkpoint, we consume checkpoint_completion_target *
   *    number of segments consumed between checkpoints.
   *-------
   */
  target = (double) ConvertToXSegs(max_wal_size_mb, wal_segment_size) /
    (1.0 + CheckPointCompletionTarget);

  /* round down */
  CheckPointSegments = (int) target;

  if (CheckPointSegments < 1)
    CheckPointSegments = 1;
}

void
assign_max_wal_size(int newval, void *extra)
{
  max_wal_size_mb = newval;
  CalculateCheckpointSegments();
}

void
assign_checkpoint_completion_target(double newval, void *extra)
{
  CheckPointCompletionTarget = newval;
  CalculateCheckpointSegments();
}

/*
 * At a checkpoint, how many WAL segments to recycle as preallocated future
 * XLOG segments? Returns the highest segment that should be preallocated.
 */
static XLogSegNo
XLOGfileslop(XLogRecPtr RedoRecPtr)
{
  XLogSegNo minSegNo;
  XLogSegNo maxSegNo;
  double    distance;
  XLogSegNo recycleSegNo;

  /*
   * Calculate the segment numbers that min_wal_size_mb and max_wal_size_mb
   * correspond to. Always recycle enough segments to meet the minimum, and
   * remove enough segments to stay below the maximum.
   */
  minSegNo = RedoRecPtr / wal_segment_size +
    ConvertToXSegs(min_wal_size_mb, wal_segment_size) - 1;
  maxSegNo = RedoRecPtr / wal_segment_size +
    ConvertToXSegs(max_wal_size_mb, wal_segment_size) - 1;

  /*
   * Between those limits, recycle enough segments to get us through to the
   * estimated end of next checkpoint.
   *
   * To estimate where the next checkpoint will finish, assume that the
   * system runs steadily consuming CheckPointDistanceEstimate bytes between
   * every checkpoint.
   */
  distance = (1.0 + CheckPointCompletionTarget) * CheckPointDistanceEstimate;
  /* add 10% for good measure. */
  distance *= 1.10;

  recycleSegNo = (XLogSegNo) ceil(((double) RedoRecPtr + distance) /
                  wal_segment_size);

  if (recycleSegNo < minSegNo)
    recycleSegNo = minSegNo;
  if (recycleSegNo > maxSegNo)
    recycleSegNo = maxSegNo;

  return recycleSegNo;
}

/*
 * Check whether we've consumed enough xlog space that a checkpoint is needed.
 *
 * new_segno indicates a log file that has just been filled up (or read
 * during recovery). We measure the distance from RedoRecPtr to new_segno
 * and see if that exceeds CheckPointSegments.
 *
 * Note: it is caller's responsibility that RedoRecPtr is up-to-date.
 */
static bool
XLogCheckpointNeeded(XLogSegNo new_segno)
{
  XLogSegNo old_segno;

  XLByteToSeg(RedoRecPtr, old_segno, wal_segment_size);

  if (new_segno >= old_segno + (uint64) (CheckPointSegments - 1))
    return true;
  return false;
}

/*
 * Write and/or fsync the log at least as far as WriteRqst indicates.
 *
 * If flexible == true, we don't have to write as far as WriteRqst, but
 * may stop at any convenient boundary (such as a cache or logfile boundary).
 * This option allows us to avoid uselessly issuing multiple writes when a
 * single one would do.
 *
 * Must be called with WALWriteLock held. WaitXLogInsertionsToFinish(WriteRqst)
 * must be called before grabbing the lock, to make sure the data is ready to
 * write.
 */
static void
XLogWrite(XLogwrtRqst WriteRqst, bool flexible)
{
  bool    ispartialpage;
  bool    last_iteration;
  bool    finishing_seg;
  bool    use_existent;
  int     curridx;
  int     npages;
  int     startidx;
  uint32    startoffset;

  /* We should always be inside a critical section here */
  Assert(CritSectionCount > 0);

  /*
   * Update local LogwrtResult (caller probably did this already, but...)
   */
  LogwrtResult = XLogCtl->LogwrtResult;

  /*
   * Since successive pages in the xlog cache are consecutively allocated,
   * we can usually gather multiple pages together and issue just one
   * write() call.  npages is the number of pages we have determined can be
   * written together; startidx is the cache block index of the first one,
   * and startoffset is the file offset at which it should go. The latter
   * two variables are only valid when npages > 0, but we must initialize
   * all of them to keep the compiler quiet.
   */
  npages = 0;
  startidx = 0;
  startoffset = 0;

  /*
   * Within the loop, curridx is the cache block index of the page to
   * consider writing.  Begin at the buffer containing the next unwritten
   * page, or last partially written page.
   */
  curridx = XLogRecPtrToBufIdx(LogwrtResult.Write);

  while (LogwrtResult.Write < WriteRqst.Write)
  {
    /*
     * Make sure we're not ahead of the insert process.  This could happen
     * if we're passed a bogus WriteRqst.Write that is past the end of the
     * last page that's been initialized by AdvanceXLInsertBuffer.
     */
    XLogRecPtr  EndPtr = XLogCtl->xlblocks[curridx];

    if (LogwrtResult.Write >= EndPtr)
      elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
         (uint32) (LogwrtResult.Write >> 32),
         (uint32) LogwrtResult.Write,
         (uint32) (EndPtr >> 32), (uint32) EndPtr);

    /* Advance LogwrtResult.Write to end of current buffer page */
    LogwrtResult.Write = EndPtr;
    ispartialpage = WriteRqst.Write < LogwrtResult.Write;

    if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo,
               wal_segment_size))
    {
      /*
       * Switch to new logfile segment.  We cannot have any pending
       * pages here (since we dump what we have at segment end).
       */
      Assert(npages == 0);
      if (openLogFile >= 0)
        XLogFileClose();
      XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo,
              wal_segment_size);

      /* create/use new log file */
      use_existent = true;
      openLogFile = XLogFileInit(openLogSegNo, &use_existent, true);
      openLogOff = 0;
    }

    /* Make sure we have the current logfile open */
    if (openLogFile < 0)
    {
      XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo,
              wal_segment_size);
      openLogFile = XLogFileOpen(openLogSegNo);
      openLogOff = 0;
    }

    /* Add current page to the set of pending pages-to-dump */
    if (npages == 0)
    {
      /* first of group */
      startidx = curridx;
      startoffset = XLogSegmentOffset(LogwrtResult.Write - XLOG_BLCKSZ,
                      wal_segment_size);
    }
    npages++;

    /*
     * Dump the set if this will be the last loop iteration, or if we are
     * at the last page of the cache area (since the next page won't be
     * contiguous in memory), or if we are at the end of the logfile
     * segment.
     */
    last_iteration = WriteRqst.Write <= LogwrtResult.Write;

    finishing_seg = !ispartialpage &&
      (startoffset + npages * 1XLOG_BLCKSZ1) >= wal_segment_size;

    if (last_iteration ||
      curridx == XLogCtl->XLogCacheBlck1 ||
      finishing_seg1)
    {
      char     *from;
      Size    nbytes;
      Size    nleft;
      int     written;

      /* Need to seek in the file? */
      if (openLogOff != startoffset)
      {
        if (lseek(openLogFile, (off_t) startoffset, SEEK_SET) < 0)
          ereport(PANIC,
              (errcode_for_file_access(),
               errmsg("could not seek in log file %s to offset %u: %m",
                  XLogFileNameP(ThisTimeLineID, openLogSegNo),
                  startoffset)));
        openLogOff = startoffset;
      }

      /* OK to write the page(s) */
      from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
      nbytes = npages * (Size) XLOG_BLCKSZ;
      nleft = nbytes;
      do
      {
        errno = 0;
        pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
        written = write(openLogFile, from, nleft);
        pgstat_report_wait_end();
        if (written <= 0)
        {
          if (errno == EINTR)
            continue;
          ereport(PANIC,
              (errcode_for_file_access(),
               errmsg("could not write to log file %s "
                  "at offset %u, length %zu: %m",
                  XLogFileNameP(ThisTimeLineID, openLogSegNo),
                  openLogOff, nbytes)));
        }
        nleft -= written;
        from += written;
      } while (nleft > 0);

      /* Update state for write */
      openLogOff += nbytes;
      npages = 0;

      /*
       * If we just wrote the whole last page of a logfile segment,
       * fsync the segment immediately.  This avoids having to go back
       * and re-open prior segments when an fsync request comes along
       * later. Doing it here ensures that one and only one backend will
       * perform this fsync.
       *
       * This is also the right place to notify the Archiver that the
       * segment is ready to copy to archival storage, and to update the
       * timer for archive_timeout, and to signal for a checkpoint if
       * too many logfile segments have been used since the last
       * checkpoint.
       */
      if (finishing_seg)
      {
        issue_xlog_fsync(openLogFile, openLogSegNo);

        /* signal that we need to wakeup walsenders later */
        WalSndWakeupRequest();

        LogwrtResult.Flush = LogwrtResult.Write;  /* end of page */

        if (XLogArchivingActive())
          XLogArchiveNotifySeg(openLogSegNo);

        XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);
        XLogCtl->lastSegSwitchLSN = LogwrtResult.Flush;

        /*
         * Request a checkpoint if we've consumed too much xlog since
         * the last one.  For speed, we first check using the local
         * copy of RedoRecPtr, which might be out of date; if it looks
         * like a checkpoint is needed, forcibly update RedoRecPtr and
         * recheck.
         */
        if (IsUnderPostmaster && XLogCheckpointNeeded(openLogSegNo))
        {
          (void) GetRedoRecPtr();
          if (XLogCheckpointNeeded(openLogSegNo))
            RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
        }
      }
    }

    if (ispartialpage)
    {
      /* Only asked to write a partial page */
      LogwrtResult.Write = WriteRqst.Write;
      break;
    }
    curridx = NextBufIdx(curridx);

    /* If flexible, break out of loop as soon as we wrote something */
    if (flexible && npages == 00)
      break;
  }

  Assert(npages == 0);

  /*
   * If asked to flush, do so
   */
  if (LogwrtResult.Flush < WriteRqst.Flush &&
    LogwrtResult.Flush < LogwrtResult.Write)

  {
    /*
     * Could get here without iterating above loop, in which case we might
     * have no open file or the wrong one.  However, we do not need to
     * fsync more than one file.
     */
    if (sync_method != SYNC_METHOD_OPEN &&
      sync_method != SYNC_METHOD_OPEN_DSYNC)
    {
      if (openLogFile >= 0 &&
        !XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo,
                 wal_segment_size))
        XLogFileClose();
      if (openLogFile < 0)
      {
        XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo,
                wal_segment_size);
        openLogFile = XLogFileOpen(openLogSegNo);
        openLogOff = 0;
      }

      issue_xlog_fsync(openLogFile, openLogSegNo);
    }

    /* signal that we need to wakeup walsenders later */
    WalSndWakeupRequest();

    LogwrtResult.Flush = LogwrtResult.Write;
  }

  /*
   * Update shared-memory status
   *
   * We make sure that the shared 'request' values do not fall behind the
   * 'result' values.  This is not absolutely essential, but it saves some
   * code in a couple of places.
   */
  {
    SpinLockAcquire(&XLogCtl->info_lck);
    XLogCtl->LogwrtResult = LogwrtResult;
    if (XLogCtl->LogwrtRqst.Write < LogwrtResult.Write)
      XLogCtl->LogwrtRqst.Write = LogwrtResult.Write;
    if (XLogCtl->LogwrtRqst.Flush < LogwrtResult.Flush)
      XLogCtl->LogwrtRqst.Flush = LogwrtResult.Flush;
    SpinLockRelease(&XLogCtl->info_lck);
  }
}

/*
 * Record the LSN for an asynchronous transaction commit/abort
 * and nudge the WALWriter if there is work for it to do.
 * (This should not be called for synchronous commits.)
 */
void
XLogSetAsyncXactLSN(XLogRecPtr asyncXactLSN)
{
  XLogRecPtr  WriteRqstPtr = asyncXactLSN;
  bool    sleeping;

  SpinLockAcquire(&XLogCtl->info_lck);
  LogwrtResult = XLogCtl->LogwrtResult;
  sleeping = XLogCtl->WalWriterSleeping;
  if (XLogCtl->asyncXactLSN < asyncXactLSN)
    XLogCtl->asyncXactLSN = asyncXactLSN;
  SpinLockRelease(&XLogCtl->info_lck);

  /*
   * If the WALWriter is sleeping, we should kick it to make it come out of
   * low-power mode.  Otherwise, determine whether there's a full page of
   * WAL available to write.
   */
  if (!sleeping)
  {
    /* back off to last completed page boundary */
    WriteRqstPtr -= WriteRqstPtr % XLOG_BLCKSZ;

    /* if we have already flushed that far, we're done */
    if (WriteRqstPtr <= LogwrtResult.Flush)
      return;
  }

  /*
   * Nudge the WALWriter: it has a full page of WAL to write, or we want it
   * to come out of low-power mode so that this async commit will reach disk
   * within the expected amount of time.
   */
  if (ProcGlobal->walwriterLatch)
    SetLatch(ProcGlobal->walwriterLatch);
}

/*
 * Record the LSN up to which we can remove WAL because it's not required by
 * any replication slot.
 */
void
XLogSetReplicationSlotMinimumLSN(XLogRecPtr lsn)
{
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->replicationSlotMinLSN = lsn;
  SpinLockRelease(&XLogCtl->info_lck);
}


/*
 * Return the oldest LSN we must retain to satisfy the needs of some
 * replication slot.
 */
static XLogRecPtr
XLogGetReplicationSlotMinimumLSN(void)
{
  XLogRecPtr  retval;

  SpinLockAcquire(&XLogCtl->info_lck);
  retval = XLogCtl->replicationSlotMinLSN;
  SpinLockRelease(&XLogCtl->info_lck);

  return retval;
}

/*
 * Advance minRecoveryPoint in control file.
 *
 * If we crash during recovery, we must reach this point again before the
 * database is consistent.
 *
 * If 'force' is true, 'lsn' argument is ignored. Otherwise, minRecoveryPoint
 * is only updated if it's not already greater than or equal to 'lsn'.
 */
static void
UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force)
{
  /* Quick check using our local copy of the variable */
  if (!updateMinRecoveryPoint || (!force && lsn <= minRecoveryPoint))
    return;

  /*
   * An invalid minRecoveryPoint means that we need to recover all the WAL,
   * i.e., we're doing crash recovery.  We never modify the control file's
   * value in that case, so we can short-circuit future checks here too. The
   * local values of minRecoveryPoint and minRecoveryPointTLI should not be
   * updated until crash recovery finishes.  We only do this for the startup
   * process as it should not update its own reference of minRecoveryPoint
   * until it has finished crash recovery to make sure that all WAL
   * available is replayed in this case.  This also saves from extra locks
   * taken on the control file from the startup process.
   */
  if (XLogRecPtrIsInvalid(minRecoveryPoint) && InRecovery)
  {
    updateMinRecoveryPoint = false;
    return;
  }

  LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

  /* update local copy */
  minRecoveryPoint = ControlFile->minRecoveryPoint;
  minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

  if (XLogRecPtrIsInvalid(minRecoveryPoint))
    updateMinRecoveryPoint = false;
  else if (force || minRecoveryPoint < lsn)
  {
    XLogRecPtr  newMinRecoveryPoint;
    TimeLineID  newMinRecoveryPointTLI;

    /*
     * To avoid having to update the control file too often, we update it
     * all the way to the last record being replayed, even though 'lsn'
     * would suffice for correctness.  This also allows the 'force' case
     * to not need a valid 'lsn' value.
     *
     * Another important reason for doing it this way is that the passed
     * 'lsn' value could be bogus, i.e., past the end of available WAL, if
     * the caller got it from a corrupted heap page.  Accepting such a
     * value as the min recovery point would prevent us from coming up at
     * all.  Instead, we just log a warning and continue with recovery.
     * (See also the comments about corrupt LSNs in XLogFlush.)
     */
    SpinLockAcquire(&XLogCtl->info_lck);
    newMinRecoveryPoint = XLogCtl->replayEndRecPtr;
    newMinRecoveryPointTLI = XLogCtl->replayEndTLI;
    SpinLockRelease(&XLogCtl->info_lck);

    if (!force && newMinRecoveryPoint < lsn)
      elog(WARNING,
         "xlog min recovery request %X/%X is past current point %X/%X",
         (uint32) (lsn >> 32), (uint32) lsn,
         (uint32) (newMinRecoveryPoint >> 32),
         (uint32) newMinRecoveryPoint);

    /* update control file */
    if (ControlFile->minRecoveryPoint < newMinRecoveryPoint)
    {
      ControlFile->minRecoveryPoint = newMinRecoveryPoint;
      ControlFile->minRecoveryPointTLI = newMinRecoveryPointTLI;
      UpdateControlFile();
      minRecoveryPoint = newMinRecoveryPoint;
      minRecoveryPointTLI = newMinRecoveryPointTLI;

      ereport(DEBUG2,
          (errmsg("updated min recovery point to %X/%X on timeline %u",
              (uint32) (minRecoveryPoint >> 32),
              (uint32) minRecoveryPoint,
              newMinRecoveryPointTLI)));
    }
  }
  LWLockRelease(ControlFileLock);
}

/*
 * Ensure that all XLOG data through the given position is flushed to disk.
 *
 * NOTE: this differs from XLogWrite mainly in that the WALWriteLock is not
 * already held, and we try to avoid acquiring it if possible.
 */
void
XLogFlush(XLogRecPtr record)
{
  XLogRecPtr  WriteRqstPtr;
  XLogwrtRqst WriteRqst;

  /*
   * During REDO, we are reading not writing WAL.  Therefore, instead of
   * trying to flush the WAL, we should update minRecoveryPoint instead. We
   * test XLogInsertAllowed(), not InRecovery, because we need checkpointer
   * to act this way too, and because when it tries to write the
   * end-of-recovery checkpoint, it should indeed flush.
   */
  if (!XLogInsertAllowed())
  {
    UpdateMinRecoveryPoint(record, false);
    return;
  }

  /* Quick exit if already known flushed */
  if (record <= LogwrtResult.Flush)
    return;

#ifdef WAL_DEBUG
  if (XLOG_DEBUG)
    elog(LOG, "xlog flush request %X/%X; write %X/%X; flush %X/%X",
       (uint32) (record >> 32), (uint32) record,
       (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
       (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
#endif

  START_CRIT_SECTION();

  /*
   * Since fsync is usually a horribly expensive operation, we try to
   * piggyback as much data as we can on each fsync: if we see any more data
   * entered into the xlog buffer, we'll write and fsync that too, so that
   * the final value of LogwrtResult.Flush is as large as possible. This
   * gives us some chance of avoiding another fsync immediately after.
   */

  /* initialize to given target; may increase below */
  WriteRqstPtr = record;

  /*
   * Now wait until we get the write lock, or someone else does the flush
   * for us.
   */
  for (;;)
  {
    XLogRecPtr  insertpos;

    /* read LogwrtResult and update local state */
    SpinLockAcquire(&XLogCtl->info_lck);
    if (WriteRqstPtr < XLogCtl->LogwrtRqst.Write)
      WriteRqstPtr = XLogCtl->LogwrtRqst.Write;
    LogwrtResult = XLogCtl->LogwrtResult;
    SpinLockRelease(&XLogCtl->info_lck);

    /* done already? */
    if (record <= LogwrtResult.Flush)
      break;

    /*
     * Before actually performing the write, wait for all in-flight
     * insertions to the pages we're about to write to finish.
     */
    insertpos = WaitXLogInsertionsToFinish(WriteRqstPtr);

    /*
     * Try to get the write lock. If we can't get it immediately, wait
     * until it's released, and recheck if we still need to do the flush
     * or if the backend that held the lock did it for us already. This
     * helps to maintain a good rate of group committing when the system
     * is bottlenecked by the speed of fsyncing.
     */
    if (!LWLockAcquireOrWait(WALWriteLock, LW_EXCLUSIVE))
    {
      /*
       * The lock is now free, but we didn't acquire it yet. Before we
       * do, loop back to check if someone else flushed the record for
       * us already.
       */
      continue;
    }

    /* Got the lock; recheck whether request is satisfied */
    LogwrtResult = XLogCtl->LogwrtResult;
    if (record <= LogwrtResult.Flush)
    {
      LWLockRelease(WALWriteLock);
      break;
    }

    /*
     * Sleep before flush! By adding a delay here, we may give further
     * backends the opportunity to join the backlog of group commit
     * followers; this can significantly improve transaction throughput,
     * at the risk of increasing transaction latency.
     *
     * We do not sleep if enableFsync is not turned on, nor if there are
     * fewer than CommitSiblings other backends with active transactions.
     */
    if (CommitDelay > 0 && enableFsync0 &&
      MinimumActiveBackends(CommitSiblings)0)
    {
      pg_usleep(CommitDelay);

      /*
       * Re-check how far we can now flush the WAL. It's generally not
       * safe to call WaitXLogInsertionsToFinish while holding
       * WALWriteLock, because an in-progress insertion might need to
       * also grab WALWriteLock to make progress. But we know that all
       * the insertions up to insertpos have already finished, because
       * that's what the earlier WaitXLogInsertionsToFinish() returned.
       * We're only calling it again to allow insertpos to be moved
       * further forward, not to actually wait for anyone.
       */
      insertpos = WaitXLogInsertionsToFinish(insertpos);
    }

    /* try to write/flush later additions to XLOG as well */
    WriteRqst.Write = insertpos;
    WriteRqst.Flush = insertpos;

    XLogWrite(WriteRqst, false);

    LWLockRelease(WALWriteLock);
    /* done */
    break;
  }

  END_CRIT_SECTION();

  /* wake up walsenders now that we've released heavily contended locks */
  WalSndWakeupProcessRequests();

  /*
   * If we still haven't flushed to the request point then we have a
   * problem; most likely, the requested flush point is past end of XLOG.
   * This has been seen to occur when a disk page has a corrupted LSN.
   *
   * Formerly we treated this as a PANIC condition, but that hurts the
   * system's robustness rather than helping it: we do not want to take down
   * the whole system due to corruption on one data page.  In particular, if
   * the bad page is encountered again during recovery then we would be
   * unable to restart the database at all!  (This scenario actually
   * happened in the field several times with 7.1 releases.)  As of 8.4, bad
   * LSNs encountered during recovery are UpdateMinRecoveryPoint's problem;
   * the only time we can reach here during recovery is while flushing the
   * end-of-recovery checkpoint record, and we don't expect that to have a
   * bad LSN.
   *
   * Note that for calls from xact.c, the ERROR will be promoted to PANIC
   * since xact.c calls this routine inside a critical section.  However,
   * calls from bufmgr.c are not within critical sections and so we will not
   * force a restart for a bad LSN on a data page.
   */
  if (LogwrtResult.Flush < record)
    elog(ERROR,
       "xlog flush request %X/%X is not satisfied --- flushed only to %X/%X",
       (uint32) (record >> 32), (uint32) record,
       (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
}

/*
 * Write & flush xlog, but without specifying exactly where to.
 *
 * We normally write only completed blocks; but if there is nothing to do on
 * that basis, we check for unwritten async commits in the current incomplete
 * block, and write through the latest one of those.  Thus, if async commits
 * are not being used, we will write complete blocks only.
 *
 * If, based on the above, there's anything to write we do so immediately. But
 * to avoid calling fsync, fdatasync et. al. at a rate that'd impact
 * concurrent IO, we only flush WAL every wal_writer_delay ms, or if there's
 * more than wal_writer_flush_after unflushed blocks.
 *
 * We can guarantee that async commits reach disk after at most three
 * wal_writer_delay cycles. (When flushing complete blocks, we allow XLogWrite
 * to write "flexibly", meaning it can stop at the end of the buffer ring;
 * this makes a difference only with very high load or long wal_writer_delay,
 * but imposes one extra cycle for the worst case for async commits.)
 *
 * This routine is invoked periodically by the background walwriter process.
 *
 * Returns true if there was any work to do, even if we skipped flushing due
 * to wal_writer_delay/wal_writer_flush_after.
 */
bool
XLogBackgroundFlush(void)
{
  XLogwrtRqst WriteRqst;
  bool    flexible = true;
  static TimestampTz lastflush;
  TimestampTz now;
  int     flushbytes;

  /* XLOG doesn't need flushing during recovery */
  if (RecoveryInProgress())
    return false;

  /* read LogwrtResult and update local state */
  SpinLockAcquire(&XLogCtl->info_lck);
  LogwrtResult = XLogCtl->LogwrtResult;
  WriteRqst = XLogCtl->LogwrtRqst;
  SpinLockRelease(&XLogCtl->info_lck);

  /* back off to last completed page boundary */
  WriteRqst.Write -= WriteRqst.Write % XLOG_BLCKSZ;

  /* if we have already flushed that far, consider async commit records */
  if (WriteRqst.Write <= LogwrtResult.Flush)
  {
    SpinLockAcquire(&XLogCtl->info_lck);
    WriteRqst.Write = XLogCtl->asyncXactLSN;
    SpinLockRelease(&XLogCtl->info_lck);
    flexible = false;   /* ensure it all gets written */
  }

  /*
   * If already known flushed, we're done. Just need to check if we are
   * holding an open file handle to a logfile that's no longer in use,
   * preventing the file from being deleted.
   */
  if (WriteRqst.Write <= LogwrtResult.Flush)
  {
    if (openLogFile >= 0)
    {
      if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo,
                 wal_segment_size))
      {
        XLogFileClose();
      }
    }
    return false;
  }

  /*
   * Determine how far to flush WAL, based on the wal_writer_delay and
   * wal_writer_flush_after GUCs.
   */
  now = GetCurrentTimestamp();
  flushbytes =
    WriteRqst.Write / XLOG_BLCKSZ - LogwrtResult.Flush / XLOG_BLCKSZ;

  if (WalWriterFlushAfter == 0 || lastflush == 0)
  {
    /* first call, or block based limits disabled */
    WriteRqst.Flush = WriteRqst.Write;
    lastflush = now;
  }
  else if (TimestampDifferenceExceeds(lastflush, now, WalWriterDelay))
  {
    /*
     * Flush the writes at least every WalWriteDelay ms. This is important
     * to bound the amount of time it takes for an asynchronous commit to
     * hit disk.
     */
    WriteRqst.Flush = WriteRqst.Write;
    lastflush = now;
  }
  else if (flushbytes >= WalWriterFlushAfter)
  {
    /* exceeded wal_writer_flush_after blocks, flush */
    WriteRqst.Flush = WriteRqst.Write;
    lastflush = now;
  }
  else
  {
    /* no flushing, this time round */
    WriteRqst.Flush = 0;
  }

#ifdef WAL_DEBUG
  if (XLOG_DEBUG)
    elog(LOG, "xlog bg flush request write %X/%X; flush: %X/%X, current is write %X/%X; flush %X/%X",
       (uint32) (WriteRqst.Write >> 32), (uint32) WriteRqst.Write,
       (uint32) (WriteRqst.Flush >> 32), (uint32) WriteRqst.Flush,
       (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
       (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
#endif

  START_CRIT_SECTION();

  /* now wait for any in-progress insertions to finish and get write lock */
  WaitXLogInsertionsToFinish(WriteRqst.Write);
  LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
  LogwrtResult = XLogCtl->LogwrtResult;
  if (WriteRqst.Write > LogwrtResult.Write ||
    WriteRqst.Flush > LogwrtResult.Flush)
  {
    XLogWrite(WriteRqst, flexible);
  }
  LWLockRelease(WALWriteLock);

  END_CRIT_SECTION();

  /* wake up walsenders now that we've released heavily contended locks */
  WalSndWakeupProcessRequests();

  /*
   * Great, done. To take some work off the critical path, try to initialize
   * as many of the no-longer-needed WAL buffers for future use as we can.
   */
  AdvanceXLInsertBuffer(InvalidXLogRecPtr, true);

  /*
   * If we determined that we need to write data, but somebody else
   * wrote/flushed already, it should be considered as being active, to
   * avoid hibernating too early.
   */
  return true;
}

/*
 * Test whether XLOG data has been flushed up to (at least) the given position.
 *
 * Returns true if a flush is still needed.  (It may be that someone else
 * is already in process of flushing that far, however.)
 */
bool
XLogNeedsFlush(XLogRecPtr record)
{
  /*
   * During recovery, we don't flush WAL but update minRecoveryPoint
   * instead. So "needs flush" is taken to mean whether minRecoveryPoint
   * would need to be updated.
   */
  if (RecoveryInProgress())
  {
    /*
     * An invalid minRecoveryPoint means that we need to recover all the
     * WAL, i.e., we're doing crash recovery.  We never modify the control
     * file's value in that case, so we can short-circuit future checks
     * here too.  This triggers a quick exit path for the startup process,
     * which cannot update its local copy of minRecoveryPoint as long as
     * it has not replayed all WAL available when doing crash recovery.
     */
    if (XLogRecPtrIsInvalid(minRecoveryPoint) && InRecovery)
      updateMinRecoveryPoint = false;

    /* Quick exit if already known to be updated or cannot be updated */
    if (record <= minRecoveryPoint || !updateMinRecoveryPoint)
      return false;

    /*
     * Update local copy of minRecoveryPoint. But if the lock is busy,
     * just return a conservative guess.
     */
    if (!LWLockConditionalAcquire(ControlFileLock, LW_SHARED))
      return true;
    minRecoveryPoint = ControlFile->minRecoveryPoint;
    minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
    LWLockRelease(ControlFileLock);

    /*
     * Check minRecoveryPoint for any other process than the startup
     * process doing crash recovery, which should not update the control
     * file value if crash recovery is still running.
     */
    if (XLogRecPtrIsInvalid(minRecoveryPoint))
      updateMinRecoveryPoint = false;

    /* check again */
    if (record <= minRecoveryPoint || !updateMinRecoveryPoint)
      return false;
    else
      return true;
  }

  /* Quick exit if already known flushed */
  if (record <= LogwrtResult.Flush)
    return false;

  /* read LogwrtResult and update local state */
  SpinLockAcquire(&XLogCtl->info_lck);
  LogwrtResult = XLogCtl->LogwrtResult;
  SpinLockRelease(&XLogCtl->info_lck);

  /* check again */
  if (record <= LogwrtResult.Flush)
    return false;

  return true;
}

/*
 * Create a new XLOG file segment, or open a pre-existing one.
 *
 * log, seg: identify segment to be created/opened.
 *
 * *use_existent: if true, OK to use a pre-existing file (else, any
 * pre-existing file will be deleted).  On return, true if a pre-existing
 * file was used.
 *
 * use_lock: if true, acquire ControlFileLock while moving file into
 * place.  This should be true except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns FD of opened file.
 *
 * Note: errors here are ERROR not PANIC because we might or might not be
 * inside a critical section (eg, during checkpoint there is no reason to
 * take down the system on failure).  They will promote to PANIC if we are
 * in a critical section.
 */
int
XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
{
  char    path[MAXPGPATH];
  char    tmppath[MAXPGPATH];
  PGAlignedXLogBlock zbuffer;
  XLogSegNo installed_segno;
  XLogSegNo max_segno;
  int     fd;
  int     nbytes;

  XLogFilePath(path, ThisTimeLineID, logsegno, wal_segment_size);

  /*
   * Try to use existent file (checkpoint maker may have created it already)
   */
  if (*use_existent)
  {
    fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method));
    if (fd < 0)
    {
      if (errno != ENOENT)
        ereport(ERROR,
            (errcode_for_file_access(),
             errmsg("could not open file \"%s\": %m", path)));
    }
    else
      return fd;
  }

  /*
   * Initialize an empty (all zeroes) segment.  NOTE: it is possible that
   * another process is doing the same thing.  If so, we will end up
   * pre-creating an extra log segment.  That seems OK, and better than
   * holding the lock throughout this lengthy process.
   */
  elog(DEBUG2, "creating and filling new WAL file");

  snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

  unlink(tmppath);

  /* do not use get_sync_bit() here --- want to fsync only at end of fill */
  fd = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
  if (fd < 0)
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("could not create file \"%s\": %m", tmppath)));

  /*
   * Zero-fill the file.  We have to do this the hard way to ensure that all
   * the file space has really been allocated --- on platforms that allow
   * "holes" in files, just seeking to the end doesn't allocate intermediate
   * space.  This way, we know that we have all the space and (after the
   * fsync below) that all the indirect blocks are down on disk.  Therefore,
   * fdatasync(2) or O_DSYNC will be sufficient to sync future writes to the
   * log file.
   */
  memset(zbuffer.data, 0, XLOG_BLCKSZ);
  for (nbytes = 0; nbytes < wal_segment_size; nbytes += 4.10MXLOG_BLCKSZ4.10M)
  {
    errno = 0;
    pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE);
    if ((int) write(fd, zbuffer.data, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
    {
      int     save_errno = errno;

      /*
       * If we fail to make the file, delete it to release disk space
       */
      unlink(tmppath);

      close(fd);

      /* if write didn't set errno, assume problem is no disk space */
      errno = save_errno ? save_errno : ENOSPC;

      ereport(ERROR,
          (errcode_for_file_access(),
           errmsg("could not write to file \"%s\": %m", tmppath)));
    }
    pgstat_report_wait_end();
  }

  pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_SYNC);
  if (pg_fsync(fd) != 0)
  {
    int     save_errno = errno;

    close(fd);
    errno = save_errno;
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("could not fsync file \"%s\": %m", tmppath)));
  }
  pgstat_report_wait_end();

  if (close(fd))
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("could not close file \"%s\": %m", tmppath)));

  /*
   * Now move the segment into place with its final name.
   *
   * If caller didn't want to use a pre-existing file, get rid of any
   * pre-existing file.  Otherwise, cope with possibility that someone else
   * has created the file while we were filling ours: if so, use ours to
   * pre-create a future log segment.
   */
  installed_segno = logsegno;

  /*
   * XXX: What should we use as max_segno? We used to use XLOGfileslop when
   * that was a constant, but that was always a bit dubious: normally, at a
   * checkpoint, XLOGfileslop was the offset from the checkpoint record, but
   * here, it was the offset from the insert location. We can't do the
   * normal XLOGfileslop calculation here because we don't have access to
   * the prior checkpoint's redo location. So somewhat arbitrarily, just use
   * CheckPointSegments.
   */
  max_segno = logsegno + CheckPointSegments;
  if (!InstallXLogFileSegment(&installed_segno, tmppath,
                *use_existent, max_segno,
                use_lock))
  {
    /*
     * No need for any more future segments, or InstallXLogFileSegment()
     * failed to rename the file into place. If the rename failed, opening
     * the file below will fail.
     */
    unlink(tmppath);
  }

  /* Set flag to tell caller there was no existent file */
  *use_existent = false;

  /* Now open original target segment (might not be file I just made) */
  fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method));
  if (fd < 0)
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("could not open file \"%s\": %m", path)));

  elog(DEBUG2, "done creating and filling new WAL file");

  return fd;
}

/*
 * Create a new XLOG file segment by copying a pre-existing one.
 *
 * destsegno: identify segment to be created.
 *
 * srcTLI, srcsegno: identify segment to be copied (could be from
 *    a different timeline)
 *
 * upto: how much of the source file to copy (the rest is filled with
 *    zeros)
 *
 * Currently this is only used during recovery, and so there are no locking
 * considerations.  But we should be just as tense as XLogFileInit to avoid
 * emplacing a bogus file.
 */
static void
XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno,
       int upto)
{
  char    path[MAXPGPATH];
  char    tmppath[MAXPGPATH];
  PGAlignedXLogBlock buffer;
  int     srcfd;
  int     fd;
  int     nbytes;

  /*
   * Open the source file
   */
  XLogFilePath(path, srcTLI, srcsegno, wal_segment_size);
  srcfd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
  if (srcfd < 0)
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("could not open file \"%s\": %m", path)));

  /*
   * Copy into a temp file name.
   */
  snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

  unlink(tmppath);

  /* do not use get_sync_bit() here --- want to fsync only at end of fill */
  fd = OpenTransientFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
  if (fd < 0)
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("could not create file \"%s\": %m", tmppath)));

  /*
   * Do the data copying.
   */
  for (nbytes = 0; nbytes < wal_segment_size; nbytes += sizeof(buffer))
  {
    int     nread;

    nread = upto - nbytes;

    /*
     * The part that is not read from the source file is filled with
     * zeros.
     */
    if (nread < sizeof(buffer))
      memset(buffer.data, 0, sizeof(buffer));

    if (nread > 0)
    {
      if (nread > sizeof(buffer))
        nread = sizeof(buffer);
      errno = 0;
      pgstat_report_wait_start(WAIT_EVENT_WAL_COPY_READ);
      if (read(srcfd, buffer.data, nread) != nread)
      {
        if (errno != 0)
          ereport(ERROR,
              (errcode_for_file_access(),
               errmsg("could not read file \"%s\": %m",
                  path)));
        else
          ereport(ERROR,
              (errmsg("not enough data in file \"%s\"",
                  path)));
      }
      pgstat_report_wait_end();
    }
    errno = 0;
    pgstat_report_wait_start(WAIT_EVENT_WAL_COPY_WRITE);
    if ((int) write(fd, buffer.data, sizeof(buffer)) != (int) sizeof(buffer))
    {
      int     save_errno = errno;

      /*
       * If we fail to make the file, delete it to release disk space
       */
      unlink(tmppath);
      /* if write didn't set errno, assume problem is no disk space */
      errno = save_errno ? save_errno : ENOSPC;

      ereport(ERROR,
          (errcode_for_file_access(),
           errmsg("could not write to file \"%s\": %m", tmppath)));
    }
    pgstat_report_wait_end();
  }

  pgstat_report_wait_start(WAIT_EVENT_WAL_COPY_SYNC);
  if (pg_fsync(fd) != 0)
    ereport(data_sync_elevel(ERROR),
        (errcode_for_file_access(),
         errmsg("could not fsync file \"%s\": %m", tmppath)));
  pgstat_report_wait_end();

  if (CloseTransientFile(fd))
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("could not close file \"%s\": %m", tmppath)));

  CloseTransientFile(srcfd);

  /*
   * Now move the segment into place with its final name.
   */
  if (!InstallXLogFileSegment(&destsegno, tmppath, false, 0, false))
    elog(ERROR, "InstallXLogFileSegment should not have failed");
}

/*
 * Install a new XLOG segment file as a current or future log segment.
 *
 * This is used both to install a newly-created segment (which has a temp
 * filename while it's being created) and to recycle an old segment.
 *
 * *segno: identify segment to install as (or first possible target).
 * When find_free is true, this is modified on return to indicate the
 * actual installation location or last segment searched.
 *
 * tmppath: initial name of file to install.  It will be renamed into place.
 *
 * find_free: if true, install the new segment at the first empty segno
 * number at or after the passed numbers.  If false, install the new segment
 * exactly where specified, deleting any existing segment file there.
 *
 * max_segno: maximum segment number to install the new file as.  Fail if no
 * free slot is found between *segno and max_segno. (Ignored when find_free
 * is false.)
 *
 * use_lock: if true, acquire ControlFileLock while moving file into
 * place.  This should be true except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns true if the file was installed successfully.  false indicates that
 * max_segno limit was exceeded, or an error occurred while renaming the
 * file into place.
 */
static bool
InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
             bool find_free, XLogSegNo max_segno,
             bool use_lock)
{
  char    path[MAXPGPATH];
  struct stat stat_buf;

  XLogFilePath(path, ThisTimeLineID, *segno, wal_segment_size);

  /*
   * We want to be sure that only one process does this at a time.
   */
  if (use_lock)
    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

  if (!find_free)
  {
    /* Force installation: get rid of any pre-existing segment file */
    durable_unlink(path, DEBUG1);
  }
  else
  {
    /* Find a free slot to put it in */
    while (stat(path, &stat_buf) == 0)
    {
      if ((*segno) >= max_segno)
      {
        /* Failed to find a free slot within specified range */
        if (use_lock)
          LWLockRelease(ControlFileLock);
        return false;
      }
      (*segno)++;
      XLogFilePath(path, ThisTimeLineID, *segno, wal_segment_size);
    }
  }

  /*
   * Perform the rename using link if available, paranoidly trying to avoid
   * overwriting an existing file (there shouldn't be one).
   */
  if (durable_link_or_rename(tmppath, path, LOG) != 0)
  {
    if (use_lock)
      LWLockRelease(ControlFileLock);
    /* durable_link_or_rename already emitted log message */
    return false;
  }

  if (use_lock)
    LWLockRelease(ControlFileLock);

  return true;
}

/*
 * Open a pre-existing logfile segment for writing.
 */
int
XLogFileOpen(XLogSegNo segno)
{
  char    path[MAXPGPATH];
  int     fd;

  XLogFilePath(path, ThisTimeLineID, segno, wal_segment_size);

  fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method));
  if (fd < 0)
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not open write-ahead log file \"%s\": %m", path)));

  return fd;
}

/*
 * Open a logfile segment for reading (during recovery).
 *
 * If source == XLOG_FROM_ARCHIVE, the segment is retrieved from archive.
 * Otherwise, it's assumed to be already available in pg_wal.
 */
static int
XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
       int source, bool notfoundOk)
{
  char    xlogfname[MAXFNAMELEN];
  char    activitymsg[MAXFNAMELEN + 16];
  char    path[MAXPGPATH];
  int     fd;

