Flush Data at object destruction if disableWal is used.

Summary:
Added a conditional flush in ~DBImpl to flush.
There is still a chance of writes not being persisted if there is a
crash (not a clean shutdown) before the DBImpl instance is destroyed.

Test Plan: modified db_test to meet the new expectations.

Reviewers: dhruba, heyongqiang

Differential Revision: https://reviews.facebook.net/D6519

db/db_impl.cc

@@ -193,7 +193,8 @@ DBImpl::DBImpl(const Options& options, const std::string& dbname)
       stall_memtable_compaction_(0),
       stall_level0_num_files_(0),
       stall_leveln_slowdown_(0),
-      started_at_(options.env->NowMicros()) {
+      started_at_(options.env->NowMicros()),
+      flush_on_destroy_(false) {
   mem_->Ref();
   has_imm_.Release_Store(NULL);
 
@@ -226,6 +227,9 @@ DBImpl::DBImpl(const Options& options, const std::string& dbname)
 
 DBImpl::~DBImpl() {
   // Wait for background work to finish
+  if (flush_on_destroy_) {
+    FlushMemTable(FlushOptions());
+  }
     mutex_.Lock();
   shutting_down_.Release_Store(this);  // Any non-NULL value is ok
   while (bg_compaction_scheduled_ || bg_logstats_scheduled_) {
@@ -1414,6 +1418,10 @@ Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
     // into mem_.
     {
       mutex_.Unlock();
+      if (options.disableWAL) {
+        flush_on_destroy_ = true;
+      }
+
       if (!options.disableWAL) {
         status = log_->AddRecord(WriteBatchInternal::Contents(updates));
         if (status.ok() && options.sync) {

db/db_impl.h

@@ -222,6 +222,8 @@ class DBImpl : public DB {
   // Time at which this instance was started.
   const uint64_t started_at_;
 
+  bool flush_on_destroy_; // Used when disableWAL is true.
+
   // Per level compaction stats.  stats_[level] stores the stats for
   // compactions that produced data for the specified "level".
   struct CompactionStats {

db/db_test.cc

@@ -858,8 +858,8 @@ TEST(DBTest, WAL) {
   ASSERT_OK(dbfull()->Put(writeOpt, "bar", "v1"));
 
   Reopen();
-  ASSERT_EQ("NOT_FOUND", Get("foo"));
-  ASSERT_EQ("NOT_FOUND", Get("bar"));
+  ASSERT_EQ("v1", Get("foo"));
+  ASSERT_EQ("v1", Get("bar"));
 
   writeOpt.disableWAL = false;
   ASSERT_OK(dbfull()->Put(writeOpt, "bar", "v2"));
@@ -867,10 +867,9 @@ TEST(DBTest, WAL) {
   ASSERT_OK(dbfull()->Put(writeOpt, "foo", "v2"));
 
   Reopen();
-  // We garantee the 'bar' will be there
-  // because its put has WAL enabled.
-  // But 'foo' may or may not be there.
+  // Both value's should be present.
   ASSERT_EQ("v2", Get("bar"));
+  ASSERT_EQ("v2", Get("foo"));
 
   writeOpt.disableWAL = true;
   ASSERT_OK(dbfull()->Put(writeOpt, "bar", "v3"));
@@ -878,9 +877,9 @@ TEST(DBTest, WAL) {
   ASSERT_OK(dbfull()->Put(writeOpt, "foo", "v3"));
 
   Reopen();
-  // 'foo' should be there because its put
-  // has WAL enabled.
+  // again both values should be present.
   ASSERT_EQ("v3", Get("foo"));
+  ASSERT_EQ("v3", Get("bar"));
 }
 
 TEST(DBTest, CheckLock) {
@@ -895,13 +894,13 @@ TEST(DBTest, FLUSH) {
   WriteOptions writeOpt = WriteOptions();
   writeOpt.disableWAL = true;
   ASSERT_OK(dbfull()->Put(writeOpt, "foo", "v1"));
-  // this will not flush the last 2 writes
+  // this will now also flush the last 2 writes
   dbfull()->Flush(FlushOptions());
   ASSERT_OK(dbfull()->Put(writeOpt, "bar", "v1"));
 
   Reopen();
   ASSERT_EQ("v1", Get("foo"));
-  ASSERT_EQ("NOT_FOUND", Get("bar"));
+  ASSERT_EQ("v1", Get("bar"));
 
   writeOpt.disableWAL = true;
   ASSERT_OK(dbfull()->Put(writeOpt, "bar", "v2"));