Use write & rename instead of write. Also check content length.

#1377740: file_unmanaged_move() should issue rename() where possible instead of copy() & unlink()

This is a copy of what I use in AdvAgg in order to avoid multiple writes overwriting each other. If you don't do this then you can get corrupted files fairly easily in a high concurrent environment.

/**
 * Use write & rename instead of write.
 *
 * Perform the replace operation. Since there could be multiple processes
 * writing to the same file, the best option is to create a temporary file in
 * the same directory and then rename it to the destination. A temporary file
 * is needed if the directory is mounted on a separate machine; thus ensuring
 * the rename command stays local.
 *
 * @param string $destination
 *   A string containing the destination location.
 * @param string $data
 *   A string containing the contents of the file.
 *
 * @return bool
 *   True if write was successful. False if write or rename failed.
 */
function stage_file_proxy_write_file($destination, $data) {
  // Get a temporary filename in the destination directory.
  $dir = drupal_dirname($destination) . '/';
  $temporary_file = drupal_tempnam($dir, 'stage_file_proxy_');
  $temporary_file_copy = $temporary_file;

  // Get the extension of the original filename and append it to the temp file
  // name. Preserves the mime type in different stream wrapper implementations.
  $parts = pathinfo($destination);
  $extension = '.' . $parts['extension'];
  if ($extension === '.gz') {
    $parts = pathinfo($parts['filename']);
    $extension = '.' . $parts['extension'] . $extension;
  }
  // Move temp file into the destination dir if not in there.
  // Add the extension on as well.
  $temporary_file = str_replace(substr($temporary_file, 0, strpos($temporary_file, 'stage_file_proxy_')), $dir, $temporary_file) . $extension;

  // Preform the rename, adding the extension to the temp file.
  if (!@rename($temporary_file_copy, $temporary_file)) {
    // Remove if rename failed.
    @unlink($temporary_file_copy);
    return FALSE;
  }

  // Save to temporary filename in the destination directory.
  $filepath = file_unmanaged_save_data($data, $temporary_file, FILE_EXISTS_REPLACE);

  // Perform the rename operation if the write succeeded.
  if ($filepath) {
    if (!@rename($filepath, $destination)) {
      // Unlink and try again for windows. Rename on windows does not replace
      // the file if it already exists.
      @unlink($destination);
      if (!@rename($filepath, $destination)) {
        // Remove temporary_file if rename failed.
        @unlink($filepath);
      }
    }
  }

  // Final check; make sure file exists & is not empty.
  $result = FALSE;
  if (file_exists($destination) & filesize($destination) != 0 ) {
    $result = TRUE;
  }

  return $result;
}

I could use file_unmanaged_delete but that would fill the watchdog table up most likely.

Heads up that in advagg (which is where the write & rename code came from) I discovered that drupal_tempnam doesn't work as expected if the dir is a stream wrapper. The above patch already has a workaround:

+  // Move temp file into the destination dir if not in there.
+  // Add the extension on as well.
+  $temporary_file = str_replace(substr($temporary_file, 0, strpos($temporary_file, 'stage_file_proxy_')), $dir, $temporary_file) . $extension;

It works, we just might want to make it more clear in the code what is going on and why this is needed.