  XLogFileName(xlogfname, tli, segno, wal_segment_size);

  switch (source)
  {
    case XLOG_FROM_ARCHIVE:
      /* Report recovery progress in PS display */
      snprintf(activitymsg, sizeof(activitymsg), "waiting for %s",
           xlogfname);
      set_ps_display(activitymsg, false);

      restoredFromArchive = RestoreArchivedFile(path, xlogfname,
                            "RECOVERYXLOG",
                            wal_segment_size,
                            InRedo);
      if (!restoredFromArchive)
        return -1;
      break;

    case XLOG_FROM_PG_WAL:
    case XLOG_FROM_STREAM:
      XLogFilePath(path, tli, segno, wal_segment_size);
      restoredFromArchive = false;
      break;

    default:
      elog(ERROR, "invalid XLogFileRead source %d", source);
  }

  /*
   * If the segment was fetched from archival storage, replace the existing
   * xlog segment (if any) with the archival version.
   */
  if (source == XLOG_FROM_ARCHIVE)
  {
    KeepFileRestoredFromArchive(path, xlogfname);

    /*
     * Set path to point at the new file in pg_wal.
     */
    snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlogfname);
  }

  fd = BasicOpenFile(path, O_RDONLY | PG_BINARY);
  if (fd >= 0)
  {
    /* Success! */
    curFileTLI = tli;

    /* Report recovery progress in PS display */
    snprintf(activitymsg, sizeof(activitymsg), "recovering %s",
         xlogfname);
    set_ps_display(activitymsg, false);

    /* Track source of data in assorted state variables */
    readSource = source;
    XLogReceiptSource = source;
    /* In FROM_STREAM case, caller tracks receipt time, not me */
    if (source != XLOG_FROM_STREAM)
      XLogReceiptTime = GetCurrentTimestamp();

    return fd;
  }
  if (errno != ENOENT || !notfoundOk) /* unexpected failure? */
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not open file \"%s\": %m", path)));
  return -1;
}

/*
 * Open a logfile segment for reading (during recovery).
 *
 * This version searches for the segment with any TLI listed in expectedTLEs.
 */
static int
XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source)
{
  char    path[MAXPGPATH];
  ListCell   *cell;
  int     fd;
  List     *tles;

  /*
   * Loop looking for a suitable timeline ID: we might need to read any of
   * the timelines listed in expectedTLEs.
   *
   * We expect curFileTLI on entry to be the TLI of the preceding file in
   * sequence, or 0 if there was no predecessor.  We do not allow curFileTLI
   * to go backwards; this prevents us from picking up the wrong file when a
   * parent timeline extends to higher segment numbers than the child we
   * want to read.
   *
   * If we haven't read the timeline history file yet, read it now, so that
   * we know which TLIs to scan.  We don't save the list in expectedTLEs,
   * however, unless we actually find a valid segment.  That way if there is
   * neither a timeline history file nor a WAL segment in the archive, and
   * streaming replication is set up, we'll read the timeline history file
   * streamed from the master when we start streaming, instead of recovering
   * with a dummy history generated here.
   */
  if (expectedTLEs)
    tles = expectedTLEs;
  else
    tles = readTimeLineHistory(recoveryTargetTLI);

  foreach(cell, tles)
  {
    TimeLineID  tli = ((TimeLineHistoryEntry *) lfirst(cell))->tli;

    if (tli < curFileTLI)
      break;       /* don't bother looking at too-old TLIs */

    if (source == XLOG_FROM_ANY || source == XLOG_FROM_ARCHIVE)
    {
      fd = XLogFileRead(segno, emode, tli,
                XLOG_FROM_ARCHIVE, true);
      if (fd != -1)
      {
        elog(DEBUG1, "got WAL segment from archive");
        if (!expectedTLEs)
          expectedTLEs = tles;
        return fd;
      }
    }

    if (source == XLOG_FROM_ANY || source == XLOG_FROM_PG_WAL)
    {
      fd = XLogFileRead(segno, emode, tli,
                XLOG_FROM_PG_WAL, true);
      if (fd != -1)
      {
        if (!expectedTLEs)
          expectedTLEs = tles;
        return fd;
      }
    }
  }

  /* Couldn't find it.  For simplicity, complain about front timeline */
  XLogFilePath(path, recoveryTargetTLI, segno, wal_segment_size);
  errno = ENOENT;
  ereport(emode,
      (errcode_for_file_access(),
       errmsg("could not open file \"%s\": %m", path)));
  return -1;
}

/*
 * Close the current logfile segment for writing.
 */
static void
XLogFileClose(void)
{
  Assert(openLogFile >= 0);

  /*
   * WAL segment files will not be re-read in normal operation, so we advise
   * the OS to release any cached pages.  But do not do so if WAL archiving
   * or streaming is active, because archiver and walsender process could
   * use the cache to read the WAL segment.
   */
#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
  if (!XLogIsNeeded())
    (void) posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
#endif

  if (close(openLogFile))
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not close log file %s: %m",
            XLogFileNameP(ThisTimeLineID, openLogSegNo))));
  openLogFile = -1;
}

/*
 * Preallocate log files beyond the specified log endpoint.
 *
 * XXX this is currently extremely conservative, since it forces only one
 * future log segment to exist, and even that only if we are 75% done with
 * the current one.  This is only appropriate for very low-WAL-volume systems.
 * High-volume systems will be OK once they've built up a sufficient set of
 * recycled log segments, but the startup transient is likely to include
 * a lot of segment creations by foreground processes, which is not so good.
 */
static void
PreallocXlogFiles(XLogRecPtr endptr)
{
  XLogSegNo _logSegNo;
  int     lf;
  bool    use_existent;
  uint64    offset;

  XLByteToPrevSeg(endptr, _logSegNo, wal_segment_size);
  offset = XLogSegmentOffset(endptr - 1, wal_segment_size);
  if (offset >= (uint32) (0.75 * wal_segment_size))
  {
    _logSegNo++;
    use_existent = true;
    lf = XLogFileInit(_logSegNo, &use_existent, true);
    close(lf);
    if (!use_existent)
      CheckpointStats.ckpt_segs_added++;
  }
}

/*
 * Throws an error if the given log segment has already been removed or
 * recycled. The caller should only pass a segment that it knows to have
 * existed while the server has been running, as this function always
 * succeeds if no WAL segments have been removed since startup.
 * 'tli' is only used in the error message.
 *
 * Note: this function guarantees to keep errno unchanged on return.
 * This supports callers that use this to possibly deliver a better
 * error message about a missing file, while still being able to throw
 * a normal file-access error afterwards, if this does return.
 */
void
CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
{
  int     save_errno = errno;
  XLogSegNo lastRemovedSegNo;

  SpinLockAcquire(&XLogCtl->info_lck);
  lastRemovedSegNo = XLogCtl->lastRemovedSegNo;
  SpinLockRelease(&XLogCtl->info_lck);

  if (segno <= lastRemovedSegNo)
  {
    char    filename[MAXFNAMELEN];

    XLogFileName(filename, tli, segno, wal_segment_size);
    errno = save_errno;
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("requested WAL segment %s has already been removed",
            filename)));
  }
  errno = save_errno;
}

/*
 * Return the last WAL segment removed, or 0 if no segment has been removed
 * since startup.
 *
 * NB: the result can be out of date arbitrarily fast, the caller has to deal
 * with that.
 */
XLogSegNo
XLogGetLastRemovedSegno(void)
{
  XLogSegNo lastRemovedSegNo;

  SpinLockAcquire(&XLogCtl->info_lck);
  lastRemovedSegNo = XLogCtl->lastRemovedSegNo;
  SpinLockRelease(&XLogCtl->info_lck);

  return lastRemovedSegNo;
}

/*
 * Update the last removed segno pointer in shared memory, to reflect
 * that the given XLOG file has been removed.
 */
static void
UpdateLastRemovedPtr(char *filename)
{
  uint32    tli;
  XLogSegNo segno;

  XLogFromFileName(filename, &tli, &segno, wal_segment_size);

  SpinLockAcquire(&XLogCtl->info_lck);
  if (segno > XLogCtl->lastRemovedSegNo)
    XLogCtl->lastRemovedSegNo = segno;
  SpinLockRelease(&XLogCtl->info_lck);
}

/*
 * Recycle or remove all log files older or equal to passed segno.
 *
 * endptr is current (or recent) end of xlog, and RedoRecPtr is the
 * redo pointer of the last checkpoint. These are used to determine
 * whether we want to recycle rather than delete no-longer-wanted log files.
 */
static void
RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr RedoRecPtr, XLogRecPtr endptr)
{
  DIR      *xldir;
  struct dirent *xlde;
  char    lastoff[MAXFNAMELEN];

  /*
   * Construct a filename of the last segment to be kept. The timeline ID
   * doesn't matter, we ignore that in the comparison. (During recovery,
   * ThisTimeLineID isn't set, so we can't use that.)
   */
  XLogFileName(lastoff, 0, segno, wal_segment_size);

  elog(DEBUG2, "attempting to remove WAL segments older than log file %s",
     lastoff);

  xldir = AllocateDir(XLOGDIR);

  while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
  {
    /* Ignore files that are not XLOG segments */
    if (!IsXLogFileName(xlde->d_name) &&
      !7.05kIsPartialXLogFileName7.05k(xlde->d_name))
      continue;

    /*
     * We ignore the timeline part of the XLOG segment identifiers in
     * deciding whether a segment is still needed.  This ensures that we
     * won't prematurely remove a segment from a parent timeline. We could
     * probably be a little more proactive about removing segments of
     * non-parent timelines, but that would be a whole lot more
     * complicated.
     *
     * We use the alphanumeric sorting property of the filenames to decide
     * which ones are earlier than the lastoff segment.
     */
    if (strcmp(xlde->d_name + 8, lastoff + 8) <= 0)
    {
      if (XLogArchiveCheckDone(xlde->d_name))
      {
        /* Update the last removed location in shared memory first */
        UpdateLastRemovedPtr(xlde->d_name);

        RemoveXlogFile(xlde->d_name, RedoRecPtr, endptr);
      }
    }
  }

  FreeDir(xldir);
}

/*
 * Remove WAL files that are not part of the given timeline's history.
 *
 * This is called during recovery, whenever we switch to follow a new
 * timeline, and at the end of recovery when we create a new timeline. We
 * wouldn't otherwise care about extra WAL files lying in pg_wal, but they
 * might be leftover pre-allocated or recycled WAL segments on the old timeline
 * that we haven't used yet, and contain garbage. If we just leave them in
 * pg_wal, they will eventually be archived, and we can't let that happen.
 * Files that belong to our timeline history are valid, because we have
 * successfully replayed them, but from others we can't be sure.
 *
 * 'switchpoint' is the current point in WAL where we switch to new timeline,
 * and 'newTLI' is the new timeline we switch to.
 */
static void
RemoveNonParentXlogFiles(XLogRecPtr switchpoint, TimeLineID newTLI)
{
  DIR      *xldir;
  struct dirent *xlde;
  char    switchseg[MAXFNAMELEN];
  XLogSegNo endLogSegNo;

  XLByteToPrevSeg(switchpoint, endLogSegNo, wal_segment_size);

  /*
   * Construct a filename of the last segment to be kept.
   */
  XLogFileName(switchseg, newTLI, endLogSegNo, wal_segment_size);

  elog(DEBUG2, "attempting to remove WAL segments newer than log file %s",
     switchseg);

  xldir = AllocateDir(XLOGDIR);

  while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
  {
    /* Ignore files that are not XLOG segments */
    if (!IsXLogFileName(xlde->d_name))
      continue;

    /*
     * Remove files that are on a timeline older than the new one we're
     * switching to, but with a segment number >= the first segment on the
     * new timeline.
     */
    if (strncmp(xlde->d_name, switchseg, 8) < 0 &&
      strcmp(xlde->d_name + 8, switchseg + 8) > 0)
    {
      /*
       * If the file has already been marked as .ready, however, don't
       * remove it yet. It should be OK to remove it - files that are
       * not part of our timeline history are not required for recovery
       * - but seems safer to let them be archived and removed later.
       */
      if (!XLogArchiveIsReady(xlde->d_name))
        RemoveXlogFile(xlde->d_name, InvalidXLogRecPtr, switchpoint);
    }
  }

  FreeDir(xldir);
}

/*
 * Recycle or remove a log file that's no longer needed.
 *
 * endptr is current (or recent) end of xlog, and RedoRecPtr is the
 * redo pointer of the last checkpoint. These are used to determine
 * whether we want to recycle rather than delete no-longer-wanted log files.
 * If RedoRecPtr is not known, pass invalid, and the function will recycle,
 * somewhat arbitrarily, 10 future segments.
 */
static void
RemoveXlogFile(const char *segname, XLogRecPtr RedoRecPtr, XLogRecPtr endptr)
{
  char    path[MAXPGPATH];
#ifdef WIN32
  char    newpath[MAXPGPATH];
#endif
  struct stat statbuf;
  XLogSegNo endlogSegNo;
  XLogSegNo recycleSegNo;

  /*
   * Initialize info about where to try to recycle to.
   */
  XLByteToSeg(endptr, endlogSegNo, wal_segment_size);
  if (RedoRecPtr == InvalidXLogRecPtr)
    recycleSegNo = endlogSegNo + 10;
  else
    recycleSegNo = XLOGfileslop(RedoRecPtr);

  snprintf(path, MAXPGPATH, XLOGDIR "/%s", segname);

  /*
   * Before deleting the file, see if it can be recycled as a future log
   * segment. Only recycle normal files, pg_standby for example can create
   * symbolic links pointing to a separate archive directory.
   */
  if (endlogSegNo <= recycleSegNo &&
    lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
    InstallXLogFileSegment(&endlogSegNo, path,
                 true, recycleSegNo, true))
  {
    ereport(DEBUG2,
        (errmsg("recycled write-ahead log file \"%s\"",
            segname)));
    CheckpointStats.ckpt_segs_recycled++;
    /* Needn't recheck that slot on future iterations */
    endlogSegNo++;
  }
  else
  {
    /* No need for any more future segments... */
    int     rc;

    ereport(DEBUG2,
        (errmsg("removing write-ahead log file \"%s\"",
            segname)));

#ifdef WIN32

    /*
     * On Windows, if another process (e.g another backend) holds the file
     * open in FILE_SHARE_DELETE mode, unlink will succeed, but the file
     * will still show up in directory listing until the last handle is
     * closed. To avoid confusing the lingering deleted file for a live
     * WAL file that needs to be archived, rename it before deleting it.
     *
     * If another process holds the file open without FILE_SHARE_DELETE
     * flag, rename will fail. We'll try again at the next checkpoint.
     */
    snprintf(newpath, MAXPGPATH, "%s.deleted", path);
    if (rename(path, newpath) != 0)
    {
      ereport(LOG,
          (errcode_for_file_access(),
           errmsg("could not rename old write-ahead log file \"%s\": %m",
              path)));
      return;
    }
    rc = durable_unlink(newpath, LOG);
#else
    rc = durable_unlink(path, LOG);
#endif
    if (rc != 0)
    {
      /* Message already logged by durable_unlink() */
      return;
    }
    CheckpointStats.ckpt_segs_removed++;
  }

  XLogArchiveCleanup(segname);
}

/*
 * Verify whether pg_wal and pg_wal/archive_status exist.
 * If the latter does not exist, recreate it.
 *
 * It is not the goal of this function to verify the contents of these
 * directories, but to help in cases where someone has performed a cluster
 * copy for PITR purposes but omitted pg_wal from the copy.
 *
 * We could also recreate pg_wal if it doesn't exist, but a deliberate
 * policy decision was made not to.  It is fairly common for pg_wal to be
 * a symlink, and if that was the DBA's intent then automatically making a
 * plain directory would result in degraded performance with no notice.
 */
static void
ValidateXLOGDirectoryStructure(void)
{
  char    path[MAXPGPATH];
  struct stat stat_buf;

  /* Check for pg_wal; if it doesn't exist, error out */
  if (stat(XLOGDIR, &stat_buf) != 0 ||
    !S_ISDIR(stat_buf.st_mode))
    ereport(FATAL,
        (errmsg("required WAL directory \"%s\" does not exist",
            XLOGDIR)));

  /* Check for archive_status */
  snprintf(path, MAXPGPATH, XLOGDIR "/archive_status");
  if (stat(path, &stat_buf) == 0)
  {
    /* Check for weird cases where it exists but isn't a directory */
    if (!S_ISDIR(stat_buf.st_mode))
      ereport(FATAL,
          (errmsg("required WAL directory \"%s\" does not exist",
              path)));
  }
  else
  {
    ereport(LOG,
        (errmsg("creating missing WAL directory \"%s\"", path)));
    if (MakePGDirectory(path) < 0)
      ereport(FATAL,
          (errmsg("could not create missing directory \"%s\": %m",
              path)));
  }
}

/*
 * Remove previous backup history files.  This also retries creation of
 * .ready files for any backup history files for which XLogArchiveNotify
 * failed earlier.
 */
static void
CleanupBackupHistory(void)
{
  DIR      *xldir;
  struct dirent *xlde;
  char    path[MAXPGPATH + sizeof(XLOGDIR)];

  xldir = AllocateDir(XLOGDIR);

  while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
  {
    if (IsBackupHistoryFileName(xlde->d_name))
    {
      if (XLogArchiveCheckDone(xlde->d_name))
      {
        elog(DEBUG2, "removing WAL backup history file \"%s\"",
           xlde->d_name);
        snprintf(path, sizeof(path), XLOGDIR "/%s", xlde->d_name);
        unlink(path);
        XLogArchiveCleanup(xlde->d_name);
      }
    }
  }

  FreeDir(xldir);
}

/*
 * Attempt to read an XLOG record.
 *
 * If RecPtr is valid, try to read a record at that position.  Otherwise
 * try to read a record just after the last one previously read.
 *
 * If no valid record is available, returns NULL, or fails if emode is PANIC.
 * (emode must be either PANIC, LOG). In standby mode, retries until a valid
 * record is available.
 *
 * The record is copied into readRecordBuf, so that on successful return,
 * the returned record pointer always points there.
 */
static XLogRecord *
ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr, int emode,
       bool fetching_ckpt)
{
  XLogRecord *record;
  XLogPageReadPrivate *private = (XLogPageReadPrivate *) xlogreader->private_data;

  /* Pass through parameters to XLogPageRead */
  private->fetching_ckpt = fetching_ckpt;
  private->emode = emode;
  private->randAccess = (RecPtr != InvalidXLogRecPtr);

  /* This is the first attempt to read this page. */
  lastSourceFailed = false;

  for (;;)
  {
    char     *errormsg;

    record = XLogReadRecord(xlogreader, RecPtr, &errormsg);
    ReadRecPtr = xlogreader->ReadRecPtr;
    EndRecPtr = xlogreader->EndRecPtr;
    if (record == NULL)
    {
      if (readFile >= 0)
      {
        close(readFile);
        readFile = -1;
      }

      /*
       * We only end up here without a message when XLogPageRead()
       * failed - in that case we already logged something. In
       * StandbyMode that only happens if we have been triggered, so we
       * shouldn't loop anymore in that case.
       */
      if (errormsg)
        ereport(emode_for_corrupt_record(emode,
                         RecPtr ? RecPtr : EndRecPtr),
            (errmsg_internal("%s", errormsg) /* already translated */ ));
    }

    /*
     * Check page TLI is one of the expected values.
     */
    else if (!tliInHistory(xlogreader->latestPageTLI, expectedTLEs))
    {
      char    fname[MAXFNAMELEN];
      XLogSegNo segno;
      int32   offset;

      XLByteToSeg(xlogreader->latestPagePtr, segno, wal_segment_size);
      offset = XLogSegmentOffset(xlogreader->latestPagePtr,
                     wal_segment_size);
      XLogFileName(fname, xlogreader->readPageTLI, segno,
             wal_segment_size);
      ereport(emode_for_corrupt_record(emode,
                       RecPtr ? RecPtr : EndRecPtr),
          (errmsg("unexpected timeline ID %u in log segment %s, offset %u",
              xlogreader->latestPageTLI,
              fname,
              offset)));
      record = NULL;
    }

    if (record)
    {
      /* Great, got a record */
      return record;
    }
    else
    {
      /* No valid record available from this source */
      lastSourceFailed = true;

      /*
       * If archive recovery was requested, but we were still doing
       * crash recovery, switch to archive recovery and retry using the
       * offline archive. We have now replayed all the valid WAL in
       * pg_wal, so we are presumably now consistent.
       *
       * We require that there's at least some valid WAL present in
       * pg_wal, however (!fetching_ckpt).  We could recover using the
       * WAL from the archive, even if pg_wal is completely empty, but
       * we'd have no idea how far we'd have to replay to reach
       * consistency.  So err on the safe side and give up.
       */
      if (!InArchiveRecovery && ArchiveRecoveryRequested &&
        !fetching_ckpt0)
      {
        ereport(DEBUG1,
            (errmsg_internal("reached end of WAL in pg_wal, entering archive recovery")));
        InArchiveRecovery = true;
        if (StandbyModeRequested)
          StandbyMode = true;

        /* initialize minRecoveryPoint to this record */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
        if (ControlFile->minRecoveryPoint < EndRecPtr)
        {
          ControlFile->minRecoveryPoint = EndRecPtr;
          ControlFile->minRecoveryPointTLI = ThisTimeLineID;
        }
        /* update local copy */
        minRecoveryPoint = ControlFile->minRecoveryPoint;
        minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

        /*
         * The startup process can update its local copy of
         * minRecoveryPoint from this point.
         */
        updateMinRecoveryPoint = true;

        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        CheckRecoveryConsistency();

        /*
         * Before we retry, reset lastSourceFailed and currentSource
         * so that we will check the archive next.
         */
        lastSourceFailed = false;
        currentSource = 0;

        continue;
      }

      /* In standby mode, loop back to retry. Otherwise, give up. */
      if (StandbyMode && !CheckForStandbyTrigger()0)
        continue;
      else
        return NULL;
    }
  }
}

/*
 * Scan for new timelines that might have appeared in the archive since we
 * started recovery.
 *
 * If there are any, the function changes recovery target TLI to the latest
 * one and returns 'true'.
 */
static bool
rescanLatestTimeLine(void)
{
  List     *newExpectedTLEs;
  bool    found;
  ListCell   *cell;
  TimeLineID  newtarget;
  TimeLineID  oldtarget = recoveryTargetTLI;
  TimeLineHistoryEntry *currentTle = NULL;

  newtarget = findNewestTimeLine(recoveryTargetTLI);
  if (newtarget == recoveryTargetTLI)
  {
    /* No new timelines found */
    return false;
  }

  /*
   * Determine the list of expected TLIs for the new TLI
   */

  newExpectedTLEs = readTimeLineHistory(newtarget);

  /*
   * If the current timeline is not part of the history of the new timeline,
   * we cannot proceed to it.
   */
  found = false;
  foreach(cell, newExpectedTLEs)
  {
    currentTle = (TimeLineHistoryEntry *) lfirst(cell);

    if (currentTle->tli == recoveryTargetTLI)
    {
      found = true;
      break;
    }
  }
  if (!found)
  {
    ereport(LOG,
        (errmsg("new timeline %u is not a child of database system timeline %u",
            newtarget,
            ThisTimeLineID)));
    return false;
  }

  /*
   * The current timeline was found in the history file, but check that the
   * next timeline was forked off from it *after* the current recovery
   * location.
   */
  if (currentTle->end < EndRecPtr)
  {
    ereport(LOG,
        (errmsg("new timeline %u forked off current database system timeline %u before current recovery point %X/%X",
            newtarget,
            ThisTimeLineID,
            (uint32) (EndRecPtr >> 32), (uint32) EndRecPtr)));
    return false;
  }

  /* The new timeline history seems valid. Switch target */
  recoveryTargetTLI = newtarget;
  list_free_deep(expectedTLEs);
  expectedTLEs = newExpectedTLEs;

  /*
   * As in StartupXLOG(), try to ensure we have all the history files
   * between the old target and new target in pg_wal.
   */
  restoreTimeLineHistoryFiles(oldtarget + 1, newtarget);

  ereport(LOG,
      (errmsg("new target timeline is %u",
          recoveryTargetTLI)));

  return true;
}

/*
 * I/O routines for pg_control
 *
 * *ControlFile is a buffer in shared memory that holds an image of the
 * contents of pg_control.  WriteControlFile() initializes pg_control
 * given a preloaded buffer, ReadControlFile() loads the buffer from
 * the pg_control file (during postmaster or standalone-backend startup),
 * and UpdateControlFile() rewrites pg_control after we modify xlog state.
 *
 * For simplicity, WriteControlFile() initializes the fields of pg_control
 * that are related to checking backend/database compatibility, and
 * ReadControlFile() verifies they are correct.  We could split out the
 * I/O and compatibility-check functions, but there seems no need currently.
 */
static void
WriteControlFile(void)
{
  int     fd;
  char    buffer[PG_CONTROL_FILE_SIZE]; /* need not be aligned */

  /*
   * Ensure that the size of the pg_control data structure is sane.  See the
   * comments for these symbols in pg_control.h.
   */
  StaticAssertStmt(sizeof(ControlFileData) <= PG_CONTROL_MAX_SAFE_SIZE,
           "pg_control is too large for atomic disk writes");
  StaticAssertStmt(sizeof(ControlFileData) <= PG_CONTROL_FILE_SIZE,
           "sizeof(ControlFileData) exceeds PG_CONTROL_FILE_SIZE");

  /*
   * Initialize version and compatibility-check fields
   */
  ControlFile->pg_control_version = PG_CONTROL_VERSION;
  ControlFile->catalog_version_no = CATALOG_VERSION_NO;

  ControlFile->maxAlign = MAXIMUM_ALIGNOF;
  ControlFile->floatFormat = FLOATFORMAT_VALUE;

  ControlFile->blcksz = BLCKSZ;
  ControlFile->relseg_size = RELSEG_SIZE;
  ControlFile->xlog_blcksz = XLOG_BLCKSZ;
  ControlFile->xlog_seg_size = wal_segment_size;

  ControlFile->nameDataLen = NAMEDATALEN;
  ControlFile->indexMaxKeys = INDEX_MAX_KEYS;

  ControlFile->toast_max_chunk_size = TOAST_MAX_CHUNK_SIZE;
  ControlFile->loblksize = LOBLKSIZE;

  ControlFile->float4ByVal = FLOAT4PASSBYVAL;
  ControlFile->float8ByVal = FLOAT8PASSBYVAL;

  /* Contents are protected with a CRC */
  INIT_CRC32C(ControlFile->crc);
  COMP_CRC32C(ControlFile->crc,
        (char *) ControlFile,
        offsetof(ControlFileData, crc));
  FIN_CRC32C(ControlFile->crc);

  /*
   * We write out PG_CONTROL_FILE_SIZE bytes into pg_control, zero-padding
   * the excess over sizeof(ControlFileData).  This reduces the odds of
   * premature-EOF errors when reading pg_control.  We'll still fail when we
   * check the contents of the file, but hopefully with a more specific
   * error than "couldn't read pg_control".
   */
  memset(buffer, 0, PG_CONTROL_FILE_SIZE);
  memcpy(buffer, ControlFile, sizeof(ControlFileData));

  fd = BasicOpenFile(XLOG_CONTROL_FILE,
             O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
  if (fd < 0)
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not create control file \"%s\": %m",
            XLOG_CONTROL_FILE)));

  errno = 0;
  pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_WRITE);
  if (write(fd, buffer, PG_CONTROL_FILE_SIZE) != PG_CONTROL_FILE_SIZE)
  {
    /* if write didn't set errno, assume problem is no disk space */
    if (errno == 0)
      errno = ENOSPC;
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not write to control file: %m")));
  }
  pgstat_report_wait_end();

  pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_SYNC);
  if (pg_fsync(fd) != 0)
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not fsync control file: %m")));
  pgstat_report_wait_end();

  if (close(fd))
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not close control file: %m")));
}

static void
ReadControlFile(void)
{
  pg_crc32c crc;
  int     fd;
  static char wal_segsz_str[20];
  int     r;

  /*
   * Read data...
   */
  fd = BasicOpenFile(XLOG_CONTROL_FILE,
             O_RDWR | PG_BINARY);
  if (fd < 0)
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not open control file \"%s\": %m",
            XLOG_CONTROL_FILE)));

  pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_READ);
  r = read(fd, ControlFile, sizeof(ControlFileData));
  if (r != sizeof(ControlFileData))
  {
    if (r < 0)
      ereport(PANIC,
          (errcode_for_file_access(),
           errmsg("could not read from control file: %m")));
    else
      ereport(PANIC,
          (errmsg("could not read from control file: read %d bytes, expected %d", r, (int) sizeof(ControlFileData))));
  }
  pgstat_report_wait_end();

  close(fd);

  /*
   * Check for expected pg_control format version.  If this is wrong, the
   * CRC check will likely fail because we'll be checking the wrong number
   * of bytes.  Complaining about wrong version will probably be more
   * enlightening than complaining about wrong CRC.
   */

  if (ControlFile->pg_control_version != PG_CONTROL_VERSION && ControlFile->pg_control_version % 65536 == 00 && ControlFile->pg_control_version / 65536 != 00)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d (0x%08x),"
               " but the server was compiled with PG_CONTROL_VERSION %d (0x%08x).",
               ControlFile->pg_control_version, ControlFile->pg_control_version,
               PG_CONTROL_VERSION, PG_CONTROL_VERSION),
         errhint("This could be a problem of mismatched byte ordering.  It looks like you need to initdb.")));

  if (ControlFile->pg_control_version != PG_CONTROL_VERSION)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d,"
               " but the server was compiled with PG_CONTROL_VERSION %d.",
               ControlFile->pg_control_version, PG_CONTROL_VERSION),
         errhint("It looks like you need to initdb.")));

  /* Now check the CRC. */
  INIT_CRC32C(crc);
  COMP_CRC32C(crc,
        (char *) ControlFile,
        offsetof(ControlFileData, crc));
  FIN_CRC32C(crc);

  if (!EQ_CRC32C(crc, ControlFile->crc))
    ereport(FATAL,
        (errmsg("incorrect checksum in control file")));

  /*
   * Do compatibility checking immediately.  If the database isn't
   * compatible with the backend executable, we want to abort before we can
   * possibly do any damage.
   */
  if (ControlFile->catalog_version_no != CATALOG_VERSION_NO)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with CATALOG_VERSION_NO %d,"
               " but the server was compiled with CATALOG_VERSION_NO %d.",
               ControlFile->catalog_version_no, CATALOG_VERSION_NO),
         errhint("It looks like you need to initdb.")));
  if (ControlFile->maxAlign != MAXIMUM_ALIGNOF)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with MAXALIGN %d,"
               " but the server was compiled with MAXALIGN %d.",
               ControlFile->maxAlign, MAXIMUM_ALIGNOF),
         errhint("It looks like you need to initdb.")));
  if (ControlFile->floatFormat != FLOATFORMAT_VALUE)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster appears to use a different floating-point number format than the server executable."),
         errhint("It looks like you need to initdb.")));
  if (ControlFile->blcksz != BLCKSZ)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with BLCKSZ %d,"
               " but the server was compiled with BLCKSZ %d.",
               ControlFile->blcksz, BLCKSZ),
         errhint("It looks like you need to recompile or initdb.")));
  if (ControlFile->relseg_size != RELSEG_SIZE)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with RELSEG_SIZE %d,"
               " but the server was compiled with RELSEG_SIZE %d.",
               ControlFile->relseg_size, RELSEG_SIZE),
         errhint("It looks like you need to recompile or initdb.")));
  if (ControlFile->xlog_blcksz != XLOG_BLCKSZ)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with XLOG_BLCKSZ %d,"
               " but the server was compiled with XLOG_BLCKSZ %d.",
               ControlFile->xlog_blcksz, XLOG_BLCKSZ),
         errhint("It looks like you need to recompile or initdb.")));
  if (ControlFile->nameDataLen != NAMEDATALEN)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with NAMEDATALEN %d,"
               " but the server was compiled with NAMEDATALEN %d.",
               ControlFile->nameDataLen, NAMEDATALEN),
         errhint("It looks like you need to recompile or initdb.")));
  if (ControlFile->indexMaxKeys != INDEX_MAX_KEYS)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with INDEX_MAX_KEYS %d,"
               " but the server was compiled with INDEX_MAX_KEYS %d.",
               ControlFile->indexMaxKeys, INDEX_MAX_KEYS),
         errhint("It looks like you need to recompile or initdb.")));
  if (ControlFile->toast_max_chunk_size != TOAST_MAX_CHUNK_SIZE)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with TOAST_MAX_CHUNK_SIZE %d,"
               " but the server was compiled with TOAST_MAX_CHUNK_SIZE %d.",
               ControlFile->toast_max_chunk_size, (int) TOAST_MAX_CHUNK_SIZE),
         errhint("It looks like you need to recompile or initdb.")));
  if (ControlFile->loblksize != LOBLKSIZE)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with LOBLKSIZE %d,"
               " but the server was compiled with LOBLKSIZE %d.",
               ControlFile->loblksize, (int) LOBLKSIZE),
         errhint("It looks like you need to recompile or initdb.")));

#ifdef USE_FLOAT4_BYVAL
  if (ControlFile->float4ByVal != true)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized without USE_FLOAT4_BYVAL"
               " but the server was compiled with USE_FLOAT4_BYVAL."),
         errhint("It looks like you need to recompile or initdb.")));
#else
  if (ControlFile->float4ByVal != false)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with USE_FLOAT4_BYVAL"
               " but the server was compiled without USE_FLOAT4_BYVAL."),
         errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT8_BYVAL
  if (ControlFile->float8ByVal != true)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized without USE_FLOAT8_BYVAL"
               " but the server was compiled with USE_FLOAT8_BYVAL."),
         errhint("It looks like you need to recompile or initdb.")));
#else
  if (ControlFile->float8ByVal != false)
    ereport(FATAL,
        (errmsg("database files are incompatible with server"),
         errdetail("The database cluster was initialized with USE_FLOAT8_BYVAL"
               " but the server was compiled without USE_FLOAT8_BYVAL."),
         errhint("It looks like you need to recompile or initdb.")));
#endif

  wal_segment_size = ControlFile->xlog_seg_size;

  if (!IsValidWalSegSize(wal_segment_size))
    ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
            errmsg_plural("WAL segment size must be a power of two between 1 MB and 1 GB, but the control file specifies %d byte",
                    "WAL segment size must be a power of two between 1 MB and 1 GB, but the control file specifies %d bytes",
                    wal_segment_size,
                    wal_segment_size)));

  snprintf(wal_segsz_str, sizeof(wal_segsz_str), "%d", wal_segment_size);
  SetConfigOption("wal_segment_size", wal_segsz_str, PGC_INTERNAL,
          PGC_S_OVERRIDE);

  /* check and update variables dependent on wal_segment_size */
  if (ConvertToXSegs(min_wal_size_mb, wal_segment_size) < 2)
    ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
            errmsg("\"min_wal_size\" must be at least twice \"wal_segment_size\"")));

  if (ConvertToXSegs(max_wal_size_mb, wal_segment_size) < 2)
    ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
            errmsg("\"max_wal_size\" must be at least twice \"wal_segment_size\"")));

  UsableBytesInSegment =
    (wal_segment_size / XLOG_BLCKSZ * UsableBytesInPage) -
    (SizeOfXLogLongPHD - SizeOfXLogShortPHD);

  CalculateCheckpointSegments();

  /* Make the initdb settings visible as GUC variables, too */
  SetConfigOption("data_checksums", DataChecksumsEnabled() ? "yes"0 : "no",
          PGC_INTERNAL, PGC_S_OVERRIDE);
}

void
UpdateControlFile(void)
{
  int     fd;

  INIT_CRC32C(ControlFile->crc);
  COMP_CRC32C(ControlFile->crc,
        (char *) ControlFile,
        offsetof(ControlFileData, crc));
  FIN_CRC32C(ControlFile->crc);

  fd = BasicOpenFile(XLOG_CONTROL_FILE,
             O_RDWR | PG_BINARY);
  if (fd < 0)
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not open control file \"%s\": %m",
            XLOG_CONTROL_FILE)));

  errno = 0;
  pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_WRITE_UPDATE);
  if (write(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
  {
    /* if write didn't set errno, assume problem is no disk space */
    if (errno == 0)
      errno = ENOSPC;
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not write to control file: %m")));
  }
  pgstat_report_wait_end();

  pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_SYNC_UPDATE);
  if (pg_fsync(fd) != 0)
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not fsync control file: %m")));
  pgstat_report_wait_end();

  if (close(fd))
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not close control file: %m")));
}

/*
 * Returns the unique system identifier from control file.
 */
uint64
GetSystemIdentifier(void)
{
  Assert(ControlFile != NULL);
  return ControlFile->system_identifier;
}

/*
 * Returns the random nonce from control file.
 */
char *
GetMockAuthenticationNonce(void)
{
  Assert(ControlFile != NULL);
  return ControlFile->mock_authentication_nonce;
}

/*
 * Are checksums enabled for data pages?
 */
bool
DataChecksumsEnabled(void)
{
  Assert(ControlFile != NULL);
  return (ControlFile->data_checksum_version > 0);
}

/*
 * Returns a fake LSN for unlogged relations.
 *
 * Each call generates an LSN that is greater than any previous value
 * returned. The current counter value is saved and restored across clean
 * shutdowns, but like unlogged relations, does not survive a crash. This can
 * be used in lieu of real LSN values returned by XLogInsert, if you need an
 * LSN-like increasing sequence of numbers without writing any WAL.
 */
XLogRecPtr
GetFakeLSNForUnloggedRel(void)
{
  XLogRecPtr  nextUnloggedLSN;

  /* increment the unloggedLSN counter, need SpinLock */
  SpinLockAcquire(&XLogCtl->ulsn_lck);
  nextUnloggedLSN = XLogCtl->unloggedLSN++;
  SpinLockRelease(&XLogCtl->ulsn_lck);

  return nextUnloggedLSN;
}

/*
 * Auto-tune the number of XLOG buffers.
 *
 * The preferred setting for wal_buffers is about 3% of shared_buffers, with
 * a maximum of one XLOG segment (there is little reason to think that more
 * is helpful, at least so long as we force an fsync when switching log files)
 * and a minimum of 8 blocks (which was the default value prior to PostgreSQL
 * 9.1, when auto-tuning was added).
 *
 * This should not be called until NBuffers has received its final value.
 */
static int
XLOGChooseNumBuffers(void)
{
  int     xbuffers;

  xbuffers = NBuffers / 32;
  if (xbuffers > (wal_segment_size / XLOG_BLCKSZ))
    xbuffers = (wal_segment_size / XLOG_BLCKSZ);
  if (xbuffers < 8)
    xbuffers = 8;
  return xbuffers;
}

/*
 * GUC check_hook for wal_buffers
 */
bool
check_wal_buffers(int *newval, void **extra, GucSource source)
{
  /*
   * -1 indicates a request for auto-tune.
   */
  if (*newval == -1)
  {
    /*
     * If we haven't yet changed the boot_val default of -1, just let it
     * be.  We'll fix it when XLOGShmemSize is called.
     */
    if (XLOGbuffers == -1)
      return true;

    /* Otherwise, substitute the auto-tune value */
    *newval = XLOGChooseNumBuffers();
  }

  /*
   * We clamp manually-set values to at least 4 blocks.  Prior to PostgreSQL
   * 9.1, a minimum of 4 was enforced by guc.c, but since that is no longer
   * the case, we just silently treat such values as a request for the
   * minimum.  (We could throw an error instead, but that doesn't seem very
   * helpful.)
   */
  if (*newval < 4)
    *newval = 4;

  return true;
}

/*
 * Read the control file, set respective GUCs.
 *
 * This is to be called during startup, including a crash recovery cycle,
 * unless in bootstrap mode, where no control file yet exists.  As there's no
 * usable shared memory yet (its sizing can depend on the contents of the
 * control file!), first store the contents in local memory. XLOGShmemInit()
 * will then copy it to shared memory later.
 *
 * reset just controls whether previous contents are to be expected (in the
 * reset case, there's a dangling pointer into old shared memory), or not.
 */
void
LocalProcessControlFile(bool reset)
{
  Assert(reset || ControlFile == NULL);
  ControlFile = palloc(sizeof(ControlFileData));
  ReadControlFile();
}

/*
 * Initialization of shared memory for XLOG
 */
Size
XLOGShmemSize(void)
{
  Size    size;

  /*
   * If the value of wal_buffers is -1, use the preferred auto-tune value.
   * This isn't an amazingly clean place to do this, but we must wait till
   * NBuffers has received its final value, and must do it before using the
   * value of XLOGbuffers to do anything important.
   */
  if (XLOGbuffers == -1)
  {
    char    buf[32];

    snprintf(buf, sizeof(buf), "%d", XLOGChooseNumBuffers());
    SetConfigOption("wal_buffers", buf, PGC_POSTMASTER, PGC_S_OVERRIDE);
  }
  Assert(XLOGbuffers > 0);

  /* XLogCtl */
  size = sizeof(XLogCtlData);

