def poll_heartbeat(self):
        """
        Check the status of the heartbeat thread (if it is active) and indicate
        the liveness of the client. This must be called periodically after
        joining with :meth:`.ensure_active_group` to ensure that the member stays
        in the group. If an interval of time longer than the provided rebalance
        timeout (max_poll_interval_ms) expires without calling this method, then
        the client will proactively leave the group.

        Raises: RuntimeError for unexpected errors raised from the heartbeat thread
        """
        with self._lock:
            if self._heartbeat_thread is not None:
                if self._heartbeat_thread.failed:
                    # set the heartbeat thread to None and raise an exception.
                    # If the user catches it, the next call to ensure_active_group()
                    # will spawn a new heartbeat thread.
                    cause = self._heartbeat_thread.failed
                    self._heartbeat_thread = None
                    raise cause  # pylint: disable-msg=raising-bad-type

                # Awake the heartbeat thread if needed
                if self.heartbeat.should_heartbeat():
                    self._lock.notify()
                self.heartbeat.poll()