WriteUnPrepared: Fix bug in savepoints (#5703)

Summary:
Fix a bug in write unprepared savepoints. When flushing the write batch according to savepoint boundaries, we were forgetting to flush the last write batch after the last savepoint, meaning that some data was not written to DB.

Also, add a small optimization where we avoid flushing empty batches.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5703

Differential Revision: D16811996

Pulled By: lth

fbshipit-source-id: 600c7e0e520ad7a8fad32d77e11d932453e68e3f

utilities/transactions/write_unprepared_transaction_test.cc

@@ -621,6 +621,26 @@ TEST_P(WriteUnpreparedTransactionTest, IterateAndWrite) {
   }
 }
 
+TEST_P(WriteUnpreparedTransactionTest, SavePoint) {
+  WriteOptions woptions;
+  TransactionOptions txn_options;
+  txn_options.write_batch_flush_threshold = 1;
+
+  Transaction* txn = db->BeginTransaction(woptions, txn_options);
+  txn->SetSavePoint();
+  ASSERT_OK(txn->Put("a", "a"));
+  ASSERT_OK(txn->Put("b", "b"));
+  ASSERT_OK(txn->Commit());
+
+  ReadOptions roptions;
+  std::string value;
+  ASSERT_OK(txn->Get(roptions, "a", &value));
+  ASSERT_EQ(value, "a");
+  ASSERT_OK(txn->Get(roptions, "b", &value));
+  ASSERT_EQ(value, "b");
+  delete txn;
+}
+
 }  // namespace rocksdb
 
 int main(int argc, char** argv) {

utilities/transactions/write_unprepared_txn.cc

@@ -407,10 +407,13 @@ Status WriteUnpreparedTxn::FlushWriteBatchWithSavePointToDB() {
 
   size_t prev_boundary = WriteBatchInternal::kHeader;
   const bool kPrepared = true;
-  for (size_t i = 0; i < unflushed_save_points_->size(); i++) {
+  for (size_t i = 0; i < unflushed_save_points_->size() + 1; i++) {
+    bool trailing_batch = i == unflushed_save_points_->size();
     SavePointBatchHandler sp_handler(&write_batch_,
                                      *wupt_db_->GetCFHandleMap().get());
-    size_t curr_boundary = (*unflushed_save_points_)[i];
+    size_t curr_boundary = trailing_batch
+                               ? wb.GetWriteBatch()->GetDataSize()
+                               : (*unflushed_save_points_)[i];
 
     // Construct the partial write batch up to the savepoint.
     //
@@ -424,18 +427,22 @@ Status WriteUnpreparedTxn::FlushWriteBatchWithSavePointToDB() {
       return s;
     }
 
+    if (write_batch_.GetWriteBatch()->Count() > 0) {
       // Flush the write batch.
       s = FlushWriteBatchToDBInternal(!kPrepared);
       if (!s.ok()) {
         return s;
       }
+    }
 
+    if (!trailing_batch) {
       if (flushed_save_points_ == nullptr) {
         flushed_save_points_.reset(
             new autovector<WriteUnpreparedTxn::SavePoint>());
       }
       flushed_save_points_->emplace_back(
           unprep_seqs_, new ManagedSnapshot(db_impl_, wupt_db_->GetSnapshot()));
+    }
 
     prev_boundary = curr_boundary;
     const bool kClear = true;
@@ -736,7 +743,6 @@ Status WriteUnpreparedTxn::RollbackToSavePointInternal() {
   assert(flushed_save_points_->size() > 0);
   WriteUnpreparedTxn::SavePoint& top = flushed_save_points_->back();
 
-  assert(top.unprep_seqs_.size() > 0);
   assert(save_points_ != nullptr && save_points_->size() > 0);
   const TransactionKeyMap& tracked_keys = save_points_->top().new_keys_;