  /* WAL insertion locks, plus alignment */
  size = add_size(size, mul_size(sizeof(WALInsertLockPadded), NUM_XLOGINSERT_LOCKS + 1));
  /* xlblocks array */
  size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
  /* extra alignment padding for XLOG I/O buffers */
  size = add_size(size, XLOG_BLCKSZ);
  /* and the buffers themselves */
  size = add_size(size, mul_size(XLOG_BLCKSZ, XLOGbuffers));

  /*
   * Note: we don't count ControlFileData, it comes out of the "slop factor"
   * added by CreateSharedMemoryAndSemaphores.  This lets us use this
   * routine again below to compute the actual allocation size.
   */

  return size;
}

void
XLOGShmemInit(void)
{
  bool    foundCFile,
        foundXLog;
  char     *allocptr;
  int     i;
  ControlFileData *localControlFile;

#ifdef WAL_DEBUG

  /*
   * Create a memory context for WAL debugging that's exempt from the normal
   * "no pallocs in critical section" rule. Yes, that can lead to a PANIC if
   * an allocation fails, but wal_debug is not for production use anyway.
   */
  if (walDebugCxt == NULL)
  {
    walDebugCxt = AllocSetContextCreate(TopMemoryContext,
                      "WAL Debug",
                      ALLOCSET_DEFAULT_SIZES);
    MemoryContextAllowInCriticalSection(walDebugCxt, true);
  }
#endif


  XLogCtl = (XLogCtlData *)
    ShmemInitStruct("XLOG Ctl", XLOGShmemSize(), &foundXLog);

  localControlFile = ControlFile;
  ControlFile = (ControlFileData *)
    ShmemInitStruct("Control File", sizeof(ControlFileData), &foundCFile);

  if (foundCFile || foundXLog)
  {
    /* both should be present or neither */
    Assert(foundCFile && foundXLog);

    /* Initialize local copy of WALInsertLocks and register the tranche */
    WALInsertLocks = XLogCtl->Insert.WALInsertLocks;
    LWLockRegisterTranche(LWTRANCHE_WAL_INSERT,
                "wal_insert");

    if (localControlFile)
      pfree(localControlFile);
    return;
  }
  memset(XLogCtl, 0, sizeof(XLogCtlData));

  /*
   * Already have read control file locally, unless in bootstrap mode. Move
   * contents into shared memory.
   */
  if (localControlFile)
  {
    memcpy(ControlFile, localControlFile, sizeof(ControlFileData));
    pfree(localControlFile);
  }

  /*
   * Since XLogCtlData contains XLogRecPtr fields, its sizeof should be a
   * multiple of the alignment for same, so no extra alignment padding is
   * needed here.
   */
  allocptr = ((char *) XLogCtl) + sizeof(XLogCtlData);
  XLogCtl->xlblocks = (XLogRecPtr *) allocptr;
  memset(XLogCtl->xlblocks, 0, sizeof(XLogRecPtr) * XLOGbuffers);
  allocptr += sizeof(XLogRecPtr) * XLOGbuffers;


  /* WAL insertion locks. Ensure they're aligned to the full padded size */
  allocptr += sizeof(WALInsertLockPadded) -
    ((uintptr_t) allocptr) % sizeof(WALInsertLockPadded);
  WALInsertLocks = XLogCtl->Insert.WALInsertLocks =
    (WALInsertLockPadded *) allocptr;
  allocptr += sizeof(WALInsertLockPadded) * NUM_XLOGINSERT_LOCKS;

  LWLockRegisterTranche(LWTRANCHE_WAL_INSERT, "wal_insert");
  for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++64.1k)
  {
    LWLockInitialize(&WALInsertLocks[i].l.lock, LWTRANCHE_WAL_INSERT);
    WALInsertLocks[i].l.insertingAt = InvalidXLogRecPtr;
    WALInsertLocks[i].l.lastImportantAt = InvalidXLogRecPtr;
  }

  /*
   * Align the start of the page buffers to a full xlog block size boundary.
   * This simplifies some calculations in XLOG insertion. It is also
   * required for O_DIRECT.
   */
  allocptr = (char *) TYPEALIGN(XLOG_BLCKSZ, allocptr);
  XLogCtl->pages = allocptr;
  memset(XLogCtl->pages, 0, (Size) XLOG_BLCKSZ * XLOGbuffers);

  /*
   * Do basic initialization of XLogCtl shared data. (StartupXLOG will fill
   * in additional info.)
   */
  XLogCtl->XLogCacheBlck = XLOGbuffers - 1;
  XLogCtl->SharedRecoveryInProgress = true;
  XLogCtl->SharedHotStandbyActive = false;
  XLogCtl->WalWriterSleeping = false;

  SpinLockInit(&XLogCtl->Insert.insertpos_lck);
  SpinLockInit(&XLogCtl->info_lck);
  SpinLockInit(&XLogCtl->ulsn_lck);
  InitSharedLatch(&XLogCtl->recoveryWakeupLatch);
}

/*
 * This func must be called ONCE on system install.  It creates pg_control
 * and the initial XLOG segment.
 */
void
BootStrapXLOG(void)
{
  CheckPoint  checkPoint;
  char     *buffer;
  XLogPageHeader page;
  XLogLongPageHeader longpage;
  XLogRecord *record;
  char     *recptr;
  bool    use_existent;
  uint64    sysidentifier;
  char    mock_auth_nonce[MOCK_AUTH_NONCE_LEN];
  struct timeval tv;
  pg_crc32c crc;

  /*
   * Select a hopefully-unique system identifier code for this installation.
   * We use the result of gettimeofday(), including the fractional seconds
   * field, as being about as unique as we can easily get.  (Think not to
   * use random(), since it hasn't been seeded and there's no portable way
   * to seed it other than the system clock value...)  The upper half of the
   * uint64 value is just the tv_sec part, while the lower half contains the
   * tv_usec part (which must fit in 20 bits), plus 12 bits from our current
   * PID for a little extra uniqueness.  A person knowing this encoding can
   * determine the initialization time of the installation, which could
   * perhaps be useful sometimes.
   */
  gettimeofday(&tv, NULL);
  sysidentifier = ((uint64) tv.tv_sec) << 32;
  sysidentifier |= ((uint64) tv.tv_usec) << 12;
  sysidentifier |= getpid() & 0xFFF;

  /*
   * Generate a random nonce. This is used for authentication requests that
   * will fail because the user does not exist. The nonce is used to create
   * a genuine-looking password challenge for the non-existent user, in lieu
   * of an actual stored password.
   */
  if (!pg_backend_random(mock_auth_nonce, MOCK_AUTH_NONCE_LEN))
    ereport(PANIC,
        (errcode(ERRCODE_INTERNAL_ERROR),
         errmsg("could not generate secret authorization token")));

  /* First timeline ID is always 1 */
  ThisTimeLineID = 1;

  /* page buffer must be aligned suitably for O_DIRECT */
  buffer = (char *) palloc(XLOG_BLCKSZ + XLOG_BLCKSZ);
  page = (XLogPageHeader) TYPEALIGN(XLOG_BLCKSZ, buffer);
  memset(page, 0, XLOG_BLCKSZ);

  /*
   * Set up information for the initial checkpoint record
   *
   * The initial checkpoint record is written to the beginning of the WAL
   * segment with logid=0 logseg=1. The very first WAL segment, 0/0, is not
   * used, so that we can use 0/0 to mean "before any valid WAL segment".
   */
  checkPoint.redo = wal_segment_size + SizeOfXLogLongPHD;
  checkPoint.ThisTimeLineID = ThisTimeLineID;
  checkPoint.PrevTimeLineID = ThisTimeLineID;
  checkPoint.fullPageWrites = fullPageWrites;
  checkPoint.nextXidEpoch = 0;
  checkPoint.nextXid = FirstNormalTransactionId;
  checkPoint.nextOid = FirstBootstrapObjectId;
  checkPoint.nextMulti = FirstMultiXactId;
  checkPoint.nextMultiOffset = 0;
  checkPoint.oldestXid = FirstNormalTransactionId;
  checkPoint.oldestXidDB = TemplateDbOid;
  checkPoint.oldestMulti = FirstMultiXactId;
  checkPoint.oldestMultiDB = TemplateDbOid;
  checkPoint.oldestCommitTsXid = InvalidTransactionId;
  checkPoint.newestCommitTsXid = InvalidTransactionId;
  checkPoint.time = (pg_time_t) time(NULL);
  checkPoint.oldestActiveXid = InvalidTransactionId;

  ShmemVariableCache->nextXid = checkPoint.nextXid;
  ShmemVariableCache->nextOid = checkPoint.nextOid;
  ShmemVariableCache->oidCount = 0;
  MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
  AdvanceOldestClogXid(checkPoint.oldestXid);
  SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
  SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB, true);
  SetCommitTsLimit(InvalidTransactionId, InvalidTransactionId);

  /* Set up the XLOG page header */
  page->xlp_magic = XLOG_PAGE_MAGIC;
  page->xlp_info = XLP_LONG_HEADER;
  page->xlp_tli = ThisTimeLineID;
  page->xlp_pageaddr = wal_segment_size;
  longpage = (XLogLongPageHeader) page;
  longpage->xlp_sysid = sysidentifier;
  longpage->xlp_seg_size = wal_segment_size;
  longpage->xlp_xlog_blcksz = XLOG_BLCKSZ;

  /* Insert the initial checkpoint record */
  recptr = ((char *) page + SizeOfXLogLongPHD);
  record = (XLogRecord *) recptr;
  record->xl_prev = 0;
  record->xl_xid = InvalidTransactionId;
  record->xl_tot_len = SizeOfXLogRecord + SizeOfXLogRecordDataHeaderShort + sizeof(checkPoint);
  record->xl_info = XLOG_CHECKPOINT_SHUTDOWN;
  record->xl_rmid = RM_XLOG_ID;
  recptr += SizeOfXLogRecord;
  /* fill the XLogRecordDataHeaderShort struct */
  *(recptr++) = (char) XLR_BLOCK_ID_DATA_SHORT;
  *(recptr++) = sizeof(checkPoint);
  memcpy(recptr, &checkPoint, sizeof(checkPoint));
  recptr += sizeof(checkPoint);
  Assert(recptr - (char *) record == record->xl_tot_len);

  INIT_CRC32C(crc);
  COMP_CRC32C(crc, ((char *) record) + SizeOfXLogRecord, record->xl_tot_len - SizeOfXLogRecord);
  COMP_CRC32C(crc, (char *) record, offsetof(XLogRecord, xl_crc));
  FIN_CRC32C(crc);
  record->xl_crc = crc;

  /* Create first XLOG segment file */
  use_existent = false;
  openLogFile = XLogFileInit(1, &use_existent, false);

  /* Write the first page with the initial record */
  errno = 0;
  pgstat_report_wait_start(WAIT_EVENT_WAL_BOOTSTRAP_WRITE);
  if (write(openLogFile, page, XLOG_BLCKSZ) != XLOG_BLCKSZ)
  {
    /* if write didn't set errno, assume problem is no disk space */
    if (errno == 0)
      errno = ENOSPC;
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not write bootstrap write-ahead log file: %m")));
  }
  pgstat_report_wait_end();

  pgstat_report_wait_start(WAIT_EVENT_WAL_BOOTSTRAP_SYNC);
  if (pg_fsync(openLogFile) != 0)
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not fsync bootstrap write-ahead log file: %m")));
  pgstat_report_wait_end();

  if (close(openLogFile))
    ereport(PANIC,
        (errcode_for_file_access(),
         errmsg("could not close bootstrap write-ahead log file: %m")));

  openLogFile = -1;

  /* Now create pg_control */

  memset(ControlFile, 0, sizeof(ControlFileData));
  /* Initialize pg_control status fields */
  ControlFile->system_identifier = sysidentifier;
  memcpy(ControlFile->mock_authentication_nonce, mock_auth_nonce, MOCK_AUTH_NONCE_LEN);
  ControlFile->state = DB_SHUTDOWNED;
  ControlFile->time = checkPoint.time;
  ControlFile->checkPoint = checkPoint.redo;
  ControlFile->checkPointCopy = checkPoint;
  ControlFile->unloggedLSN = 1;

  /* Set important parameter values for use when replaying WAL */
  ControlFile->MaxConnections = MaxConnections;
  ControlFile->max_worker_processes = max_worker_processes;
  ControlFile->max_prepared_xacts = max_prepared_xacts;
  ControlFile->max_locks_per_xact = max_locks_per_xact;
  ControlFile->wal_level = wal_level;
  ControlFile->wal_log_hints = wal_log_hints;
  ControlFile->track_commit_timestamp = track_commit_timestamp;
  ControlFile->data_checksum_version = bootstrap_data_checksum_version;

  /* some additional ControlFile fields are set in WriteControlFile() */

  WriteControlFile();

  /* Bootstrap the commit log, too */
  BootStrapCLOG();
  BootStrapCommitTs();
  BootStrapSUBTRANS();
  BootStrapMultiXact();

  pfree(buffer);

  /*
   * Force control file to be read - in contrast to normal processing we'd
   * otherwise never run the checks and GUC related initializations therein.
   */
  ReadControlFile();
}

static char *
str_time(pg_time_t tnow)
{
  static char buf[128];

  pg_strftime(buf, sizeof(buf),
        "%Y-%m-%d %H:%M:%S %Z",
        pg_localtime(&tnow, log_timezone));

  return buf;
}

/*
 * See if there is a recovery command file (recovery.conf), and if so
 * read in parameters for archive recovery and XLOG streaming.
 *
 * The file is parsed using the main configuration parser.
 */
static void
readRecoveryCommandFile(void)
{
  FILE     *fd;
  TimeLineID  rtli = 0;
  bool    rtliGiven = false;
  ConfigVariable *item,
         *head = NULL,
         *tail = NULL;
  bool    recoveryTargetActionSet = false;


  fd = AllocateFile(RECOVERY_COMMAND_FILE, "r");
  if (fd == NULL)
  {
    if (errno == ENOENT)
      return;        /* not there, so no archive recovery */
    ereport(FATAL,
        (errcode_for_file_access(),
         errmsg("could not open recovery command file \"%s\": %m",
            RECOVERY_COMMAND_FILE)));
  }

  /*
   * Since we're asking ParseConfigFp() to report errors as FATAL, there's
   * no need to check the return value.
   */
  (void) ParseConfigFp(fd, RECOVERY_COMMAND_FILE, 0, FATAL, &head, &tail);

  FreeFile(fd);

  for (item = head; item; item = item->next)
  {
    if (strcmp(item->name, "restore_command") == 0)
    {
      recoveryRestoreCommand = pstrdup(item->value);
      ereport(DEBUG2,
          (errmsg_internal("restore_command = '%s'",
                   recoveryRestoreCommand)));
    }
    else if (strcmp(item->name, "recovery_end_command") == 0)
    {
      recoveryEndCommand = pstrdup(item->value);
      ereport(DEBUG2,
          (errmsg_internal("recovery_end_command = '%s'",
                   recoveryEndCommand)));
    }
    else if (strcmp(item->name, "archive_cleanup_command") == 0)
    {
      archiveCleanupCommand = pstrdup(item->value);
      ereport(DEBUG2,
          (errmsg_internal("archive_cleanup_command = '%s'",
                   archiveCleanupCommand)));
    }
    else if (strcmp(item->name, "recovery_target_action") == 0)
    {
      if (strcmp(item->value, "pause") == 0)
        recoveryTargetAction = RECOVERY_TARGET_ACTION_PAUSE;
      else if (strcmp(item->value, "promote") == 0)
        recoveryTargetAction = RECOVERY_TARGET_ACTION_PROMOTE;
      else if (strcmp(item->value, "shutdown") == 0)
        recoveryTargetAction = RECOVERY_TARGET_ACTION_SHUTDOWN;
      else
        ereport(ERROR,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("invalid value for recovery parameter \"%s\": \"%s\"",
                "recovery_target_action",
                item->value),
             errhint("Valid values are \"pause\", \"promote\", and \"shutdown\".")));

      ereport(DEBUG2,
          (errmsg_internal("recovery_target_action = '%s'",
                   item->value)));

      recoveryTargetActionSet = true;
    }
    else if (strcmp(item->name, "recovery_target_timeline") == 0)
    {
      rtliGiven = true;
      if (strcmp(item->value, "latest") == 0)
        rtli = 0;
      else
      {
        errno = 0;
        rtli = (TimeLineID) strtoul(item->value, NULL, 0);
        if (errno == EINVAL || errno == ERANGE)
          ereport(FATAL,
              (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
               errmsg("recovery_target_timeline is not a valid number: \"%s\"",
                  item->value)));
      }
      if (rtli)
        ereport(DEBUG2,
            (errmsg_internal("recovery_target_timeline = %u", rtli)));
      else
        ereport(DEBUG2,
            (errmsg_internal("recovery_target_timeline = latest")));
    }
    else if (strcmp(item->name, "recovery_target_xid") == 0)
    {
      errno = 0;
      recoveryTargetXid = (TransactionId) strtoul(item->value, NULL, 0);
      if (errno == EINVAL || errno == ERANGE)
        ereport(FATAL,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("recovery_target_xid is not a valid number: \"%s\"",
                item->value)));
      ereport(DEBUG2,
          (errmsg_internal("recovery_target_xid = %u",
                   recoveryTargetXid)));
      recoveryTarget = RECOVERY_TARGET_XID;
    }
    else if (strcmp(item->name, "recovery_target_time") == 0)
    {
      recoveryTarget = RECOVERY_TARGET_TIME;

      if (strcmp(item->value, "epoch") == 0 ||
        strcmp(item->value, "infinity") == 0 ||
        strcmp(item->value, "-infinity") == 0 ||
        strcmp(item->value, "now") == 0 ||
        strcmp(item->value, "today") == 0 ||
        strcmp(item->value, "tomorrow") == 0 ||
        strcmp(item->value, "yesterday") == 0)
        ereport(FATAL,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("recovery_target_time is not a valid timestamp: \"%s\"",
                item->value)));

      /*
       * Convert the time string given by the user to TimestampTz form.
       */
      recoveryTargetTime =
        DatumGetTimestampTz(DirectFunctionCall3(timestamptz_in,
                            CStringGetDatum(item->value),
                            ObjectIdGetDatum(InvalidOid),
                            Int32GetDatum(-1)));
      ereport(DEBUG2,
          (errmsg_internal("recovery_target_time = '%s'",
                   timestamptz_to_str(recoveryTargetTime))));
    }
    else if (strcmp(item->name, "recovery_target_name") == 0)
    {
      recoveryTarget = RECOVERY_TARGET_NAME;

      recoveryTargetName = pstrdup(item->value);
      if (strlen(recoveryTargetName) >= MAXFNAMELEN)
        ereport(FATAL,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("recovery_target_name is too long (maximum %d characters)",
                MAXFNAMELEN - 1)));

      ereport(DEBUG2,
          (errmsg_internal("recovery_target_name = '%s'",
                   recoveryTargetName)));
    }
    else if (strcmp(item->name, "recovery_target_lsn") == 0)
    {
      recoveryTarget = RECOVERY_TARGET_LSN;

      /*
       * Convert the LSN string given by the user to XLogRecPtr form.
       */
      recoveryTargetLSN =
        DatumGetLSN(DirectFunctionCall3(pg_lsn_in,
                        CStringGetDatum(item->value),
                        ObjectIdGetDatum(InvalidOid),
                        Int32GetDatum(-1)));
      ereport(DEBUG2,
          (errmsg_internal("recovery_target_lsn = '%X/%X'",
                   (uint32) (recoveryTargetLSN >> 32),
                   (uint32) recoveryTargetLSN)));
    }
    else if (strcmp(item->name, "recovery_target") == 0)
    {
      if (strcmp(item->value, "immediate") == 0)
        recoveryTarget = RECOVERY_TARGET_IMMEDIATE;
      else
        ereport(ERROR,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("invalid value for recovery parameter \"%s\": \"%s\"",
                "recovery_target",
                item->value),
             errhint("The only allowed value is \"immediate\".")));
      ereport(DEBUG2,
          (errmsg_internal("recovery_target = '%s'",
                   item->value)));
    }
    else if (strcmp(item->name, "recovery_target_inclusive") == 0)
    {
      /*
       * does nothing if a recovery_target is not also set
       */
      if (!parse_bool(item->value, &recoveryTargetInclusive))
        ereport(ERROR,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("parameter \"%s\" requires a Boolean value",
                "recovery_target_inclusive")));
      ereport(DEBUG2,
          (errmsg_internal("recovery_target_inclusive = %s",
                   item->value)));
    }
    else if (strcmp(item->name, "standby_mode") == 0)
    {
      if (!parse_bool(item->value, &StandbyModeRequested))
        ereport(ERROR,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("parameter \"%s\" requires a Boolean value",
                "standby_mode")));
      ereport(DEBUG2,
          (errmsg_internal("standby_mode = '%s'", item->value)));
    }
    else if (strcmp(item->name, "primary_conninfo") == 0)
    {
      PrimaryConnInfo = pstrdup(item->value);
      ereport(DEBUG2,
          (errmsg_internal("primary_conninfo = '%s'",
                   PrimaryConnInfo)));
    }
    else if (strcmp(item->name, "primary_slot_name") == 0)
    {
      ReplicationSlotValidateName(item->value, ERROR);
      PrimarySlotName = pstrdup(item->value);
      ereport(DEBUG2,
          (errmsg_internal("primary_slot_name = '%s'",
                   PrimarySlotName)));
    }
    else if (strcmp(item->name, "trigger_file") == 0)
    {
      TriggerFile = pstrdup(item->value);
      ereport(DEBUG2,
          (errmsg_internal("trigger_file = '%s'",
                   TriggerFile)));
    }
    else if (strcmp(item->name, "recovery_min_apply_delay") == 0)
    {
      const char *hintmsg;

      if (!parse_int(item->value, &recovery_min_apply_delay, GUC_UNIT_MS,
               &hintmsg))
        ereport(ERROR,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("parameter \"%s\" requires a temporal value",
                "recovery_min_apply_delay"),
             hintmsg ? errhint("%s", _(hintmsg)) : 0));
      ereport(DEBUG2,
          (errmsg_internal("recovery_min_apply_delay = '%s'", item->value)));
    }
    else
      ereport(FATAL,
          (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
           errmsg("unrecognized recovery parameter \"%s\"",
              item->name)));
  }

  /*
   * Check for compulsory parameters
   */
  if (StandbyModeRequested)
  {
    if (PrimaryConnInfo == NULL && recoveryRestoreCommand == NULL)
      ereport(WARNING,
          (errmsg("recovery command file \"%s\" specified neither primary_conninfo nor restore_command",
              RECOVERY_COMMAND_FILE),
           errhint("The database server will regularly poll the pg_wal subdirectory to check for files placed there.")));
  }
  else
  {
    if (recoveryRestoreCommand == NULL)
      ereport(FATAL,
          (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
           errmsg("recovery command file \"%s\" must specify restore_command when standby mode is not enabled",
              RECOVERY_COMMAND_FILE)));
  }

  /*
   * Override any inconsistent requests. Not that this is a change of
   * behaviour in 9.5; prior to this we simply ignored a request to pause if
   * hot_standby = off, which was surprising behaviour.
   */
  if (recoveryTargetAction == RECOVERY_TARGET_ACTION_PAUSE &&
    recoveryTargetActionSet &&
    !EnableHotStandby)
    recoveryTargetAction = RECOVERY_TARGET_ACTION_SHUTDOWN;

  /*
   * We don't support standby_mode in standalone backends; that requires
   * other processes such as the WAL receiver to be alive.
   */
  if (StandbyModeRequested && !IsUnderPostmaster)
    ereport(FATAL,
        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
         errmsg("standby mode is not supported by single-user servers")));

  /* Enable fetching from archive recovery area */
  ArchiveRecoveryRequested = true;

  /*
   * If user specified recovery_target_timeline, validate it or compute the
   * "latest" value.  We can't do this until after we've gotten the restore
   * command and set InArchiveRecovery, because we need to fetch timeline
   * history files from the archive.
   */
  if (rtliGiven)
  {
    if (rtli)
    {
      /* Timeline 1 does not have a history file, all else should */
      if (rtli != 1 && !existsTimeLineHistory(rtli))
        ereport(FATAL,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("recovery target timeline %u does not exist",
                rtli)));
      recoveryTargetTLI = rtli;
      recoveryTargetIsLatest = false;
    }
    else
    {
      /* We start the "latest" search from pg_control's timeline */
      recoveryTargetTLI = findNewestTimeLine(recoveryTargetTLI);
      recoveryTargetIsLatest = true;
    }
  }

  FreeConfigVariables(head);
}

/*
 * Exit archive-recovery state
 */
static void
exitArchiveRecovery(TimeLineID endTLI, XLogRecPtr endOfLog)
{
  char    recoveryPath[MAXPGPATH];
  char    xlogfname[MAXFNAMELEN];
  XLogSegNo endLogSegNo;
  XLogSegNo startLogSegNo;

  /* we always switch to a new timeline after archive recovery */
  Assert(endTLI != ThisTimeLineID);

  /*
   * We are no longer in archive recovery state.
   */
  InArchiveRecovery = false;

  /*
   * Update min recovery point one last time.
   */
  UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);

  /*
   * If the ending log segment is still open, close it (to avoid problems on
   * Windows with trying to rename or delete an open file).
   */
  if (readFile >= 0)
  {
    close(readFile);
    readFile = -1;
  }

  /*
   * Calculate the last segment on the old timeline, and the first segment
   * on the new timeline. If the switch happens in the middle of a segment,
   * they are the same, but if the switch happens exactly at a segment
   * boundary, startLogSegNo will be endLogSegNo + 1.
   */
  XLByteToPrevSeg(endOfLog, endLogSegNo, wal_segment_size);
  XLByteToSeg(endOfLog, startLogSegNo, wal_segment_size);

  /*
   * Initialize the starting WAL segment for the new timeline. If the switch
   * happens in the middle of a segment, copy data from the last WAL segment
   * of the old timeline up to the switch point, to the starting WAL segment
   * on the new timeline.
   */
  if (endLogSegNo == startLogSegNo)
  {
    /*
     * Make a copy of the file on the new timeline.
     *
     * Writing WAL isn't allowed yet, so there are no locking
     * considerations. But we should be just as tense as XLogFileInit to
     * avoid emplacing a bogus file.
     */
    XLogFileCopy(endLogSegNo, endTLI, endLogSegNo,
           XLogSegmentOffset(endOfLog, wal_segment_size));
  }
  else
  {
    /*
     * The switch happened at a segment boundary, so just create the next
     * segment on the new timeline.
     */
    bool    use_existent = true;
    int     fd;

    fd = XLogFileInit(startLogSegNo, &use_existent, true);

    if (close(fd))
      ereport(ERROR,
          (errcode_for_file_access(),
           errmsg("could not close log file %s: %m",
              XLogFileNameP(ThisTimeLineID, startLogSegNo))));
  }

  /*
   * Let's just make real sure there are not .ready or .done flags posted
   * for the new segment.
   */
  XLogFileName(xlogfname, ThisTimeLineID, startLogSegNo, wal_segment_size);
  XLogArchiveCleanup(xlogfname);

  /*
   * Since there might be a partial WAL segment named RECOVERYXLOG, get rid
   * of it.
   */
  snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYXLOG");
  unlink(recoveryPath);   /* ignore any error */

  /* Get rid of any remaining recovered timeline-history file, too */
  snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYHISTORY");
  unlink(recoveryPath);   /* ignore any error */

  /*
   * Rename the config file out of the way, so that we don't accidentally
   * re-enter archive recovery mode in a subsequent crash.
   */
  unlink(RECOVERY_COMMAND_DONE);
  durable_rename(RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE, FATAL);

  ereport(LOG,
      (errmsg("archive recovery complete")));
}

/*
 * Extract timestamp from WAL record.
 *
 * If the record contains a timestamp, returns true, and saves the timestamp
 * in *recordXtime. If the record type has no timestamp, returns false.
 * Currently, only transaction commit/abort records and restore points contain
 * timestamps.
 */
static bool
getRecordTimestamp(XLogReaderState *record, TimestampTz *recordXtime)
{
  uint8   info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
  uint8   xact_info = info & XLOG_XACT_OPMASK;
  uint8   rmid = XLogRecGetRmid(record);

  if (rmid == RM_XLOG_ID && info == XLOG_RESTORE_POINT)
  {
    *recordXtime = ((xl_restore_point *) XLogRecGetData(record))->rp_time;
    return true;
  }
  if (rmid == RM_XACT_ID && (xact_info == XLOG_XACT_COMMIT ||
                 xact_info == XLOG_XACT_COMMIT_PREPARED))
  {
    *recordXtime = ((xl_xact_commit *) XLogRecGetData(record))->xact_time;
    return true;
  }
  if (rmid == RM_XACT_ID && (xact_info == XLOG_XACT_ABORT ||
                 xact_info == XLOG_XACT_ABORT_PREPARED))
  {
    *recordXtime = ((xl_xact_abort *) XLogRecGetData(record))->xact_time;
    return true;
  }
  return false;
}

/*
 * For point-in-time recovery, this function decides whether we want to
 * stop applying the XLOG before the current record.
 *
 * Returns true if we are stopping, false otherwise. If stopping, some
 * information is saved in recoveryStopXid et al for use in annotating the
 * new timeline's history file.
 */
static bool
recoveryStopsBefore(XLogReaderState *record)
{
  bool    stopsHere = false;
  uint8   xact_info;
  bool    isCommit;
  TimestampTz recordXtime = 0;
  TransactionId recordXid;

  /* Check if we should stop as soon as reaching consistency */
  if (recoveryTarget == RECOVERY_TARGET_IMMEDIATE && reachedConsistency0)
  {
    ereport(LOG,
        (errmsg("recovery stopping after reaching consistency")));

    recoveryStopAfter = false;
    recoveryStopXid = InvalidTransactionId;
    recoveryStopLSN = InvalidXLogRecPtr;
    recoveryStopTime = 0;
    recoveryStopName[0] = '\0';
    return true;
  }

  /* Check if target LSN has been reached */
  if (recoveryTarget == RECOVERY_TARGET_LSN &&
    !recoveryTargetInclusive0 &&
    record->ReadRecPtr >= recoveryTargetLSN0)
  {
    recoveryStopAfter = false;
    recoveryStopXid = InvalidTransactionId;
    recoveryStopLSN = record->ReadRecPtr;
    recoveryStopTime = 0;
    recoveryStopName[0] = '\0';
    ereport(LOG,
        (errmsg("recovery stopping before WAL location (LSN) \"%X/%X\"",
            (uint32) (recoveryStopLSN >> 32),
            (uint32) recoveryStopLSN)));
    return true;
  }

  /* Otherwise we only consider stopping before COMMIT or ABORT records. */
  if (XLogRecGetRmid(record) != RM_XACT_ID)
    return false;

  xact_info = XLogRecGetInfo(record) & XLOG_XACT_OPMASK;

  if (xact_info == XLOG_XACT_COMMIT)
  {
    isCommit = true;
    recordXid = XLogRecGetXid(record);
  }
  else if (xact_info == XLOG_XACT_COMMIT_PREPARED)
  {
    xl_xact_commit *xlrec = (xl_xact_commit *) XLogRecGetData(record);
    xl_xact_parsed_commit parsed;

    isCommit = true;
    ParseCommitRecord(XLogRecGetInfo(record),
              xlrec,
              &parsed);
    recordXid = parsed.twophase_xid;
  }
  else if (xact_info == XLOG_XACT_ABORT)
  {
    isCommit = false;
    recordXid = XLogRecGetXid(record);
  }
  else if (xact_info == XLOG_XACT_ABORT_PREPARED)
  {
    xl_xact_abort *xlrec = (xl_xact_abort *) XLogRecGetData(record);
    xl_xact_parsed_abort parsed;

    isCommit = true;
    ParseAbortRecord(XLogRecGetInfo(record),
             xlrec,
             &parsed);
    recordXid = parsed.twophase_xid;
  }
  else
    return false;

  if (recoveryTarget == RECOVERY_TARGET_XID && !recoveryTargetInclusive)
  {
    /*
     * There can be only one transaction end record with this exact
     * transactionid
     *
     * when testing for an xid, we MUST test for equality only, since
     * transactions are numbered in the order they start, not the order
     * they complete. A higher numbered xid will complete before you about
     * 50% of the time...
     */
    stopsHere = (recordXid == recoveryTargetXid);
  }

  if (recoveryTarget == RECOVERY_TARGET_TIME &&
    getRecordTimestamp(record, &recordXtime))
  {
    /*
     * There can be many transactions that share the same commit time, so
     * we stop after the last one, if we are inclusive, or stop at the
     * first one if we are exclusive
     */
    if (recoveryTargetInclusive)
      stopsHere = (recordXtime > recoveryTargetTime);
    else
      stopsHere = (recordXtime >= recoveryTargetTime);
  }

  if (stopsHere)
  {
    recoveryStopAfter = false;
    recoveryStopXid = recordXid;
    recoveryStopTime = recordXtime;
    recoveryStopLSN = InvalidXLogRecPtr;
    recoveryStopName[0] = '\0';

    if (isCommit)
    {
      ereport(LOG,
          (errmsg("recovery stopping before commit of transaction %u, time %s",
              recoveryStopXid,
              timestamptz_to_str(recoveryStopTime))));
    }
    else
    {
      ereport(LOG,
          (errmsg("recovery stopping before abort of transaction %u, time %s",
              recoveryStopXid,
              timestamptz_to_str(recoveryStopTime))));
    }
  }

  return stopsHere;
}

/*
 * Same as recoveryStopsBefore, but called after applying the record.
 *
 * We also track the timestamp of the latest applied COMMIT/ABORT
 * record in XLogCtl->recoveryLastXTime.
 */
static bool
recoveryStopsAfter(XLogReaderState *record)
{
  uint8   info;
  uint8   xact_info;
  uint8   rmid;
  TimestampTz recordXtime;

  info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
  rmid = XLogRecGetRmid(record);

  /*
   * There can be many restore points that share the same name; we stop at
   * the first one.
   */
  if (recoveryTarget == RECOVERY_TARGET_NAME &&
    rmid == RM_XLOG_ID0 && info == 0XLOG_RESTORE_POINT0)
  {
    xl_restore_point *recordRestorePointData;

    recordRestorePointData = (xl_restore_point *) XLogRecGetData(record);

    if (strcmp(recordRestorePointData->rp_name, recoveryTargetName) == 0)
    {
      recoveryStopAfter = true;
      recoveryStopXid = InvalidTransactionId;
      recoveryStopLSN = InvalidXLogRecPtr;
      (void) getRecordTimestamp(record, &recoveryStopTime);
      strlcpy(recoveryStopName, recordRestorePointData->rp_name, MAXFNAMELEN);

      ereport(LOG,
          (errmsg("recovery stopping at restore point \"%s\", time %s",
              recoveryStopName,
              timestamptz_to_str(recoveryStopTime))));
      return true;
    }
  }

  /* Check if the target LSN has been reached */
  if (recoveryTarget == RECOVERY_TARGET_LSN &&
    recoveryTargetInclusive0 &&
    record->ReadRecPtr >= recoveryTargetLSN0)
  {
    recoveryStopAfter = true;
    recoveryStopXid = InvalidTransactionId;
    recoveryStopLSN = record->ReadRecPtr;
    recoveryStopTime = 0;
    recoveryStopName[0] = '\0';
    ereport(LOG,
        (errmsg("recovery stopping after WAL location (LSN) \"%X/%X\"",
            (uint32) (recoveryStopLSN >> 32),
            (uint32) recoveryStopLSN)));
    return true;
  }

  if (rmid != RM_XACT_ID)
    return false;

  xact_info = info & XLOG_XACT_OPMASK;

  if (xact_info == XLOG_XACT_COMMIT ||
    xact_info == XLOG_XACT_COMMIT_PREPARED ||
    xact_info == XLOG_XACT_ABORT ||
    xact_info == XLOG_XACT_ABORT_PREPARED)
  {
    TransactionId recordXid;

    /* Update the last applied transaction timestamp */
    if (getRecordTimestamp(record, &recordXtime))
      SetLatestXTime(recordXtime);

    /* Extract the XID of the committed/aborted transaction */
    if (xact_info == XLOG_XACT_COMMIT_PREPARED)
    {
      xl_xact_commit *xlrec = (xl_xact_commit *) XLogRecGetData(record);
      xl_xact_parsed_commit parsed;

      ParseCommitRecord(XLogRecGetInfo(record),
                xlrec,
                &parsed);
      recordXid = parsed.twophase_xid;
    }
    else if (xact_info == XLOG_XACT_ABORT_PREPARED)
    {
      xl_xact_abort *xlrec = (xl_xact_abort *) XLogRecGetData(record);
      xl_xact_parsed_abort parsed;

      ParseAbortRecord(XLogRecGetInfo(record),
               xlrec,
               &parsed);
      recordXid = parsed.twophase_xid;
    }
    else
      recordXid = XLogRecGetXid(record);

    /*
     * There can be only one transaction end record with this exact
     * transactionid
     *
     * when testing for an xid, we MUST test for equality only, since
     * transactions are numbered in the order they start, not the order
     * they complete. A higher numbered xid will complete before you about
     * 50% of the time...
     */
    if (recoveryTarget == RECOVERY_TARGET_XID && recoveryTargetInclusive &&
      recordXid == recoveryTargetXid)
    {
      recoveryStopAfter = true;
      recoveryStopXid = recordXid;
      recoveryStopTime = recordXtime;
      recoveryStopLSN = InvalidXLogRecPtr;
      recoveryStopName[0] = '\0';

      if (xact_info == XLOG_XACT_COMMIT ||
        xact_info == XLOG_XACT_COMMIT_PREPARED)
      {
        ereport(LOG,
            (errmsg("recovery stopping after commit of transaction %u, time %s",
                recoveryStopXid,
                timestamptz_to_str(recoveryStopTime))));
      }
      else if (xact_info == XLOG_XACT_ABORT ||
           xact_info == XLOG_XACT_ABORT_PREPARED)
      {
        ereport(LOG,
            (errmsg("recovery stopping after abort of transaction %u, time %s",
                recoveryStopXid,
                timestamptz_to_str(recoveryStopTime))));
      }
      return true;
    }
  }

  /* Check if we should stop as soon as reaching consistency */
  if (recoveryTarget == RECOVERY_TARGET_IMMEDIATE && reachedConsistency)
  {
    ereport(LOG,
        (errmsg("recovery stopping after reaching consistency")));

    recoveryStopAfter = true;
    recoveryStopXid = InvalidTransactionId;
    recoveryStopTime = 0;
    recoveryStopLSN = InvalidXLogRecPtr;
    recoveryStopName[0] = '\0';
    return true;
  }

  return false;
}

/*
 * Wait until shared recoveryPause flag is cleared.
 *
 * XXX Could also be done with shared latch, avoiding the pg_usleep loop.
 * Probably not worth the trouble though.  This state shouldn't be one that
 * anyone cares about server power consumption in.
 */
static void
recoveryPausesHere(void)
{
  /* Don't pause unless users can connect! */
  if (!LocalHotStandbyActive)
    return;

  ereport(LOG,
      (errmsg("recovery has paused"),
       errhint("Execute pg_wal_replay_resume() to continue.")));

  while (RecoveryIsPaused())
  {
    pg_usleep(1000000L);  /* 1000 ms */
    HandleStartupProcInterrupts();
  }
}

bool
RecoveryIsPaused(void)
{
  bool    recoveryPause;

  SpinLockAcquire(&XLogCtl->info_lck);
  recoveryPause = XLogCtl->recoveryPause;
  SpinLockRelease(&XLogCtl->info_lck);

  return recoveryPause;
}

void
SetRecoveryPause(bool recoveryPause)
{
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->recoveryPause = recoveryPause;
  SpinLockRelease(&XLogCtl->info_lck);
}

/*
 * When recovery_min_apply_delay is set, we wait long enough to make sure
 * certain record types are applied at least that interval behind the master.
 *
 * Returns true if we waited.
 *
 * Note that the delay is calculated between the WAL record log time and
 * the current time on standby. We would prefer to keep track of when this
 * standby received each WAL record, which would allow a more consistent
 * approach and one not affected by time synchronisation issues, but that
 * is significantly more effort and complexity for little actual gain in
 * usability.
 */
static bool
recoveryApplyDelay(XLogReaderState *record)
{
  uint8   xact_info;
  TimestampTz xtime;
  long    secs;
  int     microsecs;

  /* nothing to do if no delay configured */
  if (recovery_min_apply_delay <= 0)
    return false;

  /* no delay is applied on a database not yet consistent */
  if (!reachedConsistency)
    return false;

  /*
   * Is it a COMMIT record?
   *
   * We deliberately choose not to delay aborts since they have no effect on
   * MVCC. We already allow replay of records that don't have a timestamp,
   * so there is already opportunity for issues caused by early conflicts on
   * standbys.
   */
  if (XLogRecGetRmid(record) != RM_XACT_ID)
    return false;

  xact_info = XLogRecGetInfo(record) & XLOG_XACT_OPMASK;

  if (xact_info != XLOG_XACT_COMMIT &&
    xact_info != XLOG_XACT_COMMIT_PREPARED)
    return false;

  if (!getRecordTimestamp(record, &xtime))
    return false;

  recoveryDelayUntilTime =
    TimestampTzPlusMilliseconds(xtime, recovery_min_apply_delay);

