feat: add Connection::on_close to get notified on close without keeping the connection alive

quinn/src/connection.rs

@@ -518,6 +518,24 @@ impl Connection {
             .clone()
     }
 
+    /// Wait for the connection to be closed without keeping a strong reference to the connection
+    ///
+    /// Returns a future that resolves, once the connection is closed, to a tuple of
+    /// ([`ConnectionError`], [`ConnectionStats`]).
+    ///
+    /// Calling [`Self::closed`] keeps the connection alive until it is either closed locally via [`Connection::close`]
+    /// or closed by the remote peer. This function instead does not keep the connection itself alive,
+    /// so if all *other* clones of the connection are dropped, the connection will be closed implicitly even
+    /// if there are futures returned from this function still being awaited.
+    pub fn on_closed(&self) -> OnClosed {
+        let (tx, rx) = oneshot::channel();
+        self.0.state.lock("on_closed").on_closed.push(tx);
+        OnClosed {
+            conn: self.weak_handle(),
+            rx,
+        }
+    }
+
     /// If the connection is closed, the reason why.
     ///
     /// Returns `None` if the connection is still open.
@@ -1037,6 +1055,43 @@ impl Future for SendDatagram<'_> {
     }
 }
 
+/// Future returned by [`Connection::on_closed`]
+///
+/// Resolves to a tuple of ([`ConnectionError`], [`ConnectionStats`]).
+pub struct OnClosed {
+    rx: oneshot::Receiver<(ConnectionError, ConnectionStats)>,
+    conn: WeakConnectionHandle,
+}
+
+impl Drop for OnClosed {
+    fn drop(&mut self) {
+        if self.rx.is_terminated() {
+            return;
+        };
+        if let Some(conn) = self.conn.upgrade() {
+            self.rx.close();
+            conn.0
+                .state
+                .lock("OnClosed::drop")
+                .on_closed
+                .retain(|tx| !tx.is_closed());
+        }
+    }
+}
+
+impl Future for OnClosed {
+    type Output = (ConnectionError, ConnectionStats);
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.get_mut();
+        // The `expect` is safe because `State::drop` ensures that all senders are triggered
+        // before being dropped.
+        Pin::new(&mut this.rx)
+            .poll(cx)
+            .map(|x| x.expect("on_close sender is never dropped before sending"))
+    }
+}
+
 #[derive(Debug)]
 pub(crate) struct ConnectionRef(Arc<ConnectionInner>);
 
@@ -1077,6 +1132,7 @@ impl ConnectionRef {
                 send_buffer: Vec::new(),
                 buffered_transmit: None,
                 observed_external_addr: watch::Sender::new(None),
+                on_closed: Vec::new(),
             }),
             shared: Shared::default(),
         }))
@@ -1215,6 +1271,7 @@ pub(crate) struct State {
     /// Our last external address reported by the peer. When multipath is enabled, this will be the
     /// last report across all paths.
     pub(crate) observed_external_addr: watch::Sender<Option<SocketAddr>>,
+    on_closed: Vec<oneshot::Sender<(ConnectionError, ConnectionStats)>>,
 }
 
 impl State {
@@ -1475,6 +1532,12 @@ impl State {
         }
         wake_all_notify(&mut self.stopped);
         shared.closed.notify_waiters();
+
+        // Send to the registered on_closed futures.
+        let stats = self.inner.stats();
+        for tx in self.on_closed.drain(..) {
+            tx.send((reason.clone(), stats.clone())).ok();
+        }
     }
 
     fn close(&mut self, error_code: VarInt, reason: Bytes, shared: &Shared) {
@@ -1508,6 +1571,15 @@ impl Drop for State {
                 .endpoint_events
                 .send((self.handle, proto::EndpointEvent::drained()));
         }
+
+        if !self.on_closed.is_empty() {
+            // Ensure that all on_closed oneshot senders are triggered before dropping.
+            let reason = self.error.as_ref().expect("closed without error reason");
+            let stats = self.inner.stats();
+            for tx in self.on_closed.drain(..) {
+                tx.send((reason.clone(), stats.clone())).ok();
+            }
+        }
     }
 }
 

quinn/src/lib.rs

@@ -76,8 +76,8 @@ pub use rustls;
 pub use udp;
 
