Move checksum calculation ahead of memory copy (#6844)

Summary:
Originally, the checksum of appended data in writable file writer is calculated after the data is copied to the buffer. It will not be able to catch the bit flip happens during copy. Move the checksum calculation before it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6844

Test Plan: pass make asan_check

Reviewed By: cheng-chang

Differential Revision: D21576726

Pulled By: zhichao-cao

fbshipit-source-id: 0a062a1f19886f6ea0d4e3f557e6f4b799773254

file/writable_file_writer.cc

@@ -30,6 +30,9 @@ IOStatus WritableFileWriter::Append(const Slice& data) {
   TEST_KILL_RANDOM("WritableFileWriter::Append:0",
                    rocksdb_kill_odds * REDUCE_ODDS2);
 
+  // Calculate the checksum of appended data
+  CalculateFileChecksum(data);
+
   {
     IOSTATS_TIMER_GUARD(prepare_write_nanos);
     TEST_SYNC_POINT("WritableFileWriter::Append:BeforePrepareWrite");
@@ -89,7 +92,6 @@ IOStatus WritableFileWriter::Append(const Slice& data) {
   TEST_KILL_RANDOM("WritableFileWriter::Append:1", rocksdb_kill_odds);
   if (s.ok()) {
     filesize_ += data.size();
-    CalculateFileChecksum(data);
   }
   return s;
 }