  /*
   * Exit without arming the latch if it's already past time to apply this
   * record
   */
  TimestampDifference(GetCurrentTimestamp(), recoveryDelayUntilTime,
            &secs, &microsecs);
  if (secs <= 0 && microsecs <= 0)
    return false;

  while (true)
  {
    ResetLatch(&XLogCtl->recoveryWakeupLatch);

    /* might change the trigger file's location */
    HandleStartupProcInterrupts();

    if (CheckForStandbyTrigger())
      break;

    /*
     * Wait for difference between GetCurrentTimestamp() and
     * recoveryDelayUntilTime
     */
    TimestampDifference(GetCurrentTimestamp(), recoveryDelayUntilTime,
              &secs, &microsecs);

    /* NB: We're ignoring waits below min_apply_delay's resolution. */
    if (secs <= 0 && microsecs / 1000 <= 0)
      break;

    elog(DEBUG2, "recovery apply delay %ld seconds, %d milliseconds",
       secs, microsecs / 1000);

    WaitLatch(&XLogCtl->recoveryWakeupLatch,
          WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
          secs * 1000L + microsecs / 1000,
          WAIT_EVENT_RECOVERY_APPLY_DELAY);
  }
  return true;
}

/*
 * Save timestamp of latest processed commit/abort record.
 *
 * We keep this in XLogCtl, not a simple static variable, so that it can be
 * seen by processes other than the startup process.  Note in particular
 * that CreateRestartPoint is executed in the checkpointer.
 */
static void
SetLatestXTime(TimestampTz xtime)
{
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->recoveryLastXTime = xtime;
  SpinLockRelease(&XLogCtl->info_lck);
}

/*
 * Fetch timestamp of latest processed commit/abort record.
 */
TimestampTz
GetLatestXTime(void)
{
  TimestampTz xtime;

  SpinLockAcquire(&XLogCtl->info_lck);
  xtime = XLogCtl->recoveryLastXTime;
  SpinLockRelease(&XLogCtl->info_lck);

  return xtime;
}

/*
 * Save timestamp of the next chunk of WAL records to apply.
 *
 * We keep this in XLogCtl, not a simple static variable, so that it can be
 * seen by all backends.
 */
static void
SetCurrentChunkStartTime(TimestampTz xtime)
{
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->currentChunkStartTime = xtime;
  SpinLockRelease(&XLogCtl->info_lck);
}

/*
 * Fetch timestamp of latest processed commit/abort record.
 * Startup process maintains an accurate local copy in XLogReceiptTime
 */
TimestampTz
GetCurrentChunkReplayStartTime(void)
{
  TimestampTz xtime;

  SpinLockAcquire(&XLogCtl->info_lck);
  xtime = XLogCtl->currentChunkStartTime;
  SpinLockRelease(&XLogCtl->info_lck);

  return xtime;
}

/*
 * Returns time of receipt of current chunk of XLOG data, as well as
 * whether it was received from streaming replication or from archives.
 */
void
GetXLogReceiptTime(TimestampTz *rtime, bool *fromStream)
{
  /*
   * This must be executed in the startup process, since we don't export the
   * relevant state to shared memory.
   */
  Assert(InRecovery);

  *rtime = XLogReceiptTime;
  *fromStream = (XLogReceiptSource == XLOG_FROM_STREAM);
}

/*
 * Note that text field supplied is a parameter name and does not require
 * translation
 */
#define RecoveryRequiresIntParameter(param_name, currValue, minValue) \
do { \
  if ((currValue) < (minValue)) \
    ereport(ERROR, \
        (errcode(ERRCODE_INVALID_PARAMETER_VALUE), \
         errmsg("hot standby is not possible because " \
            "%s = %d is a lower setting than on the master server " \
            "(its value was %d)", \
            param_name, \
            currValue, \
            minValue))); \
} while(0)

/*
 * Check to see if required parameters are set high enough on this server
 * for various aspects of recovery operation.
 *
 * Note that all the parameters which this function tests need to be
 * listed in Administrator's Overview section in high-availability.sgml.
 * If you change them, don't forget to update the list.
 */
static void
CheckRequiredParameterValues(void)
{
  /*
   * For archive recovery, the WAL must be generated with at least 'replica'
   * wal_level.
   */
  if (ArchiveRecoveryRequested && ControlFile->wal_level == WAL_LEVEL_MINIMAL0)
  {
    ereport(WARNING,
        (errmsg("WAL was generated with wal_level=minimal, data may be missing"),
         errhint("This happens if you temporarily set wal_level=minimal without taking a new base backup.")));
  }

  /*
   * For Hot Standby, the WAL must be generated with 'replica' mode, and we
   * must have at least as many backend slots as the primary.
   */
  if (ArchiveRecoveryRequested && EnableHotStandby0)
  {
    if (ControlFile->wal_level < WAL_LEVEL_REPLICA)
      ereport(ERROR,
          (errmsg("hot standby is not possible because wal_level was not set to \"replica\" or higher on the master server"),
           errhint("Either set wal_level to \"replica\" on the master, or turn off hot_standby here.")));

    /* We ignore autovacuum_max_workers when we make this test. */
    RecoveryRequiresIntParameter("max_connections",
                   MaxConnections,
                   ControlFile->MaxConnections);
    RecoveryRequiresIntParameter("max_worker_processes",
                   max_worker_processes,
                   ControlFile->max_worker_processes);
    RecoveryRequiresIntParameter("max_prepared_transactions",
                   max_prepared_xacts,
                   ControlFile->max_prepared_xacts);
    RecoveryRequiresIntParameter("max_locks_per_transaction",
                   max_locks_per_xact,
                   ControlFile->max_locks_per_xact);
  }
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
  XLogCtlInsert *Insert;
  CheckPoint  checkPoint;
  bool    wasShutdown;
  bool    reachedStopPoint = false;
  bool    haveBackupLabel = false;
  bool    haveTblspcMap = false;
  XLogRecPtr  RecPtr,
        checkPointLoc,
        EndOfLog;
  TimeLineID  EndOfLogTLI;
  TimeLineID  PrevTimeLineID;
  XLogRecord *record;
  TransactionId oldestActiveXID;
  bool    backupEndRequired = false;
  bool    backupFromStandby = false;
  DBState   dbstate_at_startup;
  XLogReaderState *xlogreader;
  XLogPageReadPrivate private;
  bool    fast_promoted = false;
  struct stat st;

  /*
   * Verify XLOG status looks valid.
   */
  if (ControlFile->state < DB_SHUTDOWNED ||
    ControlFile->state > DB_IN_PRODUCTION ||
    !XRecOffIsValid(ControlFile->checkPoint))
    ereport(FATAL,
        (errmsg("control file contains invalid data")));

  if (ControlFile->state == DB_SHUTDOWNED)
  {
    /* This is the expected case, so don't be chatty in standalone mode */
    ereport(IsPostmasterEnvironment ? LOG : NOTICE,
        (errmsg("database system was shut down at %s",
            str_time(ControlFile->time))));
  }
  else if (ControlFile->state == DB_SHUTDOWNED_IN_RECOVERY)
    ereport(LOG,
        (errmsg("database system was shut down in recovery at %s",
            str_time(ControlFile->time))));
  else if (ControlFile->state == DB_SHUTDOWNING)
    ereport(LOG,
        (errmsg("database system shutdown was interrupted; last known up at %s",
            str_time(ControlFile->time))));
  else if (ControlFile->state == DB_IN_CRASH_RECOVERY)
    ereport(LOG,
        (errmsg("database system was interrupted while in recovery at %s",
            str_time(ControlFile->time)),
         errhint("This probably means that some data is corrupted and"
             " you will have to use the last backup for recovery.")));
  else if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY)
    ereport(LOG,
        (errmsg("database system was interrupted while in recovery at log time %s",
            str_time(ControlFile->checkPointCopy.time)),
         errhint("If this has occurred more than once some data might be corrupted"
             " and you might need to choose an earlier recovery target.")));
  else if (ControlFile->state == DB_IN_PRODUCTION)
    ereport(LOG,
        (errmsg("database system was interrupted; last known up at %s",
            str_time(ControlFile->time))));

  /* This is just to allow attaching to startup process with a debugger */
#ifdef XLOG_REPLAY_DELAY
  if (ControlFile->state != DB_SHUTDOWNED)
    pg_usleep(60000000L);
#endif

  /*
   * Verify that pg_wal and pg_wal/archive_status exist.  In cases where
   * someone has performed a copy for PITR, these directories may have been
   * excluded and need to be re-created.
   */
  ValidateXLOGDirectoryStructure();

  /*
   * If we previously crashed, there might be data which we had written,
   * intending to fsync it, but which we had not actually fsync'd yet.
   * Therefore, a power failure in the near future might cause earlier
   * unflushed writes to be lost, even though more recent data written to
   * disk from here on would be persisted.  To avoid that, fsync the entire
   * data directory.
   */
  if (ControlFile->state != DB_SHUTDOWNED &&
    ControlFile->state != DB_SHUTDOWNED_IN_RECOVERY5)
    SyncDataDirectory();

  /*
   * Initialize on the assumption we want to recover to the latest timeline
   * that's active according to pg_control.
   */
  if (ControlFile->minRecoveryPointTLI >
    ControlFile->checkPointCopy.ThisTimeLineID)
    recoveryTargetTLI = ControlFile->minRecoveryPointTLI;
  else
    recoveryTargetTLI = ControlFile->checkPointCopy.ThisTimeLineID;

  /*
   * Check for recovery control file, and if so set up state for offline
   * recovery
   */
  readRecoveryCommandFile();

  /*
   * Save archive_cleanup_command in shared memory so that other processes
   * can see it.
   */
  strlcpy(XLogCtl->archiveCleanupCommand,
      archiveCleanupCommand ? archiveCleanupCommand : "",
      sizeof(XLogCtl->archiveCleanupCommand));

  if (ArchiveRecoveryRequested)
  {
    if (StandbyModeRequested)
      ereport(LOG,
          (errmsg("entering standby mode")));
    else if (recoveryTarget == RECOVERY_TARGET_XID)
      ereport(LOG,
          (errmsg("starting point-in-time recovery to XID %u",
              recoveryTargetXid)));
    else if (recoveryTarget == RECOVERY_TARGET_TIME)
      ereport(LOG,
          (errmsg("starting point-in-time recovery to %s",
              timestamptz_to_str(recoveryTargetTime))));
    else if (recoveryTarget == RECOVERY_TARGET_NAME)
      ereport(LOG,
          (errmsg("starting point-in-time recovery to \"%s\"",
              recoveryTargetName)));
    else if (recoveryTarget == RECOVERY_TARGET_LSN)
      ereport(LOG,
          (errmsg("starting point-in-time recovery to WAL location (LSN) \"%X/%X\"",
              (uint32) (recoveryTargetLSN >> 32),
              (uint32) recoveryTargetLSN)));
    else if (recoveryTarget == RECOVERY_TARGET_IMMEDIATE)
      ereport(LOG,
          (errmsg("starting point-in-time recovery to earliest consistent point")));
    else
      ereport(LOG,
          (errmsg("starting archive recovery")));
  }

  /*
   * Take ownership of the wakeup latch if we're going to sleep during
   * recovery.
   */
  if (StandbyModeRequested)
    OwnLatch(&XLogCtl->recoveryWakeupLatch);

  /* Set up XLOG reader facility */
  MemSet(&private, 0, sizeof(XLogPageReadPrivate));
  xlogreader = XLogReaderAllocate(wal_segment_size, &XLogPageRead, &private);
  if (!xlogreader)
    ereport(ERROR,
        (errcode(ERRCODE_OUT_OF_MEMORY),
         errmsg("out of memory"),
         errdetail("Failed while allocating a WAL reading processor.")));
  xlogreader->system_identifier = ControlFile->system_identifier;

  /*
   * Allocate two page buffers dedicated to WAL consistency checks.  We do
   * it this way, rather than just making static arrays, for two reasons:
   * (1) no need to waste the storage in most instantiations of the backend;
   * (2) a static char array isn't guaranteed to have any particular
   * alignment, whereas palloc() will provide MAXALIGN'd storage.
   */
  replay_image_masked = (char *) palloc(BLCKSZ);
  master_image_masked = (char *) palloc(BLCKSZ);

  if (read_backup_label(&checkPointLoc, &backupEndRequired,
              &backupFromStandby))
  {
    List     *tablespaces = NIL;

    /*
     * Archive recovery was requested, and thanks to the backup label
     * file, we know how far we need to replay to reach consistency. Enter
     * archive recovery directly.
     */
    InArchiveRecovery = true;
    if (StandbyModeRequested)
      StandbyMode = true;

    /*
     * When a backup_label file is present, we want to roll forward from
     * the checkpoint it identifies, rather than using pg_control.
     */
    record = ReadCheckpointRecord(xlogreader, checkPointLoc, 0, true);
    if (record != NULL)
    {
      memcpy(&checkPoint, XLogRecGetData(xlogreader), sizeof(CheckPoint));
      wasShutdown = ((record->xl_info & ~XLR_INFO_MASK) == XLOG_CHECKPOINT_SHUTDOWN);
      ereport(DEBUG1,
          (errmsg("checkpoint record is at %X/%X",
              (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
      InRecovery = true;  /* force recovery even if SHUTDOWNED */

      /*
       * Make sure that REDO location exists. This may not be the case
       * if there was a crash during an online backup, which left a
       * backup_label around that references a WAL segment that's
       * already been archived.
       */
      if (checkPoint.redo < checkPointLoc)
      {
        if (!ReadRecord(xlogreader, checkPoint.redo, LOG, false))
          ereport(FATAL,
              (errmsg("could not find redo location referenced by checkpoint record"),
               errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
      }
    }
    else
    {
      ereport(FATAL,
          (errmsg("could not locate required checkpoint record"),
           errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
      wasShutdown = false;  /* keep compiler quiet */
    }

    /* read the tablespace_map file if present and create symlinks. */
    if (read_tablespace_map(&tablespaces))
    {
      ListCell   *lc;

      foreach(lc, tablespaces)
      {
        tablespaceinfo *ti = lfirst(lc);
        char     *linkloc;

        linkloc = psprintf("pg_tblspc/%s", ti->oid);

        /*
         * Remove the existing symlink if any and Create the symlink
         * under PGDATA.
         */
        remove_tablespace_symlink(linkloc);

        if (symlink(ti->path, linkloc) < 0)
          ereport(ERROR,
              (errcode_for_file_access(),
               errmsg("could not create symbolic link \"%s\": %m",
                  linkloc)));

        pfree(ti->oid);
        pfree(ti->path);
        pfree(ti);
      }

      /* set flag to delete it later */
      haveTblspcMap = true;
    }

    /* set flag to delete it later */
    haveBackupLabel = true;
  }
  else
  {
    /*
     * If tablespace_map file is present without backup_label file, there
     * is no use of such file.  There is no harm in retaining it, but it
     * is better to get rid of the map file so that we don't have any
     * redundant file in data directory and it will avoid any sort of
     * confusion.  It seems prudent though to just rename the file out of
     * the way rather than delete it completely, also we ignore any error
     * that occurs in rename operation as even if map file is present
     * without backup_label file, it is harmless.
     */
    if (stat(TABLESPACE_MAP, &st) == 0)
    {
      unlink(TABLESPACE_MAP_OLD);
      if (durable_rename(TABLESPACE_MAP, TABLESPACE_MAP_OLD, DEBUG1) == 0)
        ereport(LOG,
            (errmsg("ignoring file \"%s\" because no file \"%s\" exists",
                TABLESPACE_MAP, BACKUP_LABEL_FILE),
             errdetail("File \"%s\" was renamed to \"%s\".",
                   TABLESPACE_MAP, TABLESPACE_MAP_OLD)));
      else
        ereport(LOG,
            (errmsg("ignoring file \"%s\" because no file \"%s\" exists",
                TABLESPACE_MAP, BACKUP_LABEL_FILE),
             errdetail("Could not rename file \"%s\" to \"%s\": %m.",
                   TABLESPACE_MAP, TABLESPACE_MAP_OLD)));
    }

    /*
     * It's possible that archive recovery was requested, but we don't
     * know how far we need to replay the WAL before we reach consistency.
     * This can happen for example if a base backup is taken from a
     * running server using an atomic filesystem snapshot, without calling
     * pg_start/stop_backup. Or if you just kill a running master server
     * and put it into archive recovery by creating a recovery.conf file.
     *
     * Our strategy in that case is to perform crash recovery first,
     * replaying all the WAL present in pg_wal, and only enter archive
     * recovery after that.
     *
     * But usually we already know how far we need to replay the WAL (up
     * to minRecoveryPoint, up to backupEndPoint, or until we see an
     * end-of-backup record), and we can enter archive recovery directly.
     */
    if (ArchiveRecoveryRequested &&
      (0ControlFile->minRecoveryPoint != 0InvalidXLogRecPtr0 ||
       ControlFile->backupEndRequired ||
       ControlFile->backupEndPoint != InvalidXLogRecPtr ||
       ControlFile->state == DB_SHUTDOWNED))
    {
      InArchiveRecovery = true;
      if (StandbyModeRequested)
        StandbyMode = true;
    }

    /* Get the last valid checkpoint record. */
    checkPointLoc = ControlFile->checkPoint;
    RedoStartLSN = ControlFile->checkPointCopy.redo;
    record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, true);
    if (record != NULL)
    {
      ereport(DEBUG1,
          (errmsg("checkpoint record is at %X/%X",
              (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
    }
    else
    {
      /*
       * We used to attempt to go back to a secondary checkpoint record
       * here, but only when not in standby_mode. We now just fail if we
       * can't read the last checkpoint because this allows us to
       * simplify processing around checkpoints.
       */
      ereport(PANIC,
          (errmsg("could not locate a valid checkpoint record")));
    }
    memcpy(&checkPoint, XLogRecGetData(xlogreader), sizeof(CheckPoint));
    wasShutdown = ((record->xl_info & ~XLR_INFO_MASK) == XLOG_CHECKPOINT_SHUTDOWN);
  }

  /*
   * Clear out any old relcache cache files.  This is *necessary* if we do
   * any WAL replay, since that would probably result in the cache files
   * being out of sync with database reality.  In theory we could leave them
   * in place if the database had been cleanly shut down, but it seems
   * safest to just remove them always and let them be rebuilt during the
   * first backend startup.  These files needs to be removed from all
   * directories including pg_tblspc, however the symlinks are created only
   * after reading tablespace_map file in case of archive recovery from
   * backup, so needs to clear old relcache files here after creating
   * symlinks.
   */
  RelationCacheInitFileRemove();

  /*
   * If the location of the checkpoint record is not on the expected
   * timeline in the history of the requested timeline, we cannot proceed:
   * the backup is not part of the history of the requested timeline.
   */
  Assert(expectedTLEs);   /* was initialized by reading checkpoint
                 * record */
  if (tliOfPointInHistory(checkPointLoc, expectedTLEs) !=
    checkPoint.ThisTimeLineID)
  {
    XLogRecPtr  switchpoint;

    /*
     * tliSwitchPoint will throw an error if the checkpoint's timeline is
     * not in expectedTLEs at all.
     */
    switchpoint = tliSwitchPoint(ControlFile->checkPointCopy.ThisTimeLineID, expectedTLEs, NULL);
    ereport(FATAL,
        (errmsg("requested timeline %u is not a child of this server's history",
            recoveryTargetTLI),
         errdetail("Latest checkpoint is at %X/%X on timeline %u, but in the history of the requested timeline, the server forked off from that timeline at %X/%X.",
               (uint32) (ControlFile->checkPoint >> 32),
               (uint32) ControlFile->checkPoint,
               ControlFile->checkPointCopy.ThisTimeLineID,
               (uint32) (switchpoint >> 32),
               (uint32) switchpoint)));
  }

  /*
   * The min recovery point should be part of the requested timeline's
   * history, too.
   */
  if (!XLogRecPtrIsInvalid(ControlFile->minRecoveryPoint) &&
    tliOfPointInHistory(ControlFile->minRecoveryPoint - 1, expectedTLEs) !=
    ControlFile->minRecoveryPointTLI)
    ereport(FATAL,
        (errmsg("requested timeline %u does not contain minimum recovery point %X/%X on timeline %u",
            recoveryTargetTLI,
            (uint32) (ControlFile->minRecoveryPoint >> 32),
            (uint32) ControlFile->minRecoveryPoint,
            ControlFile->minRecoveryPointTLI)));

  LastRec = RecPtr = checkPointLoc;

  ereport(DEBUG1,
      (errmsg_internal("redo record is at %X/%X; shutdown %s",
               (uint32) (checkPoint.redo >> 32), (uint32) checkPoint.redo,
               wasShutdown ? "true" : "false")));
  ereport(DEBUG1,
      (errmsg_internal("next transaction ID: %u:%u; next OID: %u",
               checkPoint.nextXidEpoch, checkPoint.nextXid,
               checkPoint.nextOid)));
  ereport(DEBUG1,
      (errmsg_internal("next MultiXactId: %u; next MultiXactOffset: %u",
               checkPoint.nextMulti, checkPoint.nextMultiOffset)));
  ereport(DEBUG1,
      (errmsg_internal("oldest unfrozen transaction ID: %u, in database %u",
               checkPoint.oldestXid, checkPoint.oldestXidDB)));
  ereport(DEBUG1,
      (errmsg_internal("oldest MultiXactId: %u, in database %u",
               checkPoint.oldestMulti, checkPoint.oldestMultiDB)));
  ereport(DEBUG1,
      (errmsg_internal("commit timestamp Xid oldest/newest: %u/%u",
               checkPoint.oldestCommitTsXid,
               checkPoint.newestCommitTsXid)));
  if (!TransactionIdIsNormal(checkPoint.nextXid))
    ereport(PANIC,
        (errmsg("invalid next transaction ID")));

  /* initialize shared memory variables from the checkpoint record */
  ShmemVariableCache->nextXid = checkPoint.nextXid;
  ShmemVariableCache->nextOid = checkPoint.nextOid;
  ShmemVariableCache->oidCount = 0;
  MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
  AdvanceOldestClogXid(checkPoint.oldestXid);
  SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
  SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB, true);
  SetCommitTsLimit(checkPoint.oldestCommitTsXid,
           checkPoint.newestCommitTsXid);
  XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
  XLogCtl->ckptXid = checkPoint.nextXid;

  /*
   * Initialize replication slots, before there's a chance to remove
   * required resources.
   */
  StartupReplicationSlots();

  /*
   * Startup logical state, needs to be setup now so we have proper data
   * during crash recovery.
   */
  StartupReorderBuffer();

  /*
   * Startup MultiXact. We need to do this early to be able to replay
   * truncations.
   */
  StartupMultiXact();

  /*
   * Ditto for commit timestamps.  Activate the facility if the setting is
   * enabled in the control file, as there should be no tracking of commit
   * timestamps done when the setting was disabled.  This facility can be
   * started or stopped when replaying a XLOG_PARAMETER_CHANGE record.
   */
  if (ControlFile->track_commit_timestamp)
    StartupCommitTs();

  /*
   * Recover knowledge about replay progress of known replication partners.
   */
  StartupReplicationOrigin();

  /*
   * Initialize unlogged LSN. On a clean shutdown, it's restored from the
   * control file. On recovery, all unlogged relations are blown away, so
   * the unlogged LSN counter can be reset too.
   */
  if (ControlFile->state == DB_SHUTDOWNED)
    XLogCtl->unloggedLSN = ControlFile->unloggedLSN;
  else
    XLogCtl->unloggedLSN = 1;

  /*
   * We must replay WAL entries using the same TimeLineID they were created
   * under, so temporarily adopt the TLI indicated by the checkpoint (see
   * also xlog_redo()).
   */
  ThisTimeLineID = checkPoint.ThisTimeLineID;

  /*
   * Copy any missing timeline history files between 'now' and the recovery
   * target timeline from archive to pg_wal. While we don't need those files
   * ourselves - the history file of the recovery target timeline covers all
   * the previous timelines in the history too - a cascading standby server
   * might be interested in them. Or, if you archive the WAL from this
   * server to a different archive than the master, it'd be good for all the
   * history files to get archived there after failover, so that you can use
   * one of the old timelines as a PITR target. Timeline history files are
   * small, so it's better to copy them unnecessarily than not copy them and
   * regret later.
   */
  restoreTimeLineHistoryFiles(ThisTimeLineID, recoveryTargetTLI);

  /*
   * Before running in recovery, scan pg_twophase and fill in its status to
   * be able to work on entries generated by redo.  Doing a scan before
   * taking any recovery action has the merit to discard any 2PC files that
   * are newer than the first record to replay, saving from any conflicts at
   * replay.  This avoids as well any subsequent scans when doing recovery
   * of the on-disk two-phase data.
   */
  restoreTwoPhaseData();

  lastFullPageWrites = checkPoint.fullPageWrites;

  RedoRecPtr = XLogCtl->RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;
  doPageWrites = lastFullPageWrites;

  if (RecPtr < checkPoint.redo)
    ereport(PANIC,
        (errmsg("invalid redo in checkpoint record")));

  /*
   * Check whether we need to force recovery from WAL.  If it appears to
   * have been a clean shutdown and we did not have a recovery.conf file,
   * then assume no recovery needed.
   */
  if (checkPoint.redo < RecPtr)
  {
    if (wasShutdown)
      ereport(PANIC,
          (errmsg("invalid redo record in shutdown checkpoint")));
    InRecovery = true;
  }
  else if (ControlFile->state != DB_SHUTDOWNED)
    InRecovery = true;
  else if (ArchiveRecoveryRequested)
  {
    /* force recovery due to presence of recovery.conf */
    InRecovery = true;
  }

  /* REDO */
  if (InRecovery)
  {
    int     rmid;

    /*
     * Update pg_control to show that we are recovering and to show the
     * selected checkpoint as the place we are starting from. We also mark
     * pg_control with any minimum recovery stop point obtained from a
     * backup history file.
     */
    dbstate_at_startup = ControlFile->state;
    if (InArchiveRecovery)
      ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
    else
    {
      ereport(LOG,
          (errmsg("database system was not properly shut down; "
              "automatic recovery in progress")));
      if (recoveryTargetTLI > ControlFile->checkPointCopy.ThisTimeLineID)
        ereport(LOG,
            (errmsg("crash recovery starts in timeline %u "
                "and has target timeline %u",
                ControlFile->checkPointCopy.ThisTimeLineID,
                recoveryTargetTLI)));
      ControlFile->state = DB_IN_CRASH_RECOVERY;
    }
    ControlFile->checkPoint = checkPointLoc;
    ControlFile->checkPointCopy = checkPoint;
    if (InArchiveRecovery)
    {
      /* initialize minRecoveryPoint if not set yet */
      if (ControlFile->minRecoveryPoint < checkPoint.redo)
      {
        ControlFile->minRecoveryPoint = checkPoint.redo;
        ControlFile->minRecoveryPointTLI = checkPoint.ThisTimeLineID;
      }
    }

    /*
     * Set backupStartPoint if we're starting recovery from a base backup.
     *
     * Also set backupEndPoint and use minRecoveryPoint as the backup end
     * location if we're starting recovery from a base backup which was
     * taken from a standby. In this case, the database system status in
     * pg_control must indicate that the database was already in recovery.
     * Usually that will be DB_IN_ARCHIVE_RECOVERY but also can be
     * DB_SHUTDOWNED_IN_RECOVERY if recovery previously was interrupted
     * before reaching this point; e.g. because restore_command or
     * primary_conninfo were faulty.
     *
     * Any other state indicates that the backup somehow became corrupted
     * and we can't sensibly continue with recovery.
     */
    if (haveBackupLabel)
    {
      ControlFile->backupStartPoint = checkPoint.redo;
      ControlFile->backupEndRequired = backupEndRequired;

      if (backupFromStandby)
      {
        if (dbstate_at_startup != DB_IN_ARCHIVE_RECOVERY &&
          dbstate_at_startup != DB_SHUTDOWNED_IN_RECOVERY)
          ereport(FATAL,
              (errmsg("backup_label contains data inconsistent with control file"),
               errhint("This means that the backup is corrupted and you will "
                   "have to use another backup for recovery.")));
        ControlFile->backupEndPoint = ControlFile->minRecoveryPoint;
      }
    }
    ControlFile->time = (pg_time_t) time(NULL);
    /* No need to hold ControlFileLock yet, we aren't up far enough */
    UpdateControlFile();

    /*
     * Initialize our local copy of minRecoveryPoint.  When doing crash
     * recovery we want to replay up to the end of WAL.  Particularly, in
     * the case of a promoted standby minRecoveryPoint value in the
     * control file is only updated after the first checkpoint.  However,
     * if the instance crashes before the first post-recovery checkpoint
     * is completed then recovery will use a stale location causing the
     * startup process to think that there are still invalid page
     * references when checking for data consistency.
     */
    if (InArchiveRecovery)
    {
      minRecoveryPoint = ControlFile->minRecoveryPoint;
      minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
    }
    else
    {
      minRecoveryPoint = InvalidXLogRecPtr;
      minRecoveryPointTLI = 0;
    }

    /*
     * Reset pgstat data, because it may be invalid after recovery.
     */
    pgstat_reset_all();

    /*
     * If there was a backup label file, it's done its job and the info
     * has now been propagated into pg_control.  We must get rid of the
     * label file so that if we crash during recovery, we'll pick up at
     * the latest recovery restartpoint instead of going all the way back
     * to the backup start point.  It seems prudent though to just rename
     * the file out of the way rather than delete it completely.
     */
    if (haveBackupLabel)
    {
      unlink(BACKUP_LABEL_OLD);
      durable_rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD, FATAL);
    }

    /*
     * If there was a tablespace_map file, it's done its job and the
     * symlinks have been created.  We must get rid of the map file so
     * that if we crash during recovery, we don't create symlinks again.
     * It seems prudent though to just rename the file out of the way
     * rather than delete it completely.
     */
    if (haveTblspcMap)
    {
      unlink(TABLESPACE_MAP_OLD);
      durable_rename(TABLESPACE_MAP, TABLESPACE_MAP_OLD, FATAL);
    }

    /* Check that the GUCs used to generate the WAL allow recovery */
    CheckRequiredParameterValues();

    /*
     * We're in recovery, so unlogged relations may be trashed and must be
     * reset.  This should be done BEFORE allowing Hot Standby
     * connections, so that read-only backends don't try to read whatever
     * garbage is left over from before.
     */
    ResetUnloggedRelations(UNLOGGED_RELATION_CLEANUP);

    /*
     * Likewise, delete any saved transaction snapshot files that got left
     * behind by crashed backends.
     */
    DeleteAllExportedSnapshotFiles();

    /*
     * Initialize for Hot Standby, if enabled. We won't let backends in
     * yet, not until we've reached the min recovery point specified in
     * control file and we've established a recovery snapshot from a
     * running-xacts WAL record.
     */
    if (ArchiveRecoveryRequested && EnableHotStandby0)
    {
      TransactionId *xids;
      int     nxids;

      ereport(DEBUG1,
          (errmsg("initializing for hot standby")));

      InitRecoveryTransactionEnvironment();

      if (wasShutdown)
        oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);
      else
        oldestActiveXID = checkPoint.oldestActiveXid;
      Assert(TransactionIdIsValid(oldestActiveXID));

      /* Tell procarray about the range of xids it has to deal with */
      ProcArrayInitRecovery(ShmemVariableCache->nextXid);

      /*
       * Startup commit log and subtrans only.  MultiXact and commit
       * timestamp have already been started up and other SLRUs are not
       * maintained during recovery and need not be started yet.
       */
      StartupCLOG();
      StartupSUBTRANS(oldestActiveXID);

      /*
       * If we're beginning at a shutdown checkpoint, we know that
       * nothing was running on the master at this point. So fake-up an
       * empty running-xacts record and use that here and now. Recover
       * additional standby state for prepared transactions.
       */
      if (wasShutdown)
      {
        RunningTransactionsData running;
        TransactionId latestCompletedXid;

        /*
         * Construct a RunningTransactions snapshot representing a
         * shut down server, with only prepared transactions still
         * alive. We're never overflowed at this point because all
         * subxids are listed with their parent prepared transactions.
         */
        running.xcnt = nxids;
        running.subxcnt = 0;
        running.subxid_overflow = false;
        running.nextXid = checkPoint.nextXid;
        running.oldestRunningXid = oldestActiveXID;
        latestCompletedXid = checkPoint.nextXid;
        TransactionIdRetreat(latestCompletedXid);
        Assert(TransactionIdIsNormal(latestCompletedXid));
        running.latestCompletedXid = latestCompletedXid;
        running.xids = xids;

        ProcArrayApplyRecoveryInfo(&running);

        StandbyRecoverPreparedTransactions();
      }
    }

    /* Initialize resource managers */
    for (rmid = 0; 5rmid <= RM_MAX_ID; rmid++110)
    {
      if (RmgrTable[rmid].rm_startup != NULL)
        RmgrTable[rmid].rm_startup();
    }

    /*
     * Initialize shared variables for tracking progress of WAL replay, as
     * if we had just replayed the record before the REDO location (or the
     * checkpoint record itself, if it's a shutdown checkpoint).
     */
    SpinLockAcquire(&XLogCtl->info_lck);
    if (checkPoint.redo < RecPtr)
      XLogCtl->replayEndRecPtr = checkPoint.redo;
    else
      XLogCtl->replayEndRecPtr = EndRecPtr;
    XLogCtl->replayEndTLI = ThisTimeLineID;
    XLogCtl->lastReplayedEndRecPtr = XLogCtl->replayEndRecPtr;
    XLogCtl->lastReplayedTLI = XLogCtl->replayEndTLI;
    XLogCtl->recoveryLastXTime = 0;
    XLogCtl->currentChunkStartTime = 0;
    XLogCtl->recoveryPause = false;
    SpinLockRelease(&XLogCtl->info_lck);

    /* Also ensure XLogReceiptTime has a sane value */
    XLogReceiptTime = GetCurrentTimestamp();

    /*
     * Let postmaster know we've started redo now, so that it can launch
     * checkpointer to perform restartpoints.  We don't bother during
     * crash recovery as restartpoints can only be performed during
     * archive recovery.  And we'd like to keep crash recovery simple, to
     * avoid introducing bugs that could affect you when recovering after
     * crash.
     *
     * After this point, we can no longer assume that we're the only
     * process in addition to postmaster!  Also, fsync requests are
     * subsequently to be handled by the checkpointer, not locally.
     */
    if (ArchiveRecoveryRequested && IsUnderPostmaster0)
    {
      PublishStartupProcessInformation();
      SetForwardFsyncRequests();
      SendPostmasterSignal(PMSIGNAL_RECOVERY_STARTED);
      bgwriterLaunched = true;
    }

    /*
     * Allow read-only connections immediately if we're consistent
     * already.
     */
    CheckRecoveryConsistency();

    /*
     * Find the first record that logically follows the checkpoint --- it
     * might physically precede it, though.
     */
    if (checkPoint.redo < RecPtr)
    {
      /* back up to find the record */
      record = ReadRecord(xlogreader, checkPoint.redo, PANIC, false);
    }
    else
    {
      /* just have to read next record after CheckPoint */
      record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
    }

    if (record != NULL)
    {
      ErrorContextCallback errcallback;
      TimestampTz xtime;

      InRedo = true;

      ereport(LOG,
          (errmsg("redo starts at %X/%X",
              (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr)));

      /*
       * main redo apply loop
       */
      do
      {
        bool    switchedTLI = false;

#ifdef WAL_DEBUG
        if (XLOG_DEBUG ||
          (rmid == RM_XACT_ID && trace_recovery_messages <= DEBUG2) ||
          (rmid != RM_XACT_ID && trace_recovery_messages <= DEBUG3))
        {
          StringInfoData buf;

          initStringInfo(&buf);
          appendStringInfo(&buf, "REDO @ %X/%X; LSN %X/%X: ",
                   (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr,
                   (uint32) (EndRecPtr >> 32), (uint32) EndRecPtr);
          xlog_outrec(&buf, xlogreader);
          appendStringInfoString(&buf, " - ");
          xlog_outdesc(&buf, xlogreader);
          elog(LOG, "%s", buf.data);
          pfree(buf.data);
        }
#endif

        /* Handle interrupt signals of startup process */
        HandleStartupProcInterrupts();

        /*
         * Pause WAL replay, if requested by a hot-standby session via
         * SetRecoveryPause().
         *
         * Note that we intentionally don't take the info_lck spinlock
         * here.  We might therefore read a slightly stale value of
         * the recoveryPause flag, but it can't be very stale (no
         * worse than the last spinlock we did acquire).  Since a
         * pause request is a pretty asynchronous thing anyway,
         * possibly responding to it one WAL record later than we
         * otherwise would is a minor issue, so it doesn't seem worth
         * adding another spinlock cycle to prevent that.
         */
        if (((volatile XLogCtlData *) XLogCtl)->recoveryPause)
          recoveryPausesHere();

        /*
         * Have we reached our recovery target?
         */
        if (recoveryStopsBefore(xlogreader))
        {
          reachedStopPoint = true;  /* see below */
          break;
        }

        /*
         * If we've been asked to lag the master, wait on latch until
         * enough time has passed.
         */
        if (recoveryApplyDelay(xlogreader))
        {
          /*
           * We test for paused recovery again here. If user sets
           * delayed apply, it may be because they expect to pause
           * recovery in case of problems, so we must test again
           * here otherwise pausing during the delay-wait wouldn't
           * work.
           */
          if (((volatile XLogCtlData *) XLogCtl)->recoveryPause)
            recoveryPausesHere();
        }

        /* Setup error traceback support for ereport() */
        errcallback.callback = rm_redo_error_callback;
        errcallback.arg = (void *) xlogreader;
        errcallback.previous = error_context_stack;
        error_context_stack = &errcallback;

        /*
         * ShmemVariableCache->nextXid must be beyond record's xid.
         *
         * We don't expect anyone else to modify nextXid, hence we
         * don't need to hold a lock while examining it.  We still
         * acquire the lock to modify it, though.
         */
        if (TransactionIdFollowsOrEquals(record->xl_xid,
                         ShmemVariableCache->nextXid))
        {
          LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
          ShmemVariableCache->nextXid = record->xl_xid;
          TransactionIdAdvance(ShmemVariableCache->nextXid);
          LWLockRelease(XidGenLock);
        }

        /*
         * Before replaying this record, check if this record causes
         * the current timeline to change. The record is already
         * considered to be part of the new timeline, so we update
         * ThisTimeLineID before replaying it. That's important so
         * that replayEndTLI, which is recorded as the minimum
         * recovery point's TLI if recovery stops after this record,
         * is set correctly.
         */
        if (record->xl_rmid == RM_XLOG_ID)
        {
          TimeLineID  newTLI = ThisTimeLineID;
          TimeLineID  prevTLI = ThisTimeLineID;
          uint8   info = record->xl_info & ~XLR_INFO_MASK;

          if (info == XLOG_CHECKPOINT_SHUTDOWN)
          {
            CheckPoint  checkPoint;

            memcpy(&checkPoint, XLogRecGetData(xlogreader), sizeof(CheckPoint));
            newTLI = checkPoint.ThisTimeLineID;
            prevTLI = checkPoint.PrevTimeLineID;
          }
          else if (info == XLOG_END_OF_RECOVERY)
          {
            xl_end_of_recovery xlrec;

            memcpy(&xlrec, XLogRecGetData(xlogreader), sizeof(xl_end_of_recovery));
            newTLI = xlrec.ThisTimeLineID;
            prevTLI = xlrec.PrevTimeLineID;
          }

          if (newTLI != ThisTimeLineID)
          {
            /* Check that it's OK to switch to this TLI */
            checkTimeLineSwitch(EndRecPtr, newTLI, prevTLI);

            /* Following WAL records should be run with new TLI */
            ThisTimeLineID = newTLI;
            switchedTLI = true;
          }
        }

        /*
         * Update shared replayEndRecPtr before replaying this record,
         * so that XLogFlush will update minRecoveryPoint correctly.
         */
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->replayEndRecPtr = EndRecPtr;
        XLogCtl->replayEndTLI = ThisTimeLineID;
        SpinLockRelease(&XLogCtl->info_lck);

        /*
         * If we are attempting to enter Hot Standby mode, process
         * XIDs we see
         */
        if (standbyState >= STANDBY_INITIALIZED &&
          TransactionIdIsValid0(record->xl_xid))
          RecordKnownAssignedTransactionIds(record->xl_xid);

        /* Now apply the WAL record itself */
        RmgrTable[record->xl_rmid].rm_redo(xlogreader);

        /*
         * After redo, check whether the backup pages associated with
         * the WAL record are consistent with the existing pages. This
         * check is done only if consistency check is enabled for this
         * record.
         */
        if ((record->xl_info & XLR_CHECK_CONSISTENCY) != 0)
          checkXLogConsistency(xlogreader);

        /* Pop the error context stack */
        error_context_stack = errcallback.previous;

        /*
         * Update lastReplayedEndRecPtr after this record has been
         * successfully replayed.
         */
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->lastReplayedEndRecPtr = EndRecPtr;
        XLogCtl->lastReplayedTLI = ThisTimeLineID;
        SpinLockRelease(&XLogCtl->info_lck);

        /*
         * If rm_redo called XLogRequestWalReceiverReply, then we wake
         * up the receiver so that it notices the updated
         * lastReplayedEndRecPtr and sends a reply to the master.
         */
        if (doRequestWalReceiverReply)
        {
          doRequestWalReceiverReply = false;
          WalRcvForceReply();
        }

