/Users/deen/code/yugabyte-db/build/debugcov-clang-dynamic-arm64-ninja/postgres_build/src/backend/port/pg_shmem.c

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/*-------------------------------------------------------------------------
 *
 * sysv_shmem.c
 *    Implement shared memory using SysV facilities
 *
 * These routines used to be a fairly thin layer on top of SysV shared
 * memory functionality.  With the addition of anonymous-shmem logic,
 * they're a bit fatter now.  We still require a SysV shmem block to
 * exist, though, because mmap'd shmem provides no way to find out how
 * many processes are attached, which we need for interlocking purposes.
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/port/sysv_shmem.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/mman.h>
#include <sys/stat.h>
#ifdef HAVE_SYS_IPC_H
#include <sys/ipc.h>
#endif
#ifdef HAVE_SYS_SHM_H
#include <sys/shm.h>
#endif

#include "miscadmin.h"
#include "portability/mem.h"
#include "storage/dsm.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/pg_shmem.h"
#include "utils/guc.h"
#include "utils/pidfile.h"


/*
 * As of PostgreSQL 9.3, we normally allocate only a very small amount of
 * System V shared memory, and only for the purposes of providing an
 * interlock to protect the data directory.  The real shared memory block
 * is allocated using mmap().  This works around the problem that many
 * systems have very low limits on the amount of System V shared memory
 * that can be allocated.  Even a limit of a few megabytes will be enough
 * to run many copies of PostgreSQL without needing to adjust system settings.
 *
 * We assume that no one will attempt to run PostgreSQL 9.3 or later on
 * systems that are ancient enough that anonymous shared memory is not
 * supported, such as pre-2.4 versions of Linux.  If that turns out to be
 * false, we might need to add compile and/or run-time tests here and do this
 * only if the running kernel supports it.
 *
 * However, we must always disable this logic in the EXEC_BACKEND case, and
 * fall back to the old method of allocating the entire segment using System V
 * shared memory, because there's no way to attach an anonymous mmap'd segment
 * to a process after exec().  Since EXEC_BACKEND is intended only for
 * developer use, this shouldn't be a big problem.  Because of this, we do
 * not worry about supporting anonymous shmem in the EXEC_BACKEND cases below.
 *
 * As of PostgreSQL 12, we regained the ability to use a large System V shared
 * memory region even in non-EXEC_BACKEND builds, if shared_memory_type is set
 * to sysv (though this is not the default).
 */


typedef key_t IpcMemoryKey;   /* shared memory key passed to shmget(2) */
typedef int IpcMemoryId;    /* shared memory ID returned by shmget(2) */

/*
 * How does a given IpcMemoryId relate to this PostgreSQL process?
 *
 * One could recycle unattached segments of different data directories if we
 * distinguished that case from other SHMSTATE_FOREIGN cases.  Doing so would
 * cause us to visit less of the key space, making us less likely to detect a
 * SHMSTATE_ATTACHED key.  It would also complicate the concurrency analysis,
 * in that postmasters of different data directories could simultaneously
 * attempt to recycle a given key.  We'll waste keys longer in some cases, but
 * avoiding the problems of the alternative justifies that loss.
 */
typedef enum
{
  SHMSTATE_ANALYSIS_FAILURE,  /* unexpected failure to analyze the ID */
  SHMSTATE_ATTACHED,      /* pertinent to DataDir, has attached PIDs */
  SHMSTATE_ENOENT,      /* no segment of that ID */
  SHMSTATE_FOREIGN,     /* exists, but not pertinent to DataDir */
  SHMSTATE_UNATTACHED     /* pertinent to DataDir, no attached PIDs */
} IpcMemoryState;


unsigned long UsedShmemSegID = 0;
void     *UsedShmemSegAddr = NULL;

static Size AnonymousShmemSize;
static void *AnonymousShmem = NULL;

static void *InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size);
static void IpcMemoryDetach(int status, Datum shmaddr);
static void IpcMemoryDelete(int status, Datum shmId);
static IpcMemoryState PGSharedMemoryAttach(IpcMemoryId shmId,
           PGShmemHeader **addr);


