Function poll_peer_set_on_notify 

async fn poll_peer_set_on_notify<S>(
    peer_set: S,
    queued_request_waker: Arc<QueuedRequestWaker>,
) -> Result<(), BoxError>
where
    S: Service<Request, Response = Response, Error = BoxError> + Send + Clone + 'static,
    S::Future: Send + 'static,

Polls peer_set with a Request::PollPeerSet each time queued_request_waker is notified, so queued requests are routed when peers become ready or their serving-only wait expires.

peer_set must be the service that wraps the PeerSet that owns queued_request_waker. See PeerSet::into_buffer.

Only returns if the peer set fails.