From ac95ae1b5d9b58606160dc856d04ed7bb5c06ac5 Mon Sep 17 00:00:00 2001
From: sdong <siying.d@fb.com>
Date: Wed, 5 Nov 2014 18:07:22 -0800
Subject: [PATCH] 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        | 21 +++++++++++++++++++++
 include/rocksdb/db.h |  2 ++
 2 files changed, 23 insertions(+)

diff --git a/db/db_test.cc b/db/db_test.cc
index 7aea863f8..b81b7c08e 100644
--- a/db/db_test.cc
+++ b/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"));
diff --git a/include/rocksdb/db.h b/include/rocksdb/db.h
index 21fa43838..65b517f54 100644
--- a/include/rocksdb/db.h
+++ b/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;