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rust-rocksdb/librocksdb-sys/rocksdb/utilities/transactions/write_prepared_txn_db.h

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Squashed 'librocksdb-sys/rocksdb/' content from commit 217f76421 git-subtree-dir: librocksdb-sys/rocksdb git-subtree-split: 217f7642119aa60a9fa64bc64e02d7f78813d911
1 year ago
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#include <cinttypes>
#include <mutex>
#include <queue>
#include <set>
#include <string>
#include <unordered_map>
#include <vector>
#include "db/db_iter.h"
#include "db/pre_release_callback.h"
#include "db/read_callback.h"
#include "db/snapshot_checker.h"
#include "logging/logging.h"
#include "rocksdb/db.h"
#include "rocksdb/options.h"
#include "rocksdb/utilities/transaction_db.h"
#include "util/cast_util.h"
#include "util/set_comparator.h"
#include "util/string_util.h"
#include "utilities/transactions/pessimistic_transaction.h"
#include "utilities/transactions/pessimistic_transaction_db.h"
#include "utilities/transactions/write_prepared_txn.h"
namespace ROCKSDB_NAMESPACE {
enum SnapshotBackup : bool { kUnbackedByDBSnapshot, kBackedByDBSnapshot };
// A PessimisticTransactionDB that writes data to DB after prepare phase of 2PC.
// In this way some data in the DB might not be committed. The DB provides
// mechanisms to tell such data apart from committed data.
class WritePreparedTxnDB : public PessimisticTransactionDB {
public:
explicit WritePreparedTxnDB(DB* db,
const TransactionDBOptions& txn_db_options)
: PessimisticTransactionDB(db, txn_db_options),
SNAPSHOT_CACHE_BITS(txn_db_options.wp_snapshot_cache_bits),
SNAPSHOT_CACHE_SIZE(static_cast<size_t>(1ull << SNAPSHOT_CACHE_BITS)),
COMMIT_CACHE_BITS(txn_db_options.wp_commit_cache_bits),
COMMIT_CACHE_SIZE(static_cast<size_t>(1ull << COMMIT_CACHE_BITS)),
FORMAT(COMMIT_CACHE_BITS) {
Init(txn_db_options);
}
explicit WritePreparedTxnDB(StackableDB* db,
const TransactionDBOptions& txn_db_options)
: PessimisticTransactionDB(db, txn_db_options),
SNAPSHOT_CACHE_BITS(txn_db_options.wp_snapshot_cache_bits),
SNAPSHOT_CACHE_SIZE(static_cast<size_t>(1ull << SNAPSHOT_CACHE_BITS)),
COMMIT_CACHE_BITS(txn_db_options.wp_commit_cache_bits),
COMMIT_CACHE_SIZE(static_cast<size_t>(1ull << COMMIT_CACHE_BITS)),
FORMAT(COMMIT_CACHE_BITS) {
Init(txn_db_options);
}
virtual ~WritePreparedTxnDB();
virtual Status Initialize(
const std::vector<size_t>& compaction_enabled_cf_indices,
const std::vector<ColumnFamilyHandle*>& handles) override;
Transaction* BeginTransaction(const WriteOptions& write_options,
const TransactionOptions& txn_options,
Transaction* old_txn) override;
using TransactionDB::Write;
Status Write(const WriteOptions& opts, WriteBatch* updates) override;
// Optimized version of ::Write that receives more optimization request such
// as skip_concurrency_control.
using PessimisticTransactionDB::Write;
Status Write(const WriteOptions& opts, const TransactionDBWriteOptimizations&,
WriteBatch* updates) override;
// Write the batch to the underlying DB and mark it as committed. Could be
// used by both directly from TxnDB or through a transaction.
Status WriteInternal(const WriteOptions& write_options, WriteBatch* batch,
size_t batch_cnt, WritePreparedTxn* txn);
using DB::Get;
virtual Status Get(const ReadOptions& options,
ColumnFamilyHandle* column_family, const Slice& key,
PinnableSlice* value) override;
using DB::MultiGet;
virtual std::vector<Status> MultiGet(
const ReadOptions& options,
const std::vector<ColumnFamilyHandle*>& column_family,
const std::vector<Slice>& keys,
std::vector<std::string>* values) override;
using DB::NewIterator;
virtual Iterator* NewIterator(const ReadOptions& options,
ColumnFamilyHandle* column_family) override;
using DB::NewIterators;
virtual Status NewIterators(
const ReadOptions& options,
const std::vector<ColumnFamilyHandle*>& column_families,
std::vector<Iterator*>* iterators) override;
// Check whether the transaction that wrote the value with sequence number seq
// is visible to the snapshot with sequence number snapshot_seq.
// Returns true if commit_seq <= snapshot_seq
// If the snapshot_seq is already released and snapshot_seq <= max, sets
// *snap_released to true and returns true as well.
inline bool IsInSnapshot(uint64_t prep_seq, uint64_t snapshot_seq,
uint64_t min_uncommitted = kMinUnCommittedSeq,
bool* snap_released = nullptr) const {
ROCKS_LOG_DETAILS(info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64
" min_uncommitted %" PRIu64,
prep_seq, snapshot_seq, min_uncommitted);
assert(min_uncommitted >= kMinUnCommittedSeq);
// Caller is responsible to initialize snap_released.
assert(snap_released == nullptr || *snap_released == false);
// Here we try to infer the return value without looking into prepare list.
// This would help avoiding synchronization over a shared map.
// TODO(myabandeh): optimize this. This sequence of checks must be correct
// but not necessary efficient
if (prep_seq == 0) {
// Compaction will output keys to bottom-level with sequence number 0 if
// it is visible to the earliest snapshot.
