Fix bug that flush doesn't respond to fsync result

Summary:
With a regression bug was introduced two years ago, by https://github.com/facebook/rocksdb/commit/6e9fbeb27c38329f33ae541302c44c8db8374f8c , we fail to check return status of fsync call. This can cause we miss the information from the file system and can potentially cause corrupted data which we could have been detected.
Closes https://github.com/facebook/rocksdb/pull/2495

Reviewed By: ajkr

Differential Revision: D5321949

Pulled By: siying

fbshipit-source-id: c68117914bb40700198fc37d0e4c63163a8a1031

HISTORY.md

@@ -21,6 +21,9 @@
 * Introduce WriteOptions.low_pri. If it is true, low priority writes will be throttled if the compaction is behind.
 * `DB::IngestExternalFile()` now supports ingesting files into a database containing range deletions.
 
+### Bu Fixes
+* Shouldn't ignore return value of fsync() in flush.
+
 ## 5.5.0 (05/17/2017)
 ### New Features
 * FIFO compaction to support Intra L0 compaction too with CompactionOptionsFIFO.allow_compaction=true.

db/builder.cc

@@ -179,7 +179,7 @@ Status BuildTable(
     // Finish and check for file errors
     if (s.ok() && !empty) {
       StopWatch sw(env, ioptions.statistics, TABLE_SYNC_MICROS);
-      file_writer->Sync(ioptions.use_fsync);
+      s = file_writer->Sync(ioptions.use_fsync);
     }
     if (s.ok() && !empty) {
       s = file_writer->Close();