/*
 *  InternalIpcMemoryCreate(memKey, size)
 *
 * Attempt to create a new shared memory segment with the specified key.
 * Will fail (return NULL) if such a segment already exists.  If successful,
 * attach the segment to the current process and return its attached address.
 * On success, callbacks are registered with on_shmem_exit to detach and
 * delete the segment when on_shmem_exit is called.
 *
 * If we fail with a failure code other than collision-with-existing-segment,
 * print out an error and abort.  Other types of errors are not recoverable.
 */
static void *
InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
{
  IpcMemoryId shmid;
  void     *requestedAddress = NULL;
  void     *memAddress;

  /*
   * Normally we just pass requestedAddress = NULL to shmat(), allowing the
   * system to choose where the segment gets mapped.  But in an EXEC_BACKEND
   * build, it's possible for whatever is chosen in the postmaster to not
   * work for backends, due to variations in address space layout.  As a
   * rather klugy workaround, allow the user to specify the address to use
   * via setting the environment variable PG_SHMEM_ADDR.  (If this were of
   * interest for anything except debugging, we'd probably create a cleaner
   * and better-documented way to set it, such as a GUC.)
   */
#ifdef EXEC_BACKEND
  {
    char     *pg_shmem_addr = getenv("PG_SHMEM_ADDR");

    if (pg_shmem_addr)
      requestedAddress = (void *) strtoul(pg_shmem_addr, NULL, 0);
  }
#endif

  shmid = shmget(memKey, size, IPC_CREAT | IPC_EXCL | IPCProtection);

  if (shmid < 0)
  {
    int     shmget_errno = errno;

    /*
     * Fail quietly if error indicates a collision with existing segment.
     * One would expect EEXIST, given that we said IPC_EXCL, but perhaps
     * we could get a permission violation instead?  Also, EIDRM might
     * occur if an old seg is slated for destruction but not gone yet.
     */
    if (shmget_errno == EEXIST || shmget_errno == EACCES
#ifdef EIDRM
      || shmget_errno == EIDRM
#endif
      )
      return NULL;

    /*
     * Some BSD-derived kernels are known to return EINVAL, not EEXIST, if
     * there is an existing segment but it's smaller than "size" (this is
     * a result of poorly-thought-out ordering of error tests). To
     * distinguish between collision and invalid size in such cases, we
     * make a second try with size = 0.  These kernels do not test size
     * against SHMMIN in the preexisting-segment case, so we will not get
     * EINVAL a second time if there is such a segment.
     */
    if (shmget_errno == EINVAL)
    {
      shmid = shmget(memKey, 0, IPC_CREAT | IPC_EXCL | IPCProtection);

      if (shmid < 0)
      {
        /* As above, fail quietly if we verify a collision */
        if (errno == EEXIST || errno == EACCES
#ifdef EIDRM
          || errno == EIDRM
#endif
          )
          return NULL;
        /* Otherwise, fall through to report the original error */
      }
      else
      {
        /*
         * On most platforms we cannot get here because SHMMIN is
         * greater than zero.  However, if we do succeed in creating a
         * zero-size segment, free it and then fall through to report
         * the original error.
         */
        if (shmctl(shmid, IPC_RMID, NULL) < 0)
          elog(LOG, "shmctl(%d, %d, 0) failed: %m",
             (int) shmid, IPC_RMID);
      }
    }