ROCKS_LOG_DETAILS(
info_log_, "IsInSnapshot %" PRIu64 " in %" PRIu64 " returns %" PRId32,
prep_seq, snapshot_seq, 1);
return true;
}
if (snapshot_seq < prep_seq) {
// snapshot_seq < prep_seq <= commit_seq => snapshot_seq < commit_seq
ROCKS_LOG_DETAILS(
info_log_, "IsInSnapshot %" PRIu64 " in %" PRIu64 " returns %" PRId32,
prep_seq, snapshot_seq, 0);
return false;
}
if (prep_seq < min_uncommitted) {
ROCKS_LOG_DETAILS(info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64
" returns %" PRId32
" because of min_uncommitted %" PRIu64,
prep_seq, snapshot_seq, 1, min_uncommitted);
return true;
}
// Commit of delayed prepared has two non-atomic steps: add to commit cache,
// remove from delayed prepared. Our reads from these two is also
// non-atomic. By looking into commit cache first thus we might not find the
// prep_seq neither in commit cache not in delayed_prepared_. To fix that i)
// we check if there was any delayed prepared BEFORE looking into commit
// cache, ii) if there was, we complete the search steps to be these: i)
// commit cache, ii) delayed prepared, commit cache again. In this way if
// the first query to commit cache missed the commit, the 2nd will catch it.
bool was_empty;
SequenceNumber max_evicted_seq_lb, max_evicted_seq_ub;
CommitEntry64b dont_care;
auto indexed_seq = prep_seq % COMMIT_CACHE_SIZE;
size_t repeats = 0;
do {
repeats++;
assert(repeats < 100);
if (UNLIKELY(repeats >= 100)) {
throw std::runtime_error(
"The read was intrupted 100 times by update to max_evicted_seq_. "
"This is unexpected in all setups");
}
max_evicted_seq_lb = max_evicted_seq_.load(std::memory_order_acquire);
TEST_SYNC_POINT(
"WritePreparedTxnDB::IsInSnapshot:max_evicted_seq_:pause");
TEST_SYNC_POINT(
"WritePreparedTxnDB::IsInSnapshot:max_evicted_seq_:resume");
was_empty = delayed_prepared_empty_.load(std::memory_order_acquire);
TEST_SYNC_POINT(
"WritePreparedTxnDB::IsInSnapshot:delayed_prepared_empty_:pause");
TEST_SYNC_POINT(
"WritePreparedTxnDB::IsInSnapshot:delayed_prepared_empty_:resume");
CommitEntry cached;
bool exist = GetCommitEntry(indexed_seq, &dont_care, &cached);
TEST_SYNC_POINT("WritePreparedTxnDB::IsInSnapshot:GetCommitEntry:pause");
TEST_SYNC_POINT("WritePreparedTxnDB::IsInSnapshot:GetCommitEntry:resume");
if (exist && prep_seq == cached.prep_seq) {
// It is committed and also not evicted from commit cache
ROCKS_LOG_DETAILS(
info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64 " returns %" PRId32,
prep_seq, snapshot_seq, cached.commit_seq <= snapshot_seq);
return cached.commit_seq <= snapshot_seq;
}
// else it could be committed but not inserted in the map which could
// happen after recovery, or it could be committed and evicted by another
// commit, or never committed.
// At this point we don't know if it was committed or it is still prepared
max_evicted_seq_ub = max_evicted_seq_.load(std::memory_order_acquire);
if (UNLIKELY(max_evicted_seq_lb != max_evicted_seq_ub)) {
continue;
}
// Note: max_evicted_seq_ when we did GetCommitEntry <= max_evicted_seq_ub
if (max_evicted_seq_ub < prep_seq) {
// Not evicted from cache and also not present, so must be still
// prepared
ROCKS_LOG_DETAILS(info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64
" returns %" PRId32,
prep_seq, snapshot_seq, 0);
return false;
}
TEST_SYNC_POINT("WritePreparedTxnDB::IsInSnapshot:prepared_mutex_:pause");
TEST_SYNC_POINT(
"WritePreparedTxnDB::IsInSnapshot:prepared_mutex_:resume");
if (!was_empty) {
// We should not normally reach here
WPRecordTick(TXN_PREPARE_MUTEX_OVERHEAD);
ReadLock rl(&prepared_mutex_);
ROCKS_LOG_WARN(
info_log_, "prepared_mutex_ overhead %" PRIu64 " for %" PRIu64,
static_cast<uint64_t>(delayed_prepared_.size()), prep_seq);
if (delayed_prepared_.find(prep_seq) != delayed_prepared_.end()) {
// This is the order: 1) delayed_prepared_commits_ update, 2) publish
// 3) delayed_prepared_ clean up. So check if it is the case of a late
// clenaup.
auto it = delayed_prepared_commits_.find(prep_seq);
if (it == delayed_prepared_commits_.end()) {
// Then it is not committed yet
ROCKS_LOG_DETAILS(info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64
" returns %" PRId32,
prep_seq, snapshot_seq, 0);
return false;
} else {
ROCKS_LOG_DETAILS(info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64
" commit: %" PRIu64 " returns %" PRId32,
prep_seq, snapshot_seq, it->second,
snapshot_seq <= it->second);
return it->second <= snapshot_seq;
}
} else {
// 2nd query to commit cache. Refer to was_empty comment above.
exist = GetCommitEntry(indexed_seq, &dont_care, &cached);
if (exist && prep_seq == cached.prep_seq) {
ROCKS_LOG_DETAILS(
info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64 " returns %" PRId32,
prep_seq, snapshot_seq, cached.commit_seq <= snapshot_seq);
return cached.commit_seq <= snapshot_seq;
}
max_evicted_seq_ub = max_evicted_seq_.load(std::memory_order_acquire);
}
}
} while (UNLIKELY(max_evicted_seq_lb != max_evicted_seq_ub));
// When advancing max_evicted_seq_, we move older entires from prepared to
// delayed_prepared_. Also we move evicted entries from commit cache to
// old_commit_map_ if it overlaps with any snapshot. Since prep_seq <=
// max_evicted_seq_, we have three cases: i) in delayed_prepared_, ii) in
// old_commit_map_, iii) committed with no conflict with any snapshot. Case
// (i) delayed_prepared_ is checked above
if (max_evicted_seq_ub < snapshot_seq) { // then (ii) cannot be the case
// only (iii) is the case: committed
// commit_seq <= max_evicted_seq_ < snapshot_seq => commit_seq <
// snapshot_seq
ROCKS_LOG_DETAILS(
info_log_, "IsInSnapshot %" PRIu64 " in %" PRIu64 " returns %" PRId32,
prep_seq, snapshot_seq, 1);
return true;
}
// else (ii) might be the case: check the commit data saved for this
// snapshot. If there was no overlapping commit entry, then it is committed
// with a commit_seq lower than any live snapshot, including snapshot_seq.
if (old_commit_map_empty_.load(std::memory_order_acquire)) {
ROCKS_LOG_DETAILS(info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64
" returns %" PRId32 " released=1",
prep_seq, snapshot_seq, 0);
assert(snap_released);
// This snapshot is not valid anymore. We cannot tell if prep_seq is
// committed before or after the snapshot. Return true but also set
// snap_released to true.