        /* Remember this record as the last-applied one */
        LastRec = ReadRecPtr;

        /* Allow read-only connections if we're consistent now */
        CheckRecoveryConsistency();

        /* Is this a timeline switch? */
        if (switchedTLI)
        {
          /*
           * Before we continue on the new timeline, clean up any
           * (possibly bogus) future WAL segments on the old
           * timeline.
           */
          RemoveNonParentXlogFiles(EndRecPtr, ThisTimeLineID);

          /*
           * Wake up any walsenders to notice that we are on a new
           * timeline.
           */
          if (switchedTLI && AllowCascadeReplication())
            WalSndWakeup();
        }

        /* Exit loop if we reached inclusive recovery target */
        if (recoveryStopsAfter(xlogreader))
        {
          reachedStopPoint = true;
          break;
        }

        /* Else, try to fetch the next WAL record */
        record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
      } while (record != NULL);

      /*
       * end of main redo apply loop
       */

      if (reachedStopPoint)
      {
        if (!reachedConsistency)
          ereport(FATAL,
              (errmsg("requested recovery stop point is before consistent recovery point")));

        /*
         * This is the last point where we can restart recovery with a
         * new recovery target, if we shutdown and begin again. After
         * this, Resource Managers may choose to do permanent
         * corrective actions at end of recovery.
         */
        switch (recoveryTargetAction)
        {
          case RECOVERY_TARGET_ACTION_SHUTDOWN:

            /*
             * exit with special return code to request shutdown
             * of postmaster.  Log messages issued from
             * postmaster.
             */
            proc_exit(3);

          case RECOVERY_TARGET_ACTION_PAUSE:
            SetRecoveryPause(true);
            recoveryPausesHere();

            /* drop into promote */
            switch_fallthrough();

          case RECOVERY_TARGET_ACTION_PROMOTE:
            break;
        }
      }

      /* Allow resource managers to do any required cleanup. */
      for (rmid = 0; 1rmid <= RM_MAX_ID; rmid++22)
      {
        if (RmgrTable[rmid].rm_cleanup != NULL)
          RmgrTable[rmid].rm_cleanup();
      }

      ereport(LOG,
          (errmsg("redo done at %X/%X",
              (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr)));
      xtime = GetLatestXTime();
      if (xtime)
        ereport(LOG,
            (errmsg("last completed transaction was at log time %s",
                timestamptz_to_str(xtime))));

      InRedo = false;
    }
    else
    {
      /* there are no WAL records following the checkpoint */
      ereport(LOG,
          (errmsg("redo is not required")));
    }
  }

  /*
   * Kill WAL receiver, if it's still running, before we continue to write
   * the startup checkpoint record. It will trump over the checkpoint and
   * subsequent records if it's still alive when we start writing WAL.
   */
  ShutdownWalRcv();

  /*
   * Reset unlogged relations to the contents of their INIT fork. This is
   * done AFTER recovery is complete so as to include any unlogged relations
   * created during recovery, but BEFORE recovery is marked as having
   * completed successfully. Otherwise we'd not retry if any of the post
   * end-of-recovery steps fail.
   */
  if (InRecovery)
    ResetUnloggedRelations(UNLOGGED_RELATION_INIT);

  /*
   * We don't need the latch anymore. It's not strictly necessary to disown
   * it, but let's do it for the sake of tidiness.
   */
  if (StandbyModeRequested)
    DisownLatch(&XLogCtl->recoveryWakeupLatch);

  /*
   * We are now done reading the xlog from stream. Turn off streaming
   * recovery to force fetching the files (which would be required at end of
   * recovery, e.g., timeline history file) from archive or pg_wal.
   */
  StandbyMode = false;

  /*
   * Re-fetch the last valid or last applied record, so we can identify the
   * exact endpoint of what we consider the valid portion of WAL.
   */
  record = ReadRecord(xlogreader, LastRec, PANIC, false);
  EndOfLog = EndRecPtr;

  /*
   * EndOfLogTLI is the TLI in the filename of the XLOG segment containing
   * the end-of-log. It could be different from the timeline that EndOfLog
   * nominally belongs to, if there was a timeline switch in that segment,
   * and we were reading the old WAL from a segment belonging to a higher
   * timeline.
   */
  EndOfLogTLI = xlogreader->readPageTLI;

  /*
   * Complain if we did not roll forward far enough to render the backup
   * dump consistent.  Note: it is indeed okay to look at the local variable
   * minRecoveryPoint here, even though ControlFile->minRecoveryPoint might
   * be further ahead --- ControlFile->minRecoveryPoint cannot have been
   * advanced beyond the WAL we processed.
   */
  if (InRecovery &&
    (5EndOfLog < minRecoveryPoint5 ||
     !XLogRecPtrIsInvalid(ControlFile->backupStartPoint)))
  {
    /*
     * Ran off end of WAL before reaching end-of-backup WAL record, or
     * minRecoveryPoint. That's usually a bad sign, indicating that you
     * tried to recover from an online backup but never called
     * pg_stop_backup(), or you didn't archive all the WAL up to that
     * point. However, this also happens in crash recovery, if the system
     * crashes while an online backup is in progress. We must not treat
     * that as an error, or the database will refuse to start up.
     */
    if (ArchiveRecoveryRequested || ControlFile->backupEndRequired)
    {
      if (ControlFile->backupEndRequired)
        ereport(FATAL,
            (errmsg("WAL ends before end of online backup"),
             errhint("All WAL generated while online backup was taken must be available at recovery.")));
      else if (!XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
        ereport(FATAL,
            (errmsg("WAL ends before end of online backup"),
             errhint("Online backup started with pg_start_backup() must be ended with pg_stop_backup(), and all WAL up to that point must be available at recovery.")));
      else
        ereport(FATAL,
            (errmsg("WAL ends before consistent recovery point")));
    }
  }

  /*
   * Pre-scan prepared transactions to find out the range of XIDs present.
   * This information is not quite needed yet, but it is positioned here so
   * as potential problems are detected before any on-disk change is done.
   */
  oldestActiveXID = PrescanPreparedTransactions(NULL, NULL);

  /*
   * Consider whether we need to assign a new timeline ID.
   *
   * If we are doing an archive recovery, we always assign a new ID.  This
   * handles a couple of issues.  If we stopped short of the end of WAL
   * during recovery, then we are clearly generating a new timeline and must
   * assign it a unique new ID.  Even if we ran to the end, modifying the
   * current last segment is problematic because it may result in trying to
   * overwrite an already-archived copy of that segment, and we encourage
   * DBAs to make their archive_commands reject that.  We can dodge the
   * problem by making the new active segment have a new timeline ID.
   *
   * In a normal crash recovery, we can just extend the timeline we were in.
   */
  PrevTimeLineID = ThisTimeLineID;
  if (ArchiveRecoveryRequested)
  {
    char    reason[200];

    Assert(InArchiveRecovery);

    ThisTimeLineID = findNewestTimeLine(recoveryTargetTLI) + 1;
    ereport(LOG,
        (errmsg("selected new timeline ID: %u", ThisTimeLineID)));

    /*
     * Create a comment for the history file to explain why and where
     * timeline changed.
     */
    if (recoveryTarget == RECOVERY_TARGET_XID)
      snprintf(reason, sizeof(reason),
           "%s transaction %u",
           recoveryStopAfter ? "after" : "before",
           recoveryStopXid);
    else if (recoveryTarget == RECOVERY_TARGET_TIME)
      snprintf(reason, sizeof(reason),
           "%s %s\n",
           recoveryStopAfter ? "after" : "before",
           timestamptz_to_str(recoveryStopTime));
    else if (recoveryTarget == RECOVERY_TARGET_LSN)
      snprintf(reason, sizeof(reason),
           "%s LSN %X/%X\n",
           recoveryStopAfter ? "after" : "before",
           (uint32) (recoveryStopLSN >> 32),
           (uint32) recoveryStopLSN);
    else if (recoveryTarget == RECOVERY_TARGET_NAME)
      snprintf(reason, sizeof(reason),
           "at restore point \"%s\"",
           recoveryStopName);
    else if (recoveryTarget == RECOVERY_TARGET_IMMEDIATE)
      snprintf(reason, sizeof(reason), "reached consistency");
    else
      snprintf(reason, sizeof(reason), "no recovery target specified");

    /*
     * We are now done reading the old WAL.  Turn off archive fetching if
     * it was active, and make a writable copy of the last WAL segment.
     * (Note that we also have a copy of the last block of the old WAL in
     * readBuf; we will use that below.)
     */
    exitArchiveRecovery(EndOfLogTLI, EndOfLog);

    /*
     * Write the timeline history file, and have it archived. After this
     * point (or rather, as soon as the file is archived), the timeline
     * will appear as "taken" in the WAL archive and to any standby
     * servers.  If we crash before actually switching to the new
     * timeline, standby servers will nevertheless think that we switched
     * to the new timeline, and will try to connect to the new timeline.
     * To minimize the window for that, try to do as little as possible
     * between here and writing the end-of-recovery record.
     */
    writeTimeLineHistory(ThisTimeLineID, recoveryTargetTLI,
               EndRecPtr, reason);
  }

  /* Save the selected TimeLineID in shared memory, too */
  XLogCtl->ThisTimeLineID = ThisTimeLineID;
  XLogCtl->PrevTimeLineID = PrevTimeLineID;

  /*
   * Prepare to write WAL starting at EndOfLog location, and init xlog
   * buffer cache using the block containing the last record from the
   * previous incarnation.
   */
  Insert = &XLogCtl->Insert;
  Insert->PrevBytePos = XLogRecPtrToBytePos(LastRec);
  Insert->CurrBytePos = XLogRecPtrToBytePos(EndOfLog);

  /*
   * Tricky point here: readBuf contains the *last* block that the LastRec
   * record spans, not the one it starts in.  The last block is indeed the
   * one we want to use.
   */
  if (EndOfLog % XLOG_BLCKSZ != 0)
  {
    char     *page;
    int     len;
    int     firstIdx;
    XLogRecPtr  pageBeginPtr;

    pageBeginPtr = EndOfLog - (EndOfLog % XLOG_BLCKSZ);
    Assert(readOff == XLogSegmentOffset(pageBeginPtr, wal_segment_size));

    firstIdx = XLogRecPtrToBufIdx(EndOfLog);

    /* Copy the valid part of the last block, and zero the rest */
    page = &XLogCtl->pages[firstIdx * XLOG_BLCKSZ];
    len = EndOfLog % XLOG_BLCKSZ;
    memcpy(page, xlogreader->readBuf, len);
    memset(page + len, 0, XLOG_BLCKSZ - len);

    XLogCtl->xlblocks[firstIdx] = pageBeginPtr + XLOG_BLCKSZ;
    XLogCtl->InitializedUpTo = pageBeginPtr + XLOG_BLCKSZ;
  }
  else
  {
    /*
     * There is no partial block to copy. Just set InitializedUpTo, and
     * let the first attempt to insert a log record to initialize the next
     * buffer.
     */
    XLogCtl->InitializedUpTo = EndOfLog;
  }

  LogwrtResult.Write = LogwrtResult.Flush = EndOfLog;

  XLogCtl->LogwrtResult = LogwrtResult;

  XLogCtl->LogwrtRqst.Write = EndOfLog;
  XLogCtl->LogwrtRqst.Flush = EndOfLog;

  /*
   * Update full_page_writes in shared memory and write an XLOG_FPW_CHANGE
   * record before resource manager writes cleanup WAL records or checkpoint
   * record is written.
   */
  Insert->fullPageWrites = lastFullPageWrites;
  LocalSetXLogInsertAllowed();
  UpdateFullPageWrites();
  LocalXLogInsertAllowed = -1;

  if (InRecovery)
  {
    /*
     * Perform a checkpoint to update all our recovery activity to disk.
     *
     * Note that we write a shutdown checkpoint rather than an on-line
     * one. This is not particularly critical, but since we may be
     * assigning a new TLI, using a shutdown checkpoint allows us to have
     * the rule that TLI only changes in shutdown checkpoints, which
     * allows some extra error checking in xlog_redo.
     *
     * In fast promotion, only create a lightweight end-of-recovery record
     * instead of a full checkpoint. A checkpoint is requested later,
     * after we're fully out of recovery mode and already accepting
     * queries.
     */
    if (bgwriterLaunched)
    {
      if (fast_promote)
      {
        checkPointLoc = ControlFile->checkPoint;

        /*
         * Confirm the last checkpoint is available for us to recover
         * from if we fail.
         */
        record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, false);
        if (record != NULL)
        {
          fast_promoted = true;

          /*
           * Insert a special WAL record to mark the end of
           * recovery, since we aren't doing a checkpoint. That
           * means that the checkpointer process may likely be in
           * the middle of a time-smoothed restartpoint and could
           * continue to be for minutes after this. That sounds
           * strange, but the effect is roughly the same and it
           * would be stranger to try to come out of the
           * restartpoint and then checkpoint. We request a
           * checkpoint later anyway, just for safety.
           */
          CreateEndOfRecoveryRecord();
        }
      }

      if (!fast_promoted)
        RequestCheckpoint(CHECKPOINT_END_OF_RECOVERY |
                  CHECKPOINT_IMMEDIATE |
                  CHECKPOINT_WAIT);
    }
    else
      CreateCheckPoint(CHECKPOINT_END_OF_RECOVERY | CHECKPOINT_IMMEDIATE);

    /*
     * And finally, execute the recovery_end_command, if any.
     */
    if (recoveryEndCommand)
      ExecuteRecoveryCommand(recoveryEndCommand,
                   "recovery_end_command",
                   true);
  }

  if (ArchiveRecoveryRequested)
  {
    /*
     * We switched to a new timeline. Clean up segments on the old
     * timeline.
     *
     * If there are any higher-numbered segments on the old timeline,
     * remove them. They might contain valid WAL, but they might also be
     * pre-allocated files containing garbage. In any case, they are not
     * part of the new timeline's history so we don't need them.
     */
    RemoveNonParentXlogFiles(EndOfLog, ThisTimeLineID);

    /*
     * If the switch happened in the middle of a segment, what to do with
     * the last, partial segment on the old timeline? If we don't archive
     * it, and the server that created the WAL never archives it either
     * (e.g. because it was hit by a meteor), it will never make it to the
     * archive. That's OK from our point of view, because the new segment
     * that we created with the new TLI contains all the WAL from the old
     * timeline up to the switch point. But if you later try to do PITR to
     * the "missing" WAL on the old timeline, recovery won't find it in
     * the archive. It's physically present in the new file with new TLI,
     * but recovery won't look there when it's recovering to the older
     * timeline. On the other hand, if we archive the partial segment, and
     * the original server on that timeline is still running and archives
     * the completed version of the same segment later, it will fail. (We
     * used to do that in 9.4 and below, and it caused such problems).
     *
     * As a compromise, we rename the last segment with the .partial
     * suffix, and archive it. Archive recovery will never try to read
     * .partial segments, so they will normally go unused. But in the odd
     * PITR case, the administrator can copy them manually to the pg_wal
     * directory (removing the suffix). They can be useful in debugging,
     * too.
     *
     * If a .done or .ready file already exists for the old timeline,
     * however, we had already determined that the segment is complete, so
     * we can let it be archived normally. (In particular, if it was
     * restored from the archive to begin with, it's expected to have a
     * .done file).
     */
    if (XLogSegmentOffset(EndOfLog, wal_segment_size) != 0 &&
      XLogArchivingActive())
    {
      char    origfname[MAXFNAMELEN];
      XLogSegNo endLogSegNo;

      XLByteToPrevSeg(EndOfLog, endLogSegNo, wal_segment_size);
      XLogFileName(origfname, EndOfLogTLI, endLogSegNo, wal_segment_size);

      if (!XLogArchiveIsReadyOrDone(origfname))
      {
        char    origpath[MAXPGPATH];
        char    partialfname[MAXFNAMELEN];
        char    partialpath[MAXPGPATH];

        XLogFilePath(origpath, EndOfLogTLI, endLogSegNo, wal_segment_size);
        snprintf(partialfname, MAXFNAMELEN, "%s.partial", origfname);
        snprintf(partialpath, MAXPGPATH, "%s.partial", origpath);

        /*
         * Make sure there's no .done or .ready file for the .partial
         * file.
         */
        XLogArchiveCleanup(partialfname);

        durable_rename(origpath, partialpath, ERROR);
        XLogArchiveNotify(partialfname);
      }
    }
  }

  /*
   * Preallocate additional log files, if wanted.
   */
  PreallocXlogFiles(EndOfLog);

  /*
   * Okay, we're officially UP.
   */
  InRecovery = false;

  /* start the archive_timeout timer and LSN running */
  XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);
  XLogCtl->lastSegSwitchLSN = EndOfLog;

  /* also initialize latestCompletedXid, to nextXid - 1 */
  LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
  ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
  TransactionIdRetreat(ShmemVariableCache->latestCompletedXid);
  LWLockRelease(ProcArrayLock);

  /*
   * Start up the commit log and subtrans, if not already done for hot
   * standby.  (commit timestamps are started below, if necessary.)
   */
  if (standbyState == STANDBY_DISABLED)
  {
    StartupCLOG();
    StartupSUBTRANS(oldestActiveXID);
  }

  /*
   * Perform end of recovery actions for any SLRUs that need it.
   */
  TrimCLOG();
  TrimMultiXact();

  /* Reload shared-memory state for prepared transactions */
  RecoverPreparedTransactions();

  /*
   * Shutdown the recovery environment. This must occur after
   * RecoverPreparedTransactions(), see notes for lock_twophase_recover()
   */
  if (standbyState != STANDBY_DISABLED)
    ShutdownRecoveryTransactionEnvironment();

  /* Shut down xlogreader */
  if (readFile >= 0)
  {
    close(readFile);
    readFile = -1;
  }
  XLogReaderFree(xlogreader);

  /*
   * If any of the critical GUCs have changed, log them before we allow
   * backends to write WAL.
   */
  LocalSetXLogInsertAllowed();
  XLogReportParameters();

  /*
   * Local WAL inserts enabled, so it's time to finish initialization of
   * commit timestamp.
   */
  CompleteCommitTsInitialization();

  /*
   * All done with end-of-recovery actions.
   *
   * Now allow backends to write WAL and update the control file status in
   * consequence.  The boolean flag allowing backends to write WAL is
   * updated while holding ControlFileLock to prevent other backends to look
   * at an inconsistent state of the control file in shared memory.  There
   * is still a small window during which backends can write WAL and the
   * control file is still referring to a system not in DB_IN_PRODUCTION
   * state while looking at the on-disk control file.
   *
   * Also, although the boolean flag to allow WAL is probably atomic in
   * itself, we use the info_lck here to ensure that there are no race
   * conditions concerning visibility of other recent updates to shared
   * memory.
   */
  LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
  ControlFile->state = DB_IN_PRODUCTION;
  ControlFile->time = (pg_time_t) time(NULL);

  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->SharedRecoveryInProgress = false;
  SpinLockRelease(&XLogCtl->info_lck);

  UpdateControlFile();
  LWLockRelease(ControlFileLock);

  /*
   * If there were cascading standby servers connected to us, nudge any wal
   * sender processes to notice that we've been promoted.
   */
  WalSndWakeup();

  /*
   * If this was a fast promotion, request an (online) checkpoint now. This
   * isn't required for consistency, but the last restartpoint might be far
   * back, and in case of a crash, recovering from it might take a longer
   * than is appropriate now that we're not in standby mode anymore.
   */
  if (fast_promoted)
    RequestCheckpoint(CHECKPOINT_FORCE);
}

/*
 * Checks if recovery has reached a consistent state. When consistency is
 * reached and we have a valid starting standby snapshot, tell postmaster
 * that it can start accepting read-only connections.
 */
static void
CheckRecoveryConsistency(void)
{
  XLogRecPtr  lastReplayedEndRecPtr;

  /*
   * During crash recovery, we don't reach a consistent state until we've
   * replayed all the WAL.
   */
  if (XLogRecPtrIsInvalid(minRecoveryPoint))
    return;

  Assert(InArchiveRecovery);

  /*
   * assume that we are called in the startup process, and hence don't need
   * a lock to read lastReplayedEndRecPtr
   */
  lastReplayedEndRecPtr = XLogCtl->lastReplayedEndRecPtr;

  /*
   * Have we reached the point where our base backup was completed?
   */
  if (!XLogRecPtrIsInvalid(ControlFile->backupEndPoint) &&
    ControlFile->backupEndPoint <= lastReplayedEndRecPtr)
  {
    /*
     * We have reached the end of base backup, as indicated by pg_control.
     * The data on disk is now consistent. Reset backupStartPoint and
     * backupEndPoint, and update minRecoveryPoint to make sure we don't
     * allow starting up at an earlier point even if recovery is stopped
     * and restarted soon after this.
     */
    elog(DEBUG1, "end of backup reached");

    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

    if (ControlFile->minRecoveryPoint < lastReplayedEndRecPtr)
      ControlFile->minRecoveryPoint = lastReplayedEndRecPtr;

    ControlFile->backupStartPoint = InvalidXLogRecPtr;
    ControlFile->backupEndPoint = InvalidXLogRecPtr;
    ControlFile->backupEndRequired = false;
    UpdateControlFile();

    LWLockRelease(ControlFileLock);
  }

  /*
   * Have we passed our safe starting point? Note that minRecoveryPoint is
   * known to be incorrectly set if ControlFile->backupEndRequired, until
   * the XLOG_BACKUP_END arrives to advise us of the correct
   * minRecoveryPoint. All we know prior to that is that we're not
   * consistent yet.
   */
  if (!reachedConsistency && !ControlFile->backupEndRequired &&
    minRecoveryPoint <= lastReplayedEndRecPtr &&
    XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
  {
    /*
     * Check to see if the XLOG sequence contained any unresolved
     * references to uninitialized pages.
     */
    XLogCheckInvalidPages();

    reachedConsistency = true;
    ereport(LOG,
        (errmsg("consistent recovery state reached at %X/%X",
            (uint32) (lastReplayedEndRecPtr >> 32),
            (uint32) lastReplayedEndRecPtr)));
  }

  /*
   * Have we got a valid starting snapshot that will allow queries to be
   * run? If so, we can tell postmaster that the database is consistent now,
   * enabling connections.
   */
  if (standbyState == STANDBY_SNAPSHOT_READY &&
    !LocalHotStandbyActive &&
    reachedConsistency &&
    IsUnderPostmaster)
  {
    SpinLockAcquire(&XLogCtl->info_lck);
    XLogCtl->SharedHotStandbyActive = true;
    SpinLockRelease(&XLogCtl->info_lck);

    LocalHotStandbyActive = true;

    SendPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY);
  }
}

/*
 * Is the system still in recovery?
 *
 * Unlike testing InRecovery, this works in any process that's connected to
 * shared memory.
 *
 * As a side-effect, we initialize the local TimeLineID and RedoRecPtr
 * variables the first time we see that recovery is finished.
 */
bool
RecoveryInProgress(void)
{
  /*
   * We check shared state each time only until we leave recovery mode. We
   * can't re-enter recovery, so there's no need to keep checking after the
   * shared variable has once been seen false.
   */
  if (!LocalRecoveryInProgress)
    return false;
  else
  {
    /*
     * use volatile pointer to make sure we make a fresh read of the
     * shared variable.
     */
    volatile XLogCtlData *xlogctl = XLogCtl;

    LocalRecoveryInProgress = xlogctl->SharedRecoveryInProgress;

    /*
     * Initialize TimeLineID and RedoRecPtr when we discover that recovery
     * is finished. InitPostgres() relies upon this behaviour to ensure
     * that InitXLOGAccess() is called at backend startup.  (If you change
     * this, see also LocalSetXLogInsertAllowed.)
     */
    if (!LocalRecoveryInProgress)
    {
      /*
       * If we just exited recovery, make sure we read TimeLineID and
       * RedoRecPtr after SharedRecoveryInProgress (for machines with
       * weak memory ordering).
       */
      pg_memory_barrier();
      InitXLOGAccess();
    }

    /*
     * Note: We don't need a memory barrier when we're still in recovery.
     * We might exit recovery immediately after return, so the caller
     * can't rely on 'true' meaning that we're still in recovery anyway.
     */

    return LocalRecoveryInProgress;
  }
}

/*
 * Is HotStandby active yet? This is only important in special backends
 * since normal backends won't ever be able to connect until this returns
 * true. Postmaster knows this by way of signal, not via shared memory.
 *
 * Unlike testing standbyState, this works in any process that's connected to
 * shared memory.  (And note that standbyState alone doesn't tell the truth
 * anyway.)
 */
bool
HotStandbyActive(void)
{
  /*
   * We check shared state each time only until Hot Standby is active. We
   * can't de-activate Hot Standby, so there's no need to keep checking
   * after the shared variable has once been seen true.
   */
  if (LocalHotStandbyActive)
    return true;
  else
  {
    /* spinlock is essential on machines with weak memory ordering! */
    SpinLockAcquire(&XLogCtl->info_lck);
    LocalHotStandbyActive = XLogCtl->SharedHotStandbyActive;
    SpinLockRelease(&XLogCtl->info_lck);

    return LocalHotStandbyActive;
  }
}

/*
 * Like HotStandbyActive(), but to be used only in WAL replay code,
 * where we don't need to ask any other process what the state is.
 */
bool
HotStandbyActiveInReplay(void)
{
  Assert(AmStartupProcess() || !IsPostmasterEnvironment);
  return LocalHotStandbyActive;
}

/*
 * Is this process allowed to insert new WAL records?
 *
 * Ordinarily this is essentially equivalent to !RecoveryInProgress().
 * But we also have provisions for forcing the result "true" or "false"
 * within specific processes regardless of the global state.
 */
bool
XLogInsertAllowed(void)
{
  /*
   * If value is "unconditionally true" or "unconditionally false", just
   * return it.  This provides the normal fast path once recovery is known
   * done.
   */
  if (LocalXLogInsertAllowed >= 0)
    return (bool) LocalXLogInsertAllowed;

  /*
   * Else, must check to see if we're still in recovery.
   */
  if (RecoveryInProgress())
    return false;

  /*
   * On exit from recovery, reset to "unconditionally true", since there is
   * no need to keep checking.
   */
  LocalXLogInsertAllowed = 1;
  return true;
}

/*
 * Make XLogInsertAllowed() return true in the current process only.
 *
 * Note: it is allowed to switch LocalXLogInsertAllowed back to -1 later,
 * and even call LocalSetXLogInsertAllowed() again after that.
 */
static void
LocalSetXLogInsertAllowed(void)
{
  Assert(LocalXLogInsertAllowed == -1);
  LocalXLogInsertAllowed = 1;

  /* Initialize as RecoveryInProgress() would do when switching state */
  InitXLOGAccess();
}

/*
 * Subroutine to try to fetch and validate a prior checkpoint record.
 *
 * whichChkpt identifies the checkpoint (merely for reporting purposes).
 * 1 for "primary", 0 for "other" (backup_label)
 */
static XLogRecord *
ReadCheckpointRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
           int whichChkpt, bool report)
{
  XLogRecord *record;
  uint8   info;

  if (!XRecOffIsValid(RecPtr))
  {
    if (!report)
      return NULL;

    switch (whichChkpt)
    {
      case 1:
        ereport(LOG,
            (errmsg("invalid primary checkpoint link in control file")));
        break;
      default:
        ereport(LOG,
            (errmsg("invalid checkpoint link in backup_label file")));
        break;
    }
    return NULL;
  }

  record = ReadRecord(xlogreader, RecPtr, LOG, true);

  if (record == NULL)
  {
    if (!report)
      return NULL;

    switch (whichChkpt)
    {
      case 1:
        ereport(LOG,
            (errmsg("invalid primary checkpoint record")));
        break;
      default:
        ereport(LOG,
            (errmsg("invalid checkpoint record")));
        break;
    }
    return NULL;
  }
  if (record->xl_rmid != RM_XLOG_ID)
  {
    switch (whichChkpt)
    {
      case 1:
        ereport(LOG,
            (errmsg("invalid resource manager ID in primary checkpoint record")));
        break;
      default:
        ereport(LOG,
            (errmsg("invalid resource manager ID in checkpoint record")));
        break;
    }
    return NULL;
  }
  info = record->xl_info & ~XLR_INFO_MASK;
  if (info != XLOG_CHECKPOINT_SHUTDOWN &&
    info != 1XLOG_CHECKPOINT_ONLINE1)
  {
    switch (whichChkpt)
    {
      case 1:
        ereport(LOG,
            (errmsg("invalid xl_info in primary checkpoint record")));
        break;
      default:
        ereport(LOG,
            (errmsg("invalid xl_info in checkpoint record")));
        break;
    }
    return NULL;
  }
  if (record->xl_tot_len != SizeOfXLogRecord + SizeOfXLogRecordDataHeaderShort + sizeof(CheckPoint))
  {
    switch (whichChkpt)
    {
      case 1:
        ereport(LOG,
            (errmsg("invalid length of primary checkpoint record")));
        break;
      default:
        ereport(LOG,
            (errmsg("invalid length of checkpoint record")));
        break;
    }
    return NULL;
  }
  return record;
}

/*
 * This must be called in a backend process before creating WAL records
 * (except in a standalone backend, which does StartupXLOG instead).  We need
 * to initialize the local copies of ThisTimeLineID and RedoRecPtr.
 *
 * Note: before Postgres 8.0, we went to some effort to keep the postmaster
 * process's copies of ThisTimeLineID and RedoRecPtr valid too.  This was
 * unnecessary however, since the postmaster itself never touches XLOG anyway.
 */
void
InitXLOGAccess(void)
{
  XLogCtlInsert *Insert = &XLogCtl->Insert;

  /* ThisTimeLineID doesn't change so we need no lock to copy it */
  ThisTimeLineID = XLogCtl->ThisTimeLineID;
  Assert(ThisTimeLineID != 0 || IsBootstrapProcessingMode());

  /* set wal_segment_size */
  wal_segment_size = ControlFile->xlog_seg_size;

  /* Use GetRedoRecPtr to copy the RedoRecPtr safely */
  (void) GetRedoRecPtr();
  /* Also update our copy of doPageWrites. */
  doPageWrites = (Insert->fullPageWrites || Insert->forcePageWrites0);

  /* Also initialize the working areas for constructing WAL records */
  InitXLogInsert();
}

/*
 * Return the current Redo pointer from shared memory.
 *
 * As a side-effect, the local RedoRecPtr copy is updated.
 */
XLogRecPtr
GetRedoRecPtr(void)
{
  XLogRecPtr  ptr;

  /*
   * The possibly not up-to-date copy in XlogCtl is enough. Even if we
   * grabbed a WAL insertion lock to read the master copy, someone might
   * update it just after we've released the lock.
   */
  SpinLockAcquire(&XLogCtl->info_lck);
  ptr = XLogCtl->RedoRecPtr;
  SpinLockRelease(&XLogCtl->info_lck);

  if (RedoRecPtr < ptr)
    RedoRecPtr = ptr;

  return RedoRecPtr;
}

/*
 * Return information needed to decide whether a modified block needs a
 * full-page image to be included in the WAL record.
 *
 * The returned values are cached copies from backend-private memory, and
 * possibly out-of-date.  XLogInsertRecord will re-check them against
 * up-to-date values, while holding the WAL insert lock.
 */
void
GetFullPageWriteInfo(XLogRecPtr *RedoRecPtr_p, bool *doPageWrites_p)
{
  *RedoRecPtr_p = RedoRecPtr;
  *doPageWrites_p = doPageWrites;
}

/*
 * GetInsertRecPtr -- Returns the current insert position.
 *
 * NOTE: The value *actually* returned is the position of the last full
 * xlog page. It lags behind the real insert position by at most 1 page.
 * For that, we don't need to scan through WAL insertion locks, and an
 * approximation is enough for the current usage of this function.
 */
XLogRecPtr
GetInsertRecPtr(void)
{
  XLogRecPtr  recptr;

  SpinLockAcquire(&XLogCtl->info_lck);
  recptr = XLogCtl->LogwrtRqst.Write;
  SpinLockRelease(&XLogCtl->info_lck);

  return recptr;
}

/*
 * GetFlushRecPtr -- Returns the current flush position, ie, the last WAL
 * position known to be fsync'd to disk.
 */
XLogRecPtr
GetFlushRecPtr(void)
{
  SpinLockAcquire(&XLogCtl->info_lck);
  LogwrtResult = XLogCtl->LogwrtResult;
  SpinLockRelease(&XLogCtl->info_lck);

  return LogwrtResult.Flush;
}

/*
 * GetLastImportantRecPtr -- Returns the LSN of the last important record
 * inserted. All records not explicitly marked as unimportant are considered
 * important.
 *
 * The LSN is determined by computing the maximum of
 * WALInsertLocks[i].lastImportantAt.
 */
XLogRecPtr
GetLastImportantRecPtr(void)
{
  XLogRecPtr  res = InvalidXLogRecPtr;
  int     i;

  for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++20.5k)
  {
    XLogRecPtr  last_important;

    /*
     * Need to take a lock to prevent torn reads of the LSN, which are
     * possible on some of the supported platforms. WAL insert locks only
     * support exclusive mode, so we have to use that.
     */
    LWLockAcquire(&WALInsertLocks[i].l.lock, LW_EXCLUSIVE);
    last_important = WALInsertLocks[i].l.lastImportantAt;
    LWLockRelease(&WALInsertLocks[i].l.lock);

    if (res < last_important)
      res = last_important;
  }

  return res;
}

/*
 * Get the time and LSN of the last xlog segment switch
 */
pg_time_t
GetLastSegSwitchData(XLogRecPtr *lastSwitchLSN)
{
  pg_time_t result;

  /* Need WALWriteLock, but shared lock is sufficient */
  LWLockAcquire(WALWriteLock, LW_SHARED);
  result = XLogCtl->lastSegSwitchTime;
  *lastSwitchLSN = XLogCtl->lastSegSwitchLSN;
  LWLockRelease(WALWriteLock);

  return result;
}

/*
 * GetNextXidAndEpoch - get the current nextXid value and associated epoch
 *
 * This is exported for use by code that would like to have 64-bit XIDs.
 * We don't really support such things, but all XIDs within the system
 * can be presumed "close to" the result, and thus the epoch associated
 * with them can be determined.
 */
void
GetNextXidAndEpoch(TransactionId *xid, uint32 *epoch)
{
  uint32    ckptXidEpoch;
  TransactionId ckptXid;
  TransactionId nextXid;

  /* Must read checkpoint info first, else have race condition */
  SpinLockAcquire(&XLogCtl->info_lck);
  ckptXidEpoch = XLogCtl->ckptXidEpoch;
  ckptXid = XLogCtl->ckptXid;
  SpinLockRelease(&XLogCtl->info_lck);

  /* Now fetch current nextXid */
  nextXid = ReadNewTransactionId();

  /*
   * nextXid is certainly logically later than ckptXid.  So if it's
   * numerically less, it must have wrapped into the next epoch.
   */
  if (nextXid < ckptXid)
    ckptXidEpoch++;

  *xid = nextXid;
  *epoch = ckptXidEpoch;
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownXLOG(int code, Datum arg)
{
  /* Don't be chatty in standalone mode */
  ereport(IsPostmasterEnvironment ? LOG : NOTICE,
      (errmsg("shutting down")));

  /*
   * Signal walsenders to move to stopping state.
   */
  WalSndInitStopping();

  /*
   * Wait for WAL senders to be in stopping state.  This prevents commands
   * from writing new WAL.
   */
  WalSndWaitStopping();

  if (RecoveryInProgress())
    CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
  else
  {
    /*
     * If archiving is enabled, rotate the last XLOG file so that all the
     * remaining records are archived (postmaster wakes up the archiver
     * process one more time at the end of shutdown). The checkpoint
     * record will go to the next XLOG file and won't be archived (yet).
     */
    if (XLogArchivingActive() && XLogArchiveCommandSet0())
      RequestXLogSwitch(false);

    CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
  }
  ShutdownCLOG();
  ShutdownCommitTs();
  ShutdownSUBTRANS();
  ShutdownMultiXact();
}

/*
 * Log start of a checkpoint.
 */
static void
LogCheckpointStart(int flags, bool restartpoint)
{
  elog(LOG, "%s starting:%s%s%s%s%s%s%s%s",
     restartpoint ? "restartpoint" : "checkpoint",
     (flags & CHECKPOINT_IS_SHUTDOWN) ? " shutdown" : "",
     (flags & CHECKPOINT_END_OF_RECOVERY) ? " end-of-recovery" : "",
     (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
     (flags & CHECKPOINT_FORCE) ? " force" : "",
     (flags & CHECKPOINT_WAIT) ? " wait" : "",
     (flags & CHECKPOINT_CAUSE_XLOG) ? " xlog" : "",
     (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "",
     (flags & CHECKPOINT_FLUSH_ALL) ? " flush-all" : "");
}

/*
 * Log end of a checkpoint.
 */
static void
LogCheckpointEnd(bool restartpoint)
{
  long    write_secs,
        sync_secs,
        total_secs,
        longest_secs,
        average_secs;
  int     write_usecs,
        sync_usecs,
        total_usecs,
        longest_usecs,
        average_usecs;
  uint64    average_sync_time;

  CheckpointStats.ckpt_end_t = GetCurrentTimestamp();

  TimestampDifference(CheckpointStats.ckpt_write_t,
            CheckpointStats.ckpt_sync_t,
            &write_secs, &write_usecs);

  TimestampDifference(CheckpointStats.ckpt_sync_t,
            CheckpointStats.ckpt_sync_end_t,
            &sync_secs, &sync_usecs);

  /* Accumulate checkpoint timing summary data, in milliseconds. */
  BgWriterStats.m_checkpoint_write_time +=
    write_secs * 1000 + write_usecs / 1000;
  BgWriterStats.m_checkpoint_sync_time +=
    sync_secs * 1000 + sync_usecs / 1000;

  /*
   * All of the published timing statistics are accounted for.  Only
   * continue if a log message is to be written.
   */
  if (!log_checkpoints)
    return;

  TimestampDifference(CheckpointStats.ckpt_start_t,
            CheckpointStats.ckpt_end_t,
            &total_secs, &total_usecs);

  /*
   * Timing values returned from CheckpointStats are in microseconds.
   * Convert to the second plus microsecond form that TimestampDifference
   * returns for homogeneous printing.
   */
  longest_secs = (long) (CheckpointStats.ckpt_longest_sync / 1000000);
  longest_usecs = CheckpointStats.ckpt_longest_sync -
    (uint64) longest_secs * 1000000;

  average_sync_time = 0;
  if (CheckpointStats.ckpt_sync_rels > 0)
    average_sync_time = CheckpointStats.ckpt_agg_sync_time /
      CheckpointStats.ckpt_sync_rels;
  average_secs = (long) (average_sync_time / 1000000);
  average_usecs = average_sync_time - (uint64) average_secs * 1000000;

  elog(LOG, "%s complete: wrote %d buffers (%.1f%%); "
     "%d WAL file(s) added, %d removed, %d recycled; "
     "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
     "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s; "
     "distance=%d kB, estimate=%d kB",
     restartpoint ? "restartpoint" : "checkpoint",
     CheckpointStats.ckpt_bufs_written,
     (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
     CheckpointStats.ckpt_segs_added,
     CheckpointStats.ckpt_segs_removed,
     CheckpointStats.ckpt_segs_recycled,
     write_secs, write_usecs / 1000,
     sync_secs, sync_usecs / 1000,
     total_secs, total_usecs / 1000,
     CheckpointStats.ckpt_sync_rels,
     longest_secs, longest_usecs / 1000,
     average_secs, average_usecs / 1000,
     (int) (PrevCheckPointDistance / 1024.0),
     (int) (CheckPointDistanceEstimate / 1024.0));
}

