YugabyteDB (2.13.1.0-b60, 21121d69985fbf76aa6958d8f04a9bfa936293b5)

// Licensed to the Apache Software Foundation (ASF) under one

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// distributed with this work for additional information

// regarding copyright ownership.  The ASF licenses this file

// to you under the Apache License, Version 2.0 (the

// "License"); you may not use this file except in compliance

// with the License.  You may obtain a copy of the License at

//

//   http://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing,

// software distributed under the License is distributed on an

// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY

// KIND, either express or implied.  See the License for the

// specific language governing permissions and limitations

// under the License.

//

// The following only applies to changes made to this file as part of YugaByte development.

//

// Portions Copyright (c) YugaByte, Inc.

//

// Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except

// in compliance with the License.  You may obtain a copy of the License at

//

// http://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing, software distributed under the License

// is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express

// or implied.  See the License for the specific language governing permissions and limitations

// under the License.

//

#include "yb/tablet/tablet_bootstrap.h"

#include <map>

#include <set>

#include <boost/preprocessor/cat.hpp>

#include <boost/preprocessor/stringize.hpp>

#include "yb/common/common_fwd.h"

#include "yb/common/schema.h"

#include "yb/common/wire_protocol.h"

#include "yb/consensus/consensus.h"

#include "yb/consensus/consensus.pb.h"

#include "yb/consensus/consensus_meta.h"

#include "yb/consensus/consensus_util.h"

#include "yb/consensus/log.h"

#include "yb/consensus/log_anchor_registry.h"

#include "yb/consensus/log_index.h"

#include "yb/consensus/log_reader.h"

#include "yb/consensus/log_util.h"

#include "yb/consensus/opid_util.h"

#include "yb/consensus/retryable_requests.h"

#include "yb/docdb/consensus_frontier.h"

#include "yb/docdb/value_type.h"

#include "yb/gutil/casts.h"

#include "yb/gutil/ref_counted.h"

#include "yb/gutil/strings/substitute.h"

#include "yb/gutil/thread_annotations.h"

#include "yb/rpc/rpc_fwd.h"

#include "yb/tablet/tablet_fwd.h"

#include "yb/tablet/mvcc.h"

#include "yb/tablet/operations/change_metadata_operation.h"

#include "yb/tablet/operations/history_cutoff_operation.h"

#include "yb/tablet/operations/snapshot_operation.h"

#include "yb/tablet/operations/split_operation.h"

#include "yb/tablet/operations/truncate_operation.h"

#include "yb/tablet/operations/update_txn_operation.h"

#include "yb/tablet/operations/write_operation.h"

#include "yb/tablet/snapshot_coordinator.h"

#include "yb/tablet/tablet.h"

#include "yb/tablet/tablet_metadata.h"

#include "yb/tablet/tablet_options.h"

#include "yb/tablet/tablet_snapshots.h"

#include "yb/tablet/tablet_splitter.h"

#include "yb/tablet/transaction_coordinator.h"

#include "yb/tablet/transaction_participant.h"

#include "yb/tserver/backup.pb.h"

#include "yb/util/atomic.h"

#include "yb/util/env_util.h"

#include "yb/util/fault_injection.h"

#include "yb/util/flag_tags.h"

#include "yb/util/format.h"

#include "yb/util/logging.h"

#include "yb/util/metric_entity.h"

#include "yb/util/monotime.h"

#include "yb/util/opid.h"

#include "yb/util/scope_exit.h"

#include "yb/util/status.h"

#include "yb/util/status_format.h"

#include "yb/util/stopwatch.h"

DEFINE_bool(skip_remove_old_recovery_dir, false,

            "Skip removing WAL recovery dir after startup. (useful for debugging)");

TAG_FLAG(skip_remove_old_recovery_dir, hidden);

DEFINE_bool(skip_wal_rewrite, true,

            "Skip rewriting WAL files during bootstrap.");

TAG_FLAG(skip_wal_rewrite, experimental);

TAG_FLAG(skip_wal_rewrite, runtime);

DEFINE_test_flag(double, fault_crash_during_log_replay, 0.0,

                 "Fraction of the time when the tablet will crash immediately "

                 "after processing a log entry during log replay.");

DECLARE_uint64(max_clock_sync_error_usec);

DEFINE_bool(force_recover_flushed_frontier, false,

            "Could be used to ignore the flushed frontier metadata from RocksDB manifest and "

            "recover it from the log instead.");

TAG_FLAG(force_recover_flushed_frontier, hidden);

TAG_FLAG(force_recover_flushed_frontier, advanced);

DEFINE_bool(skip_flushed_entries, true,

            "Only replay WAL entries that are not flushed to RocksDB or within the retryable "

            "request timeout.");

DECLARE_int32(retryable_request_timeout_secs);

DEFINE_uint64(transaction_status_tablet_log_segment_size_bytes, 4_MB,

              "The segment size for transaction status tablet log roll-overs, in bytes.");

DEFINE_test_flag(int32, tablet_bootstrap_delay_ms, 0,

                 "Time (in ms) to delay tablet bootstrap by.");

DEFINE_test_flag(bool, dump_docdb_before_tablet_bootstrap, false,

                 "Dump the contents of DocDB before tablet bootstrap. Should only be used when "

                 "data is small.")