*snap_released = true;
return true;
}
{
// We should not normally reach here unless sapshot_seq is old. This is a
// rare case and it is ok to pay the cost of mutex ReadLock for such old,
// reading transactions.
WPRecordTick(TXN_OLD_COMMIT_MAP_MUTEX_OVERHEAD);
ReadLock rl(&old_commit_map_mutex_);
auto prep_set_entry = old_commit_map_.find(snapshot_seq);
bool found = prep_set_entry != old_commit_map_.end();
if (found) {
auto& vec = prep_set_entry->second;
found = std::binary_search(vec.begin(), vec.end(), prep_seq);
} else {
// coming from compaction
ROCKS_LOG_DETAILS(info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64
" returns %" PRId32 " released=1",
prep_seq, snapshot_seq, 0);
// This snapshot is not valid anymore. We cannot tell if prep_seq is
// committed before or after the snapshot. Return true but also set
// snap_released to true.
assert(snap_released);
*snap_released = true;
return true;
}
if (!found) {
ROCKS_LOG_DETAILS(info_log_,
"IsInSnapshot %" PRIu64 " in %" PRIu64
" returns %" PRId32,
prep_seq, snapshot_seq, 1);
return true;
}
}
// (ii) it the case: it is committed but after the snapshot_seq
ROCKS_LOG_DETAILS(
info_log_, "IsInSnapshot %" PRIu64 " in %" PRIu64 " returns %" PRId32,
prep_seq, snapshot_seq, 0);
return false;
}
// Add the transaction with prepare sequence seq to the prepared list.
// Note: must be called serially with increasing seq on each call.
// locked is true if prepared_mutex_ is already locked.
void AddPrepared(uint64_t seq, bool locked = false);
// Check if any of the prepared txns are less than new max_evicted_seq_. Must
// be called with prepared_mutex_ write locked.
void CheckPreparedAgainstMax(SequenceNumber new_max, bool locked);
// Remove the transaction with prepare sequence seq from the prepared list
void RemovePrepared(const uint64_t seq, const size_t batch_cnt = 1);
// Add the transaction with prepare sequence prepare_seq and commit sequence
// commit_seq to the commit map. loop_cnt is to detect infinite loops.
// Note: must be called serially.
void AddCommitted(uint64_t prepare_seq, uint64_t commit_seq,
uint8_t loop_cnt = 0);
struct CommitEntry {
uint64_t prep_seq;
uint64_t commit_seq;
CommitEntry() : prep_seq(0), commit_seq(0) {}
CommitEntry(uint64_t ps, uint64_t cs) : prep_seq(ps), commit_seq(cs) {}
bool operator==(const CommitEntry& rhs) const {
return prep_seq == rhs.prep_seq && commit_seq == rhs.commit_seq;
}
};
struct CommitEntry64bFormat {
explicit CommitEntry64bFormat(size_t index_bits)
: INDEX_BITS(index_bits),
PREP_BITS(static_cast<size_t>(64 - PAD_BITS - INDEX_BITS)),
COMMIT_BITS(static_cast<size_t>(64 - PREP_BITS)),
COMMIT_FILTER(static_cast<uint64_t>((1ull << COMMIT_BITS) - 1)),
DELTA_UPPERBOUND(static_cast<uint64_t>((1ull << COMMIT_BITS))) {}
// Number of higher bits of a sequence number that is not used. They are
// used to encode the value type, ...
const size_t PAD_BITS = static_cast<size_t>(8);
// Number of lower bits from prepare seq that can be skipped as they are
// implied by the index of the entry in the array
const size_t INDEX_BITS;
// Number of bits we use to encode the prepare seq
const size_t PREP_BITS;
// Number of bits we use to encode the commit seq.
const size_t COMMIT_BITS;
// Filter to encode/decode commit seq
const uint64_t COMMIT_FILTER;
// The value of commit_seq - prepare_seq + 1 must be less than this bound
const uint64_t DELTA_UPPERBOUND;
};
// Prepare Seq (64 bits) = PAD ... PAD PREP PREP ... PREP INDEX INDEX ...
// INDEX Delta Seq (64 bits) = 0 0 0 0 0 0 0 0 0 0 0 0 DELTA DELTA ...
// DELTA DELTA Encoded Value = PREP PREP .... PREP PREP DELTA DELTA
// ... DELTA DELTA PAD: first bits of a seq that is reserved for tagging and
// hence ignored PREP/INDEX: the used bits in a prepare seq number INDEX: the
// bits that do not have to be encoded (will be provided externally) DELTA:
// prep seq - commit seq + 1 Number of DELTA bits should be equal to number of
// index bits + PADs
struct CommitEntry64b {
constexpr CommitEntry64b() noexcept : rep_(0) {}
CommitEntry64b(const CommitEntry& entry, const CommitEntry64bFormat& format)
: CommitEntry64b(entry.prep_seq, entry.commit_seq, format) {}
CommitEntry64b(const uint64_t ps, const uint64_t cs,
const CommitEntry64bFormat& format) {
assert(ps < static_cast<uint64_t>(
(1ull << (format.PREP_BITS + format.INDEX_BITS))));
assert(ps <= cs);
uint64_t delta = cs - ps + 1; // make initialized delta always >= 1
// zero is reserved for uninitialized entries
assert(0 < delta);
assert(delta < format.DELTA_UPPERBOUND);
if (delta >= format.DELTA_UPPERBOUND) {
throw std::runtime_error(
"commit_seq >> prepare_seq. The allowed distance is " +
std::to_string(format.DELTA_UPPERBOUND) + " commit_seq is " +
std::to_string(cs) + " prepare_seq is " + std::to_string(ps));
}
rep_ = (ps << format.PAD_BITS) & ~format.COMMIT_FILTER;
rep_ = rep_ | delta;
}
// Return false if the entry is empty
bool Parse(const uint64_t indexed_seq, CommitEntry* entry,
const CommitEntry64bFormat& format) {
uint64_t delta = rep_ & format.COMMIT_FILTER;
// zero is reserved for uninitialized entries
assert(delta < static_cast<uint64_t>((1ull << format.COMMIT_BITS)));
if (delta == 0) {
return false; // initialized entry would have non-zero delta
}
assert(indexed_seq < static_cast<uint64_t>((1ull << format.INDEX_BITS)));
uint64_t prep_up = rep_ & ~format.COMMIT_FILTER;
prep_up >>= format.PAD_BITS;
const uint64_t& prep_low = indexed_seq;
entry->prep_seq = prep_up | prep_low;
entry->commit_seq = entry->prep_seq + delta - 1;
return true;
}
private:
uint64_t rep_;
};
// Struct to hold ownership of snapshot and read callback for cleanup.