    /*
     * Else complain and abort.
     *
     * Note: at this point EINVAL should mean that either SHMMIN or SHMMAX
     * is violated.  SHMALL violation might be reported as either ENOMEM
     * (BSDen) or ENOSPC (Linux); the Single Unix Spec fails to say which
     * it should be.  SHMMNI violation is ENOSPC, per spec.  Just plain
     * not-enough-RAM is ENOMEM.
     */
    errno = shmget_errno;
    ereport(FATAL,
        (errmsg("could not create shared memory segment: %m"),
         errdetail("Failed system call was shmget(key=%lu, size=%zu, 0%o).",
               (unsigned long) memKey, size,
               IPC_CREAT | IPC_EXCL | IPCProtection),
         (shmget_errno == EINVAL) ?
         errhint("This error usually means that PostgreSQL's request for a shared memory "
             "segment exceeded your kernel's SHMMAX parameter, or possibly that "
             "it is less than "
             "your kernel's SHMMIN parameter.\n"
             "The PostgreSQL documentation contains more information about shared "
             "memory configuration.") : 0,
         (shmget_errno == ENOMEM) ?
         errhint("This error usually means that PostgreSQL's request for a shared "
             "memory segment exceeded your kernel's SHMALL parameter.  You might need "
             "to reconfigure the kernel with larger SHMALL.\n"
             "The PostgreSQL documentation contains more information about shared "
             "memory configuration.") : 0,
         (shmget_errno == ENOSPC) ?
         errhint("This error does *not* mean that you have run out of disk space.  "
             "It occurs either if all available shared memory IDs have been taken, "
             "in which case you need to raise the SHMMNI parameter in your kernel, "
             "or because the system's overall limit for shared memory has been "
             "reached.\n"
             "The PostgreSQL documentation contains more information about shared "
             "memory configuration.") : 0));
  }

  /* Register on-exit routine to delete the new segment */
  on_shmem_exit(IpcMemoryDelete, Int32GetDatum(shmid));

  /* OK, should be able to attach to the segment */
  memAddress = shmat(shmid, requestedAddress, PG_SHMAT_FLAGS);

  if (memAddress == (void *) -1)
    elog(FATAL, "shmat(id=%d, addr=%p, flags=0x%x) failed: %m",
       shmid, requestedAddress, PG_SHMAT_FLAGS);

  /* Register on-exit routine to detach new segment before deleting */
  on_shmem_exit(IpcMemoryDetach, PointerGetDatum(memAddress));

  /*
   * Store shmem key and ID in data directory lockfile.  Format to try to
   * keep it the same length always (trailing junk in the lockfile won't
   * hurt, but might confuse humans).
   */
  {
    char    line[64];

    sprintf(line, "%9lu %9lu",
        (unsigned long) memKey, (unsigned long) shmid);
    AddToDataDirLockFile(LOCK_FILE_LINE_SHMEM_KEY, line);
  }

  return memAddress;
}

/****************************************************************************/
/*  IpcMemoryDetach(status, shmaddr)  removes a shared memory segment   */
/*                    from process' address space     */
/*  (called as an on_shmem_exit callback, hence funny argument list)    */
/****************************************************************************/
static void
IpcMemoryDetach(int status, Datum shmaddr)
{
  /* Detach System V shared memory block. */
  if (shmdt(DatumGetPointer(shmaddr)) < 0)
    elog(LOG, "shmdt(%p) failed: %m", DatumGetPointer(shmaddr));
}

/****************************************************************************/
/*  IpcMemoryDelete(status, shmId)    deletes a shared memory segment   */
/*  (called as an on_shmem_exit callback, hence funny argument list)    */
/****************************************************************************/
static void
IpcMemoryDelete(int status, Datum shmId)
{
  if (shmctl(DatumGetInt32(shmId), IPC_RMID, NULL) < 0)
    elog(LOG, "shmctl(%d, %d, 0) failed: %m",
       DatumGetInt32(shmId), IPC_RMID);
}