DEFINE_test_flag(bool, dump_docdb_after_tablet_bootstrap, false,

                 "Dump the contents of DocDB after tablet bootstrap. Should only be used when "

                 "data is small.")

namespace yb {

namespace tablet {

using namespace std::literals; // NOLINT

using namespace std::placeholders;

using std::shared_ptr;

using log::Log;

using log::LogEntryPB;

using log::LogOptions;

using log::LogReader;

using log::ReadableLogSegment;

using log::LogEntryMetadata;

using log::LogIndex;

using log::CreateNewSegment;

using log::SegmentSequence;

using consensus::ChangeConfigRecordPB;

using consensus::RaftConfigPB;

using consensus::ConsensusBootstrapInfo;

using consensus::ConsensusMetadata;

using consensus::MinimumOpId;

using consensus::OpIdEquals;

using consensus::OpIdToString;

using consensus::ReplicateMsg;

using consensus::MakeOpIdPB;

using strings::Substitute;

using tserver::WriteRequestPB;

using tserver::TabletSnapshotOpRequestPB;

static string DebugInfo(const string& tablet_id,

                        uint64_t segment_seqno,

                        size_t entry_idx,

                        const string& segment_path,

                        const LogEntryPB* entry) {

  // Truncate the debug string to a reasonable length for logging.  Otherwise, glog will truncate

  // for us and we may miss important information which came after this long string.

  string debug_str = entry ? entry->ShortDebugString() : "<nullptr>"s;

  if (debug_str.size() > 500) {

    debug_str.resize(500);

    debug_str.append("...");

  }

  return Substitute("Debug Info: Error playing entry $0 of segment $1 of tablet $2. "

                    "Segment path: $3. Entry: $4", entry_idx, segment_seqno, tablet_id,

                    segment_path, debug_str);

}

// ================================================================================================

// Class ReplayState.

// ================================================================================================

struct Entry {

  std::unique_ptr<log::LogEntryPB> entry;

  RestartSafeCoarseTimePoint entry_time;

  std::string ToString() const {

    return Format("{ entry: $0 entry_time: $1 }", entry, entry_time);

  }

};

typedef std::map<int64_t, Entry> OpIndexToEntryMap;

// State kept during replay.

struct ReplayState {

  ReplayState(

      const DocDbOpIds& op_ids_,

      const std::string& log_prefix_)

      : stored_op_ids(op_ids_),

        log_prefix(log_prefix_) {

  }

  // Return true if 'b' is allowed to immediately follow 'a' in the log.

  static bool IsValidSequence(const OpId& a, const OpId& b);

  // Return a Corruption status if 'id' seems to be out-of-sequence in the log.

  Status CheckSequentialReplicateId(const consensus::ReplicateMsg& msg);

  void UpdateCommittedOpId(const OpId& id);

  // half_limit is half the limit on the number of entries added

  void AddEntriesToStrings(

      const OpIndexToEntryMap& entries, std::vector<std::string>* strings, size_t half_limit) const;

  // half_limit is half the limit on the number of entries to be dumped

  void DumpReplayStateToStrings(std::vector<std::string>* strings, int half_limit) const;

  bool CanApply(log::LogEntryPB* entry);

  const std::string& LogPrefix() const { return log_prefix; }

  void UpdateCommittedFromStored();

  // Determines the lowest possible OpId we have to replay. This is based on OpIds of operations

  // flushed to regular and intents RocksDBs. Also logs some diagnostics.

  OpId GetLowestOpIdToReplay(bool has_intents_db, const char* extra_log_prefix) const;

  // ----------------------------------------------------------------------------------------------

  // ReplayState member fields

  // ----------------------------------------------------------------------------------------------

  // The last replicate message's ID.

  OpId prev_op_id;

  // The last operation known to be committed. All other operations with lower IDs are also

  // committed.

  OpId committed_op_id;

  // All REPLICATE entries that have not been applied to RocksDB yet. We decide what entries are

  // safe to apply and delete from this map based on the commit index included into each REPLICATE

  // message.

  //

  // The key in this map is the Raft index.

  OpIndexToEntryMap pending_replicates;

  // ----------------------------------------------------------------------------------------------

  // State specific to RocksDB-backed tables (not transaction status table)

  const DocDbOpIds stored_op_ids;

  // Total number of log entries applied to RocksDB.

  int64_t num_entries_applied_to_rocksdb = 0;

  // If we encounter the last entry flushed to a RocksDB SSTable (as identified by the max

  // persistent sequence number), we remember the hybrid time of that entry in this field.

  // We guarantee that we'll either see that entry or a latter entry we know is committed into Raft

  // during log replay. This is crucial for properly setting safe time at bootstrap.

  HybridTime max_committed_hybrid_time = HybridTime::kMin;

  const std::string log_prefix;

};

void ReplayState::UpdateCommittedFromStored() {

  if (stored_op_ids.regular > committed_op_id) {

    committed_op_id = stored_op_ids.regular;

  }

  if (stored_op_ids.intents > committed_op_id) {

    committed_op_id = stored_op_ids.intents;

  }

}

// Return true if 'b' is allowed to immediately follow 'a' in the log.

bool ReplayState::IsValidSequence(const OpId& a, const OpId& b) {

  if (a.term == 0 && 
a.index == 02.79k
) {

    // Not initialized - can start with any opid.

    return true;

  }

  // Within the same term, we should never skip entries.

  // We can, however go backwards (see KUDU-783 for an example)

  if (b.term == a.term && 
b.index > a.index + 11.35M
) {

    return false;

  }

  // TODO: check that the term does not decrease.

  // https://github.com/yugabyte/yugabyte-db/issues/5115

  return true;

}

// Return a Corruption status if 'id' seems to be out-of-sequence in the log.