struct IteratorState;
std::shared_ptr<std::map<uint32_t, const Comparator*>> GetCFComparatorMap() {
return cf_map_;
}
std::shared_ptr<std::map<uint32_t, ColumnFamilyHandle*>> GetCFHandleMap() {
return handle_map_;
}
void UpdateCFComparatorMap(
const std::vector<ColumnFamilyHandle*>& handles) override;
void UpdateCFComparatorMap(ColumnFamilyHandle* handle) override;
virtual const Snapshot* GetSnapshot() override;
SnapshotImpl* GetSnapshotInternal(bool for_ww_conflict_check);
protected:
virtual Status VerifyCFOptions(
const ColumnFamilyOptions& cf_options) override;
// Assign the min and max sequence numbers for reading from the db. A seq >
// max is not valid, and a seq < min is valid, and a min <= seq < max requires
// further checking. Normally max is defined by the snapshot and min is by
// minimum uncommitted seq.
inline SnapshotBackup AssignMinMaxSeqs(const Snapshot* snapshot,
SequenceNumber* min,
SequenceNumber* max);
// Validate is a snapshot sequence number is still valid based on the latest
// db status. backed_by_snapshot specifies if the number is baked by an actual
// snapshot object. order specified the memory order with which we load the
// atomic variables: relax is enough for the default since we care about last
// value seen by same thread.
inline bool ValidateSnapshot(
const SequenceNumber snap_seq, const SnapshotBackup backed_by_snapshot,
std::memory_order order = std::memory_order_relaxed);
// Get a dummy snapshot that refers to kMaxSequenceNumber
Snapshot* GetMaxSnapshot() { return &dummy_max_snapshot_; }
bool ShouldRollbackWithSingleDelete(ColumnFamilyHandle* column_family,
const Slice& key) {
return rollback_deletion_type_callback_
? rollback_deletion_type_callback_(this, column_family, key)
: false;
}
std::function<bool(TransactionDB*, ColumnFamilyHandle*, const Slice&)>
rollback_deletion_type_callback_;
private:
friend class AddPreparedCallback;
friend class PreparedHeap_BasicsTest_Test;
friend class PreparedHeap_Concurrent_Test;
friend class PreparedHeap_EmptyAtTheEnd_Test;
friend class SnapshotConcurrentAccessTest_SnapshotConcurrentAccess_Test;
friend class WritePreparedCommitEntryPreReleaseCallback;
friend class WritePreparedTransactionTestBase;
friend class WritePreparedTxn;
friend class WritePreparedTxnDBMock;
friend class WritePreparedTransactionTest_AddPreparedBeforeMax_Test;
friend class WritePreparedTransactionTest_AdvanceMaxEvictedSeqBasic_Test;
friend class
WritePreparedTransactionTest_AdvanceMaxEvictedSeqWithDuplicates_Test;
friend class WritePreparedTransactionTest_AdvanceSeqByOne_Test;
friend class WritePreparedTransactionTest_BasicRecovery_Test;
friend class WritePreparedTransactionTest_CheckAgainstSnapshots_Test;
friend class WritePreparedTransactionTest_CleanupSnapshotEqualToMax_Test;
friend class WritePreparedTransactionTest_ConflictDetectionAfterRecovery_Test;
friend class WritePreparedTransactionTest_CommitMap_Test;
friend class WritePreparedTransactionTest_DoubleSnapshot_Test;
friend class WritePreparedTransactionTest_IsInSnapshotEmptyMap_Test;
friend class WritePreparedTransactionTest_IsInSnapshotReleased_Test;
friend class WritePreparedTransactionTest_IsInSnapshot_Test;
friend class WritePreparedTransactionTest_NewSnapshotLargerThanMax_Test;
friend class WritePreparedTransactionTest_MaxCatchupWithNewSnapshot_Test;
friend class WritePreparedTransactionTest_MaxCatchupWithUnbackedSnapshot_Test;
friend class
WritePreparedTransactionTest_NonAtomicCommitOfDelayedPrepared_Test;
friend class
WritePreparedTransactionTest_NonAtomicUpdateOfDelayedPrepared_Test;
friend class WritePreparedTransactionTest_NonAtomicUpdateOfMaxEvictedSeq_Test;
friend class WritePreparedTransactionTest_OldCommitMapGC_Test;
friend class WritePreparedTransactionTest_Rollback_Test;
friend class WritePreparedTransactionTest_SmallestUnCommittedSeq_Test;
friend class WriteUnpreparedTxn;
friend class WriteUnpreparedTxnDB;
friend class WriteUnpreparedTransactionTest_RecoveryTest_Test;
friend class MultiOpsTxnsStressTest;
void Init(const TransactionDBOptions& txn_db_opts);
void WPRecordTick(uint32_t ticker_type) const {
RecordTick(db_impl_->immutable_db_options_.statistics.get(), ticker_type);
}
// A heap with the amortized O(1) complexity for erase. It uses one extra heap
// to keep track of erased entries that are not yet on top of the main heap.
class PreparedHeap {
// The mutex is required for push and pop from PreparedHeap. ::erase will
// use external synchronization via prepared_mutex_.
port::Mutex push_pop_mutex_;
std::deque<uint64_t> heap_;
std::priority_queue<uint64_t, std::vector<uint64_t>, std::greater<uint64_t>>
erased_heap_;
std::atomic<uint64_t> heap_top_ = {kMaxSequenceNumber};
// True when testing crash recovery
bool TEST_CRASH_ = false;
friend class WritePreparedTxnDB;
public:
~PreparedHeap() {
if (!TEST_CRASH_) {
assert(heap_.empty());
assert(erased_heap_.empty());
}
}
port::Mutex* push_pop_mutex() { return &push_pop_mutex_; }
inline bool empty() { return top() == kMaxSequenceNumber; }
// Returns kMaxSequenceNumber if empty() and the smallest otherwise.
