WritePrepared: commit of delayed prepared entries (#4894)

Summary:
Here is the order of ops in a commit: 1) update commit cache 2) publish seq, 3) RemovePrepared. In case of a delayed prepared, there will be a gap between when the commit is visible to snapshots until delayed_prepared_ is cleaned up. To tell apart this case from a delayed uncommitted txn from, the commit entry of a delayed prepared is also stored in delayed_prepared_commits_, which is updated before publishing the commit.
Also logic in GetSnapshotInternal that ensures that each new snapshot is always larger than max_evicted_seq_ is updated to check against the upcoming value of max_evicted_seq_ rather than its current one. This is because AdvanceMaxEvictedSeq gets the list of snapshots lower than the new max, before updating max_evicted_seq_.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4894

Differential Revision: D13726988

Pulled By: maysamyabandeh

fbshipit-source-id: 1e70d78061b50c944c9816bf4b6dac405ab4ccd3

utilities/transactions/write_prepared_transaction_test.cc

@@ -2557,6 +2557,81 @@ TEST_P(WritePreparedTransactionTest, IteratorRefreshNotSupported) {
   delete iter;
 }
 
+// When an old prepared entry gets committed, there is a gap between the time
+// that it is published and when it is cleaned up from old_prepared_. This test
+// stresses such cacese.
+TEST_P(WritePreparedTransactionTest, CommitOfOldPrepared) {
+  const size_t snapshot_cache_bits = 7;  // same as default
+  for (const size_t commit_cache_bits : {0, 2, 3}) {
+    for (const size_t sub_batch_cnt : {1, 2, 3}) {
+      DestroyAndReopenWithExtraOptions(snapshot_cache_bits, commit_cache_bits);
+      std::atomic<const Snapshot*> snap = {nullptr};
+      std::atomic<SequenceNumber> exp_prepare = {0};
+      // Value is synchronized via snap
+      PinnableSlice value;
+      // Take a snapshot after publish and before RemovePrepared:Start
+      auto callback = [&](void* param) {
+        SequenceNumber prep_seq = *((SequenceNumber*)param);
+        if (prep_seq == exp_prepare.load()) {  // only for write_thread
+          ASSERT_EQ(nullptr, snap.load());
+          snap.store(db->GetSnapshot());
+          ReadOptions roptions;
+          roptions.snapshot = snap.load();
+          auto s = db->Get(roptions, db->DefaultColumnFamily(), "key", &value);
+          ASSERT_OK(s);
+        }
+      };
+      SyncPoint::GetInstance()->SetCallBack("RemovePrepared:Start", callback);
+      SyncPoint::GetInstance()->EnableProcessing();
+      // Thread to cause frequent evictions
+      rocksdb::port::Thread eviction_thread([&]() {
+        // Too many txns might cause commit_seq - prepare_seq in another thread
+        // to go beyond DELTA_UPPERBOUND
+        for (int i = 0; i < 25 * (1 << commit_cache_bits); i++) {
+          db->Put(WriteOptions(), Slice("key1"), Slice("value1"));
+        }
+      });
+      rocksdb::port::Thread write_thread([&]() {
+        for (int i = 0; i < 25 * (1 << commit_cache_bits); i++) {
+          Transaction* txn =
+              db->BeginTransaction(WriteOptions(), TransactionOptions());
+          ASSERT_OK(txn->SetName("xid"));
+          std::string val_str = "value" + ToString(i);
+          for (size_t b = 0; b < sub_batch_cnt; b++) {
+            ASSERT_OK(txn->Put(Slice("key"), val_str));
+          }
+          ASSERT_OK(txn->Prepare());
+          // Let an eviction to kick in
+          std::this_thread::yield();
+
+          exp_prepare.store(txn->GetId());
+          ASSERT_OK(txn->Commit());
+          delete txn;
+
+          // Read with the snapshot taken before delayed_prepared_ cleanup
+          ReadOptions roptions;
+          roptions.snapshot = snap.load();
+          ASSERT_NE(nullptr, roptions.snapshot);
+          PinnableSlice value2;
+          auto s = db->Get(roptions, db->DefaultColumnFamily(), "key", &value2);
+          ASSERT_OK(s);
+          // It should see its own write
+          ASSERT_TRUE(val_str == value2);
+          // The value read by snapshot should not change
+          ASSERT_STREQ(value2.ToString().c_str(), value.ToString().c_str());
+
+          db->ReleaseSnapshot(roptions.snapshot);
+          snap.store(nullptr);
+        }
+      });
+      write_thread.join();
+      eviction_thread.join();
+    }
+    rocksdb::SyncPoint::GetInstance()->DisableProcessing();
+    rocksdb::SyncPoint::GetInstance()->ClearAllCallBacks();
+  }
+}
+
 // Test that updating the commit map will not affect the existing snapshots
 TEST_P(WritePreparedTransactionTest, AtomicCommit) {
   for (bool skip_prepare : {true, false}) {