Status ReplayState::CheckSequentialReplicateId(const ReplicateMsg& msg) {

  SCHECK(msg.has_id(), Corruption, "A REPLICATE message must have an id");

  const auto msg_op_id = OpId::FromPB(msg.id());

  if (PREDICT_FALSE(!IsValidSequence(prev_op_id, msg_op_id))) {

    string op_desc = Format(

        "$0 REPLICATE (Type: $1)", msg_op_id, OperationType_Name(msg.op_type()));

    return STATUS_FORMAT(Corruption,

                         "Unexpected op id following op id $0. Operation: $1",

                         prev_op_id, op_desc);

  }

  prev_op_id = msg_op_id;

  return Status::OK();

}

void ReplayState::UpdateCommittedOpId(const OpId& id) {

  if (committed_op_id < id) {

    
VLOG_WITH_PREFIX4
(1) << "Updating committed op id to " << id4
;

    committed_op_id = id;

  }

}

void ReplayState::AddEntriesToStrings(const OpIndexToEntryMap& entries,

                                      std::vector<std::string>* strings,

                                      size_t half_limit) const {

  const auto n = entries.size();

  const bool overflow = n > 2 * half_limit;

  size_t index = 0;

  for (const auto& entry : entries) {

    if (!overflow || 
(0
index < half_limit0
 || 
index >= n - half_limit0
)) {

      const auto& replicate = entry.second.entry.get()->replicate();

      strings->push_back(Format(

          "    [$0] op_id: $1 hybrid_time: $2 op_type: $3 committed_op_id: $4",

          index + 1,

          OpId::FromPB(replicate.id()),

          replicate.hybrid_time(),

          replicate.op_type(),

          OpId::FromPB(replicate.committed_op_id())));

    }

    if (overflow && 
index == half_limit - 10
) {

      strings->push_back(Format("($0 lines skipped)", n - 2 * half_limit));

    }

    index++;

  }

}

void ReplayState::DumpReplayStateToStrings(

    std::vector<std::string>* strings,

    int half_limit) const {

  strings->push_back(Format(

      "ReplayState: "

      "Previous OpId: $0, "

      "Committed OpId: $1, "

      "Pending Replicates: $2, "

      "Flushed Regular: $3, "

      "Flushed Intents: $4",

      prev_op_id,

      committed_op_id,

      pending_replicates.size(),

      stored_op_ids.regular,

      stored_op_ids.intents));

  if (num_entries_applied_to_rocksdb > 0) {

    strings->push_back(Substitute("Log entries applied to RocksDB: $0",

                                  num_entries_applied_to_rocksdb));

  }

  if (!pending_replicates.empty()) {

    strings->push_back(Substitute("Dumping REPLICATES ($0 items):", pending_replicates.size()));

    AddEntriesToStrings(pending_replicates, strings, half_limit);

  }

}

bool ReplayState::CanApply(LogEntryPB* entry) {

  return OpId::FromPB(entry->replicate().id()) <= committed_op_id;

}

OpId ReplayState::GetLowestOpIdToReplay(bool has_intents_db, const char* extra_log_prefix) const {

  const auto op_id_replay_lowest =

      has_intents_db ? 
std::min(stored_op_ids.regular, stored_op_ids.intents)422

                     : 
stored_op_ids.regular2.37k
;

  LOG_WITH_PREFIX(INFO)

      << extra_log_prefix

      << "op_id_replay_lowest=" << op_id_replay_lowest

      << " (regular_op_id=" << stored_op_ids.regular

      << ", intents_op_id=" << stored_op_ids.intents

      << ", has_intents_db=" << has_intents_db << ")";

  return op_id_replay_lowest;

}

// ================================================================================================

// Class TabletBootstrap.

// ================================================================================================

namespace {

struct ReplayDecision {

  bool should_replay = false;

  // This is true for transaction update operations that have already been applied to the regular

  // RocksDB but not to the intents RocksDB.

  AlreadyAppliedToRegularDB already_applied_to_regular_db = AlreadyAppliedToRegularDB::kFalse;

  std::string ToString() const {

    return YB_STRUCT_TO_STRING(should_replay, already_applied_to_regular_db);

  }

};

ReplayDecision ShouldReplayOperation(

    consensus::OperationType op_type,

    const int64_t index,

    const int64_t regular_flushed_index,

    const int64_t intents_flushed_index,

    TransactionStatus txn_status,

    bool write_op_has_transaction) {

  // In most cases we assume that intents_flushed_index <= regular_flushed_index but here we are

  // trying to be resilient to violations of that assumption.

  if (index <= std::min(regular_flushed_index, intents_flushed_index)) {

    // Never replay anyting that is flushed to both regular and intents RocksDBs in a transactional

    // table.

    VLOG_WITH_FUNC(3) << "index: " << index << " "

                      << "regular_flushed_index: " << regular_flushed_index

                      << " intents_flushed_index: " << intents_flushed_index;

    return {false};

  }

  if (op_type == consensus::UPDATE_TRANSACTION_OP) {

    if (txn_status == TransactionStatus::APPLYING &&

        
intents_flushed_index < index31.0k
 && 
index <= regular_flushed_index31.0k
) {

      // Intents were applied/flushed to regular RocksDB, but not flushed into the intents RocksDB.

      
VLOG_WITH_FUNC0
(3) << "index: " << index << " "

                        << "regular_flushed_index: " << regular_flushed_index

                        << " intents_flushed_index: " << intents_flushed_index;

      return {true, AlreadyAppliedToRegularDB::kTrue};

    }

    // For other types of transaction updates, we ignore them if they have been flushed to the

    // regular RocksDB.