inline uint64_t top() { return heap_top_.load(std::memory_order_acquire); }
inline void push(uint64_t v) {
push_pop_mutex_.AssertHeld();
if (heap_.empty()) {
heap_top_.store(v, std::memory_order_release);
} else {
assert(heap_top_.load() < v);
}
heap_.push_back(v);
}
void pop(bool locked = false) {
if (!locked) {
push_pop_mutex()->Lock();
}
push_pop_mutex_.AssertHeld();
heap_.pop_front();
while (!heap_.empty() && !erased_heap_.empty() &&
// heap_.top() > erased_heap_.top() could happen if we have erased
// a non-existent entry. Ideally the user should not do that but we
// should be resilient against it.
heap_.front() >= erased_heap_.top()) {
if (heap_.front() == erased_heap_.top()) {
heap_.pop_front();
}
uint64_t erased __attribute__((__unused__));
erased = erased_heap_.top();
erased_heap_.pop();
// No duplicate prepare sequence numbers
assert(erased_heap_.empty() || erased_heap_.top() != erased);
}
while (heap_.empty() && !erased_heap_.empty()) {
erased_heap_.pop();
}
heap_top_.store(!heap_.empty() ? heap_.front() : kMaxSequenceNumber,
std::memory_order_release);
if (!locked) {
push_pop_mutex()->Unlock();
}
}
// Concurrrent calls needs external synchronization. It is safe to be called
// concurrent to push and pop though.
void erase(uint64_t seq) {
if (!empty()) {
auto top_seq = top();
if (seq < top_seq) {
// Already popped, ignore it.
} else if (top_seq == seq) {
pop();
#ifndef NDEBUG
MutexLock ml(push_pop_mutex());
assert(heap_.empty() || heap_.front() != seq);
#endif
} else { // top() > seq
// Down the heap, remember to pop it later
erased_heap_.push(seq);
}
}
}
};
void TEST_Crash() override { prepared_txns_.TEST_CRASH_ = true; }
// Get the commit entry with index indexed_seq from the commit table. It
// returns true if such entry exists.
bool GetCommitEntry(const uint64_t indexed_seq, CommitEntry64b* entry_64b,
CommitEntry* entry) const;
// Rewrite the entry with the index indexed_seq in the commit table with the
// commit entry <prep_seq, commit_seq>. If the rewrite results into eviction,
// sets the evicted_entry and returns true.
bool AddCommitEntry(const uint64_t indexed_seq, const CommitEntry& new_entry,
CommitEntry* evicted_entry);
// Rewrite the entry with the index indexed_seq in the commit table with the
// commit entry new_entry only if the existing entry matches the
// expected_entry. Returns false otherwise.
bool ExchangeCommitEntry(const uint64_t indexed_seq,
CommitEntry64b& expected_entry,
const CommitEntry& new_entry);
// Increase max_evicted_seq_ from the previous value prev_max to the new
// value. This also involves taking care of prepared txns that are not
// committed before new_max, as well as updating the list of live snapshots at
// the time of updating the max. Thread-safety: this function can be called
// concurrently. The concurrent invocations of this function is equivalent to
// a serial invocation in which the last invocation is the one with the
// largest new_max value.
void AdvanceMaxEvictedSeq(const SequenceNumber& prev_max,
const SequenceNumber& new_max);
inline SequenceNumber SmallestUnCommittedSeq() {
// Note: We have two lists to look into, but for performance reasons they
// are not read atomically. Since CheckPreparedAgainstMax copies the entry
// to delayed_prepared_ before removing it from prepared_txns_, to ensure
// that a prepared entry will not go unmissed, we look into them in opposite
// order: first read prepared_txns_ and then delayed_prepared_.
// This must be called before calling ::top. This is because the concurrent
// thread would call ::RemovePrepared before updating
// GetLatestSequenceNumber(). Reading then in opposite order here guarantees
// that the ::top that we read would be lower the ::top if we had otherwise
// update/read them atomically.
auto next_prepare = db_impl_->GetLatestSequenceNumber() + 1;
auto min_prepare = prepared_txns_.top();
// Since we update the prepare_heap always from the main write queue via
// PreReleaseCallback, the prepared_txns_.top() indicates the smallest
// prepared data in 2pc transactions. For non-2pc transactions that are
// written in two steps, we also update prepared_txns_ at the first step
// (via the same mechanism) so that their uncommitted data is reflected in
// SmallestUnCommittedSeq.
if (!delayed_prepared_empty_.load()) {
ReadLock rl(&prepared_mutex_);
if (!delayed_prepared_.empty()) {
return *delayed_prepared_.begin();
}
}
bool empty = min_prepare == kMaxSequenceNumber;
if (empty) {
// Since GetLatestSequenceNumber is updated
// after prepared_txns_ are, the value of GetLatestSequenceNumber would
// reflect any uncommitted data that is not added to prepared_txns_ yet.
// Otherwise, if there is no concurrent txn, this value simply reflects
// that latest value in the memtable.
return next_prepare;
} else {
return std::min(min_prepare, next_prepare);
}
}
// Enhance the snapshot object by recording in it the smallest uncommitted seq
inline void EnhanceSnapshot(SnapshotImpl* snapshot,
SequenceNumber min_uncommitted) {
assert(snapshot);
assert(min_uncommitted <= snapshot->number_ + 1);
snapshot->min_uncommitted_ = min_uncommitted;
}
virtual const std::vector<SequenceNumber> GetSnapshotListFromDB(
SequenceNumber max);
// Will be called by the public ReleaseSnapshot method. Does the maintenance
// internal to WritePreparedTxnDB
void ReleaseSnapshotInternal(const SequenceNumber snap_seq);
// Update the list of snapshots corresponding to the soon-to-be-updated
// max_evicted_seq_. Thread-safety: this function can be called concurrently.
