Add a callback to be executed when the operation is complete.

        If the operation is not already complete, this will start a helper
        thread to poll for the status of the operation in the background.

        Args:
            fn (Callable[Future]): The callback to execute when the operation
                is complete.

def add_done_callback(self, fn):
        """Add a callback to be executed when the operation is complete.

        If the operation is not already complete, this will start a helper
        thread to poll for the status of the operation in the background.

        Args:
            fn (Callable[Future]): The callback to execute when the operation
                is complete.
        """
        if self._result_set:
            _helpers.safe_invoke_callback(fn, self)
            return

        self._done_callbacks.append(fn)

        if self._polling_thread is None:
            # The polling thread will exit on its own as soon as the operation
            # is done.
            self._polling_thread = _helpers.start_daemon_thread(
                target=self._blocking_poll
            )