    
VLOG_WITH_FUNC0
(3) << "index: " << index << " > "

                      << "regular_flushed_index: " << regular_flushed_index;

    return {index > regular_flushed_index};

  }

  if (op_type == consensus::WRITE_OP && 
write_op_has_transaction1.11M
) {

    // Write intents that have not been flushed into the intents DB.

    
VLOG_WITH_FUNC0
(3) << "index: " << index << " > "

                      << "intents_flushed_index: " << intents_flushed_index;

    return {index > intents_flushed_index};

  }

  VLOG_WITH_FUNC(3) << "index: " << index << " > "

                    << "regular_flushed_index: " << regular_flushed_index;

  return {index > regular_flushed_index};

}

bool WriteOpHasTransaction(const ReplicateMsg& replicate) {

  if (!replicate.has_write()) {

    return false;

  }

  const auto& write_request = replicate.write();

  if (!write_request.has_write_batch()) {

    return false;

  }

  const auto& write_batch = write_request.write_batch();

  if (write_batch.has_transaction()) {

    return true;

  }

  
for (const auto& pair : write_batch.write_pairs())1.20M
 {

    if (
!pair.key().empty()7.15M
 && pair.key()[0] == docdb::ValueTypeAsChar::kExternalTransactionId) {

      return true;

    }

  }

  return false;

}

}  // anonymous namespace

YB_STRONGLY_TYPED_BOOL(NeedsRecovery);

// Bootstraps an existing tablet by opening the metadata from disk, and rebuilding soft state by

// playing log segments. A bootstrapped tablet can then be added to an existing consensus

// configuration as a LEARNER, which will bring its state up to date with the rest of the consensus

// configuration, or it can start serving the data itself, after it has been appointed LEADER of

// that particular consensus configuration.

//

// NOTE: this does not handle pulling data from other replicas in the cluster. That is handled by

// the 'RemoteBootstrap' classes, which copy blocks and metadata locally before invoking this local

// bootstrap functionality.

//

// This class is not thread-safe.

class TabletBootstrap {

 public:

  explicit TabletBootstrap(const BootstrapTabletData& data)

      : data_(data),

        meta_(data.tablet_init_data.metadata),

        mem_tracker_(data.tablet_init_data.parent_mem_tracker),

        listener_(data.listener),

        append_pool_(data.append_pool),

        allocation_pool_(data.allocation_pool),

      skip_wal_rewrite_(FLAGS_skip_wal_rewrite) ,

        test_hooks_(data.test_hooks) {

  }

  ~TabletBootstrap() {}

  CHECKED_STATUS Bootstrap(

      TabletPtr* rebuilt_tablet,

      scoped_refptr<log::Log>* rebuilt_log,

      consensus::ConsensusBootstrapInfo* consensus_info) {

    const string tablet_id = meta_->raft_group_id();

    // Replay requires a valid Consensus metadata file to exist in order to compare the committed

    // consensus configuration seqno with the log entries and also to persist committed but

    // unpersisted changes.

    RETURN_NOT_OK_PREPEND(ConsensusMetadata::Load(meta_->fs_manager(), tablet_id,

                                                  meta_->fs_manager()->uuid(), &cmeta_),

                          "Unable to load Consensus metadata");

    // Make sure we don't try to locally bootstrap a tablet that was in the middle of a remote

    // bootstrap. It's likely that not all files were copied over successfully.

    TabletDataState tablet_data_state = meta_->tablet_data_state();

    if (!CanServeTabletData(tablet_data_state)) {

      return STATUS(Corruption, "Unable to locally bootstrap tablet " + tablet_id + ": " +

                                "RaftGroupMetadata bootstrap state is " +

                                TabletDataState_Name(tablet_data_state));

    }

    listener_->StatusMessage("Bootstrap starting.");

    if (VLOG_IS_ON(1)) {

      RaftGroupReplicaSuperBlockPB super_block;

      meta_->ToSuperBlock(&super_block);

      VLOG_WITH_PREFIX(1) << "Tablet Metadata: " << super_block.DebugString();

    }

    const bool has_blocks = 
VERIFY_RESULT150k
(OpenTablet());150k

    if (FLAGS_TEST_dump_docdb_before_tablet_bootstrap) {

      LOG_WITH_PREFIX(INFO) << "DEBUG: DocDB dump before tablet bootstrap:";

      tablet_->TEST_DocDBDumpToLog(IncludeIntents::kTrue);

    }

    const auto needs_recovery = VERIFY_RESULT(PrepareToReplay());

    if (needs_recovery && 
!skip_wal_rewrite_2.79k
) {

      RETURN_NOT_OK(OpenLogReader());

    }

    // This is a new tablet, nothing left to do.

    if (!has_blocks && 
!needs_recovery149k
) {

      LOG_WITH_PREFIX(INFO) << "No blocks or log segments found. Creating new log.";

      RETURN_NOT_OK_PREPEND(OpenNewLog(CreateNewSegment::kTrue), "Failed to open new log");

      RETURN_NOT_OK(FinishBootstrap("No bootstrap required, opened a new log",

                                    rebuilt_log,

                                    rebuilt_tablet));

      consensus_info->last_id = MinimumOpId();

      consensus_info->last_committed_id = MinimumOpId();

      return Status::OK();

    }

    // Only sleep if this isn't a new tablet, since we only want to delay on restart when testing.

    if (PREDICT_FALSE(FLAGS_TEST_tablet_bootstrap_delay_ms > 0)) {

      SleepFor(MonoDelta::FromMilliseconds(FLAGS_TEST_tablet_bootstrap_delay_ms));