utilities/transactions/write_prepared_txn_db.cc

@@ -394,16 +394,22 @@ void WritePreparedTxnDB::Init(const TransactionDBOptions& /* unused */) {
 }
 
 void WritePreparedTxnDB::AddPrepared(uint64_t seq) {
-  ROCKS_LOG_DETAILS(info_log_, "Txn %" PRIu64 " Prepareing", seq);
-  assert(seq > max_evicted_seq_);
-  if (seq <= max_evicted_seq_) {
-    throw std::runtime_error(
-        "Added prepare_seq is larger than max_evicted_seq_: " + ToString(seq) +
-        " <= " + ToString(max_evicted_seq_.load()));
-  }
+  ROCKS_LOG_DETAILS(info_log_, "Txn %" PRIu64 " Prepareing with max %" PRIu64,
+                    seq, max_evicted_seq_.load());
   WriteLock wl(&prepared_mutex_);
+  if (UNLIKELY(seq <= max_evicted_seq_)) {
+    // This should not happen in normal case
+    ROCKS_LOG_ERROR(
+        info_log_,
+        "Added prepare_seq is not larger than max_evicted_seq_: %" PRIu64
+        " <= %" PRIu64,
+        seq, max_evicted_seq_.load());
+    delayed_prepared_.insert(seq);
+    delayed_prepared_empty_.store(false, std::memory_order_release);
+  } else {
     prepared_txns_.push(seq);
   }
+}
 
 void WritePreparedTxnDB::AddCommitted(uint64_t prepare_seq, uint64_t commit_seq,
                                       uint8_t loop_cnt) {
@@ -443,6 +449,21 @@ void WritePreparedTxnDB::AddCommitted(uint64_t prepare_seq, uint64_t commit_seq,
     // After each eviction from commit cache, check if the commit entry should
     // be kept around because it overlaps with a live snapshot.
     CheckAgainstSnapshots(evicted);
+    if (UNLIKELY(!delayed_prepared_empty_.load(std::memory_order_acquire))) {
+      WriteLock wl(&prepared_mutex_);
+      for (auto dp : delayed_prepared_) {
+        if (dp == evicted.prep_seq) {
+          // This is a rare case that txn is committed but prepared_txns_ is not
+          // cleaned up yet. Refer to delayed_prepared_commits_ definition for
+          // why it should be kept updated.
+          delayed_prepared_commits_[evicted.prep_seq] = evicted.commit_seq;
+          ROCKS_LOG_DEBUG(info_log_,
+                          "delayed_prepared_commits_[%" PRIu64 "]=%" PRIu64,
+                          evicted.prep_seq, evicted.commit_seq);
+          break;
+        }
+      }
+    }
   }
   bool succ =
       ExchangeCommitEntry(indexed_seq, evicted_64b, {prepare_seq, commit_seq});
@@ -465,12 +486,24 @@ void WritePreparedTxnDB::AddCommitted(uint64_t prepare_seq, uint64_t commit_seq,
 