// The concurrent invocations of this function is equivalent to a serial
// invocation in which the last invocation is the one with the largest
// version value.
void UpdateSnapshots(const std::vector<SequenceNumber>& snapshots,
const SequenceNumber& version);
// Check the new list of new snapshots against the old one to see if any of
// the snapshots are released and to do the cleanup for the released snapshot.
void CleanupReleasedSnapshots(
const std::vector<SequenceNumber>& new_snapshots,
const std::vector<SequenceNumber>& old_snapshots);
// Check an evicted entry against live snapshots to see if it should be kept
// around or it can be safely discarded (and hence assume committed for all
// snapshots). Thread-safety: this function can be called concurrently. If it
// is called concurrently with multiple UpdateSnapshots, the result is the
// same as checking the intersection of the snapshot list before updates with
// the snapshot list of all the concurrent updates.
void CheckAgainstSnapshots(const CommitEntry& evicted);
// Add a new entry to old_commit_map_ if prep_seq <= snapshot_seq <
// commit_seq. Return false if checking the next snapshot(s) is not needed.
// This is the case if none of the next snapshots could satisfy the condition.
// next_is_larger: the next snapshot will be a larger value
bool MaybeUpdateOldCommitMap(const uint64_t& prep_seq,
const uint64_t& commit_seq,
const uint64_t& snapshot_seq,
const bool next_is_larger);
// A trick to increase the last visible sequence number by one and also wait
// for the in-flight commits to be visible.
void AdvanceSeqByOne();
// The list of live snapshots at the last time that max_evicted_seq_ advanced.
// The list stored into two data structures: in snapshot_cache_ that is
// efficient for concurrent reads, and in snapshots_ if the data does not fit
// into snapshot_cache_. The total number of snapshots in the two lists
std::atomic<size_t> snapshots_total_ = {};
// The list sorted in ascending order. Thread-safety for writes is provided
// with snapshots_mutex_ and concurrent reads are safe due to std::atomic for
// each entry. In x86_64 architecture such reads are compiled to simple read
// instructions.
const size_t SNAPSHOT_CACHE_BITS;
const size_t SNAPSHOT_CACHE_SIZE;
std::unique_ptr<std::atomic<SequenceNumber>[]> snapshot_cache_;
// 2nd list for storing snapshots. The list sorted in ascending order.
// Thread-safety is provided with snapshots_mutex_.
std::vector<SequenceNumber> snapshots_;
// The list of all snapshots: snapshots_ + snapshot_cache_. This list although
// redundant but simplifies CleanupOldSnapshots implementation.
// Thread-safety is provided with snapshots_mutex_.
std::vector<SequenceNumber> snapshots_all_;
// The version of the latest list of snapshots. This can be used to avoid
// rewriting a list that is concurrently updated with a more recent version.
SequenceNumber snapshots_version_ = 0;
// A heap of prepared transactions. Thread-safety is provided with
// prepared_mutex_.
PreparedHeap prepared_txns_;
const size_t COMMIT_CACHE_BITS;
const size_t COMMIT_CACHE_SIZE;
const CommitEntry64bFormat FORMAT;
// commit_cache_ must be initialized to zero to tell apart an empty index from
// a filled one. Thread-safety is provided with commit_cache_mutex_.
std::unique_ptr<std::atomic<CommitEntry64b>[]> commit_cache_;
// The largest evicted *commit* sequence number from the commit_cache_. If a
// seq is smaller than max_evicted_seq_ is might or might not be present in
// commit_cache_. So commit_cache_ must first be checked before consulting
// with max_evicted_seq_.
std::atomic<uint64_t> max_evicted_seq_ = {};
// Order: 1) update future_max_evicted_seq_ = new_max, 2)
// GetSnapshotListFromDB(new_max), max_evicted_seq_ = new_max. Since
// GetSnapshotInternal guarantess that the snapshot seq is larger than
// future_max_evicted_seq_, this guarantes that if a snapshot is not larger
// than max has already being looked at via a GetSnapshotListFromDB(new_max).
std::atomic<uint64_t> future_max_evicted_seq_ = {};
// Advance max_evicted_seq_ by this value each time it needs an update. The
// larger the value, the less frequent advances we would have. We do not want
// it to be too large either as it would cause stalls by doing too much
// maintenance work under the lock.
size_t INC_STEP_FOR_MAX_EVICTED = 1;
// A map from old snapshots (expected to be used by a few read-only txns) to
// prepared sequence number of the evicted entries from commit_cache_ that
// overlaps with such snapshot. These are the prepared sequence numbers that
// the snapshot, to which they are mapped, cannot assume to be committed just
// because it is no longer in the commit_cache_. The vector must be sorted
// after each update.
// Thread-safety is provided with old_commit_map_mutex_.
std::map<SequenceNumber, std::vector<SequenceNumber>> old_commit_map_;
// A set of long-running prepared transactions that are not finished by the
// time max_evicted_seq_ advances their sequence number. This is expected to
// be empty normally. Thread-safety is provided with prepared_mutex_.
std::set<uint64_t> delayed_prepared_;
// Commit of a delayed prepared: 1) update commit cache, 2) update
// delayed_prepared_commits_, 3) publish seq, 3) clean up delayed_prepared_.
// delayed_prepared_commits_ will help us tell apart the unprepared txns from
// the ones that are committed but not cleaned up yet.
std::unordered_map<SequenceNumber, SequenceNumber> delayed_prepared_commits_;
// Update when delayed_prepared_.empty() changes. Expected to be true
// normally.
std::atomic<bool> delayed_prepared_empty_ = {true};
// Update when old_commit_map_.empty() changes. Expected to be true normally.
std::atomic<bool> old_commit_map_empty_ = {true};
mutable port::RWMutex prepared_mutex_;
mutable port::RWMutex old_commit_map_mutex_;
mutable port::RWMutex commit_cache_mutex_;
mutable port::RWMutex snapshots_mutex_;
// A cache of the cf comparators
// Thread safety: since it is a const it is safe to read it concurrently
std::shared_ptr<std::map<uint32_t, const Comparator*>> cf_map_;
// A cache of the cf handles
// Thread safety: since the handle is read-only object it is a const it is
// safe to read it concurrently
std::shared_ptr<std::map<uint32_t, ColumnFamilyHandle*>> handle_map_;
// A dummy snapshot object that refers to kMaxSequenceNumber
SnapshotImpl dummy_max_snapshot_;
};
class WritePreparedTxnReadCallback : public ReadCallback {
public:
WritePreparedTxnReadCallback(WritePreparedTxnDB* db, SequenceNumber snapshot)
: ReadCallback(snapshot),
db_(db),
backed_by_snapshot_(kBackedByDBSnapshot) {}
WritePreparedTxnReadCallback(WritePreparedTxnDB* db, SequenceNumber snapshot,
SequenceNumber min_uncommitted,
SnapshotBackup backed_by_snapshot)
: ReadCallback(snapshot, min_uncommitted),
db_(db),
backed_by_snapshot_(backed_by_snapshot) {
(void)backed_by_snapshot_; // to silence unused private field warning
}
virtual ~WritePreparedTxnReadCallback() {
// If it is not backed by snapshot, the caller must check validity
assert(valid_checked_ || backed_by_snapshot_ == kBackedByDBSnapshot);
}
// Will be called to see if the seq number visible; if not it moves on to
// the next seq number.