/*
 * PGSharedMemoryIsInUse
 *
 * Is a previously-existing shmem segment still existing and in use?
 *
 * The point of this exercise is to detect the case where a prior postmaster
 * crashed, but it left child backends that are still running.  Therefore
 * we only care about shmem segments that are associated with the intended
 * DataDir.  This is an important consideration since accidental matches of
 * shmem segment IDs are reasonably common.
 */
bool
PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
{
  PGShmemHeader *memAddress;
  IpcMemoryState state;

  state = PGSharedMemoryAttach((IpcMemoryId) id2, &memAddress);
  if (memAddress && shmdt(memAddress) < 0)
    elog(LOG, "shmdt(%p) failed: %m", memAddress);
  switch (state)
  {
    case SHMSTATE_ENOENT:
    case SHMSTATE_FOREIGN:
    case SHMSTATE_UNATTACHED:
      return false;
    case SHMSTATE_ANALYSIS_FAILURE:
    case SHMSTATE_ATTACHED:
      return true;
  }
  return true;
}

/* See comment at IpcMemoryState. */
static IpcMemoryState
PGSharedMemoryAttach(IpcMemoryId shmId,
           PGShmemHeader **addr)
{
  struct shmid_ds shmStat;
  struct stat statbuf;
  PGShmemHeader *hdr;

  *addr = NULL;

  /*
   * We detect whether a shared memory segment is in use by seeing whether
   * it (a) exists and (b) has any processes attached to it.
   */
  if (shmctl(shmId, IPC_STAT, &shmStat) < 0)
  {
    /*
     * EINVAL actually has multiple possible causes documented in the
     * shmctl man page, but we assume it must mean the segment no longer
     * exists.
     */
    if (errno == EINVAL)
      return SHMSTATE_ENOENT;

    /*
     * EACCES implies we have no read permission, which means it is not a
     * Postgres shmem segment (or at least, not one that is relevant to
     * our data directory).
     */
    if (errno == EACCES)
      return SHMSTATE_FOREIGN;

    /*
     * Some Linux kernel versions (in fact, all of them as of July 2007)
     * sometimes return EIDRM when EINVAL is correct.  The Linux kernel
     * actually does not have any internal state that would justify
     * returning EIDRM, so we can get away with assuming that EIDRM is
     * equivalent to EINVAL on that platform.
     */
#ifdef HAVE_LINUX_EIDRM_BUG
    if (errno == EIDRM)
      return SHMSTATE_ENOENT;
#endif

    /*
     * Otherwise, we had better assume that the segment is in use. The
     * only likely case is EIDRM, which implies that the segment has been
     * IPC_RMID'd but there are still processes attached to it.
     */
    return SHMSTATE_ANALYSIS_FAILURE;
  }

  /*
   * Try to attach to the segment and see if it matches our data directory.
   * This avoids shmid-conflict problems on machines that are running
   * several postmasters under the same userid.
   */
  if (stat(DataDir, &statbuf) < 0)
    return SHMSTATE_ANALYSIS_FAILURE; /* can't stat; be conservative */

  /*
   * Attachment fails if we have no write permission.  Since that will never
   * happen with Postgres IPCProtection, such a failure shows the segment is
   * not a Postgres segment.  If attachment fails for some other reason, be
   * conservative.
   */
  hdr = (PGShmemHeader *) shmat(shmId, UsedShmemSegAddr, PG_SHMAT_FLAGS);
  if (hdr == (PGShmemHeader *) -1)
  {
    if (errno == EACCES)
      return SHMSTATE_FOREIGN;
    else
      return SHMSTATE_ANALYSIS_FAILURE;
  }
  *addr = hdr;

  if (hdr->magic != PGShmemMagic ||
    hdr->device != statbuf.st_dev ||
    hdr->inode != statbuf.st_ino)
  {
    /*
     * It's either not a Postgres segment, or not one for my data
     * directory.
     */
    return SHMSTATE_FOREIGN;
  }