 void WritePreparedTxnDB::RemovePrepared(const uint64_t prepare_seq,
                                         const size_t batch_cnt) {
+  TEST_SYNC_POINT_CALLBACK(
+      "RemovePrepared:Start",
+      const_cast<void*>(reinterpret_cast<const void*>(&prepare_seq)));
+  ROCKS_LOG_DETAILS(info_log_,
+                    "RemovePrepared %" PRIu64 " cnt: %" ROCKSDB_PRIszt,
+                    prepare_seq, batch_cnt);
   WriteLock wl(&prepared_mutex_);
   for (size_t i = 0; i < batch_cnt; i++) {
     prepared_txns_.erase(prepare_seq + i);
     bool was_empty = delayed_prepared_.empty();
     if (!was_empty) {
       delayed_prepared_.erase(prepare_seq + i);
+      auto it = delayed_prepared_commits_.find(prepare_seq + i);
+      if (it != delayed_prepared_commits_.end()) {
+        ROCKS_LOG_DETAILS(info_log_, "delayed_prepared_commits_.erase %" PRIu64,
+                          prepare_seq + i);
+        delayed_prepared_commits_.erase(it);
+      }
       bool is_empty = delayed_prepared_.empty();
       if (was_empty != is_empty) {
         delayed_prepared_empty_.store(is_empty, std::memory_order_release);
@@ -511,8 +544,19 @@ bool WritePreparedTxnDB::ExchangeCommitEntry(const uint64_t indexed_seq,
 void WritePreparedTxnDB::AdvanceMaxEvictedSeq(const SequenceNumber& prev_max,
                                               const SequenceNumber& new_max) {
   ROCKS_LOG_DETAILS(info_log_,
-                    "AdvanceMaxEvictedSeq overhead %" PRIu64 " => %" PRIu64,
-                    prev_max, new_max);
+                    "AdvanceMaxEvictedSeq overhead %" PRIu64
+                    " => %" PRIu64 prev_max,
+                    new_max);
+  // Declare the intention before getting snapshot from the DB. This helps a
+  // concurrent GetSnapshot to wait to catch up with future_max_evicted_seq_ if
+  // it has not already. Otherwise the new snapshot is when we ask DB for
+  // snapshots smaller than future max.
+  auto updated_future_max = prev_max;
+  while (updated_future_max < new_max &&
+         !future_max_evicted_seq_.compare_exchange_weak(
+             updated_future_max, new_max, std::memory_order_acq_rel,
+             std::memory_order_relaxed)) {
+  };
   // When max_evicted_seq_ advances, move older entries from prepared_txns_
   // to delayed_prepared_. This guarantees that if a seq is lower than max,
   // then it is not in prepared_txns_ ans save an expensive, synchronized
@@ -520,6 +564,11 @@ void WritePreparedTxnDB::AdvanceMaxEvictedSeq(const SequenceNumber& prev_max,
   // normal cases.
   {
     WriteLock wl(&prepared_mutex_);
+    ROCKS_LOG_DETAILS(
+        info_log_,
+        "AdvanceMaxEvictedSeq prepared_txns_.empty() %d top: %" PRIu64,
+        prepared_txns_.empty(),
+        prepared_txns_.empty() ? 0 : prepared_txns_.top());
     while (!prepared_txns_.empty() && prepared_txns_.top() <= new_max) {
       auto to_be_popped = prepared_txns_.top();
       delayed_prepared_.insert(to_be_popped);
@@ -587,15 +636,21 @@ SnapshotImpl* WritePreparedTxnDB::GetSnapshotInternal(
   SnapshotImpl* snap_impl = db_impl_->GetSnapshotImpl(for_ww_conflict_check);
   assert(snap_impl);
   SequenceNumber snap_seq = snap_impl->GetSequenceNumber();
-  if (UNLIKELY(snap_seq != 0 && snap_seq <= max_evicted_seq_)) {
+  // Note: Check against future_max_evicted_seq_ (in contrast with
+  // max_evicted_seq_) in case there is a concurrent AdvanceMaxEvictedSeq.
+  if (UNLIKELY(snap_seq != 0 && snap_seq <= future_max_evicted_seq_)) {
     // There is a very rare case in which the commit entry evicts another commit
     // entry that is not published yet thus advancing max evicted seq beyond the
     // last published seq. This case is not likely in real-world setup so we
     // handle it with a few retries.
     size_t retry = 0;
-    while (snap_impl->GetSequenceNumber() <= max_evicted_seq_ && retry < 100) {
-      ROCKS_LOG_WARN(info_log_, "GetSnapshot retry %" PRIu64,
-                     snap_impl->GetSequenceNumber());
+    SequenceNumber max;
+    while ((max = future_max_evicted_seq_.load()) != 0 &&
+           snap_impl->GetSequenceNumber() <= max && retry < 100) {
+      ROCKS_LOG_WARN(info_log_,
+                     "GetSnapshot snap: %" PRIu64 " max: %" PRIu64
+                     " retry %" ROCKSDB_PRIszt,
+                     snap_impl->GetSequenceNumber(), max, retry);
       ReleaseSnapshot(snap_impl);
       // Wait for last visible seq to catch up with max, and also go beyond it
       // by one.
@@ -604,20 +659,19 @@ SnapshotImpl* WritePreparedTxnDB::GetSnapshotInternal(
       assert(snap_impl);
       retry++;
     }
-    assert(snap_impl->GetSequenceNumber() > max_evicted_seq_);
-    if (snap_impl->GetSequenceNumber() <= max_evicted_seq_) {
-      throw std::runtime_error("Snapshot seq " +
-                               ToString(snap_impl->GetSequenceNumber()) +
+    assert(snap_impl->GetSequenceNumber() > max);
+    if (snap_impl->GetSequenceNumber() <= max) {
+      throw std::runtime_error(
+          "Snapshot seq " + ToString(snap_impl->GetSequenceNumber()) +
           " after " + ToString(retry) +
-                               " retries is still less than max_evicted_seq_" +
-                               ToString(max_evicted_seq_.load()));
+          " retries is still less than futre_max_evicted_seq_" + ToString(max));
     }
   }
   EnhanceSnapshot(snap_impl, min_uncommitted);
   ROCKS_LOG_DETAILS(
       db_impl_->immutable_db_options().info_log,
       "GetSnapshot %" PRIu64 " ww:%" PRIi32 " min_uncommitted: %" PRIu64,
-      for_ww_conflict_check, snap_impl->GetSequenceNumber(), min_uncommitted);
+      snap_impl->GetSequenceNumber(), for_ww_conflict_check, min_uncommitted);
   return snap_impl;
 }
 

