Poll and wait for the Future to be resolved.

        Args:
            timeout (int):
                How long (in seconds) to wait for the operation to complete.
                If None, wait indefinitely.

def _blocking_poll(self, timeout=None):
        """Poll and wait for the Future to be resolved.

        Args:
            timeout (int):
                How long (in seconds) to wait for the operation to complete.
                If None, wait indefinitely.
        """
        if self._result_set:
            return

        retry_ = self._retry.with_deadline(timeout)

        try:
            retry_(self._done_or_raise)()
        except exceptions.RetryError:
            raise concurrent.futures.TimeoutError(
                "Operation did not complete within the designated " "timeout."
            )