  return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
}

#ifdef MAP_HUGETLB

/*
 * Identify the huge page size to use.
 *
 * Some Linux kernel versions have a bug causing mmap() to fail on requests
 * that are not a multiple of the hugepage size.  Versions without that bug
 * instead silently round the request up to the next hugepage multiple ---
 * and then munmap() fails when we give it a size different from that.
 * So we have to round our request up to a multiple of the actual hugepage
 * size to avoid trouble.
 *
 * Doing the round-up ourselves also lets us make use of the extra memory,
 * rather than just wasting it.  Currently, we just increase the available
 * space recorded in the shmem header, which will make the extra usable for
 * purposes such as additional locktable entries.  Someday, for very large
 * hugepage sizes, we might want to think about more invasive strategies,
 * such as increasing shared_buffers to absorb the extra space.
 *
 * Returns the (real or assumed) page size into *hugepagesize,
 * and the hugepage-related mmap flags to use into *mmap_flags.
 *
 * Currently *mmap_flags is always just MAP_HUGETLB.  Someday, on systems
 * that support it, we might OR in additional bits to specify a particular
 * non-default huge page size.
 */
static void
GetHugePageSize(Size *hugepagesize, int *mmap_flags)
{
  /*
   * If we fail to find out the system's default huge page size, assume it
   * is 2MB.  This will work fine when the actual size is less.  If it's
   * more, we might get mmap() or munmap() failures due to unaligned
   * requests; but at this writing, there are no reports of any non-Linux
   * systems being picky about that.
   */
  *hugepagesize = 2 * 1024 * 1024;
  *mmap_flags = MAP_HUGETLB;

  /*
   * System-dependent code to find out the default huge page size.
   *
   * On Linux, read /proc/meminfo looking for a line like "Hugepagesize:
   * nnnn kB".  Ignore any failures, falling back to the preset default.
   */
#ifdef __linux__
  {
    FILE     *fp = AllocateFile("/proc/meminfo", "r");
    char    buf[128];
    unsigned int sz;
    char    ch;

    if (fp)
    {
      while (fgets(buf, sizeof(buf), fp))
      {
        if (sscanf(buf, "Hugepagesize: %u %c", &sz, &ch) == 2)
        {
          if (ch == 'k')
          {
            *hugepagesize = sz * (Size) 1024;
            break;
          }
          /* We could accept other units besides kB, if needed */
        }
      }
      FreeFile(fp);
    }
  }
#endif              /* __linux__ */
}

#endif              /* MAP_HUGETLB */

/*
 * Creates an anonymous mmap()ed shared memory segment.
 *
 * Pass the requested size in *size.  This function will modify *size to the
 * actual size of the allocation, if it ends up allocating a segment that is
 * larger than requested.
 */
static void *
CreateAnonymousSegment(Size *size)
{
  Size    allocsize = *size;
  void     *ptr = MAP_FAILED;
  int     mmap_errno = 0;

#ifndef MAP_HUGETLB
  /* PGSharedMemoryCreate should have dealt with this case */
  Assert(huge_pages != HUGE_PAGES_ON);
#else
  if (huge_pages == HUGE_PAGES_ON || huge_pages == HUGE_PAGES_TRY)
  {
    /*
     * Round up the request size to a suitable large value.
     */
    Size    hugepagesize;
    int     mmap_flags;

    GetHugePageSize(&hugepagesize, &mmap_flags);

    if (allocsize % hugepagesize != 0)
      allocsize += hugepagesize - (allocsize % hugepagesize);

    ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
           PG_MMAP_FLAGS | mmap_flags, -1, 0);
    mmap_errno = errno;
    if (huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED)
      elog(DEBUG1, "mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",
         allocsize);
  }
#endif

  if (ptr == MAP_FAILED && huge_pages != HUGE_PAGES_ON)
  {
    /*
     * Use the original size, not the rounded-up value, when falling back
     * to non-huge pages.
     */
    allocsize = *size;
    ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
           PG_MMAP_FLAGS, -1, 0);
    mmap_errno = errno;
  }

  if (ptr == MAP_FAILED)
  {
    errno = mmap_errno;
    ereport(FATAL,
        (errmsg("could not map anonymous shared memory: %m"),
         (mmap_errno == ENOMEM) ?
         errhint("This error usually means that PostgreSQL's request "
             "for a shared memory segment exceeded available memory, "
             "swap space, or huge pages. To reduce the request size "
             "(currently %zu bytes), reduce PostgreSQL's shared "
             "memory usage, perhaps by reducing shared_buffers or "
             "max_connections.",
             *size) : 0));
  }