utilities/transactions/write_prepared_txn_db.h

@@ -153,21 +153,6 @@ class WritePreparedTxnDB : public PessimisticTransactionDB {
                         prep_seq, snapshot_seq, 1, min_uncommitted);
       return true;
     }
-    if (!delayed_prepared_empty_.load(std::memory_order_acquire)) {
-      // We should not normally reach here
-      WPRecordTick(TXN_PREPARE_MUTEX_OVERHEAD);
-      ReadLock rl(&prepared_mutex_);
-      ROCKS_LOG_WARN(info_log_, "prepared_mutex_ overhead %" PRIu64,
-                     static_cast<uint64_t>(delayed_prepared_.size()));
-      if (delayed_prepared_.find(prep_seq) != delayed_prepared_.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;
-      }
-    }
     auto indexed_seq = prep_seq % COMMIT_CACHE_SIZE;
     CommitEntry64b dont_care;
     CommitEntry cached;
@@ -193,6 +178,34 @@ class WritePreparedTxnDB : public PessimisticTransactionDB {
           prep_seq, snapshot_seq, 0);
       return false;
     }
+    if (!delayed_prepared_empty_.load(std::memory_order_acquire)) {
+      // We should not normally reach here
+      WPRecordTick(TXN_PREPARE_MUTEX_OVERHEAD);
+      ReadLock rl(&prepared_mutex_);
+      ROCKS_LOG_WARN(info_log_, "prepared_mutex_ overhead %" PRIu64,
+                     static_cast<uint64_t>(delayed_prepared_.size()));
+      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;
+        }
+      }
+    }
     // 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 <=
@@ -267,7 +280,7 @@ class WritePreparedTxnDB : public PessimisticTransactionDB {
   void AddPrepared(uint64_t seq);
   // 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
+  // Add the transaction with prepare sequence prepare_seq and comtit sequence
   // commit_seq to the commit map. loop_cnt is to detect infinite loops.
   void AddCommitted(uint64_t prepare_seq, uint64_t commit_seq,
                     uint8_t loop_cnt = 0);
@@ -383,6 +396,7 @@ class WritePreparedTxnDB : public PessimisticTransactionDB {
       const ColumnFamilyOptions& cf_options) override;
 
  private:
+  friend class WritePreparedCommitEntryPreReleaseCallback;
   friend class WritePreparedTransactionTest_IsInSnapshotTest_Test;
   friend class WritePreparedTransactionTest_CheckAgainstSnapshotsTest_Test;
   friend class WritePreparedTransactionTest_CommitMapTest_Test;
@@ -614,6 +628,12 @@ class WritePreparedTxnDB : public PessimisticTransactionDB {
   // 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
@@ -631,6 +651,11 @@ class WritePreparedTxnDB : public PessimisticTransactionDB {
   // 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};