WritePrepared Txn: ValidateSnapshot

Summary:
Implements ValidateSnapshot for WritePrepared txns and also adds a unit test to clarify the contract of this function.
Closes https://github.com/facebook/rocksdb/pull/3101

Differential Revision: D6199405

Pulled By: maysamyabandeh

fbshipit-source-id: ace509934c307ea5d26f4bbac5f836d7c80fd240

db/db_impl.cc

@@ -1644,8 +1644,8 @@ void DBImpl::ReleaseSnapshot(const Snapshot* s) {
     uint64_t oldest_snapshot;
     if (snapshots_.empty()) {
       oldest_snapshot = concurrent_prepare_ && seq_per_batch_
-                              ? versions_->LastToBeWrittenSequence()
-                              : versions_->LastSequence();
+                            ? versions_->LastToBeWrittenSequence()
+                            : versions_->LastSequence();
     } else {
       oldest_snapshot = snapshots_.oldest()->number_;
     }

utilities/transactions/pessimistic_transaction.h

@@ -145,6 +145,7 @@ class PessimisticTransaction : public TransactionBaseImpl {
   uint64_t expiration_time_;
 
  private:
+  friend class TransactionTest_ValidateSnapshotTest_Test;
   // Used to create unique ids for transactions.
   static std::atomic<TransactionID> txn_id_counter_;
 
@@ -179,8 +180,9 @@ class PessimisticTransaction : public TransactionBaseImpl {
   // Whether to perform deadlock detection or not.
   int64_t deadlock_detect_depth_;
 
-  Status ValidateSnapshot(ColumnFamilyHandle* column_family, const Slice& key,
-                          SequenceNumber prev_seqno, SequenceNumber* new_seqno);
+  virtual Status ValidateSnapshot(ColumnFamilyHandle* column_family,
+                                  const Slice& key, SequenceNumber prev_seqno,
+                                  SequenceNumber* new_seqno);
 
   void UnlockGetForUpdate(ColumnFamilyHandle* column_family,
                           const Slice& key) override;
@@ -208,9 +210,6 @@ class WriteCommittedTxn : public PessimisticTransaction {
 
   Status RollbackInternal() override;
 
-  Status ValidateSnapshot(ColumnFamilyHandle* column_family, const Slice& key,
-                          SequenceNumber prev_seqno, SequenceNumber* new_seqno);
-
   // No copying allowed
   WriteCommittedTxn(const WriteCommittedTxn&);
   void operator=(const WriteCommittedTxn&);

utilities/transactions/transaction_test.cc

@@ -110,6 +110,47 @@ TEST_P(TransactionTest, SuccessTest) {
   delete txn;
 }
 
+// This test clarifies the contract of ValidateSnapshot
+TEST_P(TransactionTest, ValidateSnapshotTest) {
+  for (bool with_2pc : {true, false}) {
+    ReOpen();
+    WriteOptions write_options;
+    ReadOptions read_options;
+    string value;
+    Status s;
+
+    Transaction* txn1 =
+        db->BeginTransaction(write_options, TransactionOptions());
+    ASSERT_TRUE(txn1);
+    s = txn1->Put(Slice("foo"), Slice("bar1"));
+    ASSERT_OK(s);
+    if (with_2pc) {
+      s = txn1->SetName("xid1");
+      ASSERT_OK(s);
+      s = txn1->Prepare();
+      ASSERT_OK(s);
+    }
+
+    Transaction* txn2 =
+        db->BeginTransaction(write_options, TransactionOptions());
+    ASSERT_TRUE(txn2);
+    txn2->SetSnapshot();
+
+    s = txn1->Commit();
+    ASSERT_OK(s);
+    delete txn1;
+
+    SequenceNumber dont_care;
+    auto pes_txn2 = dynamic_cast<PessimisticTransaction*>(txn2);
+    // Test the simple case where the key is not tracked yet
+    auto trakced_seq = kMaxSequenceNumber;
+    s = pes_txn2->ValidateSnapshot(db->DefaultColumnFamily(), "foo",
+                                   trakced_seq, &dont_care);
+    ASSERT_TRUE(s.IsBusy());
+    delete txn2;
+  }
+}
+
 TEST_P(TransactionTest, WaitingTxn) {
   WriteOptions write_options;
   ReadOptions read_options;

utilities/transactions/transaction_util.cc

@@ -22,11 +22,9 @@
 
 namespace rocksdb {
 
-Status TransactionUtil::CheckKeyForConflicts(DBImpl* db_impl,
-                                             ColumnFamilyHandle* column_family,
-                                             const std::string& key,
-                                             SequenceNumber key_seq,
-                                             bool cache_only) {
+Status TransactionUtil::CheckKeyForConflicts(
+    DBImpl* db_impl, ColumnFamilyHandle* column_family, const std::string& key,
+    SequenceNumber snap_seq, bool cache_only, ReadCallback* snap_checker) {
   Status result;
 
   auto cfh = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family);
@@ -42,7 +40,8 @@ Status TransactionUtil::CheckKeyForConflicts(DBImpl* db_impl,
     SequenceNumber earliest_seq =
         db_impl->GetEarliestMemTableSequenceNumber(sv, true);
 
-    result = CheckKey(db_impl, sv, earliest_seq, key_seq, key, cache_only);
+    result = CheckKey(db_impl, sv, earliest_seq, snap_seq, key, cache_only,
+                      snap_checker);
 
     db_impl->ReturnAndCleanupSuperVersion(cfd, sv);
   }
@@ -52,8 +51,9 @@ Status TransactionUtil::CheckKeyForConflicts(DBImpl* db_impl,
 