  *size = allocsize;
  return ptr;
}

/*
 * AnonymousShmemDetach --- detach from an anonymous mmap'd block
 * (called as an on_shmem_exit callback, hence funny argument list)
 */
static void
AnonymousShmemDetach(int status, Datum arg)
{
  /* Release anonymous shared memory block, if any. */
  if (AnonymousShmem != NULL)
  {
    if (munmap(AnonymousShmem, AnonymousShmemSize) < 0)
      elog(LOG, "munmap(%p, %zu) failed: %m",
         AnonymousShmem, AnonymousShmemSize);
    AnonymousShmem = NULL;
  }
}

/*
 * PGSharedMemoryCreate
 *
 * Create a shared memory segment of the given size and initialize its
 * standard header.  Also, register an on_shmem_exit callback to release
 * the storage.
 *
 * Dead Postgres segments pertinent to this DataDir are recycled if found, but
 * we do not fail upon collision with foreign shmem segments.  The idea here
 * is to detect and re-use keys that may have been assigned by a crashed
 * postmaster or backend.
 *
 * The port number is passed for possible use as a key (for SysV, we use
 * it to generate the starting shmem key).
 */
PGShmemHeader *
PGSharedMemoryCreate(Size size, int port,
           PGShmemHeader **shim)
{
  IpcMemoryKey NextShmemSegID;
  void     *memAddress;
  PGShmemHeader *hdr;
  struct stat statbuf;
  Size    sysvsize;

  /* Complain if hugepages demanded but we can't possibly support them */
#if !defined(MAP_HUGETLB)
  if (huge_pages == HUGE_PAGES_ON)
    ereport(ERROR,
        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
         errmsg("huge pages not supported on this platform")));
#endif

  /* Room for a header? */
  Assert(size > MAXALIGN(sizeof(PGShmemHeader)));

  if (shared_memory_type == SHMEM_TYPE_MMAP)
  {
    AnonymousShmem = CreateAnonymousSegment(&size);
    AnonymousShmemSize = size;

    /* Register on-exit routine to unmap the anonymous segment */
    on_shmem_exit(AnonymousShmemDetach, (Datum) 0);

    /* Now we need only allocate a minimal-sized SysV shmem block. */
    sysvsize = sizeof(PGShmemHeader);
  }
  else
    sysvsize = size;

  /* Make sure PGSharedMemoryAttach doesn't fail without need */
  UsedShmemSegAddr = NULL;

  /*
   * Loop till we find a free IPC key.  Trust CreateDataDirLockFile() to
   * ensure no more than one postmaster per data directory can enter this
   * loop simultaneously.  (CreateDataDirLockFile() does not ensure that,
   * but prefer fixing it over coping here.)
   */
  NextShmemSegID = 1 + port * 1000;

  for (;;)
  {
    IpcMemoryId shmid;
    PGShmemHeader *oldhdr;
    IpcMemoryState state;

    /* Try to create new segment */
    memAddress = InternalIpcMemoryCreate(NextShmemSegID, sysvsize);
    if (memAddress)
      break;       /* successful create and attach */