/*
 * Update the estimate of distance between checkpoints.
 *
 * The estimate is used to calculate the number of WAL segments to keep
 * preallocated, see XLOGFileSlop().
 */
static void
UpdateCheckPointDistanceEstimate(uint64 nbytes)
{
  /*
   * To estimate the number of segments consumed between checkpoints, keep a
   * moving average of the amount of WAL generated in previous checkpoint
   * cycles. However, if the load is bursty, with quiet periods and busy
   * periods, we want to cater for the peak load. So instead of a plain
   * moving average, let the average decline slowly if the previous cycle
   * used less WAL than estimated, but bump it up immediately if it used
   * more.
   *
   * When checkpoints are triggered by max_wal_size, this should converge to
   * CheckpointSegments * wal_segment_size,
   *
   * Note: This doesn't pay any attention to what caused the checkpoint.
   * Checkpoints triggered manually with CHECKPOINT command, or by e.g.
   * starting a base backup, are counted the same as those created
   * automatically. The slow-decline will largely mask them out, if they are
   * not frequent. If they are frequent, it seems reasonable to count them
   * in as any others; if you issue a manual checkpoint every 5 minutes and
   * never let a timed checkpoint happen, it makes sense to base the
   * preallocation on that 5 minute interval rather than whatever
   * checkpoint_timeout is set to.
   */
  PrevCheckPointDistance = nbytes;
  if (CheckPointDistanceEstimate < nbytes)
    CheckPointDistanceEstimate = nbytes;
  else
    CheckPointDistanceEstimate =
      (0.90 * CheckPointDistanceEstimate + 0.10 * (double) nbytes);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 *
 * flags is a bitwise OR of the following:
 *  CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
 *  CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
 *  CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
 *    ignoring checkpoint_completion_target parameter.
 *  CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occurred
 *    since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
 *    CHECKPOINT_END_OF_RECOVERY).
 *  CHECKPOINT_FLUSH_ALL: also flush buffers of unlogged tables.
 *
 * Note: flags contains other bits, of interest here only for logging purposes.
 * In particular note that this routine is synchronous and does not pay
 * attention to CHECKPOINT_WAIT.
 *
 * If !shutdown then we are writing an online checkpoint. This is a very special
 * kind of operation and WAL record because the checkpoint action occurs over
 * a period of time yet logically occurs at just a single LSN. The logical
 * position of the WAL record (redo ptr) is the same or earlier than the
 * physical position. When we replay WAL we locate the checkpoint via its
 * physical position then read the redo ptr and actually start replay at the
 * earlier logical position. Note that we don't write *anything* to WAL at
 * the logical position, so that location could be any other kind of WAL record.
 * All of this mechanism allows us to continue working while we checkpoint.
 * As a result, timing of actions is critical here and be careful to note that
 * this function will likely take minutes to execute on a busy system.
 */
void
CreateCheckPoint(int flags)
{
  bool    shutdown;
  CheckPoint  checkPoint;
  XLogRecPtr  recptr;
  XLogSegNo _logSegNo;
  XLogCtlInsert *Insert = &XLogCtl->Insert;
  uint32    freespace;
  XLogRecPtr  PriorRedoPtr;
  XLogRecPtr  curInsert;
  XLogRecPtr  last_important_lsn;
  VirtualTransactionId *vxids;
  int     nvxids;

  /*
   * An end-of-recovery checkpoint is really a shutdown checkpoint, just
   * issued at a different time.
   */
  if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
    shutdown = true;
  else
    shutdown = false;

  /* sanity check */
  if (RecoveryInProgress() && (flags & 5CHECKPOINT_END_OF_RECOVERY5) == 0)
    elog(ERROR, "can't create a checkpoint during recovery");

  /*
   * Initialize InitXLogInsert working areas before entering the critical
   * section.  Normally, this is done by the first call to
   * RecoveryInProgress() or LocalSetXLogInsertAllowed(), but when creating
   * an end-of-recovery checkpoint, the LocalSetXLogInsertAllowed call is
   * done below in a critical section, and InitXLogInsert cannot be called
   * in a critical section.
   */
  InitXLogInsert();

  /*
   * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
   * (This is just pro forma, since in the present system structure there is
   * only one process that is allowed to issue checkpoints at any given
   * time.)
   */
  LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

  /*
   * Prepare to accumulate statistics.
   *
   * Note: because it is possible for log_checkpoints to change while a
   * checkpoint proceeds, we always accumulate stats, even if
   * log_checkpoints is currently off.
   */
  MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
  CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

  /*
   * Use a critical section to force system panic if we have trouble.
   */
  START_CRIT_SECTION();

  if (shutdown)
  {
    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
    ControlFile->state = DB_SHUTDOWNING;
    ControlFile->time = (pg_time_t) time(NULL);
    UpdateControlFile();
    LWLockRelease(ControlFileLock);
  }

  /*
   * Let smgr prepare for checkpoint; this has to happen before we determine
   * the REDO pointer.  Note that smgr must not do anything that'd have to
   * be undone if we decide no checkpoint is needed.
   */
  smgrpreckpt();

  /* Begin filling in the checkpoint WAL record */
  MemSet(&checkPoint, 0, sizeof(checkPoint));
  checkPoint.time = (pg_time_t) time(NULL);

  /*
   * For Hot Standby, derive the oldestActiveXid before we fix the redo
   * pointer. This allows us to begin accumulating changes to assemble our
   * starting snapshot of locks and transactions.
   */
  if (!shutdown && XLogStandbyInfoActive562())
    checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
  else
    checkPoint.oldestActiveXid = InvalidTransactionId;

  /*
   * Get location of last important record before acquiring insert locks (as
   * GetLastImportantRecPtr() also locks WAL locks).
   */
  last_important_lsn = GetLastImportantRecPtr();

  /*
   * We must block concurrent insertions while examining insert state to
   * determine the checkpoint REDO pointer.
   */
  WALInsertLockAcquireExclusive();
  curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);

  /*
   * If this isn't a shutdown or forced checkpoint, and if there has been no
   * WAL activity requiring a checkpoint, skip it.  The idea here is to
   * avoid inserting duplicate checkpoints when the system is idle.
   */
  if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
          CHECKPOINT_FORCE)) == 0)
  {
    if (last_important_lsn == ControlFile->checkPoint)
    {
      WALInsertLockRelease();
      LWLockRelease(CheckpointLock);
      END_CRIT_SECTION();
      ereport(DEBUG1,
          (errmsg("checkpoint skipped because system is idle")));
      return;
    }
  }

  /*
   * An end-of-recovery checkpoint is created before anyone is allowed to
   * write WAL. To allow us to write the checkpoint record, temporarily
   * enable XLogInsertAllowed.  (This also ensures ThisTimeLineID is
   * initialized, which we need here and in AdvanceXLInsertBuffer.)
   */
  if (flags & CHECKPOINT_END_OF_RECOVERY)
    LocalSetXLogInsertAllowed();

  checkPoint.ThisTimeLineID = ThisTimeLineID;
  if (flags & CHECKPOINT_END_OF_RECOVERY)
    checkPoint.PrevTimeLineID = XLogCtl->PrevTimeLineID;
  else
    checkPoint.PrevTimeLineID = ThisTimeLineID;

  checkPoint.fullPageWrites = Insert->fullPageWrites;

  /*
   * Compute new REDO record ptr = location of next XLOG record.
   *
   * NB: this is NOT necessarily where the checkpoint record itself will be,
   * since other backends may insert more XLOG records while we're off doing
   * the buffer flush work.  Those XLOG records are logically after the
   * checkpoint, even though physically before it.  Got that?
   */
  freespace = INSERT_FREESPACE(curInsert);
  if (freespace == 0)
  {
    if (XLogSegmentOffset(curInsert, wal_segment_size) == 0)
      curInsert += SizeOfXLogLongPHD;
    else
      curInsert += SizeOfXLogShortPHD;
  }
  checkPoint.redo = curInsert;

  /*
   * Here we update the shared RedoRecPtr for future XLogInsert calls; this
   * must be done while holding all the insertion locks.
   *
   * Note: if we fail to complete the checkpoint, RedoRecPtr will be left
   * pointing past where it really needs to point.  This is okay; the only
   * consequence is that XLogInsert might back up whole buffers that it
   * didn't really need to.  We can't postpone advancing RedoRecPtr because
   * XLogInserts that happen while we are dumping buffers must assume that
   * their buffer changes are not included in the checkpoint.
   */
  RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;

  /*
   * Now we can release the WAL insertion locks, allowing other xacts to
   * proceed while we are flushing disk buffers.
   */
  WALInsertLockRelease();

  /* Update the info_lck-protected copy of RedoRecPtr as well */
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->RedoRecPtr = checkPoint.redo;
  SpinLockRelease(&XLogCtl->info_lck);

  /*
   * If enabled, log checkpoint start.  We postpone this until now so as not
   * to log anything if we decided to skip the checkpoint.
   */
  if (log_checkpoints)
    LogCheckpointStart(flags, false);

  TRACE_POSTGRESQL_CHECKPOINT_START(flags);

  /*
   * Get the other info we need for the checkpoint record.
   *
   * We don't need to save oldestClogXid in the checkpoint, it only matters
   * for the short period in which clog is being truncated, and if we crash
   * during that we'll redo the clog truncation and fix up oldestClogXid
   * there.
   */
  LWLockAcquire(XidGenLock, LW_SHARED);
  checkPoint.nextXid = ShmemVariableCache->nextXid;
  checkPoint.oldestXid = ShmemVariableCache->oldestXid;
  checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
  LWLockRelease(XidGenLock);

  LWLockAcquire(CommitTsLock, LW_SHARED);
  checkPoint.oldestCommitTsXid = ShmemVariableCache->oldestCommitTsXid;
  checkPoint.newestCommitTsXid = ShmemVariableCache->newestCommitTsXid;
  LWLockRelease(CommitTsLock);

  /* Increase XID epoch if we've wrapped around since last checkpoint */
  checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
  if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
    checkPoint.nextXidEpoch++;

  LWLockAcquire(OidGenLock, LW_SHARED);
  checkPoint.nextOid = ShmemVariableCache->nextOid;
  if (!shutdown)
    checkPoint.nextOid += ShmemVariableCache->oidCount;
  LWLockRelease(OidGenLock);

  MultiXactGetCheckptMulti(shutdown,
               &checkPoint.nextMulti,
               &checkPoint.nextMultiOffset,
               &checkPoint.oldestMulti,
               &checkPoint.oldestMultiDB);

  /*
   * Having constructed the checkpoint record, ensure all shmem disk buffers
   * and commit-log buffers are flushed to disk.
   *
   * This I/O could fail for various reasons.  If so, we will fail to
   * complete the checkpoint, but there is no reason to force a system
   * panic. Accordingly, exit critical section while doing it.
   */
  END_CRIT_SECTION();

  /*
   * In some cases there are groups of actions that must all occur on one
   * side or the other of a checkpoint record. Before flushing the
   * checkpoint record we must explicitly wait for any backend currently
   * performing those groups of actions.
   *
   * One example is end of transaction, so we must wait for any transactions
   * that are currently in commit critical sections.  If an xact inserted
   * its commit record into XLOG just before the REDO point, then a crash
   * restart from the REDO point would not replay that record, which means
   * that our flushing had better include the xact's update of pg_xact.  So
   * we wait till he's out of his commit critical section before proceeding.
   * See notes in RecordTransactionCommit().
   *
   * Because we've already released the insertion locks, this test is a bit
   * fuzzy: it is possible that we will wait for xacts we didn't really need
   * to wait for.  But the delay should be short and it seems better to make
   * checkpoint take a bit longer than to hold off insertions longer than
   * necessary. (In fact, the whole reason we have this issue is that xact.c
   * does commit record XLOG insertion and clog update as two separate steps
   * protected by different locks, but again that seems best on grounds of
   * minimizing lock contention.)
   *
   * A transaction that has not yet set delayChkpt when we look cannot be at
   * risk, since he's not inserted his commit record yet; and one that's
   * already cleared it is not at risk either, since he's done fixing clog
   * and we will correctly flush the update below.  So we cannot miss any
   * xacts we need to wait for.
   */
  vxids = GetVirtualXIDsDelayingChkpt(&nvxids);
  if (nvxids > 0)
  {
    do
    {
      pg_usleep(10000L);  /* wait for 10 msec */
    } while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids));
  }
  pfree(vxids);

  CheckPointGuts(checkPoint.redo, flags);

  /*
   * Take a snapshot of running transactions and write this to WAL. This
   * allows us to reconstruct the state of running transactions during
   * archive recovery, if required. Skip, if this info disabled.
   *
   * If we are shutting down, or Startup process is completing crash
   * recovery we don't need to write running xact data.
   */
  if (!shutdown && XLogStandbyInfoActive340())
    LogStandbySnapshot();

  START_CRIT_SECTION();

  /*
   * Now insert the checkpoint record into XLOG.
   */
  XLogBeginInsert();
  XLogRegisterData((char *) (&checkPoint), sizeof(checkPoint));
  recptr = XLogInsert(RM_XLOG_ID,
            shutdown ? XLOG_CHECKPOINT_SHUTDOWN2.01k :
            XLOG_CHECKPOINT_ONLINE340);

  XLogFlush(recptr);

  /*
   * We mustn't write any new WAL after a shutdown checkpoint, or it will be
   * overwritten at next startup.  No-one should even try, this just allows
   * sanity-checking.  In the case of an end-of-recovery checkpoint, we want
   * to just temporarily disable writing until the system has exited
   * recovery.
   */
  if (shutdown)
  {
    if (flags & CHECKPOINT_END_OF_RECOVERY)
      LocalXLogInsertAllowed = -1;  /* return to "check" state */
    else
      LocalXLogInsertAllowed = 0; /* never again write WAL */
  }

  /*
   * We now have ProcLastRecPtr = start of actual checkpoint record, recptr
   * = end of actual checkpoint record.
   */
  if (shutdown && checkPoint.redo != ProcLastRecPtr2.01k)
    ereport(PANIC,
        (errmsg("concurrent write-ahead log activity while database system is shutting down")));

  /*
   * Remember the prior checkpoint's redo ptr for
   * UpdateCheckPointDistanceEstimate()
   */
  PriorRedoPtr = ControlFile->checkPointCopy.redo;

  /*
   * Update the control file.
   */
  LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
  if (shutdown)
    ControlFile->state = DB_SHUTDOWNED;
  ControlFile->checkPoint = ProcLastRecPtr;
  ControlFile->checkPointCopy = checkPoint;
  ControlFile->time = (pg_time_t) time(NULL);
  /* crash recovery should always recover to the end of WAL */
  ControlFile->minRecoveryPoint = InvalidXLogRecPtr;
  ControlFile->minRecoveryPointTLI = 0;

  /*
   * Persist unloggedLSN value. It's reset on crash recovery, so this goes
   * unused on non-shutdown checkpoints, but seems useful to store it always
   * for debugging purposes.
   */
  SpinLockAcquire(&XLogCtl->ulsn_lck);
  ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
  SpinLockRelease(&XLogCtl->ulsn_lck);

  UpdateControlFile();
  LWLockRelease(ControlFileLock);

  /* Update shared-memory copy of checkpoint XID/epoch */
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
  XLogCtl->ckptXid = checkPoint.nextXid;
  SpinLockRelease(&XLogCtl->info_lck);

  /*
   * We are now done with critical updates; no need for system panic if we
   * have trouble while fooling with old log segments.
   */
  END_CRIT_SECTION();

  /*
   * Let smgr do post-checkpoint cleanup (eg, deleting old files).
   */
  smgrpostckpt();

  /*
   * Update the average distance between checkpoints if the prior checkpoint
   * exists.
   */
  if (PriorRedoPtr != InvalidXLogRecPtr)
    UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);

  /*
   * Delete old log files, those no longer needed for last checkpoint to
   * prevent the disk holding the xlog from growing full.
   */
  XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size);
  KeepLogSeg(recptr, &_logSegNo);
  _logSegNo--;
  RemoveOldXlogFiles(_logSegNo, RedoRecPtr, recptr);

  /*
   * Make more log segments if needed.  (Do this after recycling old log
   * segments, since that may supply some of the needed files.)
   */
  if (!shutdown)
    PreallocXlogFiles(recptr);

  /*
   * Truncate pg_subtrans if possible.  We can throw away all data before
   * the oldest XMIN of any running transaction.  No future transaction will
   * attempt to reference any pg_subtrans entry older than that (see Asserts
   * in subtrans.c).  During recovery, though, we mustn't do this because
   * StartupSUBTRANS hasn't been called yet.
   */
  if (!RecoveryInProgress())
    TruncateSUBTRANS(GetOldestXmin(NULL, PROCARRAY_FLAGS_DEFAULT));

  /* Real work is done, but log and update stats before releasing lock. */
  LogCheckpointEnd(false);

  TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
                   NBuffers,
                   CheckpointStats.ckpt_segs_added,
                   CheckpointStats.ckpt_segs_removed,
                   CheckpointStats.ckpt_segs_recycled);

  LWLockRelease(CheckpointLock);
}

/*
 * Mark the end of recovery in WAL though without running a full checkpoint.
 * We can expect that a restartpoint is likely to be in progress as we
 * do this, though we are unwilling to wait for it to complete. So be
 * careful to avoid taking the CheckpointLock anywhere here.
 *
 * CreateRestartPoint() allows for the case where recovery may end before
 * the restartpoint completes so there is no concern of concurrent behaviour.
 */
static void
CreateEndOfRecoveryRecord(void)
{
  xl_end_of_recovery xlrec;
  XLogRecPtr  recptr;

  /* sanity check */
  if (!RecoveryInProgress())
    elog(ERROR, "can only be used to end recovery");

  xlrec.end_time = GetCurrentTimestamp();

  WALInsertLockAcquireExclusive();
  xlrec.ThisTimeLineID = ThisTimeLineID;
  xlrec.PrevTimeLineID = XLogCtl->PrevTimeLineID;
  WALInsertLockRelease();

  LocalSetXLogInsertAllowed();

  START_CRIT_SECTION();

  XLogBeginInsert();
  XLogRegisterData((char *) &xlrec, sizeof(xl_end_of_recovery));
  recptr = XLogInsert(RM_XLOG_ID, XLOG_END_OF_RECOVERY);

  XLogFlush(recptr);

  /*
   * Update the control file so that crash recovery can follow the timeline
   * changes to this point.
   */
  LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
  ControlFile->time = (pg_time_t) time(NULL);
  ControlFile->minRecoveryPoint = recptr;
  ControlFile->minRecoveryPointTLI = ThisTimeLineID;
  UpdateControlFile();
  LWLockRelease(ControlFileLock);

  END_CRIT_SECTION();

  LocalXLogInsertAllowed = -1;  /* return to "check" state */
}

/*
 * Flush all data in shared memory to disk, and fsync
 *
 * This is the common code shared between regular checkpoints and
 * recovery restartpoints.
 */
static void
CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
{
  CheckPointCLOG();
  CheckPointCommitTs();
  CheckPointSUBTRANS();
  CheckPointMultiXact();
  CheckPointPredicate();
  CheckPointRelationMap();
  CheckPointReplicationSlots();
  CheckPointSnapBuild();
  CheckPointLogicalRewriteHeap();
  CheckPointBuffers(flags); /* performs all required fsyncs */
  CheckPointReplicationOrigin();
  /* We deliberately delay 2PC checkpointing as long as possible */
  CheckPointTwoPhase(checkPointRedo);
}

/*
 * Save a checkpoint for recovery restart if appropriate
 *
 * This function is called each time a checkpoint record is read from XLOG.
 * It must determine whether the checkpoint represents a safe restartpoint or
 * not.  If so, the checkpoint record is stashed in shared memory so that
 * CreateRestartPoint can consult it.  (Note that the latter function is
 * executed by the checkpointer, while this one will be executed by the
 * startup process.)
 */
static void
RecoveryRestartPoint(const CheckPoint *checkPoint)
{
  /*
   * Also refrain from creating a restartpoint if we have seen any
   * references to non-existent pages. Restarting recovery from the
   * restartpoint would not see the references, so we would lose the
   * cross-check that the pages belonged to a relation that was dropped
   * later.
   */
  if (XLogHaveInvalidPages())
  {
    elog(trace_recovery(DEBUG2),
       "could not record restart point at %X/%X because there "
       "are unresolved references to invalid pages",
       (uint32) (checkPoint->redo >> 32),
       (uint32) checkPoint->redo);
    return;
  }

  /*
   * Copy the checkpoint record to shared memory, so that checkpointer can
   * work out the next time it wants to perform a restartpoint.
   */
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->lastCheckPointRecPtr = ReadRecPtr;
  XLogCtl->lastCheckPointEndPtr = EndRecPtr;
  XLogCtl->lastCheckPoint = *checkPoint;
  SpinLockRelease(&XLogCtl->info_lck);
}

/*
 * Establish a restartpoint if possible.
 *
 * This is similar to CreateCheckPoint, but is used during WAL recovery
 * to establish a point from which recovery can roll forward without
 * replaying the entire recovery log.
 *
 * Returns true if a new restartpoint was established. We can only establish
 * a restartpoint if we have replayed a safe checkpoint record since last
 * restartpoint.
 */
bool
CreateRestartPoint(int flags)
{
  XLogRecPtr  lastCheckPointRecPtr;
  XLogRecPtr  lastCheckPointEndPtr;
  CheckPoint  lastCheckPoint;
  XLogRecPtr  PriorRedoPtr;
  XLogRecPtr  receivePtr;
  XLogRecPtr  replayPtr;
  TimeLineID  replayTLI;
  XLogRecPtr  endptr;
  XLogSegNo _logSegNo;
  TimestampTz xtime;

  /*
   * Acquire CheckpointLock to ensure only one restartpoint or checkpoint
   * happens at a time.
   */
  LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

  /* Get a local copy of the last safe checkpoint record. */
  SpinLockAcquire(&XLogCtl->info_lck);
  lastCheckPointRecPtr = XLogCtl->lastCheckPointRecPtr;
  lastCheckPointEndPtr = XLogCtl->lastCheckPointEndPtr;
  lastCheckPoint = XLogCtl->lastCheckPoint;
  SpinLockRelease(&XLogCtl->info_lck);

  /*
   * Check that we're still in recovery mode. It's ok if we exit recovery
   * mode after this check, the restart point is valid anyway.
   */
  if (!RecoveryInProgress())
  {
    ereport(DEBUG2,
        (errmsg("skipping restartpoint, recovery has already ended")));
    LWLockRelease(CheckpointLock);
    return false;
  }

  /*
   * If the last checkpoint record we've replayed is already our last
   * restartpoint, we can't perform a new restart point. We still update
   * minRecoveryPoint in that case, so that if this is a shutdown restart
   * point, we won't start up earlier than before. That's not strictly
   * necessary, but when hot standby is enabled, it would be rather weird if
   * the database opened up for read-only connections at a point-in-time
   * before the last shutdown. Such time travel is still possible in case of
   * immediate shutdown, though.
   *
   * We don't explicitly advance minRecoveryPoint when we do create a
   * restartpoint. It's assumed that flushing the buffers will do that as a
   * side-effect.
   */
  if (XLogRecPtrIsInvalid(lastCheckPointRecPtr) ||
    lastCheckPoint.redo <= ControlFile->checkPointCopy.redo)
  {
    ereport(DEBUG2,
        (errmsg("skipping restartpoint, already performed at %X/%X",
            (uint32) (lastCheckPoint.redo >> 32),
            (uint32) lastCheckPoint.redo)));

    UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);
    if (flags & CHECKPOINT_IS_SHUTDOWN)
    {
      LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
      ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
      ControlFile->time = (pg_time_t) time(NULL);
      UpdateControlFile();
      LWLockRelease(ControlFileLock);
    }
    LWLockRelease(CheckpointLock);
    return false;
  }

  /*
   * Update the shared RedoRecPtr so that the startup process can calculate
   * the number of segments replayed since last restartpoint, and request a
   * restartpoint if it exceeds CheckPointSegments.
   *
   * Like in CreateCheckPoint(), hold off insertions to update it, although
   * during recovery this is just pro forma, because no WAL insertions are
   * happening.
   */
  WALInsertLockAcquireExclusive();
  RedoRecPtr = XLogCtl->Insert.RedoRecPtr = lastCheckPoint.redo;
  WALInsertLockRelease();

  /* Also update the info_lck-protected copy */
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->RedoRecPtr = lastCheckPoint.redo;
  SpinLockRelease(&XLogCtl->info_lck);

  /*
   * Prepare to accumulate statistics.
   *
   * Note: because it is possible for log_checkpoints to change while a
   * checkpoint proceeds, we always accumulate stats, even if
   * log_checkpoints is currently off.
   */
  MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
  CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

  if (log_checkpoints)
    LogCheckpointStart(flags, true);

  CheckPointGuts(lastCheckPoint.redo, flags);

  /*
   * Remember the prior checkpoint's redo ptr for
   * UpdateCheckPointDistanceEstimate()
   */
  PriorRedoPtr = ControlFile->checkPointCopy.redo;

  /*
   * Update pg_control, using current time.  Check that it still shows
   * DB_IN_ARCHIVE_RECOVERY state and an older checkpoint, else do nothing;
   * this is a quick hack to make sure nothing really bad happens if somehow
   * we get here after the end-of-recovery checkpoint.
   */
  LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
  if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&
    ControlFile->checkPointCopy.redo < lastCheckPoint.redo)
  {
    ControlFile->checkPoint = lastCheckPointRecPtr;
    ControlFile->checkPointCopy = lastCheckPoint;
    ControlFile->time = (pg_time_t) time(NULL);

    /*
     * Ensure minRecoveryPoint is past the checkpoint record.  Normally,
     * this will have happened already while writing out dirty buffers,
     * but not necessarily - e.g. because no buffers were dirtied.  We do
     * this because a non-exclusive base backup uses minRecoveryPoint to
     * determine which WAL files must be included in the backup, and the
     * file (or files) containing the checkpoint record must be included,
     * at a minimum. Note that for an ordinary restart of recovery there's
     * no value in having the minimum recovery point any earlier than this
     * anyway, because redo will begin just after the checkpoint record.
     */
    if (ControlFile->minRecoveryPoint < lastCheckPointEndPtr)
    {
      ControlFile->minRecoveryPoint = lastCheckPointEndPtr;
      ControlFile->minRecoveryPointTLI = lastCheckPoint.ThisTimeLineID;

      /* update local copy */
      minRecoveryPoint = ControlFile->minRecoveryPoint;
      minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
    }
    if (flags & CHECKPOINT_IS_SHUTDOWN)
      ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
    UpdateControlFile();
  }
  LWLockRelease(ControlFileLock);

  /*
   * Update the average distance between checkpoints/restartpoints if the
   * prior checkpoint exists.
   */
  if (PriorRedoPtr != InvalidXLogRecPtr)
    UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);

  /*
   * Delete old log files, those no longer needed for last restartpoint to
   * prevent the disk holding the xlog from growing full.
   */
  XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size);

  /*
   * Retreat _logSegNo using the current end of xlog replayed or received,
   * whichever is later.
   */
  receivePtr = GetWalRcvWriteRecPtr(NULL, NULL);
  replayPtr = GetXLogReplayRecPtr(&replayTLI);
  endptr = (receivePtr < replayPtr) ? replayPtr : receivePtr;
  KeepLogSeg(endptr, &_logSegNo);
  _logSegNo--;

  /*
   * Try to recycle segments on a useful timeline. If we've been promoted
   * since the beginning of this restartpoint, use the new timeline chosen
   * at end of recovery (RecoveryInProgress() sets ThisTimeLineID in that
   * case). If we're still in recovery, use the timeline we're currently
   * replaying.
   *
   * There is no guarantee that the WAL segments will be useful on the
   * current timeline; if recovery proceeds to a new timeline right after
   * this, the pre-allocated WAL segments on this timeline will not be used,
   * and will go wasted until recycled on the next restartpoint. We'll live
   * with that.
   */
  if (RecoveryInProgress())
    ThisTimeLineID = replayTLI;

  RemoveOldXlogFiles(_logSegNo, RedoRecPtr, endptr);

  /*
   * Make more log segments if needed.  (Do this after recycling old log
   * segments, since that may supply some of the needed files.)
   */
  PreallocXlogFiles(endptr);

  /*
   * ThisTimeLineID is normally not set when we're still in recovery.
   * However, recycling/preallocating segments above needed ThisTimeLineID
   * to determine which timeline to install the segments on. Reset it now,
   * to restore the normal state of affairs for debugging purposes.
   */
  if (RecoveryInProgress())
    ThisTimeLineID = 0;

  /*
   * Truncate pg_subtrans if possible.  We can throw away all data before
   * the oldest XMIN of any running transaction.  No future transaction will
   * attempt to reference any pg_subtrans entry older than that (see Asserts
   * in subtrans.c).  When hot standby is disabled, though, we mustn't do
   * this because StartupSUBTRANS hasn't been called yet.
   */
  if (EnableHotStandby)
    TruncateSUBTRANS(GetOldestXmin(NULL, PROCARRAY_FLAGS_DEFAULT));

  /* Real work is done, but log and update before releasing lock. */
  LogCheckpointEnd(true);

  xtime = GetLatestXTime();
  ereport((log_checkpoints ? LOG : DEBUG2),
      (errmsg("recovery restart point at %X/%X",
          (uint32) (lastCheckPoint.redo >> 32), (uint32) lastCheckPoint.redo),
       xtime ? errdetail("Last completed transaction was at log time %s.",
                 timestamptz_to_str(xtime)) : 0));

  LWLockRelease(CheckpointLock);

  /*
   * Finally, execute archive_cleanup_command, if any.
   */
  if (XLogCtl->archiveCleanupCommand[0])
    ExecuteRecoveryCommand(XLogCtl->archiveCleanupCommand,
                 "archive_cleanup_command",
                 false);

  return true;
}

/*
 * Retreat *logSegNo to the last segment that we need to retain because of
 * either wal_keep_segments or replication slots.
 *
 * This is calculated by subtracting wal_keep_segments from the given xlog
 * location, recptr and by making sure that that result is below the
 * requirement of replication slots.
 */
static void
KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
{
  XLogSegNo segno;
  XLogRecPtr  keep;

  XLByteToSeg(recptr, segno, wal_segment_size);
  keep = XLogGetReplicationSlotMinimumLSN();

  /* compute limit for wal_keep_segments first */
  if (wal_keep_segments > 0)
  {
    /* avoid underflow, don't go below 1 */
    if (segno <= wal_keep_segments)
      segno = 1;
    else
      segno = segno - wal_keep_segments;
  }

  /* then check whether slots limit removal further */
  if (max_replication_slots > 0 && keep != InvalidXLogRecPtr)
  {
    XLogSegNo slotSegNo;

    XLByteToSeg(keep, slotSegNo, wal_segment_size);

    if (slotSegNo <= 0)
      segno = 1;
    else if (slotSegNo < segno)
      segno = slotSegNo;
  }

  /* don't delete WAL segments newer than the calculated segment */
  if (segno < *logSegNo)
    *logSegNo = segno;
}

/*
 * Write a NEXTOID log record
 */
void
XLogPutNextOid(Oid nextOid)
{
  XLogBeginInsert();
  XLogRegisterData((char *) (&nextOid), sizeof(Oid));
  (void) XLogInsert(RM_XLOG_ID, XLOG_NEXTOID);

  /*
   * We need not flush the NEXTOID record immediately, because any of the
   * just-allocated OIDs could only reach disk as part of a tuple insert or
   * update that would have its own XLOG record that must follow the NEXTOID
   * record.  Therefore, the standard buffer LSN interlock applied to those
   * records will ensure no such OID reaches disk before the NEXTOID record
   * does.
   *
   * Note, however, that the above statement only covers state "within" the
   * database.  When we use a generated OID as a file or directory name, we
   * are in a sense violating the basic WAL rule, because that filesystem
   * change may reach disk before the NEXTOID WAL record does.  The impact
   * of this is that if a database crash occurs immediately afterward, we
   * might after restart re-generate the same OID and find that it conflicts
   * with the leftover file or directory.  But since for safety's sake we
   * always loop until finding a nonconflicting filename, this poses no real
   * problem in practice. See pgsql-hackers discussion 27-Sep-2006.
   */
}

/*
 * Write an XLOG SWITCH record.
 *
 * Here we just blindly issue an XLogInsert request for the record.
 * All the magic happens inside XLogInsert.
 *
 * The return value is either the end+1 address of the switch record,
 * or the end+1 address of the prior segment if we did not need to
 * write a switch record because we are already at segment start.
 */
XLogRecPtr
RequestXLogSwitch(bool mark_unimportant)
{
  XLogRecPtr  RecPtr;

  /* XLOG SWITCH has no data */
  XLogBeginInsert();

  if (mark_unimportant)
    XLogSetRecordFlags(XLOG_MARK_UNIMPORTANT);
  RecPtr = XLogInsert(RM_XLOG_ID, XLOG_SWITCH);

  return RecPtr;
}

/*
 * Write a RESTORE POINT record
 */
XLogRecPtr
XLogRestorePoint(const char *rpName)
{
  XLogRecPtr  RecPtr;
  xl_restore_point xlrec;

  xlrec.rp_time = GetCurrentTimestamp();
  strlcpy(xlrec.rp_name, rpName, MAXFNAMELEN);

  XLogBeginInsert();
  XLogRegisterData((char *) &xlrec, sizeof(xl_restore_point));

  RecPtr = XLogInsert(RM_XLOG_ID, XLOG_RESTORE_POINT);

  ereport(LOG,
      (errmsg("restore point \"%s\" created at %X/%X",
          rpName, (uint32) (RecPtr >> 32), (uint32) RecPtr)));

  return RecPtr;
}

/*
 * Check if any of the GUC parameters that are critical for hot standby
 * have changed, and update the value in pg_control file if necessary.
 */
static void
XLogReportParameters(void)
{
  if (wal_level != ControlFile->wal_level ||
    wal_log_hints != ControlFile->wal_log_hints ||
    MaxConnections != ControlFile->MaxConnections ||
    max_worker_processes != ControlFile->max_worker_processes3.98k ||
    max_prepared_xacts != ControlFile->max_prepared_xacts3.98k ||
    max_locks_per_xact != ControlFile->max_locks_per_xact3.98k ||
    track_commit_timestamp != ControlFile->track_commit_timestamp3.98k)
  {
    /*
     * The change in number of backend slots doesn't need to be WAL-logged
     * if archiving is not enabled, as you can't start archive recovery
     * with wal_level=minimal anyway. We don't really care about the
     * values in pg_control either if wal_level=minimal, but seems better
     * to keep them up-to-date to avoid confusion.
     */
    if (wal_level != ControlFile->wal_level || XLogIsNeeded())
    {
      xl_parameter_change xlrec;
      XLogRecPtr  recptr;

      xlrec.MaxConnections = MaxConnections;
      xlrec.max_worker_processes = max_worker_processes;
      xlrec.max_prepared_xacts = max_prepared_xacts;
      xlrec.max_locks_per_xact = max_locks_per_xact;
      xlrec.wal_level = wal_level;
      xlrec.wal_log_hints = wal_log_hints;
      xlrec.track_commit_timestamp = track_commit_timestamp;

      XLogBeginInsert();
      XLogRegisterData((char *) &xlrec, sizeof(xlrec));

      recptr = XLogInsert(RM_XLOG_ID, XLOG_PARAMETER_CHANGE);
      XLogFlush(recptr);
    }

    ControlFile->MaxConnections = MaxConnections;
    ControlFile->max_worker_processes = max_worker_processes;
    ControlFile->max_prepared_xacts = max_prepared_xacts;
    ControlFile->max_locks_per_xact = max_locks_per_xact;
    ControlFile->wal_level = wal_level;
    ControlFile->wal_log_hints = wal_log_hints;
    ControlFile->track_commit_timestamp = track_commit_timestamp;
    UpdateControlFile();
  }
}

/*
 * Update full_page_writes in shared memory, and write an
 * XLOG_FPW_CHANGE record if necessary.
 *
 * Note: this function assumes there is no other process running
 * concurrently that could update it.
 */
void
UpdateFullPageWrites(void)
{
  XLogCtlInsert *Insert = &XLogCtl->Insert;
  bool    recoveryInProgress;

  /*
   * Do nothing if full_page_writes has not been changed.
   *
   * It's safe to check the shared full_page_writes without the lock,
   * because we assume that there is no concurrently running process which
   * can update it.
   */
  if (fullPageWrites == Insert->fullPageWrites)
    return;

  /*
   * Perform this outside critical section so that the WAL insert
   * initialization done by RecoveryInProgress() doesn't trigger an
   * assertion failure.
   */
  recoveryInProgress = RecoveryInProgress();

  START_CRIT_SECTION();

  /*
   * It's always safe to take full page images, even when not strictly
   * required, but not the other round. So if we're setting full_page_writes
   * to true, first set it true and then write the WAL record. If we're
   * setting it to false, first write the WAL record and then set the global
   * flag.
   */
  if (fullPageWrites)
  {
    WALInsertLockAcquireExclusive();
    Insert->fullPageWrites = true;
    WALInsertLockRelease();
  }

  /*
   * Write an XLOG_FPW_CHANGE record. This allows us to keep track of
   * full_page_writes during archive recovery, if required.
   */
  if (XLogStandbyInfoActive() && !recoveryInProgress)
  {
    XLogBeginInsert();
    XLogRegisterData((char *) (&fullPageWrites), sizeof(bool));

    XLogInsert(RM_XLOG_ID, XLOG_FPW_CHANGE);
  }

  if (!fullPageWrites)
  {
    WALInsertLockAcquireExclusive();
    Insert->fullPageWrites = false;
    WALInsertLockRelease();
  }
  END_CRIT_SECTION();
}

/*
 * Check that it's OK to switch to new timeline during recovery.
 *
 * 'lsn' is the address of the shutdown checkpoint record we're about to
 * replay. (Currently, timeline can only change at a shutdown checkpoint).
 */
static void
checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI, TimeLineID prevTLI)
{
  /* Check that the record agrees on what the current (old) timeline is */
  if (prevTLI != ThisTimeLineID)
    ereport(PANIC,
        (errmsg("unexpected previous timeline ID %u (current timeline ID %u) in checkpoint record",
            prevTLI, ThisTimeLineID)));

  /*
   * The new timeline better be in the list of timelines we expect to see,
   * according to the timeline history. It should also not decrease.
   */
  if (newTLI < ThisTimeLineID || !tliInHistory(newTLI, expectedTLEs))
    ereport(PANIC,
        (errmsg("unexpected timeline ID %u (after %u) in checkpoint record",
            newTLI, ThisTimeLineID)));

  /*
   * If we have not yet reached min recovery point, and we're about to
   * switch to a timeline greater than the timeline of the min recovery
   * point: trouble. After switching to the new timeline, we could not
   * possibly visit the min recovery point on the correct timeline anymore.
   * This can happen if there is a newer timeline in the archive that
   * branched before the timeline the min recovery point is on, and you
   * attempt to do PITR to the new timeline.
   */
  if (!XLogRecPtrIsInvalid(minRecoveryPoint) &&
    lsn < minRecoveryPoint &&
    newTLI > minRecoveryPointTLI)
    ereport(PANIC,
        (errmsg("unexpected timeline ID %u in checkpoint record, before reaching minimum recovery point %X/%X on timeline %u",
            newTLI,
            (uint32) (minRecoveryPoint >> 32),
            (uint32) minRecoveryPoint,
            minRecoveryPointTLI)));

  /* Looks good */
}

/*
 * XLOG resource manager's routines
 *
 * Definitions of info values are in include/catalog/pg_control.h, though
 * not all record types are related to control file updates.
 */
void
xlog_redo(XLogReaderState *record)
{
  uint8   info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
  XLogRecPtr  lsn = record->EndRecPtr;

  /* in XLOG rmgr, backup blocks are only used by XLOG_FPI records */
  Assert(info == XLOG_FPI || info == XLOG_FPI_FOR_HINT ||
       !XLogRecHasAnyBlockRefs(record));

  if (info == XLOG_NEXTOID)
  {
    Oid     nextOid;

    /*
     * We used to try to take the maximum of ShmemVariableCache->nextOid
     * and the recorded nextOid, but that fails if the OID counter wraps
     * around.  Since no OID allocation should be happening during replay
     * anyway, better to just believe the record exactly.  We still take
     * OidGenLock while setting the variable, just in case.
     */
    memcpy(&nextOid, XLogRecGetData(record), sizeof(Oid));
    LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
    ShmemVariableCache->nextOid = nextOid;
    ShmemVariableCache->oidCount = 0;
    LWLockRelease(OidGenLock);
  }
  else if (info == XLOG_CHECKPOINT_SHUTDOWN)
  {
    CheckPoint  checkPoint;

    memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
    /* In a SHUTDOWN checkpoint, believe the counters exactly */
    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
    ShmemVariableCache->nextXid = checkPoint.nextXid;
    LWLockRelease(XidGenLock);
    LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
    ShmemVariableCache->nextOid = checkPoint.nextOid;
    ShmemVariableCache->oidCount = 0;
    LWLockRelease(OidGenLock);
    MultiXactSetNextMXact(checkPoint.nextMulti,
                checkPoint.nextMultiOffset);

    MultiXactAdvanceOldest(checkPoint.oldestMulti,
                 checkPoint.oldestMultiDB);

    /*
     * No need to set oldestClogXid here as well; it'll be set when we
     * redo an xl_clog_truncate if it changed since initialization.
     */
    SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);

    /*
     * If we see a shutdown checkpoint while waiting for an end-of-backup
     * record, the backup was canceled and the end-of-backup record will
     * never arrive.
     */
    if (ArchiveRecoveryRequested &&
      !XLogRecPtrIsInvalid(ControlFile->backupStartPoint) &&
      XLogRecPtrIsInvalid(ControlFile->backupEndPoint))
      ereport(PANIC,
          (errmsg("online backup was canceled, recovery cannot continue")));

    /*
     * If we see a shutdown checkpoint, we know that nothing was running
     * on the master at this point. So fake-up an empty running-xacts
     * record and use that here and now. Recover additional standby state
     * for prepared transactions.
     */
    if (standbyState >= STANDBY_INITIALIZED)
    {
      TransactionId *xids;
      int     nxids;
      TransactionId oldestActiveXID;
      TransactionId latestCompletedXid;
      RunningTransactionsData running;

      oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);

      /*
       * Construct a RunningTransactions snapshot representing a shut
       * down server, with only prepared transactions still alive. We're
       * never overflowed at this point because all subxids are listed
       * with their parent prepared transactions.
       */
      running.xcnt = nxids;
      running.subxcnt = 0;
      running.subxid_overflow = false;
      running.nextXid = checkPoint.nextXid;
      running.oldestRunningXid = oldestActiveXID;
      latestCompletedXid = checkPoint.nextXid;
      TransactionIdRetreat(latestCompletedXid);
      Assert(TransactionIdIsNormal(latestCompletedXid));
      running.latestCompletedXid = latestCompletedXid;
      running.xids = xids;