inline virtual bool IsVisibleFullCheck(SequenceNumber seq) override {
auto snapshot = max_visible_seq_;
bool snap_released = false;
auto ret =
db_->IsInSnapshot(seq, snapshot, min_uncommitted_, &snap_released);
assert(!snap_released || backed_by_snapshot_ == kUnbackedByDBSnapshot);
snap_released_ |= snap_released;
return ret;
}
inline bool valid() {
valid_checked_ = true;
return snap_released_ == false;
}
// TODO(myabandeh): override Refresh when Iterator::Refresh is supported
private:
WritePreparedTxnDB* db_;
// Whether max_visible_seq_ is backed by a snapshot
const SnapshotBackup backed_by_snapshot_;
bool snap_released_ = false;
// Safety check to ensure that the caller has checked invalid statuses
bool valid_checked_ = false;
};
class AddPreparedCallback : public PreReleaseCallback {
public:
AddPreparedCallback(WritePreparedTxnDB* db, DBImpl* db_impl,
size_t sub_batch_cnt, bool two_write_queues,
bool first_prepare_batch)
: db_(db),
db_impl_(db_impl),
sub_batch_cnt_(sub_batch_cnt),
two_write_queues_(two_write_queues),
first_prepare_batch_(first_prepare_batch) {
(void)two_write_queues_; // to silence unused private field warning
}
virtual Status Callback(SequenceNumber prepare_seq,
bool is_mem_disabled __attribute__((__unused__)),
uint64_t log_number, size_t index,
size_t total) override {
assert(index < total);
// To reduce the cost of lock acquisition competing with the concurrent
// prepare requests, lock on the first callback and unlock on the last.
const bool do_lock = !two_write_queues_ || index == 0;
const bool do_unlock = !two_write_queues_ || index + 1 == total;
// Always Prepare from the main queue
assert(!two_write_queues_ || !is_mem_disabled); // implies the 1st queue
TEST_SYNC_POINT("AddPreparedCallback::AddPrepared::begin:pause");
TEST_SYNC_POINT("AddPreparedCallback::AddPrepared::begin:resume");
if (do_lock) {
db_->prepared_txns_.push_pop_mutex()->Lock();
}
const bool kLocked = true;
for (size_t i = 0; i < sub_batch_cnt_; i++) {
db_->AddPrepared(prepare_seq + i, kLocked);
}
if (do_unlock) {
db_->prepared_txns_.push_pop_mutex()->Unlock();
}
TEST_SYNC_POINT("AddPreparedCallback::AddPrepared::end");
if (first_prepare_batch_) {
assert(log_number != 0);
db_impl_->logs_with_prep_tracker()->MarkLogAsContainingPrepSection(
log_number);
}
return Status::OK();
}
private:
WritePreparedTxnDB* db_;
DBImpl* db_impl_;
size_t sub_batch_cnt_;
bool two_write_queues_;
// It is 2PC and this is the first prepare batch. Always the case in 2PC
// unless it is WriteUnPrepared.
bool first_prepare_batch_;
};
class WritePreparedCommitEntryPreReleaseCallback : public PreReleaseCallback {
public:
// includes_data indicates that the commit also writes non-empty
// CommitTimeWriteBatch to memtable, which needs to be committed separately.
WritePreparedCommitEntryPreReleaseCallback(
WritePreparedTxnDB* db, DBImpl* db_impl, SequenceNumber prep_seq,
size_t prep_batch_cnt, size_t data_batch_cnt = 0,
SequenceNumber aux_seq = kMaxSequenceNumber, size_t aux_batch_cnt = 0)
: db_(db),
db_impl_(db_impl),
prep_seq_(prep_seq),
prep_batch_cnt_(prep_batch_cnt),
data_batch_cnt_(data_batch_cnt),
includes_data_(data_batch_cnt_ > 0),
aux_seq_(aux_seq),
aux_batch_cnt_(aux_batch_cnt),
includes_aux_batch_(aux_batch_cnt > 0) {
assert((prep_batch_cnt_ > 0) != (prep_seq == kMaxSequenceNumber)); // xor
assert(prep_batch_cnt_ > 0 || data_batch_cnt_ > 0);
assert((aux_batch_cnt_ > 0) != (aux_seq == kMaxSequenceNumber)); // xor
}
virtual Status Callback(SequenceNumber commit_seq,
bool is_mem_disabled __attribute__((__unused__)),
uint64_t, size_t /*index*/,
size_t /*total*/) override {
// Always commit from the 2nd queue
assert(!db_impl_->immutable_db_options().two_write_queues ||
is_mem_disabled);
assert(includes_data_ || prep_seq_ != kMaxSequenceNumber);
// Data batch is what accompanied with the commit marker and affects the
// last seq in the commit batch.
const uint64_t last_commit_seq = LIKELY(data_batch_cnt_ <= 1)
? commit_seq
: commit_seq + data_batch_cnt_ - 1;
if (prep_seq_ != kMaxSequenceNumber) {
for (size_t i = 0; i < prep_batch_cnt_; i++) {
db_->AddCommitted(prep_seq_ + i, last_commit_seq);
}
} // else there was no prepare phase
if (includes_aux_batch_) {
for (size_t i = 0; i < aux_batch_cnt_; i++) {
db_->AddCommitted(aux_seq_ + i, last_commit_seq);
}
}
if (includes_data_) {
assert(data_batch_cnt_);
// Commit the data that is accompanied with the commit request
for (size_t i = 0; i < data_batch_cnt_; i++) {
// For commit seq of each batch use the commit seq of the last batch.
