Let ImportantFileWriter Use fdatasync

base/files/file.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_FILES_FILE_H_
 #define BASE_FILES_FILE_H_
 
 #include "build/build_config.h"
 #if defined(OS_WIN)
 #include <windows.h>
@@ skipping 235 matching lines @@
   int WriteAtCurrentPosNoBestEffort(const char* data, int size);
 
   // Returns the current size of this file, or a negative number on failure.
   int64 GetLength();
 
   // Truncates the file to the given length. If |length| is greater than the
   // current size of the file, the file is extended with zeros. If the file
   // doesn't exist, |false| is returned.
   bool SetLength(int64 length);
 
-  // Instructs the filesystem to flush the file to disk. (POSIX: fsync, Windows:
-  // FlushFileBuffers).
+  // Instructs the filesystem to flush the file data and metadata to disk.
+  // (POSIX: fsync, Windows: FlushFileBuffers)
   bool Flush();
 
+  // Instructs the filesystem to flush the file data to disk. Not necessarily
+  // flushes metadata like modification time.
+  // (Linux: fdatasync, other platforms: same as Flush())

[rvargas (doing something else), 2015/03/20 18:56:05]

I don't think we want to end up with two methods that do almost the same thing,
especially if we are not keeping the same behavior across all platforms* (which
is one of the points of having base).

(*) as in both methods doing the same thing on Windows.

We should probably move to fdatasync and be done with it (I doubt
FlushFileBuffers guarantees that metadata is also flushed to disk). If someone
actually requires all metadata to be written then something must be done at the
directory level.

[hashimoto, 2015/03/20 19:38:56]

Good point, I thought someone might be relying on Flush() to flush metadata, but
if we've not been guaranteeing that after all, it should be OK.
Just switching to fdatasync sounds good. (maybe it might be better to make a PSA
on chromium-dev about the change?)

[rvargas (doing something else), 2015/03/20 21:43:00]

PSA sounds fine.

+  bool FlushData();
+
   // Updates the file times.
   bool SetTimes(Time last_access_time, Time last_modified_time);
 
   // Returns some basic information for the given file.
   bool GetInfo(Info* info);
 
   // Attempts to take an exclusive write lock on the file. Returns immediately
   // (i.e. does not wait for another process to unlock the file). If the lock
   // was obtained, the result will be FILE_OK. A lock only guarantees
   // that other processes may not also take a lock on the same file with the
@@ skipping 93 matching lines @@
 #endif
 
   Error error_details_;
   bool created_;
   bool async_;
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_H_