def put(self, session):
        """Return a session to the pool.

        Never blocks:  if the pool is full, raises.

        :type session: :class:`~google.cloud.spanner_v1.session.Session`
        :param session: the session being returned.

        :raises: :exc:`six.moves.queue.Full` if the queue is full.
        """
        self._sessions.put_nowait((_NOW() + self._delta, session))