      ProcArrayApplyRecoveryInfo(&running);

      StandbyRecoverPreparedTransactions();
    }

    /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
    ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
    ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

    /* Update shared-memory copy of checkpoint XID/epoch */
    SpinLockAcquire(&XLogCtl->info_lck);
    XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
    XLogCtl->ckptXid = checkPoint.nextXid;
    SpinLockRelease(&XLogCtl->info_lck);

    /*
     * We should've already switched to the new TLI before replaying this
     * record.
     */
    if (checkPoint.ThisTimeLineID != ThisTimeLineID)
      ereport(PANIC,
          (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
              checkPoint.ThisTimeLineID, ThisTimeLineID)));

    RecoveryRestartPoint(&checkPoint);
  }
  else if (info == XLOG_CHECKPOINT_ONLINE)
  {
    CheckPoint  checkPoint;

    memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
    /* In an ONLINE checkpoint, treat the XID counter as a minimum */
    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
    if (TransactionIdPrecedes(ShmemVariableCache->nextXid,
                  checkPoint.nextXid))
      ShmemVariableCache->nextXid = checkPoint.nextXid;
    LWLockRelease(XidGenLock);

    /*
     * We ignore the nextOid counter in an ONLINE checkpoint, preferring
     * to track OID assignment through XLOG_NEXTOID records.  The nextOid
     * counter is from the start of the checkpoint and might well be stale
     * compared to later XLOG_NEXTOID records.  We could try to take the
     * maximum of the nextOid counter and our latest value, but since
     * there's no particular guarantee about the speed with which the OID
     * counter wraps around, that's a risky thing to do.  In any case,
     * users of the nextOid counter are required to avoid assignment of
     * duplicates, so that a somewhat out-of-date value should be safe.
     */

    /* Handle multixact */
    MultiXactAdvanceNextMXact(checkPoint.nextMulti,
                  checkPoint.nextMultiOffset);

    /*
     * NB: This may perform multixact truncation when replaying WAL
     * generated by an older primary.
     */
    MultiXactAdvanceOldest(checkPoint.oldestMulti,
                 checkPoint.oldestMultiDB);
    if (TransactionIdPrecedes(ShmemVariableCache->oldestXid,
                  checkPoint.oldestXid))
      SetTransactionIdLimit(checkPoint.oldestXid,
                  checkPoint.oldestXidDB);
    /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
    ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
    ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

    /* Update shared-memory copy of checkpoint XID/epoch */
    SpinLockAcquire(&XLogCtl->info_lck);
    XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
    XLogCtl->ckptXid = checkPoint.nextXid;
    SpinLockRelease(&XLogCtl->info_lck);

    /* TLI should not change in an on-line checkpoint */
    if (checkPoint.ThisTimeLineID != ThisTimeLineID)
      ereport(PANIC,
          (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
              checkPoint.ThisTimeLineID, ThisTimeLineID)));

    RecoveryRestartPoint(&checkPoint);
  }
  else if (info == XLOG_END_OF_RECOVERY)
  {
    xl_end_of_recovery xlrec;

    memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_end_of_recovery));

    /*
     * For Hot Standby, we could treat this like a Shutdown Checkpoint,
     * but this case is rarer and harder to test, so the benefit doesn't
     * outweigh the potential extra cost of maintenance.
     */

    /*
     * We should've already switched to the new TLI before replaying this
     * record.
     */
    if (xlrec.ThisTimeLineID != ThisTimeLineID)
      ereport(PANIC,
          (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
              xlrec.ThisTimeLineID, ThisTimeLineID)));
  }
  else if (info == XLOG_NOOP)
  {
    /* nothing to do here */
  }
  else if (info == XLOG_SWITCH)
  {
    /* nothing to do here */
  }
  else if (info == XLOG_RESTORE_POINT)
  {
    /* nothing to do here */
  }
  else if (info == XLOG_FPI || info == XLOG_FPI_FOR_HINT)
  {
    Buffer    buffer;

    /*
     * Full-page image (FPI) records contain nothing else but a backup
     * block. The block reference must include a full-page image -
     * otherwise there would be no point in this record.
     *
     * No recovery conflicts are generated by these generic records - if a
     * resource manager needs to generate conflicts, it has to define a
     * separate WAL record type and redo routine.
     *
     * XLOG_FPI_FOR_HINT records are generated when a page needs to be
     * WAL- logged because of a hint bit update. They are only generated
     * when checksums are enabled. There is no difference in handling
     * XLOG_FPI and XLOG_FPI_FOR_HINT records, they use a different info
     * code just to distinguish them for statistics purposes.
     */
    if (XLogReadBufferForRedo(record, 0, &buffer) != BLK_RESTORED)
      elog(ERROR, "unexpected XLogReadBufferForRedo result when restoring backup block");
    UnlockReleaseBuffer(buffer);
  }
  else if (info == XLOG_BACKUP_END)
  {
    XLogRecPtr  startpoint;

    memcpy(&startpoint, XLogRecGetData(record), sizeof(startpoint));

    if (ControlFile->backupStartPoint == startpoint)
    {
      /*
       * We have reached the end of base backup, the point where
       * pg_stop_backup() was done. The data on disk is now consistent.
       * Reset backupStartPoint, and update minRecoveryPoint to make
       * sure we don't allow starting up at an earlier point even if
       * recovery is stopped and restarted soon after this.
       */
      elog(DEBUG1, "end of backup reached");

      LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

      if (ControlFile->minRecoveryPoint < lsn)
      {
        ControlFile->minRecoveryPoint = lsn;
        ControlFile->minRecoveryPointTLI = ThisTimeLineID;
      }
      ControlFile->backupStartPoint = InvalidXLogRecPtr;
      ControlFile->backupEndRequired = false;
      UpdateControlFile();

      LWLockRelease(ControlFileLock);
    }
  }
  else if (info == XLOG_PARAMETER_CHANGE)
  {
    xl_parameter_change xlrec;

    /* Update our copy of the parameters in pg_control */
    memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_parameter_change));

    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
    ControlFile->MaxConnections = xlrec.MaxConnections;
    ControlFile->max_worker_processes = xlrec.max_worker_processes;
    ControlFile->max_prepared_xacts = xlrec.max_prepared_xacts;
    ControlFile->max_locks_per_xact = xlrec.max_locks_per_xact;
    ControlFile->wal_level = xlrec.wal_level;
    ControlFile->wal_log_hints = xlrec.wal_log_hints;

    /*
     * Update minRecoveryPoint to ensure that if recovery is aborted, we
     * recover back up to this point before allowing hot standby again.
     * This is important if the max_* settings are decreased, to ensure
     * you don't run queries against the WAL preceding the change. The
     * local copies cannot be updated as long as crash recovery is
     * happening and we expect all the WAL to be replayed.
     */
    if (InArchiveRecovery)
    {
      minRecoveryPoint = ControlFile->minRecoveryPoint;
      minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
    }
    if (minRecoveryPoint != InvalidXLogRecPtr && minRecoveryPoint < lsn)
    {
      ControlFile->minRecoveryPoint = lsn;
      ControlFile->minRecoveryPointTLI = ThisTimeLineID;
    }

    CommitTsParameterChange(xlrec.track_commit_timestamp,
                ControlFile->track_commit_timestamp);
    ControlFile->track_commit_timestamp = xlrec.track_commit_timestamp;

    UpdateControlFile();
    LWLockRelease(ControlFileLock);

    /* Check to see if any changes to max_connections give problems */
    CheckRequiredParameterValues();
  }
  else if (info == XLOG_FPW_CHANGE)
  {
    bool    fpw;

    memcpy(&fpw, XLogRecGetData(record), sizeof(bool));

    /*
     * Update the LSN of the last replayed XLOG_FPW_CHANGE record so that
     * do_pg_start_backup() and do_pg_stop_backup() can check whether
     * full_page_writes has been disabled during online backup.
     */
    if (!fpw)
    {
      SpinLockAcquire(&XLogCtl->info_lck);
      if (XLogCtl->lastFpwDisableRecPtr < ReadRecPtr)
        XLogCtl->lastFpwDisableRecPtr = ReadRecPtr;
      SpinLockRelease(&XLogCtl->info_lck);
    }

    /* Keep track of full_page_writes */
    lastFullPageWrites = fpw;
  }
}

#ifdef WAL_DEBUG

static void
xlog_outrec(StringInfo buf, XLogReaderState *record)
{
  int     block_id;

  appendStringInfo(buf, "prev %X/%X; xid %u",
           (uint32) (XLogRecGetPrev(record) >> 32),
           (uint32) XLogRecGetPrev(record),
           XLogRecGetXid(record));

  appendStringInfo(buf, "; len %u",
           XLogRecGetDataLen(record));

  /* decode block references */
  for (block_id = 0; block_id <= record->max_block_id; block_id++)
  {
    RelFileNode rnode;
    ForkNumber  forknum;
    BlockNumber blk;

    if (!XLogRecHasBlockRef(record, block_id))
      continue;

    XLogRecGetBlockTag(record, block_id, &rnode, &forknum, &blk);
    if (forknum != MAIN_FORKNUM)
      appendStringInfo(buf, "; blkref #%u: rel %u/%u/%u, fork %u, blk %u",
               block_id,
               rnode.spcNode, rnode.dbNode, rnode.relNode,
               forknum,
               blk);
    else
      appendStringInfo(buf, "; blkref #%u: rel %u/%u/%u, blk %u",
               block_id,
               rnode.spcNode, rnode.dbNode, rnode.relNode,
               blk);
    if (XLogRecHasBlockImage(record, block_id))
      appendStringInfoString(buf, " FPW");
  }
}
#endif              /* WAL_DEBUG */

/*
 * Returns a string describing an XLogRecord, consisting of its identity
 * optionally followed by a colon, a space, and a further description.
 */
static void
xlog_outdesc(StringInfo buf, XLogReaderState *record)
{
  RmgrId    rmid = XLogRecGetRmid(record);
  uint8   info = XLogRecGetInfo(record);
  const char *id;

  appendStringInfoString(buf, RmgrTable[rmid].rm_name);
  appendStringInfoChar(buf, '/');

  id = RmgrTable[rmid].rm_identify(info);
  if (id == NULL)
    appendStringInfo(buf, "UNKNOWN (%X): ", info & ~XLR_INFO_MASK);
  else
    appendStringInfo(buf, "%s: ", id);

  RmgrTable[rmid].rm_desc(buf, record);
}


/*
 * Return the (possible) sync flag used for opening a file, depending on the
 * value of the GUC wal_sync_method.
 */
static int
get_sync_bit(int method)
{
  int     o_direct_flag = 0;

  /* If fsync is disabled, never open in sync mode */
  if (!enableFsync)
    return 0;

  /*
   * Optimize writes by bypassing kernel cache with O_DIRECT when using
   * O_SYNC/O_FSYNC and O_DSYNC.  But only if archiving and streaming are
   * disabled, otherwise the archive command or walsender process will read
   * the WAL soon after writing it, which is guaranteed to cause a physical
   * read if we bypassed the kernel cache. We also skip the
   * posix_fadvise(POSIX_FADV_DONTNEED) call in XLogFileClose() for the same
   * reason.
   *
   * Never use O_DIRECT in walreceiver process for similar reasons; the WAL
   * written by walreceiver is normally read by the startup process soon
   * after its written. Also, walreceiver performs unaligned writes, which
   * don't work with O_DIRECT, so it is required for correctness too.
   */
  if (!XLogIsNeeded() && !0AmWalReceiverProcess0())
    o_direct_flag = PG_O_DIRECT;

  switch (method)
  {
      /*
       * enum values for all sync options are defined even if they are
       * not supported on the current platform.  But if not, they are
       * not included in the enum option array, and therefore will never
       * be seen here.
       */
    case SYNC_METHOD_FSYNC:
    case SYNC_METHOD_FSYNC_WRITETHROUGH:
    case SYNC_METHOD_FDATASYNC:
      return 0;
#ifdef OPEN_SYNC_FLAG
    case SYNC_METHOD_OPEN:
      return OPEN_SYNC_FLAG | o_direct_flag;
#endif
#ifdef OPEN_DATASYNC_FLAG
    case SYNC_METHOD_OPEN_DSYNC:
      return OPEN_DATASYNC_FLAG | o_direct_flag;
#endif
    default:
      /* can't happen (unless we are out of sync with option array) */
      elog(ERROR, "unrecognized wal_sync_method: %d", method);
      return 0;     /* silence warning */
  }
}

/*
 * GUC support
 */
void
assign_xlog_sync_method(int new_sync_method, void *extra)
{
  if (sync_method != new_sync_method)
  {
    /*
     * To ensure that no blocks escape unsynced, force an fsync on the
     * currently open log segment (if any).  Also, if the open flag is
     * changing, close the log file so it will be reopened (with new flag
     * bit) at next use.
     */
    if (openLogFile >= 0)
    {
      pgstat_report_wait_start(WAIT_EVENT_WAL_SYNC_METHOD_ASSIGN);
      if (pg_fsync(openLogFile) != 0)
        ereport(PANIC,
            (errcode_for_file_access(),
             errmsg("could not fsync log segment %s: %m",
                XLogFileNameP(ThisTimeLineID, openLogSegNo))));
      pgstat_report_wait_end();
      if (get_sync_bit(sync_method) != get_sync_bit(new_sync_method))
        XLogFileClose();
    }
  }
}


/*
 * Issue appropriate kind of fsync (if any) for an XLOG output file.
 *
 * 'fd' is a file descriptor for the XLOG file to be fsync'd.
 * 'log' and 'seg' are for error reporting purposes.
 */
void
issue_xlog_fsync(int fd, XLogSegNo segno)
{
  switch (sync_method)
  {
    case SYNC_METHOD_FSYNC:
      if (pg_fsync_no_writethrough(fd) != 0)
        ereport(PANIC,
            (errcode_for_file_access(),
             errmsg("could not fsync log file %s: %m",
                XLogFileNameP(ThisTimeLineID, segno))));
      break;
#ifdef HAVE_FSYNC_WRITETHROUGH
    case SYNC_METHOD_FSYNC_WRITETHROUGH:
      if (pg_fsync_writethrough(fd) != 0)
        ereport(PANIC,
            (errcode_for_file_access(),
             errmsg("could not fsync write-through log file %s: %m",
                XLogFileNameP(ThisTimeLineID, segno))));
      break;
#endif
#ifdef HAVE_FDATASYNC
    case SYNC_METHOD_FDATASYNC:
      if (pg_fdatasync(fd) != 0)
        ereport(PANIC,
            (errcode_for_file_access(),
             errmsg("could not fdatasync log file %s: %m",
                XLogFileNameP(ThisTimeLineID, segno))));
      break;
#endif
    case SYNC_METHOD_OPEN:
    case SYNC_METHOD_OPEN_DSYNC:
      /* write synced it already */
      break;
    default:
      elog(PANIC, "unrecognized wal_sync_method: %d", sync_method);
      break;
  }
}

/*
 * Return the filename of given log segment, as a palloc'd string.
 */
char *
XLogFileNameP(TimeLineID tli, XLogSegNo segno)
{
  char     *result = palloc(MAXFNAMELEN);

  XLogFileName(result, tli, segno, wal_segment_size);
  return result;
}

/*
 * do_pg_start_backup is the workhorse of the user-visible pg_start_backup()
 * function. It creates the necessary starting checkpoint and constructs the
 * backup label file.
 *
 * There are two kind of backups: exclusive and non-exclusive. An exclusive
 * backup is started with pg_start_backup(), and there can be only one active
 * at a time. The backup and tablespace map files of an exclusive backup are
 * written to $PGDATA/backup_label and $PGDATA/tablespace_map, and they are
 * removed by pg_stop_backup().
 *
 * A non-exclusive backup is used for the streaming base backups (see
 * src/backend/replication/basebackup.c). The difference to exclusive backups
 * is that the backup label and tablespace map files are not written to disk.
 * Instead, their would-be contents are returned in *labelfile and *tblspcmapfile,
 * and the caller is responsible for including them in the backup archive as
 * 'backup_label' and 'tablespace_map'. There can be many non-exclusive backups
 * active at the same time, and they don't conflict with an exclusive backup
 * either.
 *
 * tblspcmapfile is required mainly for tar format in windows as native windows
 * utilities are not able to create symlinks while extracting files from tar.
 * However for consistency, the same is used for all platforms.
 *
 * needtblspcmapfile is true for the cases (exclusive backup and for
 * non-exclusive backup only when tar format is used for taking backup)
 * when backup needs to generate tablespace_map file, it is used to
 * embed escape character before newline character in tablespace path.
 *
 * Returns the minimum WAL location that must be present to restore from this
 * backup, and the corresponding timeline ID in *starttli_p.
 *
 * Every successfully started non-exclusive backup must be stopped by calling
 * do_pg_stop_backup() or do_pg_abort_backup().
 *
 * It is the responsibility of the caller of this function to verify the
 * permissions of the calling user!
 */
XLogRecPtr
do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
           StringInfo labelfile, List **tablespaces,
           StringInfo tblspcmapfile, bool infotbssize,
           bool needtblspcmapfile)
{
  bool    exclusive = (labelfile == NULL);
  bool    backup_started_in_recovery = false;
  XLogRecPtr  checkpointloc;
  XLogRecPtr  startpoint;
  TimeLineID  starttli;
  pg_time_t stamp_time;
  char    strfbuf[128];
  char    xlogfilename[MAXFNAMELEN];
  XLogSegNo _logSegNo;
  struct stat stat_buf;
  FILE     *fp;

  backup_started_in_recovery = RecoveryInProgress();

  /*
   * Currently only non-exclusive backup can be taken during recovery.
   */
  if (backup_started_in_recovery && exclusive)
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("recovery is in progress"),
         errhint("WAL control functions cannot be executed during recovery.")));

  /*
   * During recovery, we don't need to check WAL level. Because, if WAL
   * level is not sufficient, it's impossible to get here during recovery.
   */
  if (!backup_started_in_recovery && !XLogIsNeeded())
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("WAL level not sufficient for making an online backup"),
         errhint("wal_level must be set to \"replica\" or \"logical\" at server start.")));

  if (strlen(backupidstr) > MAXPGPATH)
    ereport(ERROR,
        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
         errmsg("backup label too long (max %d bytes)",
            MAXPGPATH)));

  /*
   * Mark backup active in shared memory.  We must do full-page WAL writes
   * during an on-line backup even if not doing so at other times, because
   * it's quite possible for the backup dump to obtain a "torn" (partially
   * written) copy of a database page if it reads the page concurrently with
   * our write to the same page.  This can be fixed as long as the first
   * write to the page in the WAL sequence is a full-page write. Hence, we
   * turn on forcePageWrites and then force a CHECKPOINT, to ensure there
   * are no dirty pages in shared memory that might get dumped while the
   * backup is in progress without having a corresponding WAL record.  (Once
   * the backup is complete, we need not force full-page writes anymore,
   * since we expect that any pages not modified during the backup interval
   * must have been correctly captured by the backup.)
   *
   * Note that forcePageWrites has no effect during an online backup from
   * the standby.
   *
   * We must hold all the insertion locks to change the value of
   * forcePageWrites, to ensure adequate interlocking against
   * XLogInsertRecord().
   */
  WALInsertLockAcquireExclusive();
  if (exclusive)
  {
    /*
     * At first, mark that we're now starting an exclusive backup, to
     * ensure that there are no other sessions currently running
     * pg_start_backup() or pg_stop_backup().
     */
    if (XLogCtl->Insert.exclusiveBackupState != EXCLUSIVE_BACKUP_NONE)
    {
      WALInsertLockRelease();
      ereport(ERROR,
          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg("a backup is already in progress"),
           errhint("Run pg_stop_backup() and try again.")));
    }
    XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_STARTING;
  }
  else
    XLogCtl->Insert.nonExclusiveBackups++;
  XLogCtl->Insert.forcePageWrites = true;
  WALInsertLockRelease();

  /* Ensure we release forcePageWrites if fail below */
  PG_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));
  {
    bool    gotUniqueStartpoint = false;
    DIR      *tblspcdir;
    struct dirent *de;
    tablespaceinfo *ti;
    int     datadirpathlen;

    /*
     * Force an XLOG file switch before the checkpoint, to ensure that the
     * WAL segment the checkpoint is written to doesn't contain pages with
     * old timeline IDs.  That would otherwise happen if you called
     * pg_start_backup() right after restoring from a PITR archive: the
     * first WAL segment containing the startup checkpoint has pages in
     * the beginning with the old timeline ID.  That can cause trouble at
     * recovery: we won't have a history file covering the old timeline if
     * pg_wal directory was not included in the base backup and the WAL
     * archive was cleared too before starting the backup.
     *
     * This also ensures that we have emitted a WAL page header that has
     * XLP_BKP_REMOVABLE off before we emit the checkpoint record.
     * Therefore, if a WAL archiver (such as pglesslog) is trying to
     * compress out removable backup blocks, it won't remove any that
     * occur after this point.
     *
     * During recovery, we skip forcing XLOG file switch, which means that
     * the backup taken during recovery is not available for the special
     * recovery case described above.
     */
    if (!backup_started_in_recovery)
      RequestXLogSwitch(false);

    do
    {
      bool    checkpointfpw;

      /*
       * Force a CHECKPOINT.  Aside from being necessary to prevent torn
       * page problems, this guarantees that two successive backup runs
       * will have different checkpoint positions and hence different
       * history file names, even if nothing happened in between.
       *
       * During recovery, establish a restartpoint if possible. We use
       * the last restartpoint as the backup starting checkpoint. This
       * means that two successive backup runs can have same checkpoint
       * positions.
       *
       * Since the fact that we are executing do_pg_start_backup()
       * during recovery means that checkpointer is running, we can use
       * RequestCheckpoint() to establish a restartpoint.
       *
       * We use CHECKPOINT_IMMEDIATE only if requested by user (via
       * passing fast = true).  Otherwise this can take awhile.
       */
      RequestCheckpoint(CHECKPOINT_FORCE | CHECKPOINT_WAIT |
                (fast ? CHECKPOINT_IMMEDIATE : 0));

      /*
       * Now we need to fetch the checkpoint record location, and also
       * its REDO pointer.  The oldest point in WAL that would be needed
       * to restore starting from the checkpoint is precisely the REDO
       * pointer.
       */
      LWLockAcquire(ControlFileLock, LW_SHARED);
      checkpointloc = ControlFile->checkPoint;
      startpoint = ControlFile->checkPointCopy.redo;
      starttli = ControlFile->checkPointCopy.ThisTimeLineID;
      checkpointfpw = ControlFile->checkPointCopy.fullPageWrites;
      LWLockRelease(ControlFileLock);

      if (backup_started_in_recovery)
      {
        XLogRecPtr  recptr;

        /*
         * Check to see if all WAL replayed during online backup
         * (i.e., since last restartpoint used as backup starting
         * checkpoint) contain full-page writes.
         */
        SpinLockAcquire(&XLogCtl->info_lck);
        recptr = XLogCtl->lastFpwDisableRecPtr;
        SpinLockRelease(&XLogCtl->info_lck);

        if (!checkpointfpw || startpoint <= recptr)
          ereport(ERROR,
              (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
               errmsg("WAL generated with full_page_writes=off was replayed "
                  "since last restartpoint"),
               errhint("This means that the backup being taken on the standby "
                   "is corrupt and should not be used. "
                   "Enable full_page_writes and run CHECKPOINT on the master, "
                   "and then try an online backup again.")));

        /*
         * During recovery, since we don't use the end-of-backup WAL
         * record and don't write the backup history file, the
         * starting WAL location doesn't need to be unique. This means
         * that two base backups started at the same time might use
         * the same checkpoint as starting locations.
         */
        gotUniqueStartpoint = true;
      }

      /*
       * If two base backups are started at the same time (in WAL sender
       * processes), we need to make sure that they use different
       * checkpoints as starting locations, because we use the starting
       * WAL location as a unique identifier for the base backup in the
       * end-of-backup WAL record and when we write the backup history
       * file. Perhaps it would be better generate a separate unique ID
       * for each backup instead of forcing another checkpoint, but
       * taking a checkpoint right after another is not that expensive
       * either because only few buffers have been dirtied yet.
       */
      WALInsertLockAcquireExclusive();
      if (XLogCtl->Insert.lastBackupStart < startpoint)
      {
        XLogCtl->Insert.lastBackupStart = startpoint;
        gotUniqueStartpoint = true;
      }
      WALInsertLockRelease();
    } while (!gotUniqueStartpoint);

    XLByteToSeg(startpoint, _logSegNo, wal_segment_size);
    XLogFileName(xlogfilename, starttli, _logSegNo, wal_segment_size);

    /*
     * Construct tablespace_map file
     */
    if (exclusive)
      tblspcmapfile = makeStringInfo();

    datadirpathlen = strlen(DataDir);

    /* Collect information about all tablespaces */
    tblspcdir = AllocateDir("pg_tblspc");
    while ((de = ReadDir(tblspcdir, "pg_tblspc")) != NULL)
    {
      char    fullpath[MAXPGPATH + 10];
      char    linkpath[MAXPGPATH];
      char     *relpath = NULL;
      int     rllen;
      StringInfoData buflinkpath;
      char     *s = linkpath;

      /* Skip special stuff */
      if (strcmp(de->d_name, ".") == 0 || strcmp(de->d_name, "..") == 0)
        continue;

      snprintf(fullpath, sizeof(fullpath), "pg_tblspc/%s", de->d_name);

#if defined(HAVE_READLINK) || defined(WIN32)
      rllen = readlink(fullpath, linkpath, sizeof(linkpath));
      if (rllen < 0)
      {
        ereport(WARNING,
            (errmsg("could not read symbolic link \"%s\": %m",
                fullpath)));
        continue;
      }
      else if (rllen >= sizeof(linkpath))
      {
        ereport(WARNING,
            (errmsg("symbolic link \"%s\" target is too long",
                fullpath)));
        continue;
      }
      linkpath[rllen] = '\0';

      /*
       * Add the escape character '\\' before newline in a string to
       * ensure that we can distinguish between the newline in the
       * tablespace path and end of line while reading tablespace_map
       * file during archive recovery.
       */
      initStringInfo(&buflinkpath);

      while (*s)
      {
        if ((*s == '\n' || *s == '\r') && needtblspcmapfile)
          appendStringInfoChar(&buflinkpath, '\\');
        appendStringInfoChar(&buflinkpath, *s++);
      }

      /*
       * Relpath holds the relative path of the tablespace directory
       * when it's located within PGDATA, or NULL if it's located
       * elsewhere.
       */
      if (rllen > datadirpathlen &&
        strncmp(linkpath, DataDir, datadirpathlen) == 0 &&
        IS_DIR_SEP(linkpath[datadirpathlen]))
        relpath = linkpath + datadirpathlen + 1;

      ti = palloc(sizeof(tablespaceinfo));
      ti->oid = pstrdup(de->d_name);
      ti->path = pstrdup(buflinkpath.data);
      ti->rpath = relpath ? pstrdup(relpath) : NULL;
      ti->size = infotbssize ? sendTablespace(fullpath, true) : -1;

      if (tablespaces)
        *tablespaces = lappend(*tablespaces, ti);

      appendStringInfo(tblspcmapfile, "%s %s\n", ti->oid, ti->path);

      pfree(buflinkpath.data);
#else

      /*
       * If the platform does not have symbolic links, it should not be
       * possible to have tablespaces - clearly somebody else created
       * them. Warn about it and ignore.
       */
      ereport(WARNING,
          (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
           errmsg("tablespaces are not supported on this platform")));
#endif
    }
    FreeDir(tblspcdir);

    /*
     * Construct backup label file
     */
    if (exclusive)
      labelfile = makeStringInfo();

    /* Use the log timezone here, not the session timezone */
    stamp_time = (pg_time_t) time(NULL);
    pg_strftime(strfbuf, sizeof(strfbuf),
          "%Y-%m-%d %H:%M:%S %Z",
          pg_localtime(&stamp_time, log_timezone));
    appendStringInfo(labelfile, "START WAL LOCATION: %X/%X (file %s)\n",
             (uint32) (startpoint >> 32), (uint32) startpoint, xlogfilename);
    appendStringInfo(labelfile, "CHECKPOINT LOCATION: %X/%X\n",
             (uint32) (checkpointloc >> 32), (uint32) checkpointloc);
    appendStringInfo(labelfile, "BACKUP METHOD: %s\n",
             exclusive ? "pg_start_backup" : "streamed");
    appendStringInfo(labelfile, "BACKUP FROM: %s\n",
             backup_started_in_recovery ? "standby" : "master");
    appendStringInfo(labelfile, "START TIME: %s\n", strfbuf);
    appendStringInfo(labelfile, "LABEL: %s\n", backupidstr);
    appendStringInfo(labelfile, "START TIMELINE: %u\n", starttli);

    /*
     * Okay, write the file, or return its contents to caller.
     */
    if (exclusive)
    {
      /*
       * Check for existing backup label --- implies a backup is already
       * running.  (XXX given that we checked exclusiveBackupState
       * above, maybe it would be OK to just unlink any such label
       * file?)
       */
      if (stat(BACKUP_LABEL_FILE, &stat_buf) != 0)
      {
        if (errno != ENOENT)
          ereport(ERROR,
              (errcode_for_file_access(),
               errmsg("could not stat file \"%s\": %m",
                  BACKUP_LABEL_FILE)));
      }
      else
        ereport(ERROR,
            (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
             errmsg("a backup is already in progress"),
             errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
                 BACKUP_LABEL_FILE)));

      fp = AllocateFile(BACKUP_LABEL_FILE, "w");

      if (!fp)
        ereport(ERROR,
            (errcode_for_file_access(),
             errmsg("could not create file \"%s\": %m",
                BACKUP_LABEL_FILE)));
      if (fwrite(labelfile->data, labelfile->len, 1, fp) != 1 ||
        fflush(fp) != 0 ||
        pg_fsync(fileno(fp)) != 0 ||
        ferror(fp) ||
        FreeFile(fp))
        ereport(ERROR,
            (errcode_for_file_access(),
             errmsg("could not write file \"%s\": %m",
                BACKUP_LABEL_FILE)));
      /* Allocated locally for exclusive backups, so free separately */
      pfree(labelfile->data);
      pfree(labelfile);

      /* Write backup tablespace_map file. */
      if (tblspcmapfile->len > 0)
      {
        if (stat(TABLESPACE_MAP, &stat_buf) != 0)
        {
          if (errno != ENOENT)
            ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not stat file \"%s\": %m",
                    TABLESPACE_MAP)));
        }
        else
          ereport(ERROR,
              (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
               errmsg("a backup is already in progress"),
               errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
                   TABLESPACE_MAP)));

        fp = AllocateFile(TABLESPACE_MAP, "w");

        if (!fp)
          ereport(ERROR,
              (errcode_for_file_access(),
               errmsg("could not create file \"%s\": %m",
                  TABLESPACE_MAP)));
        if (fwrite(tblspcmapfile->data, tblspcmapfile->len, 1, fp) != 1 ||
          fflush(fp) != 0 ||
          pg_fsync(fileno(fp)) != 0 ||
          ferror(fp) ||
          FreeFile(fp))
          ereport(ERROR,
              (errcode_for_file_access(),
               errmsg("could not write file \"%s\": %m",
                  TABLESPACE_MAP)));
      }

      /* Allocated locally for exclusive backups, so free separately */
      pfree(tblspcmapfile->data);
      pfree(tblspcmapfile);
    }
  }
  PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));

  /*
   * Mark that start phase has correctly finished for an exclusive backup.
   * Session-level locks are updated as well to reflect that state.
   *
   * Note that CHECK_FOR_INTERRUPTS() must not occur while updating backup
   * counters and session-level lock. Otherwise they can be updated
   * inconsistently, and which might cause do_pg_abort_backup() to fail.
   */
  if (exclusive)
  {
    WALInsertLockAcquireExclusive();
    XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;

    /* Set session-level lock */
    sessionBackupState = SESSION_BACKUP_EXCLUSIVE;
    WALInsertLockRelease();
  }
  else
    sessionBackupState = SESSION_BACKUP_NON_EXCLUSIVE;

  /*
   * We're done.  As a convenience, return the starting WAL location.
   */
  if (starttli_p)
    *starttli_p = starttli;
  return startpoint;
}

/* Error cleanup callback for pg_start_backup */
static void
pg_start_backup_callback(int code, Datum arg)
{
  bool    exclusive = DatumGetBool(arg);

  /* Update backup counters and forcePageWrites on failure */
  WALInsertLockAcquireExclusive();
  if (exclusive)
  {
    Assert(XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_STARTING);
    XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_NONE;
  }
  else
  {
    Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
    XLogCtl->Insert.nonExclusiveBackups--;
  }

  if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
    XLogCtl->Insert.nonExclusiveBackups == 0)
  {
    XLogCtl->Insert.forcePageWrites = false;
  }
  WALInsertLockRelease();
}

/*
 * Error cleanup callback for pg_stop_backup
 */
static void
pg_stop_backup_callback(int code, Datum arg)
{
  bool    exclusive = DatumGetBool(arg);

  /* Update backup status on failure */
  WALInsertLockAcquireExclusive();
  if (exclusive)
  {
    Assert(XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_STOPPING);
    XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;
  }
  WALInsertLockRelease();
}

/*
 * Utility routine to fetch the session-level status of a backup running.
 */
SessionBackupState
get_backup_status(void)
{
  return sessionBackupState;
}

/*
 * do_pg_stop_backup is the workhorse of the user-visible pg_stop_backup()
 * function.
 *
 * If labelfile is NULL, this stops an exclusive backup. Otherwise this stops
 * the non-exclusive backup specified by 'labelfile'.
 *
 * Returns the last WAL location that must be present to restore from this
 * backup, and the corresponding timeline ID in *stoptli_p.
 *
 * It is the responsibility of the caller of this function to verify the
 * permissions of the calling user!
 */
XLogRecPtr
do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
{
  bool    exclusive = (labelfile == NULL);
  bool    backup_started_in_recovery = false;
  XLogRecPtr  startpoint;
  XLogRecPtr  stoppoint;
  TimeLineID  stoptli;
  pg_time_t stamp_time;
  char    strfbuf[128];
  char    histfilepath[MAXPGPATH];
  char    startxlogfilename[MAXFNAMELEN];
  char    stopxlogfilename[MAXFNAMELEN];
  char    lastxlogfilename[MAXFNAMELEN];
  char    histfilename[MAXFNAMELEN];
  char    backupfrom[20];
  XLogSegNo _logSegNo;
  FILE     *lfp;
  FILE     *fp;
  char    ch;
  int     seconds_before_warning;
  int     waits = 0;
  bool    reported_waiting = false;
  char     *remaining;
  char     *ptr;
  uint32    hi,
        lo;

  backup_started_in_recovery = RecoveryInProgress();

  /*
   * Currently only non-exclusive backup can be taken during recovery.
   */
  if (backup_started_in_recovery && exclusive)
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("recovery is in progress"),
         errhint("WAL control functions cannot be executed during recovery.")));

  /*
   * During recovery, we don't need to check WAL level. Because, if WAL
   * level is not sufficient, it's impossible to get here during recovery.
   */
  if (!backup_started_in_recovery && !XLogIsNeeded())
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("WAL level not sufficient for making an online backup"),
         errhint("wal_level must be set to \"replica\" or \"logical\" at server start.")));

  if (exclusive)
  {
    /*
     * At first, mark that we're now stopping an exclusive backup, to
     * ensure that there are no other sessions currently running
     * pg_start_backup() or pg_stop_backup().
     */
    WALInsertLockAcquireExclusive();
    if (XLogCtl->Insert.exclusiveBackupState != EXCLUSIVE_BACKUP_IN_PROGRESS)
    {
      WALInsertLockRelease();
      ereport(ERROR,
          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg("exclusive backup not in progress")));
    }
    XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_STOPPING;
    WALInsertLockRelease();

    /*
     * Remove backup_label. In case of failure, the state for an exclusive
     * backup is switched back to in-progress.
     */
    PG_ENSURE_ERROR_CLEANUP(pg_stop_backup_callback, (Datum) BoolGetDatum(exclusive));
    {
      /*
       * Read the existing label file into memory.
       */
      struct stat statbuf;
      int     r;

      if (stat(BACKUP_LABEL_FILE, &statbuf))
      {
        /* should not happen per the upper checks */
        if (errno != ENOENT)
          ereport(ERROR,
              (errcode_for_file_access(),
               errmsg("could not stat file \"%s\": %m",
                  BACKUP_LABEL_FILE)));
        ereport(ERROR,
            (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
             errmsg("a backup is not in progress")));
      }

      lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
      if (!lfp)
      {
        ereport(ERROR,
            (errcode_for_file_access(),
             errmsg("could not read file \"%s\": %m",
                BACKUP_LABEL_FILE)));
      }
      labelfile = palloc(statbuf.st_size + 1);
      r = fread(labelfile, statbuf.st_size, 1, lfp);
      labelfile[statbuf.st_size] = '\0';

      /*
       * Close and remove the backup label file
       */
      if (r != 1 || ferror(lfp) || FreeFile(lfp))
        ereport(ERROR,
            (errcode_for_file_access(),
             errmsg("could not read file \"%s\": %m",
                BACKUP_LABEL_FILE)));
      durable_unlink(BACKUP_LABEL_FILE, ERROR);

      /*
       * Remove tablespace_map file if present, it is created only if
       * there are tablespaces.
       */
      durable_unlink(TABLESPACE_MAP, DEBUG1);
    }
    PG_END_ENSURE_ERROR_CLEANUP(pg_stop_backup_callback, (Datum) BoolGetDatum(exclusive));
  }

  /*
   * OK to update backup counters, forcePageWrites and session-level lock.
   *
   * Note that CHECK_FOR_INTERRUPTS() must not occur while updating them.
   * Otherwise they can be updated inconsistently, and which might cause
   * do_pg_abort_backup() to fail.
   */
  WALInsertLockAcquireExclusive();
  if (exclusive)
  {
    XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_NONE;
  }
  else
  {
    /*
     * The user-visible pg_start/stop_backup() functions that operate on
     * exclusive backups can be called at any time, but for non-exclusive
     * backups, it is expected that each do_pg_start_backup() call is
     * matched by exactly one do_pg_stop_backup() call.
     */
    Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
    XLogCtl->Insert.nonExclusiveBackups--;
  }

  if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
    XLogCtl->Insert.nonExclusiveBackups == 0)
  {
    XLogCtl->Insert.forcePageWrites = false;
  }

  /*
   * Clean up session-level lock.
   *
   * You might think that WALInsertLockRelease() can be called before
   * cleaning up session-level lock because session-level lock doesn't need
   * to be protected with WAL insertion lock. But since
   * CHECK_FOR_INTERRUPTS() can occur in it, session-level lock must be
   * cleaned up before it.
   */
  sessionBackupState = SESSION_BACKUP_NONE;

  WALInsertLockRelease();

  /*
   * Read and parse the START WAL LOCATION line (this code is pretty crude,
   * but we are not expecting any variability in the file format).
   */
  if (sscanf(labelfile, "START WAL LOCATION: %X/%X (file %24s)%c",
         &hi, &lo, startxlogfilename,
         &ch) != 4 || ch != '\n')
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
  startpoint = ((uint64) hi) << 32 | lo;
  remaining = strchr(labelfile, '\n') + 1;  /* %n is not portable enough */

  /*
   * Parse the BACKUP FROM line. If we are taking an online backup from the
   * standby, we confirm that the standby has not been promoted during the
   * backup.
   */
  ptr = strstr(remaining, "BACKUP FROM:");
  if (!ptr || sscanf(ptr, "BACKUP FROM: %19s\n", backupfrom) != 1)
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
  if (strcmp(backupfrom, "standby") == 0 && !backup_started_in_recovery)
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("the standby was promoted during online backup"),
         errhint("This means that the backup being taken is corrupt "
             "and should not be used. "
             "Try taking another online backup.")));

  /*
   * During recovery, we don't write an end-of-backup record. We assume that
   * pg_control was backed up last and its minimum recovery point can be
   * available as the backup end location. Since we don't have an
   * end-of-backup record, we use the pg_control value to check whether
   * we've reached the end of backup when starting recovery from this
   * backup. We have no way of checking if pg_control wasn't backed up last
   * however.
   *
   * We don't force a switch to new WAL file but it is still possible to
   * wait for all the required files to be archived if waitforarchive is
   * true. This is okay if we use the backup to start a standby and fetch
   * the missing WAL using streaming replication. But in the case of an
   * archive recovery, a user should set waitforarchive to true and wait for
   * them to be archived to ensure that all the required files are
   * available.
   *
   * We return the current minimum recovery point as the backup end
   * location. Note that it can be greater than the exact backup end
   * location if the minimum recovery point is updated after the backup of
   * pg_control. This is harmless for current uses.
   *
   * XXX currently a backup history file is for informational and debug
   * purposes only. It's not essential for an online backup. Furthermore,
   * even if it's created, it will not be archived during recovery because
   * an archiver is not invoked. So it doesn't seem worthwhile to write a
   * backup history file during recovery.
   */
  if (backup_started_in_recovery)
  {
    XLogRecPtr  recptr;