    }

    // If there were blocks, there must be segments to replay. This is required by Raft, since we

    // always need to know the term and index of the last logged op in order to vote, know how to

    // respond to AppendEntries(), etc.

    if (has_blocks && 
!needs_recovery1.24k
) {

      return STATUS(IllegalState, Substitute("Tablet $0: Found rowsets but no log "

                                            "segments could be found.",

                                            tablet_id));

    }

    RETURN_NOT_OK_PREPEND(PlaySegments(consensus_info), "Failed log replay. Reason");

    if (cmeta_->current_term() < consensus_info->last_id.term()) {

      cmeta_->set_current_term(consensus_info->last_id.term());

    }

    // Flush the consensus metadata once at the end to persist our changes, if any.

    RETURN_NOT_OK(cmeta_->Flush());

    RETURN_NOT_OK(RemoveRecoveryDir());

    if (FLAGS_force_recover_flushed_frontier) {

      RETURN_NOT_OK(tablet_->Flush(FlushMode::kSync));

      docdb::ConsensusFrontier new_consensus_frontier;

      new_consensus_frontier.set_op_id(consensus_info->last_committed_id);

      new_consensus_frontier.set_hybrid_time(tablet_->mvcc_manager()->LastReplicatedHybridTime());

      // We don't attempt to recover the history cutoff here because it will be recovered

      // automatically on the first compaction, and this is a special mode for manual

      // troubleshooting.

      LOG_WITH_PREFIX(WARNING)

          << "--force_recover_flushed_frontier specified, forcefully setting "

          << "flushed frontier after bootstrap: " << new_consensus_frontier.ToString();

      RETURN_NOT_OK(tablet_->ModifyFlushedFrontier(

          new_consensus_frontier, rocksdb::FrontierModificationMode::kForce));

    }

    RETURN_NOT_OK(FinishBootstrap("Bootstrap complete.", rebuilt_log, rebuilt_tablet));

    return Status::OK();

  }

 private:

  // Finishes bootstrap, setting 'rebuilt_log' and 'rebuilt_tablet'.

  CHECKED_STATUS FinishBootstrap(

      const std::string& message,

      scoped_refptr<log::Log>* rebuilt_log,

      TabletPtr* rebuilt_tablet) {

    tablet_->MarkFinishedBootstrapping();

    listener_->StatusMessage(message);

    if (FLAGS_TEST_dump_docdb_after_tablet_bootstrap) {

      LOG_WITH_PREFIX(INFO) << "DEBUG: DocDB debug dump after tablet bootstrap:\n";

      tablet_->TEST_DocDBDumpToLog(IncludeIntents::kTrue);

    }

    *rebuilt_tablet = std::move(tablet_);

    RETURN_NOT_OK(log_->EnsureInitialNewSegmentAllocated());

    rebuilt_log->swap(log_);

    return Status::OK();

  }

  // Sets result to true if there was any data on disk for this tablet.

  Result<bool> OpenTablet() {

    CleanupSnapshots();

    auto tablet = std::make_shared<Tablet>(data_.tablet_init_data);

    // Doing nothing for now except opening a tablet locally.

    LOG_TIMING_PREFIX(INFO, LogPrefix(), "opening tablet") {

      RETURN_NOT_OK(tablet->Open());

    }

    // In theory, an error can happen in case of tablet Shutdown or in RocksDB object replacement

    // operation like RestoreSnapshot or Truncate. However, those operations can't really be

    // happening concurrently as we haven't opened the tablet yet.

    const bool has_ss_tables = VERIFY_RESULT(tablet->HasSSTables());

    tablet_ = std::move(tablet);

    return has_ss_tables;

  }

  // Checks if a previous log recovery directory exists. If so, it deletes any files in the log dir

  // and sets 'needs_recovery' to true, meaning that the previous recovery attempt should be retried

  // from the recovery dir.

  //

  // Otherwise, if there is a log directory with log files in it, renames that log dir to the log

  // recovery dir and creates a new, empty log dir so that log replay can proceed. 'needs_recovery'

  // is also returned as true in this case.

  //

  // If no log segments are found, 'needs_recovery' is set to false.

  Result<NeedsRecovery> PrepareToReplay() {

    const string& log_dir = tablet_->metadata()->wal_dir();

    // If the recovery directory exists, then we crashed mid-recovery.  Throw away any logs from the

    // previous recovery attempt and restart the log replay process from the beginning using the

    // same recovery dir as last time.

    const string recovery_path = FsManager::GetTabletWalRecoveryDir(log_dir);

    if (GetEnv()->FileExists(recovery_path)) {

      LOG_WITH_PREFIX(INFO) << "Previous recovery directory found at " << recovery_path << ": "

                            << "Replaying log files from this location instead of " << log_dir;

      // Since we have a recovery directory, clear out the log_dir by recursively deleting it and

      // creating a new one so that we don't end up with remnants of old WAL segments or indexes

      // after replay.

      if (GetEnv()->FileExists(log_dir)) {

        LOG_WITH_PREFIX(INFO) << "Deleting old log files from previous recovery attempt in "

                              << log_dir;

        RETURN_NOT_OK_PREPEND(GetEnv()->DeleteRecursively(log_dir),

                              "Could not recursively delete old log dir " + log_dir);

      }

      RETURN_NOT_OK_PREPEND(env_util::CreateDirIfMissing(GetEnv(), DirName(log_dir)),

                            "Failed to create table log directory " + DirName(log_dir));

      RETURN_NOT_OK_PREPEND(env_util::CreateDirIfMissing(GetEnv(), log_dir),

                            "Failed to create tablet log directory " + log_dir);

      return NeedsRecovery::kTrue;

    }

    // If we made it here, there was no pre-existing recovery dir.  Now we look for log files in

    // log_dir, and if we find any then we rename the whole log_dir to a recovery dir and return

    // needs_recovery = true.