    /* Check shared memory and possibly remove and recreate */

    /*
     * shmget() failure is typically EACCES, hence SHMSTATE_FOREIGN.
     * ENOENT, a narrow possibility, implies SHMSTATE_ENOENT, but one can
     * safely treat SHMSTATE_ENOENT like SHMSTATE_FOREIGN.
     */
    shmid = shmget(NextShmemSegID, sizeof(PGShmemHeader), 0);
    if (shmid < 0)
    {
      oldhdr = NULL;
      state = SHMSTATE_FOREIGN;
    }
    else
      state = PGSharedMemoryAttach(shmid, &oldhdr);

    switch (state)
    {
      case SHMSTATE_ANALYSIS_FAILURE:
      case SHMSTATE_ATTACHED:
        ereport(FATAL,
            (errcode(ERRCODE_LOCK_FILE_EXISTS),
             errmsg("pre-existing shared memory block (key %lu, ID %lu) is still in use",
                (unsigned long) NextShmemSegID,
                (unsigned long) shmid),
             errhint("Terminate any old server processes associated with data directory \"%s\".",
                 DataDir)));
        break;
      case SHMSTATE_ENOENT:

        /*
         * To our surprise, some other process deleted since our last
         * InternalIpcMemoryCreate().  Moments earlier, we would have
         * seen SHMSTATE_FOREIGN.  Try that same ID again.
         */
        elog(LOG,
           "shared memory block (key %lu, ID %lu) deleted during startup",
           (unsigned long) NextShmemSegID,
           (unsigned long) shmid);
        break;
      case SHMSTATE_FOREIGN:
        NextShmemSegID++;
        break;
      case SHMSTATE_UNATTACHED:

        /*
         * The segment pertains to DataDir, and every process that had
         * used it has died or detached.  Zap it, if possible, and any
         * associated dynamic shared memory segments, as well.  This
         * shouldn't fail, but if it does, assume the segment belongs
         * to someone else after all, and try the next candidate.
         * Otherwise, try again to create the segment.  That may fail
         * if some other process creates the same shmem key before we
         * do, in which case we'll try the next key.
         */
        if (oldhdr->dsm_control != 0)
          dsm_cleanup_using_control_segment(oldhdr->dsm_control);
        if (shmctl(shmid, IPC_RMID, NULL) < 0)
          NextShmemSegID++;
        break;
    }

    if (oldhdr && shmdt(oldhdr) < 0)
      elog(LOG, "shmdt(%p) failed: %m", oldhdr);
  }

  /* Initialize new segment. */
  hdr = (PGShmemHeader *) memAddress;
  hdr->creatorPID = getpid();
  hdr->magic = PGShmemMagic;
  hdr->dsm_control = 0;

  /* Fill in the data directory ID info, too */
  if (stat(DataDir, &statbuf) < 0)
    ereport(FATAL,
        (errcode_for_file_access(),
         errmsg("could not stat data directory \"%s\": %m",
            DataDir)));
  hdr->device = statbuf.st_dev;
  hdr->inode = statbuf.st_ino;

  /*
   * Initialize space allocation status for segment.
   */
  hdr->totalsize = size;
  hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
  *shim = hdr;

  /* Save info for possible future use */
  UsedShmemSegAddr = memAddress;
  UsedShmemSegID = (unsigned long) NextShmemSegID;

  /*
   * If AnonymousShmem is NULL here, then we're not using anonymous shared
   * memory, and should return a pointer to the System V shared memory
   * block. Otherwise, the System V shared memory block is only a shim, and
   * we must return a pointer to the real block.
   */
  if (AnonymousShmem == NULL)
    return hdr;
  memcpy(AnonymousShmem, hdr, sizeof(PGShmemHeader));
  return (PGShmemHeader *) AnonymousShmem;
}

#ifdef EXEC_BACKEND

/*
 * PGSharedMemoryReAttach
 *
 * This is called during startup of a postmaster child process to re-attach to
 * an already existing shared memory segment.  This is needed only in the
 * EXEC_BACKEND case; otherwise postmaster children inherit the shared memory
 * segment attachment via fork().
 *
 * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
 * routine.  The caller must have already restored them to the postmaster's
 * values.
 */
void
PGSharedMemoryReAttach(void)
{
  IpcMemoryId shmid;
  PGShmemHeader *hdr;
  IpcMemoryState state;
  void     *origUsedShmemSegAddr = UsedShmemSegAddr;

