code: def _get_backend(filename):
      
      filename = os.path.abspath(filename)
  
      with _backends_lock:
          if filename not in _backends:
              _backends[filename] = _MultiprocessStorageBackend(filename)
          return _backends[filename]
docs: A helper method to get or create a backend with thread locking.
  
      This ensures that only one backend is used per-file per-process, so that
      thread and process locks are appropriately shared.
  
      Args:
          filename: The full path to the credential storage file.
  
      Returns:
          An instance of :class:`_MultiprocessStorageBackend`.