 Status TransactionUtil::CheckKey(DBImpl* db_impl, SuperVersion* sv,
                                  SequenceNumber earliest_seq,
-                                 SequenceNumber key_seq, const std::string& key,
-                                 bool cache_only) {
+                                 SequenceNumber snap_seq,
+                                 const std::string& key, bool cache_only,
+                                 ReadCallback* snap_checker) {
   Status result;
   bool need_to_read_sst = false;
 
@@ -73,9 +73,9 @@ Status TransactionUtil::CheckKey(DBImpl* db_impl, SuperVersion* sv,
       result = Status::TryAgain(
           "Transaction ould not check for conflicts as the MemTable does not "
           "countain a long enough history to check write at SequenceNumber: ",
-          ToString(key_seq));
+          ToString(snap_seq));
     }
-  } else if (key_seq < earliest_seq) {
+  } else if (snap_seq < earliest_seq) {
     need_to_read_sst = true;
 
     if (cache_only) {
@@ -91,7 +91,7 @@ Status TransactionUtil::CheckKey(DBImpl* db_impl, SuperVersion* sv,
                "max_write_buffer_number_to_maintain option could reduce the "
                "frequency "
                "of this error.",
-               key_seq, earliest_seq);
+               snap_seq, earliest_seq);
       result = Status::TryAgain(msg);
     }
   }
@@ -105,9 +105,13 @@ Status TransactionUtil::CheckKey(DBImpl* db_impl, SuperVersion* sv,
 
     if (!(s.ok() || s.IsNotFound() || s.IsMergeInProgress())) {
       result = s;
-    } else if (found_record_for_key && (seq > key_seq)) {
-      // Write Conflict
-      result = Status::Busy();
+    } else if (found_record_for_key) {
+      bool write_conflict = snap_checker == nullptr
+                                ? snap_seq < seq
+                                : !snap_checker->IsCommitted(seq);
+      if (write_conflict) {
+        result = Status::Busy();
+      }
     }
   }
 

utilities/transactions/transaction_util.h

@@ -10,6 +10,8 @@
 #include <string>
 #include <unordered_map>
 
+#include "db/read_callback.h"
+
 #include "rocksdb/db.h"
 #include "rocksdb/slice.h"
 #include "rocksdb/status.h"
@@ -40,7 +42,7 @@ class WriteBatchWithIndex;
 
 class TransactionUtil {
  public:
-  // Verifies there have been no writes to this key in the db since this
+  // Verifies there have been no commits to this key in the db since this
   // sequence number.
   //
   // If cache_only is true, then this function will not attempt to read any
@@ -52,7 +54,8 @@ class TransactionUtil {
   static Status CheckKeyForConflicts(DBImpl* db_impl,
                                      ColumnFamilyHandle* column_family,
                                      const std::string& key,
-                                     SequenceNumber key_seq, bool cache_only);
+                                     SequenceNumber snap_seq, bool cache_only,
+                                     ReadCallback* snap_checker = nullptr);
 
   // For each key,SequenceNumber pair in the TransactionKeyMap, this function
   // will verify there have been no writes to the key in the db since that
@@ -69,8 +72,9 @@ class TransactionUtil {
 
  private:
   static Status CheckKey(DBImpl* db_impl, SuperVersion* sv,
-                         SequenceNumber earliest_seq, SequenceNumber key_seq,
-                         const std::string& key, bool cache_only);
+                         SequenceNumber earliest_seq, SequenceNumber snap_seq,
+                         const std::string& key, bool cache_only,
+                         ReadCallback* snap_checker = nullptr);
 };
 
 }  // namespace rocksdb

utilities/transactions/write_prepared_txn.cc

@@ -240,6 +240,34 @@ Status WritePreparedTxn::RollbackInternal() {
   return s;
 }
 
+Status WritePreparedTxn::ValidateSnapshot(ColumnFamilyHandle* column_family,
+                                          const Slice& key,
+                                          SequenceNumber prev_seqno,
+                                          SequenceNumber* new_seqno) {
+  assert(snapshot_);
+
+  SequenceNumber snap_seq = snapshot_->GetSequenceNumber();
+  // prev_seqno is either max or the last snapshot with which this key was
+  // trackeed so there is no need to apply the IsInSnapshot to this comparison
+  // here as prev_seqno is not a prepare seq.
+  if (prev_seqno <= snap_seq) {
+    // If the key has been previous validated at a sequence number earlier
+    // than the curent snapshot's sequence number, we already know it has not
+    // been modified.
+    return Status::OK();
+  }
+
+  *new_seqno = snap_seq;
+
+  ColumnFamilyHandle* cfh =
+      column_family ? column_family : db_impl_->DefaultColumnFamily();
+
+  WritePreparedTxnReadCallback snap_checker(wpt_db_, snap_seq);
+  return TransactionUtil::CheckKeyForConflicts(
+      db_impl_, cfh, key.ToString(), snapshot_->GetSequenceNumber(),
+      false /* cache_only */, &snap_checker);
+}
+
 }  // namespace rocksdb
 
 #endif  // ROCKSDB_LITE

utilities/transactions/write_prepared_txn.h

@@ -82,12 +82,9 @@ class WritePreparedTxn : public PessimisticTransaction {
 
   Status RollbackInternal() override;
 
-  // TODO(myabandeh): verify that the current impl work with values being
-  // written with prepare sequence number too.
-  // Status ValidateSnapshot(ColumnFamilyHandle* column_family, const Slice&
-  // key,
-  //                        SequenceNumber prev_seqno, SequenceNumber*
-  //                        new_seqno);
+  virtual Status ValidateSnapshot(ColumnFamilyHandle* column_family,
+                                  const Slice& key, SequenceNumber prev_seqno,
+                                  SequenceNumber* new_seqno) override;
 
   // No copying allowed
   WritePreparedTxn(const WritePreparedTxn&);