    RETURN_NOT_OK_PREPEND(env_util::CreateDirIfMissing(GetEnv(), DirName(log_dir)),

                          "Failed to create table log directory " + DirName(log_dir));

    RETURN_NOT_OK_PREPEND(env_util::CreateDirIfMissing(GetEnv(), log_dir),

                          "Failed to create tablet log directory " + log_dir);

    vector<string> log_dir_children = VERIFY_RESULT_PREPEND(

        GetEnv()->GetChildren(log_dir, ExcludeDots::kTrue), "Couldn't list log segments.");

    // To ensure consistent order of log messages. Note: this does not affect the replay order

    // of segments, only the order of INFO log messages below.

    sort(log_dir_children.begin(), log_dir_children.end());

    bool needs_recovery = false;

    for (const string& log_dir_child : log_dir_children) {

      if (!log::IsLogFileName(log_dir_child)) {

        continue;

      }

      needs_recovery = true;

      string source_path = JoinPathSegments(log_dir, log_dir_child);

      if (
skip_wal_rewrite_10.3k
) {

        LOG_WITH_PREFIX(INFO) << "Will attempt to recover log segment " << source_path;

        continue;

      }

      string dest_path = JoinPathSegments(recovery_path, log_dir_child);

      LOG_WITH_PREFIX(INFO) << "Will attempt to recover log segment " << source_path

                            << " to " << dest_path;

    }

    if (!skip_wal_rewrite_ && 
needs_recovery0
) {

      // Atomically rename the log directory to the recovery directory and then re-create the log

      // directory.

      LOG_WITH_PREFIX(INFO) << "Moving log directory " << log_dir << " to recovery directory "

                            << recovery_path << " in preparation for log replay";

      RETURN_NOT_OK_PREPEND(GetEnv()->RenameFile(log_dir, recovery_path),

                            Substitute("Could not move log directory $0 to recovery dir $1",

                                      log_dir, recovery_path));

      RETURN_NOT_OK_PREPEND(GetEnv()->CreateDir(log_dir),

                            "Failed to recreate log directory " + log_dir);

    }

    return NeedsRecovery(needs_recovery);

  }

  // Opens the latest log segments for the Tablet that will allow to rebuild the tablet's soft

  // state. If there are existing log segments in the tablet's log directly they are moved to a

  // "log-recovery" directory which is deleted when the replay process is completed (as they have

  // been duplicated in the current log directory).

  //

  // If a "log-recovery" directory is already present, we will continue to replay from the

  // "log-recovery" directory. Tablet metadata is updated once replay has finished from the

  // "log-recovery" directory.

  Status OpenLogReader() {

    auto wal_dir = tablet_->metadata()->wal_dir();

    auto wal_path = skip_wal_rewrite_ ? wal_dir :

        meta_->fs_manager()->GetTabletWalRecoveryDir(wal_dir);

    VLOG_WITH_PREFIX(1) << "Opening log reader in log recovery dir " << wal_path;

    // Open the reader.

    scoped_refptr<LogIndex> index(nullptr);

    RETURN_NOT_OK_PREPEND(

        LogReader::Open(

            GetEnv(),

            index,

            LogPrefix(),

            wal_path,

            tablet_->GetTableMetricsEntity().get(),

            tablet_->GetTabletMetricsEntity().get(),

            &log_reader_),

        "Could not open LogReader. Reason");

    return Status::OK();

  }

  // Removes the recovery directory and all files contained therein.  Intended to be invoked after

  // log replay successfully completes.

  CHECKED_STATUS RemoveRecoveryDir() {

    const string recovery_path = FsManager::GetTabletWalRecoveryDir(tablet_->metadata()->wal_dir());

    if (!GetEnv()->FileExists(recovery_path)) {

      VLOG
(1) << "Tablet WAL recovery dir " << recovery_path << " does not exist."0
;

      if (!skip_wal_rewrite_) {

        return STATUS(IllegalState, "Expected recovery dir, none found.");

      }

      return Status::OK();

    }

    LOG_WITH_PREFIX(INFO) << "Preparing to delete log recovery files and directory "

                          << recovery_path;

    string tmp_path = Substitute("$0-$1", recovery_path, GetCurrentTimeMicros());

    LOG_WITH_PREFIX(INFO) << "Renaming log recovery dir from "  << recovery_path

                          << " to " << tmp_path;

    RETURN_NOT_OK_PREPEND(GetEnv()->RenameFile(recovery_path, tmp_path),

                          Substitute("Could not rename old recovery dir from: $0 to: $1",

                                    recovery_path, tmp_path));

    if (FLAGS_skip_remove_old_recovery_dir) {

      LOG_WITH_PREFIX(INFO) << "--skip_remove_old_recovery_dir enabled. NOT deleting " << tmp_path;

      return Status::OK();

    }

    LOG_WITH_PREFIX(INFO) << "Deleting all files from renamed log recovery directory " << tmp_path;

    RETURN_NOT_OK_PREPEND(GetEnv()->DeleteRecursively(tmp_path),

                          "Could not remove renamed recovery dir " + tmp_path);

    LOG_WITH_PREFIX(INFO) << "Completed deletion of old log recovery files and directory "

                          << tmp_path;

    return Status::OK();

  }

  // Opens a new log in the tablet's log directory.  The directory is expected to be clean.

