Make sure WAL is synced for DB::Write() if write batch is empty

Summary: This patch makes it a contract that if an empty write batch is passed to DB::Write() and WriteOptions.sync = true, fsync is called to WAL.

Test Plan: A new unit test

Reviewers: ljin, rven, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, MarkCallaghan, leveldb

Differential Revision: https://reviews.facebook.net/D28365

db/db_test.cc

@@ -1270,6 +1270,27 @@ TEST(DBTest, Empty) {
   } while (ChangeOptions());
 }
 
+TEST(DBTest, WriteEmptyBatch) {
+  Options options;
+  options.env = env_;
+  options.write_buffer_size = 100000;
+  options = CurrentOptions(options);
+  CreateAndReopenWithCF({"pikachu"}, options);
+
+  ASSERT_OK(Put(1, "foo", "bar"));
+  env_->sync_counter_.store(0);
+  WriteOptions wo;
+  wo.sync = true;
+  wo.disableWAL = false;
+  WriteBatch empty_batch;
+  ASSERT_OK(dbfull()->Write(wo, &empty_batch));
+  ASSERT_GE(env_->sync_counter_.load(), 1);
+
+  // make sure we can re-open it.
+  ASSERT_OK(TryReopenWithColumnFamilies({"default", "pikachu"}, options));
+  ASSERT_EQ("bar", Get(1, "foo"));
+}
+
 TEST(DBTest, ReadOnlyDB) {
   ASSERT_OK(Put("foo", "v1"));
   ASSERT_OK(Put("bar", "v2"));

include/rocksdb/db.h

@@ -195,6 +195,8 @@ class DB {
   }
 
   // Apply the specified updates to the database.
+  // If `updates` contains no update, WAL will still be synced if
+  // options.sync=true.
   // Returns OK on success, non-OK on failure.
   // Note: consider setting options.sync = true.
   virtual Status Write(const WriteOptions& options, WriteBatch* updates) = 0;