Add ClearSnapshot()

Summary:
MyRocks needs the ability to clear a snapshot for Read Committed support

Test Plan: transaction_test

Reviewers: anthony

Reviewed By: anthony

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D48861

include/rocksdb/utilities/transaction.h

@@ -83,10 +83,22 @@ class Transaction {
   // Returns the Snapshot created by the last call to SetSnapshot().
   //
   // REQUIRED: The returned Snapshot is only valid up until the next time
-  // SetSnapshot()/SetSnapshotOnNextSavePoint() is called or the Transaction
-  // is deleted.
+  // SetSnapshot()/SetSnapshotOnNextSavePoint() is called, ClearSnapshot()
+  // is called, or the Transaction is deleted.
   virtual const Snapshot* GetSnapshot() const = 0;
 
+  // Clears the current snapshot (i.e. no snapshot will be 'set')
+  //
+  // This removes any snapshot that currently exists or is set to be created
+  // on the next update operation (SetSnapshotOnNextOperation).
+  //
+  // Calling ClearSnapshot() has no effect on keys written before this function
+  // has been called.
+  //
+  // If a reference to a snapshot was retrieved via GetSnapshot(), it will no
+  // longer be valid and should be discarded after a call to ClearSnapshot().
+  virtual void ClearSnapshot() = 0;
+
   // Write all batched keys to the db atomically.
   //
   // Returns OK on success.

utilities/transactions/transaction_base.h

@@ -171,6 +171,11 @@ class TransactionBaseImpl : public Transaction {
   void SetSnapshot() override;
   void SetSnapshotOnNextOperation() override;
 
+  void ClearSnapshot() override {
+    snapshot_.reset();
+    snapshot_needed_ = false;
+  }
+
   void DisableIndexing() override { indexing_enabled_ = false; }
 
   void EnableIndexing() override { indexing_enabled_ = true; }

utilities/transactions/transaction_test.cc

@@ -2161,6 +2161,53 @@ TEST_F(TransactionTest, DeferSnapshotSavePointTest) {
   delete txn1;
 }
 
+TEST_F(TransactionTest, ClearSnapshotTest) {
+  WriteOptions write_options;
+  ReadOptions read_options, snapshot_read_options;
+  string value;
+  Status s;
+
+  s = db->Put(write_options, "foo", "0");
+  ASSERT_OK(s);
+
+  Transaction* txn = db->BeginTransaction(write_options);
+  ASSERT_TRUE(txn);
+
+  s = db->Put(write_options, "foo", "1");
+  ASSERT_OK(s);
+
+  snapshot_read_options.snapshot = txn->GetSnapshot();
+  ASSERT_FALSE(snapshot_read_options.snapshot);
+
+  // No snapshot created yet
+  s = txn->Get(snapshot_read_options, "foo", &value);
+  ASSERT_EQ(value, "1");
+
+  txn->SetSnapshot();
+  snapshot_read_options.snapshot = txn->GetSnapshot();
+  ASSERT_TRUE(snapshot_read_options.snapshot);
+
+  s = db->Put(write_options, "foo", "2");
+  ASSERT_OK(s);
+
+  // Snapshot was created before change to '2'
+  s = txn->Get(snapshot_read_options, "foo", &value);
+  ASSERT_EQ(value, "1");
+
+  txn->ClearSnapshot();
+  snapshot_read_options.snapshot = txn->GetSnapshot();
+  ASSERT_FALSE(snapshot_read_options.snapshot);
+
+  // Snapshot has now been cleared
+  s = txn->Get(snapshot_read_options, "foo", &value);
+  ASSERT_EQ(value, "2");
+
+  s = txn->Commit();
+  ASSERT_OK(s);
+
+  delete txn;
+}
+
 }  // namespace rocksdb
 
 int main(int argc, char** argv) {