fix(relay): wake the relay Listener on close

When closing a relayed `Listener` manually, the `TransportEvent::ListenerClosed` generated by the `relay::priv_client::Transport` is never forwarded back up to the `Swarm`, causing the `Swarm` to never remove the corresponding listener and never emitting the `SwarmEvent::ListenerClosed` event.

This happens  because, when stopping a relayed listener manually, the call to the [`close()` function](https://github.com/libp2p/rust-libp2p/blob/master/protocols/relay/src/priv_client/transport.rs#L324), is done outside the `poll` function, which mean nothing is triggering a wake up call to wake up the polling.

Unfortunately, even if the [`listeners` (`SelectAll`) is always polled](https://github.com/libp2p/rust-libp2p/blob/master/protocols/relay/src/priv_client/transport.rs#L241) after a call to the `close` method, since `SelectAll` uses a `FuturesUnordered` internally, the poll does nothing. Indeed, the `FuturesUnordered` states that:
```rust
/// This structure is optimized to manage a large number of futures.
/// Futures managed by [`FuturesUnordered`] will only be polled when they
/// generate wake-up notifications. This reduces the required amount of work
/// needed to poll large numbers of futures.
```

Since means that when closing a relayed listener manually (calling `swarm.remove_listener`), it is never removed.

This PR fixes that by triggering a `waker` when calling the `close` function.

Pull-Request: libp2p#5491.

protocols/relay/CHANGELOG.md

@@ -1,9 +1,12 @@
 ## 0.17.3
 - Use `web-time` instead of `instant`.
   See [PR 5347](https://github.com/libp2p/rust-libp2p/pull/5347).
+- Fix manual closure of relayed listener.
+  See [PR 5491](https://github.com/libp2p/rust-libp2p/pull/5491)
 - Add resource limits to `CircuitReq` to be set
   See [PR 5493](https://github.com/libp2p/rust-libp2p/pull/5493)  
 
+
 ## 0.17.2
 
 - Fix support for unlimited relay connection according to spec.

protocols/relay/src/priv_client/transport.rs

@@ -27,7 +27,6 @@ use crate::RequestId;
 use futures::channel::mpsc;
 use futures::channel::oneshot;
 use futures::future::{ready, BoxFuture, FutureExt, Ready};
-use futures::ready;
 use futures::sink::SinkExt;
 use futures::stream::SelectAll;
 use futures::stream::{Stream, StreamExt};
@@ -36,7 +35,7 @@ use libp2p_core::transport::{ListenerId, TransportError, TransportEvent};
 use libp2p_identity::PeerId;
 use std::collections::VecDeque;
 use std::pin::Pin;
-use std::task::{Context, Poll};
+use std::task::{Context, Poll, Waker};
 use thiserror::Error;
 
 /// A [`Transport`] enabling client relay capabilities.
@@ -151,6 +150,7 @@ impl libp2p_core::Transport for Transport {
             queued_events: Default::default(),
             from_behaviour,
             is_closed: false,
+            waker: None,
         };
         self.listeners.push(listener);
         Ok(())
@@ -313,6 +313,7 @@ pub(crate) struct Listener {
     /// The listener can be closed either manually with [`Transport::remove_listener`](libp2p_core::Transport) or if
     /// the sender side of the `from_behaviour` channel is dropped.
     is_closed: bool,
+    waker: Option<Waker>,
 }
 
 impl Listener {
@@ -328,6 +329,10 @@ impl Listener {
                 reason,
             });
         self.is_closed = true;
+
+        if let Some(waker) = self.waker.take() {
+            waker.wake();
+        }
     }
 }
 
@@ -337,18 +342,27 @@ impl Stream for Listener {
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         loop {
             if let Some(event) = self.queued_events.pop_front() {
+                self.waker = None;
                 return Poll::Ready(Some(event));
             }
 
             if self.is_closed {
                 // Terminate the stream if the listener closed and all remaining events have been reported.
+                self.waker = None;
                 return Poll::Ready(None);
             }
 
-            let Some(msg) = ready!(self.from_behaviour.poll_next_unpin(cx)) else {
-                // Sender of `from_behaviour` has been dropped, signaling listener to close.
-                self.close(Ok(()));
-                continue;
+            let msg = match self.from_behaviour.poll_next_unpin(cx) {
+                Poll::Ready(Some(msg)) => msg,
+                Poll::Ready(None) => {
+                    // Sender of `from_behaviour` has been dropped, signaling listener to close.
+                    self.close(Ok(()));
+                    continue;
+                }
+                Poll::Pending => {
+                    self.waker = Some(cx.waker().clone());
+                    return Poll::Pending;
+                }
             };
 
             match msg {