Fix WriteBatchWithIndex with MergeOperator bug (#5577)

Summary:
```
TEST_F(WriteBatchWithIndexTest, TestGetFromBatchAndDBMerge3) {
  DB* db;
  Options options;

  options.create_if_missing = true;
  std::string dbname = test::PerThreadDBPath("write_batch_with_index_test");

  options.merge_operator = MergeOperators::CreateFromStringId("stringappend");

  DestroyDB(dbname, options);
  Status s = DB::Open(options, dbname, &db);
  assert(s.ok());

  ReadOptions read_options;
  WriteOptions write_options;
  FlushOptions flush_options;
  std::string value;

  WriteBatchWithIndex batch;

  ASSERT_OK(db->Put(write_options, "A", "1"));
  ASSERT_OK(db->Flush(flush_options, db->DefaultColumnFamily()));
  ASSERT_OK(batch.Merge("A", "2"));

  ASSERT_OK(batch.GetFromBatchAndDB(db, read_options, "A", &value));
  ASSERT_EQ(value, "1,2");

  delete db;
  DestroyDB(dbname, options);
}
```
Fix ASSERT in batch.GetFromBatchAndDB()
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5577

Differential Revision: D16379847

fbshipit-source-id: b1320e24ec8e71350c525083cc0a16180a63f752

utilities/write_batch_with_index/write_batch_with_index.cc

@@ -917,9 +917,12 @@ Status WriteBatchWithIndex::GetFromBatchAndDB(
       }
 
       if (merge_operator) {
+        std::string merge_result;
         s = MergeHelper::TimedFullMerge(
             merge_operator, key, merge_data, merge_context.GetOperands(),
-            pinnable_val->GetSelf(), logger, statistics, env);
+            &merge_result, logger, statistics, env);
+        pinnable_val->Reset();
+        *pinnable_val->GetSelf() = std::move(merge_result);
         pinnable_val->PinSelf();
       } else {
         s = Status::InvalidArgument("Options::merge_operator must be set");

utilities/write_batch_with_index/write_batch_with_index_test.cc

@@ -1308,6 +1308,38 @@ TEST_F(WriteBatchWithIndexTest, TestGetFromBatchAndDBMerge2) {
   DestroyDB(dbname, options);
 }
 
+
+TEST_F(WriteBatchWithIndexTest, TestGetFromBatchAndDBMerge3) {
+  DB* db;
+  Options options;
+
+  options.create_if_missing = true;
+  std::string dbname = test::PerThreadDBPath("write_batch_with_index_test");
+
+  options.merge_operator = MergeOperators::CreateFromStringId("stringappend");
+
+  DestroyDB(dbname, options);
+  Status s = DB::Open(options, dbname, &db);
+  assert(s.ok());
+
+  ReadOptions read_options;
+  WriteOptions write_options;
+  FlushOptions flush_options;
+  std::string value;
+
+  WriteBatchWithIndex batch;
+
+  ASSERT_OK(db->Put(write_options, "A", "1"));
+  ASSERT_OK(db->Flush(flush_options, db->DefaultColumnFamily()));
+  ASSERT_OK(batch.Merge("A", "2"));
+
+  ASSERT_OK(batch.GetFromBatchAndDB(db, read_options, "A", &value));
+  ASSERT_EQ(value, "1,2");
+
+  delete db;
+  DestroyDB(dbname, options);
+}
+
 void AssertKey(std::string key, WBWIIterator* iter) {
   ASSERT_TRUE(iter->Valid());
   ASSERT_EQ(key, iter->Entry().key.ToString());