  CHECKED_STATUS OpenNewLog(log::CreateNewSegment create_new_segment) {

    auto log_options = LogOptions();

    const auto& metadata = *tablet_->metadata();

    log_options.retention_secs = metadata.wal_retention_secs();

    log_options.env = GetEnv();

    if (tablet_->metadata()->table_type() == TableType::TRANSACTION_STATUS_TABLE_TYPE) {

      auto log_segment_size = FLAGS_transaction_status_tablet_log_segment_size_bytes;

      if (log_segment_size) {

        log_options.segment_size_bytes = log_segment_size;

      }

    }

    RETURN_NOT_OK(Log::Open(

        log_options,

        tablet_->tablet_id(),

        metadata.wal_dir(),

        metadata.fs_manager()->uuid(),

        *tablet_->schema(),

        metadata.schema_version(),

        tablet_->GetTableMetricsEntity(),

        tablet_->GetTabletMetricsEntity(),

        append_pool_,

        allocation_pool_,

        metadata.cdc_min_replicated_index(),

        &log_,

        create_new_segment));

    // Disable sync temporarily in order to speed up appends during the bootstrap process.

    log_->DisableSync();

    return Status::OK();

  }

  // Handle the given log entry. Validates entry.type() (it can only be REPLICATE), optionally

  // injects latency in tests, and delegates to HandleReplicateMessage.

  CHECKED_STATUS HandleEntry(

      yb::log::LogEntryMetadata entry_metadata, std::unique_ptr<log::LogEntryPB>* entry_ptr) {

    auto& entry = **entry_ptr;

    
VLOG_WITH_PREFIX21
(2) << "Handling entry: " << entry.ShortDebugString()21
;

    switch (entry.type()) {

      case log::REPLICATE:

        RETURN_NOT_OK(HandleReplicateMessage(entry_metadata, entry_ptr));

        break;

      default:

        return STATUS(Corruption, Substitute("Unexpected log entry type: $0", entry.type()));

    }

    MAYBE_FAULT(FLAGS_TEST_fault_crash_during_log_replay);

    return Status::OK();

  }

  // HandleReplicateMessage implements these important pieces of logic:

  //   - Removes the "tail" of pending_replicates overwritten by a new leader's operations when

  //     encountering an entry with an index lower than or equal to the index of an operation that

  //     is already present in pending_replicates.

  //   - Ignores entries that have already been flushed into regular and intents RocksDBs.

  //   - Updates committed OpId based on the comsmited OpId from the entry and calls

  //     ApplyCommittedPendingReplicates.

  //   - Updates the "monotonic counter" used for assigning internal keys in YCQL arrays.

  CHECKED_STATUS HandleReplicateMessage(

      LogEntryMetadata entry_metadata, std::unique_ptr<log::LogEntryPB>* replicate_entry_ptr) {

    auto& replicate_entry = **replicate_entry_ptr;

    stats_.ops_read++;

    const ReplicateMsg& replicate = replicate_entry.replicate();

    VLOG_WITH_PREFIX(1) << "HandleReplicateMessage: " << entry_metadata.ToString()

                        << ", op id: " << replicate.id()

                        << ", committed op id: " << replicate.committed_op_id();

    RETURN_NOT_OK(replay_state_->CheckSequentialReplicateId(replicate));

    SCHECK(replicate.has_hybrid_time(), Corruption, "A REPLICATE message must have a hybrid time");

    UpdateClock(replicate.hybrid_time());

    // This sets the monotonic counter to at least replicate.monotonic_counter() atomically.

    tablet_->UpdateMonotonicCounter(replicate.monotonic_counter());

    const auto op_id = OpId::FromPB(replicate_entry.replicate().id());

    // Append the replicate message to the log as is if we are not skipping wal rewrite. If we are

    // skipping, set consensus_state_only to true.

    RETURN_NOT_OK(log_->Append(replicate_entry_ptr->get(), entry_metadata, skip_wal_rewrite_));

    auto iter = replay_state_->pending_replicates.lower_bound(op_id.index);

    // If there was an entry with the same or higher index as the entry we're adding, then we need

    // to delete that entry and all entries with higher indexes.

    if (iter != replay_state_->pending_replicates.end()) {

      auto& existing_entry = iter->second;

      auto& last_entry = replay_state_->pending_replicates.rbegin()->second;

      LOG_WITH_PREFIX(INFO) << "Overwriting operations starting at: "

                            << existing_entry.entry->replicate().id()

                            << " up to: " << last_entry.entry->replicate().id()

                            << " with operation: " << replicate.id();

      stats_.ops_overwritten += std::distance(iter, replay_state_->pending_replicates.end());

      if (test_hooks_) {

        // Tell the test framework about overwritten OpIds.

        for (auto callback_iter = iter;

             callback_iter != replay_state_->pending_replicates.end();

             callback_iter++) {

          test_hooks_->Overwritten(

              yb::OpId::FromPB(callback_iter->second.entry->replicate().id()));

        }

      }

      replay_state_->pending_replicates.erase(iter, replay_state_->pending_replicates.end());

    }

    // We expect entry_metadata.entry_time to always be set for newly written WAL entries. However,

    // for some very old WALs, it might be missing.