  Assert(UsedShmemSegAddr != NULL);
  Assert(IsUnderPostmaster);

#ifdef __CYGWIN__
  /* cygipc (currently) appears to not detach on exec. */
  PGSharedMemoryDetach();
  UsedShmemSegAddr = origUsedShmemSegAddr;
#endif

  elog(DEBUG3, "attaching to %p", UsedShmemSegAddr);
  shmid = shmget(UsedShmemSegID, sizeof(PGShmemHeader), 0);
  if (shmid < 0)
    state = SHMSTATE_FOREIGN;
  else
    state = PGSharedMemoryAttach(shmid, &hdr);
  if (state != SHMSTATE_ATTACHED)
    elog(FATAL, "could not reattach to shared memory (key=%d, addr=%p): %m",
       (int) UsedShmemSegID, UsedShmemSegAddr);
  if (hdr != origUsedShmemSegAddr)
    elog(FATAL, "reattaching to shared memory returned unexpected address (got %p, expected %p)",
       hdr, origUsedShmemSegAddr);
  dsm_set_control_handle(hdr->dsm_control);

  UsedShmemSegAddr = hdr;   /* probably redundant */
}

/*
 * PGSharedMemoryNoReAttach
 *
 * This is called during startup of a postmaster child process when we choose
 * *not* to re-attach to the existing shared memory segment.  We must clean up
 * to leave things in the appropriate state.  This is not used in the non
 * EXEC_BACKEND case, either.
 *
 * The child process startup logic might or might not call PGSharedMemoryDetach
 * after this; make sure that it will be a no-op if called.
 *
 * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
 * routine.  The caller must have already restored them to the postmaster's
 * values.
 */
void
PGSharedMemoryNoReAttach(void)
{
  Assert(UsedShmemSegAddr != NULL);
  Assert(IsUnderPostmaster);

#ifdef __CYGWIN__
  /* cygipc (currently) appears to not detach on exec. */
  PGSharedMemoryDetach();
#endif

  /* For cleanliness, reset UsedShmemSegAddr to show we're not attached. */
  UsedShmemSegAddr = NULL;
  /* And the same for UsedShmemSegID. */
  UsedShmemSegID = 0;
}

#endif              /* EXEC_BACKEND */

/*
 * PGSharedMemoryDetach
 *
 * Detach from the shared memory segment, if still attached.  This is not
 * intended to be called explicitly by the process that originally created the
 * segment (it will have on_shmem_exit callback(s) registered to do that).
 * Rather, this is for subprocesses that have inherited an attachment and want
 * to get rid of it.
 *
 * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
 * routine, also AnonymousShmem and AnonymousShmemSize.
 */
void
PGSharedMemoryDetach(void)
{
  if (UsedShmemSegAddr != NULL)
  {
    if ((shmdt(UsedShmemSegAddr) < 0)
#if defined(EXEC_BACKEND) && defined(__CYGWIN__)
    /* Work-around for cygipc exec bug */
      && shmdt(NULL) < 0
#endif
      )
      elog(LOG, "shmdt(%p) failed: %m", UsedShmemSegAddr);
    UsedShmemSegAddr = NULL;
  }

  if (AnonymousShmem != NULL)
  {
    if (munmap(AnonymousShmem, AnonymousShmemSize) < 0)
      elog(LOG, "munmap(%p, %zu) failed: %m",
         AnonymousShmem, AnonymousShmemSize);
    AnonymousShmem = NULL;
  }
}

YugabyteDB (2.13.0.0-b42, bfc6a6643e7399ac8a0e81d06a3ee6d6571b33ab)

Coverage Report

Created: 2022-03-09 17:30