Django1.1 file based session backend multi-threaded solution

Question

Hi, all.

I read django.contrib.sessions.backend.file today, in the save method of SessionStore there is something as the following that's used to achieve multi-threaded saving integrity:

        output_file_fd, output_file_name = tempfile.mkstemp(dir=dir,
            prefix=prefix + '_out_')
        renamed = False
        try:
            try:
                os.write(output_file_fd, self.encode(session_data))
            finally:
                os.close(output_file_fd)
            os.rename(output_file_name, session_file_name)
            renamed = True
        finally:
            if not renamed:
                os.unlink(output_file_name)

I don't quite understand how this solve the integrity problem.

Answer 1

Technically this doesn't solve the integrity problem completely. #9084 Addresses this issue.

Essentially this works by using tempfile.mkstemp which is guaranteed to be atomic, and writing the data to that file. It then calls os.rename() which will rename the temp file to the new file. In unix this will remove the old file before renaming, in windows this will raise an error. This should be fixed for django 1.1

If you looked in the revision history you'll see that they previously had locks, but changed them to this method for various reasons.