// This would make debugging easier by having all the batches having
// the same sequence number.
db_->AddCommitted(commit_seq + i, last_commit_seq);
}
}
if (db_impl_->immutable_db_options().two_write_queues) {
assert(is_mem_disabled); // implies the 2nd queue
// Publish the sequence number. We can do that here assuming the callback
// is invoked only from one write queue, which would guarantee that the
// publish sequence numbers will be in order, i.e., once a seq is
// published all the seq prior to that are also publishable.
db_impl_->SetLastPublishedSequence(last_commit_seq);
// Note RemovePrepared should be called after publishing the seq.
// Otherwise SmallestUnCommittedSeq optimization breaks.
if (prep_seq_ != kMaxSequenceNumber) {
db_->RemovePrepared(prep_seq_, prep_batch_cnt_);
} // else there was no prepare phase
if (includes_aux_batch_) {
db_->RemovePrepared(aux_seq_, aux_batch_cnt_);
}
}
// else SequenceNumber that is updated as part of the write already does the
// publishing
return Status::OK();
}
private:
WritePreparedTxnDB* db_;
DBImpl* db_impl_;
// kMaxSequenceNumber if there was no prepare phase
SequenceNumber prep_seq_;
size_t prep_batch_cnt_;
size_t data_batch_cnt_;
// Data here is the batch that is written with the commit marker, either
// because it is commit without prepare or commit has a CommitTimeWriteBatch.
bool includes_data_;
// Auxiliary batch (if there is any) is a batch that is written before, but
// gets the same commit seq as prepare batch or data batch. This is used in
// two write queues where the CommitTimeWriteBatch becomes the aux batch and
// we do a separate write to actually commit everything.
SequenceNumber aux_seq_;
size_t aux_batch_cnt_;
bool includes_aux_batch_;
};
// For two_write_queues commit both the aborted batch and the cleanup batch and
// then published the seq
class WritePreparedRollbackPreReleaseCallback : public PreReleaseCallback {
public:
WritePreparedRollbackPreReleaseCallback(WritePreparedTxnDB* db,
DBImpl* db_impl,
SequenceNumber prep_seq,
SequenceNumber rollback_seq,
size_t prep_batch_cnt)
: db_(db),
db_impl_(db_impl),
prep_seq_(prep_seq),
rollback_seq_(rollback_seq),
prep_batch_cnt_(prep_batch_cnt) {
assert(prep_seq != kMaxSequenceNumber);
assert(rollback_seq != kMaxSequenceNumber);
assert(prep_batch_cnt_ > 0);
}
Status Callback(SequenceNumber commit_seq, bool is_mem_disabled, uint64_t,
size_t /*index*/, size_t /*total*/) override {
// Always commit from the 2nd queue
assert(is_mem_disabled); // implies the 2nd queue
assert(db_impl_->immutable_db_options().two_write_queues);
#ifdef NDEBUG
(void)is_mem_disabled;
#endif
const uint64_t last_commit_seq = commit_seq;
db_->AddCommitted(rollback_seq_, last_commit_seq);
for (size_t i = 0; i < prep_batch_cnt_; i++) {
db_->AddCommitted(prep_seq_ + i, last_commit_seq);
}
db_impl_->SetLastPublishedSequence(last_commit_seq);
return Status::OK();
}
private:
WritePreparedTxnDB* db_;
DBImpl* db_impl_;
SequenceNumber prep_seq_;
SequenceNumber rollback_seq_;
size_t prep_batch_cnt_;
};
// Count the number of sub-batches inside a batch. A sub-batch does not have
// duplicate keys.
struct SubBatchCounter : public WriteBatch::Handler {
explicit SubBatchCounter(std::map<uint32_t, const Comparator*>& comparators)
: comparators_(comparators), batches_(1) {}
std::map<uint32_t, const Comparator*>& comparators_;
using CFKeys = std::set<Slice, SetComparator>;
std::map<uint32_t, CFKeys> keys_;
size_t batches_;
size_t BatchCount() { return batches_; }
void AddKey(const uint32_t cf, const Slice& key);
void InitWithComp(const uint32_t cf);
Status MarkNoop(bool) override { return Status::OK(); }
Status MarkEndPrepare(const Slice&) override { return Status::OK(); }
Status MarkCommit(const Slice&) override { return Status::OK(); }
Status PutCF(uint32_t cf, const Slice& key, const Slice&) override {
AddKey(cf, key);
return Status::OK();
}
Status DeleteCF(uint32_t cf, const Slice& key) override {
AddKey(cf, key);
return Status::OK();
}
Status SingleDeleteCF(uint32_t cf, const Slice& key) override {
AddKey(cf, key);
return Status::OK();
}
Status MergeCF(uint32_t cf, const Slice& key, const Slice&) override {
AddKey(cf, key);
return Status::OK();
}
Status MarkBeginPrepare(bool) override { return Status::OK(); }
Status MarkRollback(const Slice&) override { return Status::OK(); }
Handler::OptionState WriteAfterCommit() const override {
return Handler::OptionState::kDisabled;
}
};
SnapshotBackup WritePreparedTxnDB::AssignMinMaxSeqs(const Snapshot* snapshot,
SequenceNumber* min,
SequenceNumber* max) {
if (snapshot != nullptr) {
*min =
static_cast_with_check<const SnapshotImpl>(snapshot)->min_uncommitted_;
*max = static_cast_with_check<const SnapshotImpl>(snapshot)->number_;
// A duplicate of the check in EnhanceSnapshot().
assert(*min <= *max + 1);
return kBackedByDBSnapshot;
} else {
*min = SmallestUnCommittedSeq();
*max = 0; // to be assigned later after sv is referenced.
return kUnbackedByDBSnapshot;
}
}
bool WritePreparedTxnDB::ValidateSnapshot(
const SequenceNumber snap_seq, const SnapshotBackup backed_by_snapshot,
std::memory_order order) {
if (backed_by_snapshot == kBackedByDBSnapshot) {
return true;
} else {
SequenceNumber max = max_evicted_seq_.load(order);
// Validate that max has not advanced the snapshot seq that is not backed
// by a real snapshot. This is a very rare case that should not happen in
// real workloads.
if (UNLIKELY(snap_seq <= max && snap_seq != 0)) {
return false;
}
}
return true;
}
} // namespace ROCKSDB_NAMESPACE