 pub use crate::connection::{
-    AcceptBi, AcceptUni, Connecting, Connection, OpenBi, OpenUni, ReadDatagram, SendDatagram,
-    SendDatagramError, WeakConnectionHandle, ZeroRttAccepted,
+    AcceptBi, AcceptUni, Connecting, Connection, OnClosed, OpenBi, OpenUni, ReadDatagram,
+    SendDatagram, SendDatagramError, WeakConnectionHandle, ZeroRttAccepted,
 };
 pub use crate::endpoint::{Accept, Endpoint, EndpointStats};
 pub use crate::incoming::{Incoming, IncomingFuture, RetryError};

quinn/src/tests.rs

@@ -17,7 +17,7 @@ use std::{
 use crate::runtime::TokioRuntime;
 use crate::{Duration, Instant};
 use bytes::Bytes;
-use proto::{RandomConnectionIdGenerator, crypto::rustls::QuicClientConfig};
+use proto::{ConnectionError, RandomConnectionIdGenerator, crypto::rustls::QuicClientConfig};
 use rand::{RngCore, SeedableRng, rngs::StdRng};
 use rustls::{
     RootCertStore,
@@ -1023,3 +1023,98 @@ async fn test_multipath_observed_address() {
 
     tokio::join!(server_task, client_task);
 }
+
+#[tokio::test]
+async fn on_closed() {
+    let _guard = subscribe();
+    let endpoint = endpoint();
+    let endpoint2 = endpoint.clone();
+    let server_task = tokio::spawn(async move {
+        let conn = endpoint2
+            .accept()
+            .await
+            .expect("endpoint")
+            .await
+            .expect("connection");
+        let on_closed = conn.on_closed();
+        let cause = conn.closed().await;
+        let (cause1, _stats) = on_closed.await;
+        assert!(matches!(cause, ConnectionError::ApplicationClosed(_)));
+        assert!(matches!(cause1, ConnectionError::ApplicationClosed(_)));
+    });
+    let client_task = tokio::spawn(async move {
+        let conn = endpoint
+            .connect(endpoint.local_addr().unwrap(), "localhost")
+            .unwrap()
+            .await
+            .expect("connect");
+        let on_closed1 = conn.on_closed();
+        let on_closed2 = conn.on_closed();
+        drop(conn);
+
+        let (cause, _stats) = on_closed1.await;
+        assert_eq!(cause, ConnectionError::LocallyClosed);
+        let (cause, _stats) = on_closed2.await;
+        assert_eq!(cause, ConnectionError::LocallyClosed);
+    });
+    let (server_res, client_res) = tokio::join!(server_task, client_task);
+    server_res.expect("server task panicked");
+    client_res.expect("client task panicked");
+}
+
+#[tokio::test]
+async fn on_closed_endpoint_drop() {
+    let _guard = subscribe();
+    let factory = EndpointFactory::new();
+    let client = factory.endpoint("client");
+    let server = factory.endpoint("server");
+    let server_addr = server.local_addr().unwrap();
+    let server_task = tokio::time::timeout(
+        Duration::from_millis(500),
+        tokio::spawn(async move {
+            let conn = server
+                .accept()
+                .await
+                .expect("endpoint")
+                .await
+                .expect("accept");
+            println!("accepted");
+            let on_closed = conn.on_closed();
+            drop(conn);
+            drop(server);
+            let (cause, _stats) = on_closed.await;
+            // Depending on timing we might have received a close frame or not.
+            assert!(matches!(
+                cause,
+                ConnectionError::ApplicationClosed(_) | ConnectionError::LocallyClosed
+            ));
+        }),
+    );
+    let client_task = tokio::time::timeout(
+        Duration::from_millis(500),
+        tokio::spawn(async move {
+            let conn = client
+                .connect(server_addr, "localhost")
+                .unwrap()
+                .await
+                .expect("connect");
+            println!("connected");
+            let on_closed = conn.on_closed();
+            drop(conn);
+            drop(client);
+            let (cause, _stats) = on_closed.await;
+            // Depending on timing we might have received a close frame or not.
+            assert!(matches!(
+                cause,
+                ConnectionError::ApplicationClosed(_) | ConnectionError::LocallyClosed
+            ));
+        }),
+    );
+    let (server_res, client_res) = tokio::join!(server_task, client_task);
+    server_res
+        .expect("server timeout")
+        .expect("server task panicked");
+    client_res
+        .expect("client timeout")
+        .expect("client task panicked");
+}