    /*
     * Check to see if all WAL replayed during online backup contain
     * full-page writes.
     */
    SpinLockAcquire(&XLogCtl->info_lck);
    recptr = XLogCtl->lastFpwDisableRecPtr;
    SpinLockRelease(&XLogCtl->info_lck);

    if (startpoint <= recptr)
      ereport(ERROR,
          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg("WAL generated with full_page_writes=off was replayed "
              "during online backup"),
           errhint("This means that the backup being taken on the standby "
               "is corrupt and should not be used. "
               "Enable full_page_writes and run CHECKPOINT on the master, "
               "and then try an online backup again.")));


    LWLockAcquire(ControlFileLock, LW_SHARED);
    stoppoint = ControlFile->minRecoveryPoint;
    stoptli = ControlFile->minRecoveryPointTLI;
    LWLockRelease(ControlFileLock);
  }
  else
  {
    /*
     * Write the backup-end xlog record
     */
    XLogBeginInsert();
    XLogRegisterData((char *) (&startpoint), sizeof(startpoint));
    stoppoint = XLogInsert(RM_XLOG_ID, XLOG_BACKUP_END);
    stoptli = ThisTimeLineID;

    /*
     * Force a switch to a new xlog segment file, so that the backup is
     * valid as soon as archiver moves out the current segment file.
     */
    RequestXLogSwitch(false);

    XLByteToPrevSeg(stoppoint, _logSegNo, wal_segment_size);
    XLogFileName(stopxlogfilename, stoptli, _logSegNo, wal_segment_size);

    /* Use the log timezone here, not the session timezone */
    stamp_time = (pg_time_t) time(NULL);
    pg_strftime(strfbuf, sizeof(strfbuf),
          "%Y-%m-%d %H:%M:%S %Z",
          pg_localtime(&stamp_time, log_timezone));

    /*
     * Write the backup history file
     */
    XLByteToSeg(startpoint, _logSegNo, wal_segment_size);
    BackupHistoryFilePath(histfilepath, stoptli, _logSegNo,
                startpoint, wal_segment_size);
    fp = AllocateFile(histfilepath, "w");
    if (!fp)
      ereport(ERROR,
          (errcode_for_file_access(),
           errmsg("could not create file \"%s\": %m",
              histfilepath)));
    fprintf(fp, "START WAL LOCATION: %X/%X (file %s)\n",
        (uint32) (startpoint >> 32), (uint32) startpoint, startxlogfilename);
    fprintf(fp, "STOP WAL LOCATION: %X/%X (file %s)\n",
        (uint32) (stoppoint >> 32), (uint32) stoppoint, stopxlogfilename);

    /*
     * Transfer remaining lines including label and start timeline to
     * history file.
     */
    fprintf(fp, "%s", remaining);
    fprintf(fp, "STOP TIME: %s\n", strfbuf);
    fprintf(fp, "STOP TIMELINE: %u\n", stoptli);
    if (fflush(fp) || ferror(fp) || FreeFile(fp))
      ereport(ERROR,
          (errcode_for_file_access(),
           errmsg("could not write file \"%s\": %m",
              histfilepath)));

    /*
     * Clean out any no-longer-needed history files.  As a side effect,
     * this will post a .ready file for the newly created history file,
     * notifying the archiver that history file may be archived
     * immediately.
     */
    CleanupBackupHistory();
  }

  /*
   * If archiving is enabled, wait for all the required WAL files to be
   * archived before returning. If archiving isn't enabled, the required WAL
   * needs to be transported via streaming replication (hopefully with
   * wal_keep_segments set high enough), or some more exotic mechanism like
   * polling and copying files from pg_wal with script. We have no knowledge
   * of those mechanisms, so it's up to the user to ensure that he gets all
   * the required WAL.
   *
   * We wait until both the last WAL file filled during backup and the
   * history file have been archived, and assume that the alphabetic sorting
   * property of the WAL files ensures any earlier WAL files are safely
   * archived as well.
   *
   * We wait forever, since archive_command is supposed to work and we
   * assume the admin wanted his backup to work completely. If you don't
   * wish to wait, then either waitforarchive should be passed in as false,
   * or you can set statement_timeout.  Also, some notices are issued to
   * clue in anyone who might be doing this interactively.
   */

  if (waitforarchive &&
    ((!backup_started_in_recovery && XLogArchivingActive()) ||
     (backup_started_in_recovery && XLogArchivingAlways())))
  {
    XLByteToPrevSeg(stoppoint, _logSegNo, wal_segment_size);
    XLogFileName(lastxlogfilename, stoptli, _logSegNo, wal_segment_size);

    XLByteToSeg(startpoint, _logSegNo, wal_segment_size);
    BackupHistoryFileName(histfilename, stoptli, _logSegNo,
                startpoint, wal_segment_size);

    seconds_before_warning = 60;
    waits = 0;

    while (XLogArchiveIsBusy(lastxlogfilename) ||
         XLogArchiveIsBusy(histfilename))
    {
      CHECK_FOR_INTERRUPTS();

      if (!reported_waiting && waits > 5)
      {
        ereport(NOTICE,
            (errmsg("pg_stop_backup cleanup done, waiting for required WAL segments to be archived")));
        reported_waiting = true;
      }

      pg_usleep(1000000L);

      if (++waits >= seconds_before_warning)
      {
        seconds_before_warning *= 2;  /* This wraps in >10 years... */
        ereport(WARNING,
            (errmsg("pg_stop_backup still waiting for all required WAL segments to be archived (%d seconds elapsed)",
                waits),
             errhint("Check that your archive_command is executing properly.  "
                 "pg_stop_backup can be canceled safely, "
                 "but the database backup will not be usable without all the WAL segments.")));
      }
    }

    ereport(NOTICE,
        (errmsg("pg_stop_backup complete, all required WAL segments have been archived")));
  }
  else if (waitforarchive)
    ereport(NOTICE,
        (errmsg("WAL archiving is not enabled; you must ensure that all required WAL segments are copied through other means to complete the backup")));

  /*
   * We're done.  As a convenience, return the ending WAL location.
   */
  if (stoptli_p)
    *stoptli_p = stoptli;
  return stoppoint;
}


/*
 * do_pg_abort_backup: abort a running backup
 *
 * This does just the most basic steps of do_pg_stop_backup(), by taking the
 * system out of backup mode, thus making it a lot more safe to call from
 * an error handler.
 *
 * NB: This is only for aborting a non-exclusive backup that doesn't write
 * backup_label. A backup started with pg_start_backup() needs to be finished
 * with pg_stop_backup().
 */
void
do_pg_abort_backup(void)
{
  /*
   * Quick exit if session is not keeping around a non-exclusive backup
   * already started.
   */
  if (sessionBackupState == SESSION_BACKUP_NONE)
    return;

  WALInsertLockAcquireExclusive();
  Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
  Assert(sessionBackupState == SESSION_BACKUP_NON_EXCLUSIVE);
  XLogCtl->Insert.nonExclusiveBackups--;

  if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
    XLogCtl->Insert.nonExclusiveBackups == 0)
  {
    XLogCtl->Insert.forcePageWrites = false;
  }
  WALInsertLockRelease();
}

/*
 * Get latest redo apply position.
 *
 * Exported to allow WALReceiver to read the pointer directly.
 */
XLogRecPtr
GetXLogReplayRecPtr(TimeLineID *replayTLI)
{
  XLogRecPtr  recptr;
  TimeLineID  tli;

  SpinLockAcquire(&XLogCtl->info_lck);
  recptr = XLogCtl->lastReplayedEndRecPtr;
  tli = XLogCtl->lastReplayedTLI;
  SpinLockRelease(&XLogCtl->info_lck);

  if (replayTLI)
    *replayTLI = tli;
  return recptr;
}

/*
 * Get latest WAL insert pointer
 */
XLogRecPtr
GetXLogInsertRecPtr(void)
{
  XLogCtlInsert *Insert = &XLogCtl->Insert;
  uint64    current_bytepos;

  SpinLockAcquire(&Insert->insertpos_lck);
  current_bytepos = Insert->CurrBytePos;
  SpinLockRelease(&Insert->insertpos_lck);

  return XLogBytePosToRecPtr(current_bytepos);
}

/*
 * Get latest WAL write pointer
 */
XLogRecPtr
GetXLogWriteRecPtr(void)
{
  SpinLockAcquire(&XLogCtl->info_lck);
  LogwrtResult = XLogCtl->LogwrtResult;
  SpinLockRelease(&XLogCtl->info_lck);

  return LogwrtResult.Write;
}

/*
 * Returns the redo pointer of the last checkpoint or restartpoint. This is
 * the oldest point in WAL that we still need, if we have to restart recovery.
 */
void
GetOldestRestartPoint(XLogRecPtr *oldrecptr, TimeLineID *oldtli)
{
  LWLockAcquire(ControlFileLock, LW_SHARED);
  *oldrecptr = ControlFile->checkPointCopy.redo;
  *oldtli = ControlFile->checkPointCopy.ThisTimeLineID;
  LWLockRelease(ControlFileLock);
}

/*
 * read_backup_label: check to see if a backup_label file is present
 *
 * If we see a backup_label during recovery, we assume that we are recovering
 * from a backup dump file, and we therefore roll forward from the checkpoint
 * identified by the label file, NOT what pg_control says.  This avoids the
 * problem that pg_control might have been archived one or more checkpoints
 * later than the start of the dump, and so if we rely on it as the start
 * point, we will fail to restore a consistent database state.
 *
 * Returns true if a backup_label was found (and fills the checkpoint
 * location and its REDO location into *checkPointLoc and RedoStartLSN,
 * respectively); returns false if not. If this backup_label came from a
 * streamed backup, *backupEndRequired is set to true. If this backup_label
 * was created during recovery, *backupFromStandby is set to true.
 */
static bool
read_backup_label(XLogRecPtr *checkPointLoc, bool *backupEndRequired,
          bool *backupFromStandby)
{
  char    startxlogfilename[MAXFNAMELEN];
  TimeLineID  tli_from_walseg,
        tli_from_file;
  FILE     *lfp;
  char    ch;
  char    backuptype[20];
  char    backupfrom[20];
  char    backuplabel[MAXPGPATH];
  char    backuptime[128];
  uint32    hi,
        lo;

  *backupEndRequired = false;
  *backupFromStandby = false;

  /*
   * See if label file is present
   */
  lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
  if (!lfp)
  {
    if (errno != ENOENT)
      ereport(FATAL,
          (errcode_for_file_access(),
           errmsg("could not read file \"%s\": %m",
              BACKUP_LABEL_FILE)));
    return false;     /* it's not there, all is fine */
  }

  /*
   * Read and parse the START WAL LOCATION and CHECKPOINT lines (this code
   * is pretty crude, but we are not expecting any variability in the file
   * format).
   */
  if (fscanf(lfp, "START WAL LOCATION: %X/%X (file %08X%16s)%c",
         &hi, &lo, &tli_from_walseg, startxlogfilename, &ch) != 5 || ch != '\n')
    ereport(FATAL,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
  RedoStartLSN = ((uint64) hi) << 32 | lo;
  if (fscanf(lfp, "CHECKPOINT LOCATION: %X/%X%c",
         &hi, &lo, &ch) != 3 || ch != '\n')
    ereport(FATAL,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
  *checkPointLoc = ((uint64) hi) << 32 | lo;

  /*
   * BACKUP METHOD and BACKUP FROM lines are new in 9.2. We can't restore
   * from an older backup anyway, but since the information on it is not
   * strictly required, don't error out if it's missing for some reason.
   */
  if (fscanf(lfp, "BACKUP METHOD: %19s\n", backuptype) == 1)
  {
    if (strcmp(backuptype, "streamed") == 0)
      *backupEndRequired = true;
  }

  if (fscanf(lfp, "BACKUP FROM: %19s\n", backupfrom) == 1)
  {
    if (strcmp(backupfrom, "standby") == 0)
      *backupFromStandby = true;
  }

  /*
   * Parse START TIME and LABEL. Those are not mandatory fields for recovery
   * but checking for their presence is useful for debugging and the next
   * sanity checks. Cope also with the fact that the result buffers have a
   * pre-allocated size, hence if the backup_label file has been generated
   * with strings longer than the maximum assumed here an incorrect parsing
   * happens. That's fine as only minor consistency checks are done
   * afterwards.
   */
  if (fscanf(lfp, "START TIME: %127[^\n]\n", backuptime) == 1)
    ereport(DEBUG1,
        (errmsg("backup time %s in file \"%s\"",
            backuptime, BACKUP_LABEL_FILE)));

  if (fscanf(lfp, "LABEL: %1023[^\n]\n", backuplabel) == 1)
    ereport(DEBUG1,
        (errmsg("backup label %s in file \"%s\"",
            backuplabel, BACKUP_LABEL_FILE)));

  /*
   * START TIMELINE is new as of 11. Its parsing is not mandatory, still use
   * it as a sanity check if present.
   */
  if (fscanf(lfp, "START TIMELINE: %u\n", &tli_from_file) == 1)
  {
    if (tli_from_walseg != tli_from_file)
      ereport(FATAL,
          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE),
           errdetail("Timeline ID parsed is %u, but expected %u",
                 tli_from_file, tli_from_walseg)));

    ereport(DEBUG1,
        (errmsg("backup timeline %u in file \"%s\"",
            tli_from_file, BACKUP_LABEL_FILE)));
  }

  if (ferror(lfp) || FreeFile(lfp))
    ereport(FATAL,
        (errcode_for_file_access(),
         errmsg("could not read file \"%s\": %m",
            BACKUP_LABEL_FILE)));

  return true;
}

/*
 * read_tablespace_map: check to see if a tablespace_map file is present
 *
 * If we see a tablespace_map file during recovery, we assume that we are
 * recovering from a backup dump file, and we therefore need to create symlinks
 * as per the information present in tablespace_map file.
 *
 * Returns true if a tablespace_map file was found (and fills the link
 * information for all the tablespace links present in file); returns false
 * if not.
 */
static bool
read_tablespace_map(List **tablespaces)
{
  tablespaceinfo *ti;
  FILE     *lfp;
  char    tbsoid[MAXPGPATH];
  char     *tbslinkpath;
  char    str[MAXPGPATH];
  int     ch,
        prev_ch = -1,
        i = 0,
        n;

  /*
   * See if tablespace_map file is present
   */
  lfp = AllocateFile(TABLESPACE_MAP, "r");
  if (!lfp)
  {
    if (errno != ENOENT)
      ereport(FATAL,
          (errcode_for_file_access(),
           errmsg("could not read file \"%s\": %m",
              TABLESPACE_MAP)));
    return false;     /* it's not there, all is fine */
  }

  /*
   * Read and parse the link name and path lines from tablespace_map file
   * (this code is pretty crude, but we are not expecting any variability in
   * the file format).  While taking backup we embed escape character '\\'
   * before newline in tablespace path, so that during reading of
   * tablespace_map file, we could distinguish newline in tablespace path
   * and end of line.  Now while reading tablespace_map file, remove the
   * escape character that has been added in tablespace path during backup.
   */
  while ((ch = fgetc(lfp)) != EOF)
  {
    if ((ch == '\n' || ch == '\r') && prev_ch != '\\')
    {
      str[i] = '\0';
      if (sscanf(str, "%s %n", tbsoid, &n) != 1)
        ereport(FATAL,
            (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
             errmsg("invalid data in file \"%s\"", TABLESPACE_MAP)));
      tbslinkpath = str + n;
      i = 0;

      ti = palloc(sizeof(tablespaceinfo));
      ti->oid = pstrdup(tbsoid);
      ti->path = pstrdup(tbslinkpath);

      *tablespaces = lappend(*tablespaces, ti);
      continue;
    }
    else if ((ch == '\n' || ch == '\r') && prev_ch == '\\')
      str[i - 1] = ch;
    else
      str[i++] = ch;
    prev_ch = ch;
  }

  if (ferror(lfp) || FreeFile(lfp))
    ereport(FATAL,
        (errcode_for_file_access(),
         errmsg("could not read file \"%s\": %m",
            TABLESPACE_MAP)));

  return true;
}

/*
 * Error context callback for errors occurring during rm_redo().
 */
static void
rm_redo_error_callback(void *arg)
{
  XLogReaderState *record = (XLogReaderState *) arg;
  StringInfoData buf;

  initStringInfo(&buf);
  xlog_outdesc(&buf, record);

  /* translator: %s is a WAL record description */
  errcontext("WAL redo at %X/%X for %s",
         (uint32) (record->ReadRecPtr >> 32),
         (uint32) record->ReadRecPtr,
         buf.data);

  pfree(buf.data);
}

/*
 * BackupInProgress: check if online backup mode is active
 *
 * This is done by checking for existence of the "backup_label" file.
 */
bool
BackupInProgress(void)
{
  struct stat stat_buf;

  return (stat(BACKUP_LABEL_FILE, &stat_buf) == 0);
}

/*
 * CancelBackup: rename the "backup_label" and "tablespace_map"
 *         files to cancel backup mode
 *
 * If the "backup_label" file exists, it will be renamed to "backup_label.old".
 * Similarly, if the "tablespace_map" file exists, it will be renamed to
 * "tablespace_map.old".
 *
 * Note that this will render an online backup in progress
 * useless. To correctly finish an online backup, pg_stop_backup must be
 * called.
 */
void
CancelBackup(void)
{
  struct stat stat_buf;

  /* if the backup_label file is not there, return */
  if (stat(BACKUP_LABEL_FILE, &stat_buf) < 0)
    return;

  /* remove leftover file from previously canceled backup if it exists */
  unlink(BACKUP_LABEL_OLD);

  if (durable_rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD, DEBUG1) != 0)
  {
    ereport(WARNING,
        (errcode_for_file_access(),
         errmsg("online backup mode was not canceled"),
         errdetail("File \"%s\" could not be renamed to \"%s\": %m.",
               BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
    return;
  }

  /* if the tablespace_map file is not there, return */
  if (stat(TABLESPACE_MAP, &stat_buf) < 0)
  {
    ereport(LOG,
        (errmsg("online backup mode canceled"),
         errdetail("File \"%s\" was renamed to \"%s\".",
               BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
    return;
  }

  /* remove leftover file from previously canceled backup if it exists */
  unlink(TABLESPACE_MAP_OLD);

  if (durable_rename(TABLESPACE_MAP, TABLESPACE_MAP_OLD, DEBUG1) == 0)
  {
    ereport(LOG,
        (errmsg("online backup mode canceled"),
         errdetail("Files \"%s\" and \"%s\" were renamed to "
               "\"%s\" and \"%s\", respectively.",
               BACKUP_LABEL_FILE, TABLESPACE_MAP,
               BACKUP_LABEL_OLD, TABLESPACE_MAP_OLD)));
  }
  else
  {
    ereport(WARNING,
        (errcode_for_file_access(),
         errmsg("online backup mode canceled"),
         errdetail("File \"%s\" was renamed to \"%s\", but "
               "file \"%s\" could not be renamed to \"%s\": %m.",
               BACKUP_LABEL_FILE, BACKUP_LABEL_OLD,
               TABLESPACE_MAP, TABLESPACE_MAP_OLD)));
  }
}

/*
 * Read the XLOG page containing RecPtr into readBuf (if not read already).
 * Returns number of bytes read, if the page is read successfully, or -1
 * in case of errors.  When errors occur, they are ereport'ed, but only
 * if they have not been previously reported.
 *
 * This is responsible for restoring files from archive as needed, as well
 * as for waiting for the requested WAL record to arrive in standby mode.
 *
 * 'emode' specifies the log level used for reporting "file not found" or
 * "end of WAL" situations in archive recovery, or in standby mode when a
 * trigger file is found. If set to WARNING or below, XLogPageRead() returns
 * false in those situations, on higher log levels the ereport() won't
 * return.
 *
 * In standby mode, if after a successful return of XLogPageRead() the
 * caller finds the record it's interested in to be broken, it should
 * ereport the error with the level determined by
 * emode_for_corrupt_record(), and then set lastSourceFailed
 * and call XLogPageRead() again with the same arguments. This lets
 * XLogPageRead() to try fetching the record from another source, or to
 * sleep and retry.
 */
static int
XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
       XLogRecPtr targetRecPtr, char *readBuf, TimeLineID *readTLI)
{
  XLogPageReadPrivate *private =
  (XLogPageReadPrivate *) xlogreader->private_data;
  int     emode = private->emode;
  uint32    targetPageOff;
  XLogSegNo targetSegNo PG_USED_FOR_ASSERTS_ONLY;

  XLByteToSeg(targetPagePtr, targetSegNo, wal_segment_size);
  targetPageOff = XLogSegmentOffset(targetPagePtr, wal_segment_size);

  /*
   * See if we need to switch to a new segment because the requested record
   * is not in the currently open one.
   */
  if (readFile >= 0 &&
    !0XLByteInSeg0(targetPagePtr, readSegNo, wal_segment_size))
  {
    /*
     * Request a restartpoint if we've replayed too much xlog since the
     * last one.
     */
    if (bgwriterLaunched)
    {
      if (XLogCheckpointNeeded(readSegNo))
      {
        (void) GetRedoRecPtr();
        if (XLogCheckpointNeeded(readSegNo))
          RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
      }
    }

    close(readFile);
    readFile = -1;
    readSource = 0;
  }

  XLByteToSeg(targetPagePtr, readSegNo, wal_segment_size);

retry:
  /* See if we need to retrieve more data */
  if (readFile < 0 ||
    (0readSource == XLOG_FROM_STREAM0 &&
     receivedUpto < targetPagePtr + reqLen))
  {
    if (!WaitForWALToBecomeAvailable(targetPagePtr + reqLen,
                     private->randAccess,
                     private->fetching_ckpt,
                     targetRecPtr))
    {
      if (readFile >= 0)
        close(readFile);
      readFile = -1;
      readLen = 0;
      readSource = 0;

      return -1;
    }
  }

  /*
   * At this point, we have the right segment open and if we're streaming we
   * know the requested record is in it.
   */
  Assert(readFile != -1);

  /*
   * If the current segment is being streamed from master, calculate how
   * much of the current page we have received already. We know the
   * requested record has been received, but this is for the benefit of
   * future calls, to allow quick exit at the top of this function.
   */
  if (readSource == XLOG_FROM_STREAM)
  {
    if (((targetPagePtr) / XLOG_BLCKSZ) != (receivedUpto / XLOG_BLCKSZ))
      readLen = XLOG_BLCKSZ;
    else
      readLen = XLogSegmentOffset(receivedUpto, wal_segment_size) -
        targetPageOff;
  }
  else
    readLen = XLOG_BLCKSZ;

  /* Read the requested page */
  readOff = targetPageOff;
  if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0)
  {
    char    fname[MAXFNAMELEN];
    int     save_errno = errno;

    XLogFileName(fname, curFileTLI, readSegNo, wal_segment_size);
    errno = save_errno;
    ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
        (errcode_for_file_access(),
         errmsg("could not seek in log segment %s to offset %u: %m",
            fname, readOff)));
    goto next_record_is_invalid;
  }

  pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
  if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
  {
    char    fname[MAXFNAMELEN];
    int     save_errno = errno;

    pgstat_report_wait_end();
    XLogFileName(fname, curFileTLI, readSegNo, wal_segment_size);
    errno = save_errno;
    ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
        (errcode_for_file_access(),
         errmsg("could not read from log segment %s, offset %u: %m",
            fname, readOff)));
    goto next_record_is_invalid;
  }
  pgstat_report_wait_end();

  Assert(targetSegNo == readSegNo);
  Assert(targetPageOff == readOff);
  Assert(reqLen <= readLen);

  *readTLI = curFileTLI;

  /*
   * Check the page header immediately, so that we can retry immediately if
   * it's not valid. This may seem unnecessary, because XLogReadRecord()
   * validates the page header anyway, and would propagate the failure up to
   * ReadRecord(), which would retry. However, there's a corner case with
   * continuation records, if a record is split across two pages such that
   * we would need to read the two pages from different sources. For
   * example, imagine a scenario where a streaming replica is started up,
   * and replay reaches a record that's split across two WAL segments. The
   * first page is only available locally, in pg_wal, because it's already
   * been recycled in the master. The second page, however, is not present
   * in pg_wal, and we should stream it from the master. There is a recycled
   * WAL segment present in pg_wal, with garbage contents, however. We would
   * read the first page from the local WAL segment, but when reading the
   * second page, we would read the bogus, recycled, WAL segment. If we
   * didn't catch that case here, we would never recover, because
   * ReadRecord() would retry reading the whole record from the beginning.
   *
   * Of course, this only catches errors in the page header, which is what
   * happens in the case of a recycled WAL segment. Other kinds of errors or
   * corruption still has the same problem. But this at least fixes the
   * common case, which can happen as part of normal operation.
   *
   * Validating the page header is cheap enough that doing it twice
   * shouldn't be a big deal from a performance point of view.
   */
  if (!XLogReaderValidatePageHeader(xlogreader, targetPagePtr, readBuf))
  {
    /* reset any error XLogReaderValidatePageHeader() might have set */
    xlogreader->errormsg_buf[0] = '\0';
    goto next_record_is_invalid;
  }

  return readLen;

next_record_is_invalid:
  lastSourceFailed = true;

  if (readFile >= 0)
    close(readFile);
  readFile = -1;
  readLen = 0;
  readSource = 0;

  /* In standby-mode, keep trying */
  if (StandbyMode)
    goto retry;
  else
    return -1;
}

/*
 * Open the WAL segment containing WAL location 'RecPtr'.
 *
 * The segment can be fetched via restore_command, or via walreceiver having
 * streamed the record, or it can already be present in pg_wal. Checking
 * pg_wal is mainly for crash recovery, but it will be polled in standby mode
 * too, in case someone copies a new segment directly to pg_wal. That is not
 * documented or recommended, though.
 *
 * If 'fetching_ckpt' is true, we're fetching a checkpoint record, and should
 * prepare to read WAL starting from RedoStartLSN after this.
 *
 * 'RecPtr' might not point to the beginning of the record we're interested
 * in, it might also point to the page or segment header. In that case,
 * 'tliRecPtr' is the position of the WAL record we're interested in. It is
 * used to decide which timeline to stream the requested WAL from.
 *
 * If the record is not immediately available, the function returns false
 * if we're not in standby mode. In standby mode, waits for it to become
 * available.
 *
 * When the requested record becomes available, the function opens the file
 * containing it (if not open already), and returns true. When end of standby
 * mode is triggered by the user, and there is no more WAL available, returns
 * false.
 */
static bool
WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
              bool fetching_ckpt, XLogRecPtr tliRecPtr)
{
  static TimestampTz last_fail_time = 0;
  TimestampTz now;
  bool    streaming_reply_sent = false;

  /*-------
   * Standby mode is implemented by a state machine:
   *
   * 1. Read from either archive or pg_wal (XLOG_FROM_ARCHIVE), or just
   *    pg_wal (XLOG_FROM_PG_WAL)
   * 2. Check trigger file
   * 3. Read from primary server via walreceiver (XLOG_FROM_STREAM)
   * 4. Rescan timelines
   * 5. Sleep wal_retrieve_retry_interval milliseconds, and loop back to 1.
   *
   * Failure to read from the current source advances the state machine to
   * the next state.
   *
   * 'currentSource' indicates the current state. There are no currentSource
   * values for "check trigger", "rescan timelines", and "sleep" states,
   * those actions are taken when reading from the previous source fails, as
   * part of advancing to the next state.
   *-------
   */
  if (!InArchiveRecovery)
    currentSource = XLOG_FROM_PG_WAL;
  else if (currentSource == 0)
    currentSource = XLOG_FROM_ARCHIVE;

  for (;;)
  {
    int     oldSource = currentSource;

    /*
     * First check if we failed to read from the current source, and
     * advance the state machine if so. The failure to read might've
     * happened outside this function, e.g when a CRC check fails on a
     * record, or within this loop.
     */
    if (lastSourceFailed)
    {
      switch (currentSource)
      {
        case XLOG_FROM_ARCHIVE:
        case XLOG_FROM_PG_WAL:

          /*
           * Check to see if the trigger file exists. Note that we
           * do this only after failure, so when you create the
           * trigger file, we still finish replaying as much as we
           * can from archive and pg_wal before failover.
           */
          if (StandbyMode && CheckForStandbyTrigger())
          {
            ShutdownWalRcv();
            return false;
          }

          /*
           * Not in standby mode, and we've now tried the archive
           * and pg_wal.
           */
          if (!StandbyMode)
            return false;

          /*
           * If primary_conninfo is set, launch walreceiver to try
           * to stream the missing WAL.
           *
           * If fetching_ckpt is true, RecPtr points to the initial
           * checkpoint location. In that case, we use RedoStartLSN
           * as the streaming start position instead of RecPtr, so
           * that when we later jump backwards to start redo at
           * RedoStartLSN, we will have the logs streamed already.
           */
          if (PrimaryConnInfo)
          {
            XLogRecPtr  ptr;
            TimeLineID  tli;

            if (fetching_ckpt)
            {
              ptr = RedoStartLSN;
              tli = ControlFile->checkPointCopy.ThisTimeLineID;
            }
            else
            {
              ptr = RecPtr;

              /*
               * Use the record begin position to determine the
               * TLI, rather than the position we're reading.
               */
              tli = tliOfPointInHistory(tliRecPtr, expectedTLEs);

              if (curFileTLI > 0 && tli < curFileTLI)
                elog(ERROR, "according to history file, WAL location %X/%X belongs to timeline %u, but previous recovered WAL file came from timeline %u",
                   (uint32) (tliRecPtr >> 32),
                   (uint32) tliRecPtr,
                   tli, curFileTLI);
            }
            curFileTLI = tli;
            RequestXLogStreaming(tli, ptr, PrimaryConnInfo,
                       PrimarySlotName);
            receivedUpto = 0;
          }

          /*
           * Move to XLOG_FROM_STREAM state in either case. We'll
           * get immediate failure if we didn't launch walreceiver,
           * and move on to the next state.
           */
          currentSource = XLOG_FROM_STREAM;
          break;

        case XLOG_FROM_STREAM:

          /*
           * Failure while streaming. Most likely, we got here
           * because streaming replication was terminated, or
           * promotion was triggered. But we also get here if we
           * find an invalid record in the WAL streamed from master,
           * in which case something is seriously wrong. There's
           * little chance that the problem will just go away, but
           * PANIC is not good for availability either, especially
           * in hot standby mode. So, we treat that the same as
           * disconnection, and retry from archive/pg_wal again. The
           * WAL in the archive should be identical to what was
           * streamed, so it's unlikely that it helps, but one can
           * hope...
           */

          /*
           * Before we leave XLOG_FROM_STREAM state, make sure that
           * walreceiver is not active, so that it won't overwrite
           * WAL that we restore from archive.
           */
          if (WalRcvStreaming())
            ShutdownWalRcv();

          /*
           * Before we sleep, re-scan for possible new timelines if
           * we were requested to recover to the latest timeline.
           */
          if (recoveryTargetIsLatest)
          {
            if (rescanLatestTimeLine())
            {
              currentSource = XLOG_FROM_ARCHIVE;
              break;
            }
          }

          /*
           * XLOG_FROM_STREAM is the last state in our state
           * machine, so we've exhausted all the options for
           * obtaining the requested WAL. We're going to loop back
           * and retry from the archive, but if it hasn't been long
           * since last attempt, sleep wal_retrieve_retry_interval
           * milliseconds to avoid busy-waiting.
           */
          now = GetCurrentTimestamp();
          if (!TimestampDifferenceExceeds(last_fail_time, now,
                          wal_retrieve_retry_interval))
          {
            long    secs,
                  wait_time;
            int     usecs;

            TimestampDifference(last_fail_time, now, &secs, &usecs);
            wait_time = wal_retrieve_retry_interval -
              (secs * 1000 + usecs / 1000);

            WaitLatch(&XLogCtl->recoveryWakeupLatch,
                  WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
                  wait_time, WAIT_EVENT_RECOVERY_WAL_STREAM);
            ResetLatch(&XLogCtl->recoveryWakeupLatch);
            now = GetCurrentTimestamp();
          }
          last_fail_time = now;
          currentSource = XLOG_FROM_ARCHIVE;
          break;

        default:
          elog(ERROR, "unexpected WAL source %d", currentSource);
      }
    }
    else if (currentSource == XLOG_FROM_PG_WAL)
    {
      /*
       * We just successfully read a file in pg_wal. We prefer files in
       * the archive over ones in pg_wal, so try the next file again
       * from the archive first.
       */
      if (InArchiveRecovery)
        currentSource = XLOG_FROM_ARCHIVE;
    }

    if (currentSource != oldSource)
      elog(DEBUG2, "switched WAL source from %s to %s after %s",
         xlogSourceNames[oldSource], xlogSourceNames[currentSource],
         lastSourceFailed ? "failure" : "success");

    /*
     * We've now handled possible failure. Try to read from the chosen
     * source.
     */
    lastSourceFailed = false;

    switch (currentSource)
    {
      case XLOG_FROM_ARCHIVE:
      case XLOG_FROM_PG_WAL:
        /* Close any old file we might have open. */
        if (readFile >= 0)
        {
          close(readFile);
          readFile = -1;
        }
        /* Reset curFileTLI if random fetch. */
        if (randAccess)
          curFileTLI = 0;

        /*
         * Try to restore the file from archive, or read an existing
         * file from pg_wal.
         */
        readFile = XLogFileReadAnyTLI(readSegNo, DEBUG2,
                        currentSource == XLOG_FROM_ARCHIVE ? XLOG_FROM_ANY0 :
                        currentSource);
        if (readFile >= 0)
          return true;  /* success! */

        /*
         * Nope, not found in archive or pg_wal.
         */
        lastSourceFailed = true;
        break;

      case XLOG_FROM_STREAM:
        {
          bool    havedata;

          /*
           * Check if WAL receiver is still active.
           */
          if (!WalRcvStreaming())
          {
            lastSourceFailed = true;
            break;
          }

          /*
           * Walreceiver is active, so see if new data has arrived.
           *
           * We only advance XLogReceiptTime when we obtain fresh
           * WAL from walreceiver and observe that we had already
           * processed everything before the most recent "chunk"
           * that it flushed to disk.  In steady state where we are
           * keeping up with the incoming data, XLogReceiptTime will
           * be updated on each cycle. When we are behind,
           * XLogReceiptTime will not advance, so the grace time
           * allotted to conflicting queries will decrease.
           */
          if (RecPtr < receivedUpto)
            havedata = true;
          else
          {
            XLogRecPtr  latestChunkStart;

            receivedUpto = GetWalRcvWriteRecPtr(&latestChunkStart, &receiveTLI);
            if (RecPtr < receivedUpto && receiveTLI == curFileTLI)
            {
              havedata = true;
              if (latestChunkStart <= RecPtr)
              {
                XLogReceiptTime = GetCurrentTimestamp();
                SetCurrentChunkStartTime(XLogReceiptTime);
              }
            }
            else
              havedata = false;
          }
          if (havedata)
          {
            /*
             * Great, streamed far enough.  Open the file if it's
             * not open already.  Also read the timeline history
             * file if we haven't initialized timeline history
             * yet; it should be streamed over and present in
             * pg_wal by now.  Use XLOG_FROM_STREAM so that source
             * info is set correctly and XLogReceiptTime isn't
             * changed.
             */
            if (readFile < 0)
            {
              if (!expectedTLEs)
                expectedTLEs = readTimeLineHistory(receiveTLI);
              readFile = XLogFileRead(readSegNo, PANIC,
                          receiveTLI,
                          XLOG_FROM_STREAM, false);
              Assert(readFile >= 0);
            }
            else
            {
              /* just make sure source info is correct... */
              readSource = XLOG_FROM_STREAM;
              XLogReceiptSource = XLOG_FROM_STREAM;
              return true;
            }
            break;
          }

          /*
           * Data not here yet. Check for trigger, then wait for
           * walreceiver to wake us up when new WAL arrives.
           */
          if (CheckForStandbyTrigger())
          {
            /*
             * Note that we don't "return false" immediately here.
             * After being triggered, we still want to replay all
             * the WAL that was already streamed. It's in pg_wal
             * now, so we just treat this as a failure, and the
             * state machine will move on to replay the streamed
             * WAL from pg_wal, and then recheck the trigger and
             * exit replay.
             */
            lastSourceFailed = true;
            break;
          }

          /*
           * Since we have replayed everything we have received so
           * far and are about to start waiting for more WAL, let's
           * tell the upstream server our replay location now so
           * that pg_stat_replication doesn't show stale
           * information.
           */
          if (!streaming_reply_sent)
          {
            WalRcvForceReply();
            streaming_reply_sent = true;
          }

          /*
           * Wait for more WAL to arrive. Time out after 5 seconds
           * to react to a trigger file promptly.
           */
          WaitLatch(&XLogCtl->recoveryWakeupLatch,
                WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
                5000L, WAIT_EVENT_RECOVERY_WAL_ALL);
          ResetLatch(&XLogCtl->recoveryWakeupLatch);
          break;
        }

      default:
        elog(ERROR, "unexpected WAL source %d", currentSource);
    }

    /*
     * This possibly-long loop needs to handle interrupts of startup
     * process.
     */
    HandleStartupProcInterrupts();
  }

  return false;       /* not reached */
}

/*
 * Determine what log level should be used to report a corrupt WAL record
 * in the current WAL page, previously read by XLogPageRead().
 *
 * 'emode' is the error mode that would be used to report a file-not-found
 * or legitimate end-of-WAL situation.   Generally, we use it as-is, but if
 * we're retrying the exact same record that we've tried previously, only
 * complain the first time to keep the noise down.  However, we only do when
 * reading from pg_wal, because we don't expect any invalid records in archive
 * or in records streamed from master. Files in the archive should be complete,
 * and we should never hit the end of WAL because we stop and wait for more WAL
 * to arrive before replaying it.
 *
 * NOTE: This function remembers the RecPtr value it was last called with,
 * to suppress repeated messages about the same record. Only call this when
 * you are about to ereport(), or you might cause a later message to be
 * erroneously suppressed.
 */
static int
emode_for_corrupt_record(int emode, XLogRecPtr RecPtr)
{
  static XLogRecPtr lastComplaint = 0;

  if (readSource == XLOG_FROM_PG_WAL && emode == LOG)
  {
    if (RecPtr == lastComplaint)
      emode = DEBUG1;
    else
      lastComplaint = RecPtr;
  }
  return emode;
}

/*
 * Check to see whether the user-specified trigger file exists and whether a
 * promote request has arrived.  If either condition holds, return true.
 */
static bool
CheckForStandbyTrigger(void)
{
  struct stat stat_buf;
  static bool triggered = false;

  if (triggered)
    return true;

  if (IsPromoteTriggered())
  {
    /*
     * In 9.1 and 9.2 the postmaster unlinked the promote file inside the
     * signal handler. It now leaves the file in place and lets the
     * Startup process do the unlink. This allows Startup to know whether
     * it should create a full checkpoint before starting up (fallback
     * mode). Fast promotion takes precedence.
     */
    if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
    {
      unlink(PROMOTE_SIGNAL_FILE);
      unlink(FALLBACK_PROMOTE_SIGNAL_FILE);
      fast_promote = true;
    }
    else if (stat(FALLBACK_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
    {
      unlink(FALLBACK_PROMOTE_SIGNAL_FILE);
      fast_promote = false;
    }

    ereport(LOG, (errmsg("received promote request")));

    ResetPromoteTriggered();
    triggered = true;
    return true;
  }

  if (TriggerFile == NULL)
    return false;

  if (stat(TriggerFile, &stat_buf) == 0)
  {
    ereport(LOG,
        (errmsg("trigger file found: %s", TriggerFile)));
    unlink(TriggerFile);
    triggered = true;
    fast_promote = true;
    return true;
  }
  else if (errno != ENOENT)
    ereport(ERROR,
        (errcode_for_file_access(),
         errmsg("could not stat trigger file \"%s\": %m",
            TriggerFile)));

  return false;
}

/*
 * Remove the files signaling a standby promotion request.
 */
void
RemovePromoteSignalFiles(void)
{
  unlink(PROMOTE_SIGNAL_FILE);
  unlink(FALLBACK_PROMOTE_SIGNAL_FILE);
}

/*
 * Check to see if a promote request has arrived. Should be
 * called by postmaster after receiving SIGUSR1.
 */
bool
CheckPromoteSignal(void)
{
  struct stat stat_buf;

  if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0 ||
    stat(FALLBACK_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
    return true;

  return false;
}

/*
 * Wake up startup process to replay newly arrived WAL, or to notice that
 * failover has been requested.
 */
void
WakeupRecovery(void)
{
  SetLatch(&XLogCtl->recoveryWakeupLatch);
}

/*
 * Update the WalWriterSleeping flag.
 */
void
SetWalWriterSleeping(bool sleeping)
{
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->WalWriterSleeping = sleeping;
  SpinLockRelease(&XLogCtl->info_lck);
}

/*
 * Schedule a walreceiver wakeup in the main recovery loop.
 */
void
XLogRequestWalReceiverReply(void)
{
  doRequestWalReceiverReply = true;
}

YugabyteDB (2.13.1.0-b60, 21121d69985fbf76aa6958d8f04a9bfa936293b5)

Coverage Report

Created: 2022-03-22 16:43