    LOG_IF_WITH_PREFIX(DFATAL, entry_metadata.entry_time == RestartSafeCoarseTimePoint())

        << "Entry metadata must have a restart-safe time. OpId: " << OpId::FromPB(replicate.id());

    CHECK(replay_state_->pending_replicates.emplace(

        op_id.index, Entry{std::move(*replicate_entry_ptr), entry_metadata.entry_time}).second);

    CHECK(replicate.has_committed_op_id())

        << "Replicate message has no committed_op_id for table type "

        << TableType_Name(tablet_->table_type()) << ". Replicate message:\n"

        << replicate.DebugString();

    // We include the commit index as of the time a REPLICATE entry was added to the leader's log

    // into that entry. This allows us to decide when we can replay a REPLICATE entry during

    // bootstrap.

    replay_state_->UpdateCommittedOpId(OpId::FromPB(replicate.committed_op_id()));

    return ApplyCommittedPendingReplicates();

  }

  // Replays the given committed operation.

  CHECKED_STATUS PlayAnyRequest(

      ReplicateMsg* replicate, AlreadyAppliedToRegularDB already_applied_to_regular_db) {

    const auto op_type = replicate->op_type();

    if (test_hooks_) {

      test_hooks_->Replayed(yb::OpId::FromPB(replicate->id()), already_applied_to_regular_db);

    }

    switch (op_type) {

      case consensus::WRITE_OP:

        return PlayWriteRequest(replicate, already_applied_to_regular_db);

      case consensus::CHANGE_METADATA_OP:

        return PlayChangeMetadataRequest(replicate);

      case consensus::CHANGE_CONFIG_OP:

        return PlayChangeConfigRequest(replicate);

      case consensus::TRUNCATE_OP:

        return PlayTruncateRequest(replicate);

      case consensus::NO_OP:

        return Status::OK();  // This is why it is a no-op!

      case consensus::UPDATE_TRANSACTION_OP:

        return PlayUpdateTransactionRequest(replicate, already_applied_to_regular_db);

      case consensus::SNAPSHOT_OP:

        return PlayTabletSnapshotRequest(replicate);

      case consensus::HISTORY_CUTOFF_OP:

        return PlayHistoryCutoffRequest(replicate);

      case consensus::SPLIT_OP:

        return PlaySplitOpRequest(replicate);

      // Unexpected cases:

      case consensus::UNKNOWN_OP:

        return STATUS(IllegalState, Substitute("Unsupported operation type: $0", op_type));

    }

    LOG_WITH_PREFIX(DFATAL) << "Invalid operation type " << op_type

                            << "for a REPLICATE operation: " << replicate->ShortDebugString();

    return STATUS_FORMAT(Corruption, "Invalid operation type: $0", op_type);

  }

  CHECKED_STATUS PlayTabletSnapshotRequest(ReplicateMsg* replicate_msg) {

    TabletSnapshotOpRequestPB* const snapshot = replicate_msg->mutable_snapshot_request();

    SnapshotOperation operation(tablet_.get(), snapshot);

    operation.set_hybrid_time(HybridTime(replicate_msg->hybrid_time()));

    return operation.Replicated(/* leader_term= */ yb::OpId::kUnknownTerm);

  }

  CHECKED_STATUS PlayHistoryCutoffRequest(ReplicateMsg* replicate_msg) {

    HistoryCutoffOperation operation(

        tablet_.get(), replicate_msg->mutable_history_cutoff());

    return operation.Apply(/* leader_term= */ yb::OpId::kUnknownTerm);

  }

  CHECKED_STATUS PlaySplitOpRequest(ReplicateMsg* replicate_msg) {

    SplitTabletRequestPB* const split_request = replicate_msg->mutable_split_request();

    // We might be asked to replay SPLIT_OP even if it was applied and flushed when

    // FLAGS_force_recover_flushed_frontier is set.

    if (split_request->tablet_id() != tablet_->tablet_id()) {

      // Ignore SPLIT_OP designated for ancestor tablet(s).

      return Status::OK();

    }

    if (tablet_->metadata()->tablet_data_state() == TabletDataState::TABLET_DATA_SPLIT_COMPLETED) {

      // Ignore SPLIT_OP if tablet has been already split.

      
VLOG_WITH_PREFIX_AND_FUNC0
(1) << "Tablet has been already split."0
;

      return Status::OK();

    }

    SplitOperation operation(tablet_.get(), data_.tablet_init_data.tablet_splitter, split_request);

    operation.set_hybrid_time(HybridTime(replicate_msg->hybrid_time()));

    return data_.tablet_init_data.tablet_splitter->ApplyTabletSplit(&operation, log_.get());

    // TODO(tsplit): In scope of https://github.com/yugabyte/yugabyte-db/issues/1461 add integration

    // tests for:

    // - tablet bootstrap of original tablet which hasn't been yet split and replaying split

    // operation.

    // - tablet bootstrap of original tablet which has been already successfully split and replaying

    // split operation.

    // - tablet bootstrap of new after-split tablet replaying split operation.

  }

  void HandleRetryableRequest(

      const ReplicateMsg& replicate, RestartSafeCoarseTimePoint entry_time) {

    if (!replicate.has_write())

      return;

    if (data_.retryable_requests) {

      data_.retryable_requests->Bootstrap(replicate, entry_time);

    }

    // In a test, we might not have data_.retryable_requests, but we still want to tell the test

    // that we would submit this OpId to retryable_requests.

    if (test_hooks_) {

      test_hooks_->RetryableRequest(OpId::FromPB(replicate.id()));

    }

  }

  // Performs various checks based on the OpId, and decides whether to replay the given operation.