Merge "Flush buffer to filesystem before fsync" am: 1f48be6f am: 673bdd3a

  // Steps to ensure content of config file gets to disk:

  //

  // 1) Open and write to temp file (e.g. bt_config.conf.new).

  // 2) Sync the temp file to disk with fsync().

  // 3) Rename temp file to actual config file (e.g. bt_config.conf).

  // 2) Flush the stream buffer to the temp file.

  // 3) Sync the temp file to disk with fsync().

  // 4) Rename temp file to actual config file (e.g. bt_config.conf).

  //    This ensures atomic update.

  // 4) Sync directory that has the conf file with fsync().

  // 5) Sync directory that has the conf file with fsync().

  //    This ensures directory entries are up-to-date.

  int dir_fd = -1;

  FILE* fp = nullptr;

    goto error;

  }

  // Flush the stream buffer to the temp file.

  if (fflush(fp) < 0) {

    LOG(ERROR) << __func__ << ": unable to write flush buffer to file '"

               << temp_filename << "': " << strerror(errno);

    goto error;

  }

  // Sync written temp file out to disk. fsync() is blocking until data makes it

  // to disk.

  if (fsync(fileno(fp)) < 0) {