webrtc: Avoid future being dropped when channel is full

.gitignore

@@ -1,3 +1,2 @@
 /target
 .idea
-

src/transport/webrtc/substream.rs

@@ -24,9 +24,10 @@ use crate::{
 };
 
 use bytes::{Buf, BufMut, BytesMut};
-use futures::{Future, Stream};
+use futures::Stream;
 use parking_lot::Mutex;
 use tokio::sync::mpsc::{channel, Receiver, Sender};
+use tokio_util::sync::PollSender;
 
 use std::{
     pin::Pin,
@@ -59,16 +60,17 @@ enum State {
     SendClosed,
 }
 
-/// Channel-backedn substream.
+/// Channel-backed substream. Must be owned and polled by exactly one task at a time.
 pub struct Substream {
     /// Substream state.
     state: Arc<Mutex<State>>,
 
     /// Read buffer.
     read_buffer: BytesMut,
 
-    /// TX channel for sending messages to `peer`.
-    tx: Sender<Event>,
+    /// TX channel for sending messages to `peer`, wrapped in a [`PollSender`]
+    /// so that backpressure is driven by the caller's waker.
+    tx: PollSender<Event>,
 
     /// RX channel for receiving messages from `peer`.
     rx: Receiver<Event>,
@@ -80,6 +82,7 @@ impl Substream {
         let (outbound_tx, outbound_rx) = channel(256);
         let (inbound_tx, inbound_rx) = channel(256);
         let state = Arc::new(Mutex::new(State::Open));
+
         let handle = SubstreamHandle {
             tx: inbound_tx,
             rx: outbound_rx,
@@ -89,7 +92,7 @@ impl Substream {
         (
             Self {
                 state,
-                tx: outbound_tx,
+                tx: PollSender::new(outbound_tx),
                 rx: inbound_rx,
                 read_buffer: BytesMut::new(),
             },
@@ -102,10 +105,10 @@ impl Substream {
 pub struct SubstreamHandle {
     state: Arc<Mutex<State>>,
 
-    /// TX channel for sending messages to `peer`.
+    /// TX channel for sending inbound messages from `peer` to the associated `Substream`.
     tx: Sender<Event>,
 
-    /// RX channel for receiving messages from `peer`.
+    /// RX channel for receiving outbound messages to `peer` from the associated `Substream`.
     rx: Receiver<Event>,
 }
 
@@ -190,49 +193,45 @@ impl tokio::io::AsyncRead for Substream {
 
 impl tokio::io::AsyncWrite for Substream {
     fn poll_write(
-        self: Pin<&mut Self>,
+        mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
         buf: &[u8],
     ) -> Poll<Result<usize, std::io::Error>> {
         if let State::SendClosed = *self.state.lock() {
             return Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into()));
         }
 
-        // TODO: try to coalesce multiple calls to `poll_write()` into single `Event::Message`
-
-        let num_bytes = std::cmp::min(MAX_FRAME_SIZE, buf.len());
-        let future = self.tx.reserve();
-        futures::pin_mut!(future);
-
-        let permit = match futures::ready!(future.poll(cx)) {
+        match futures::ready!(self.tx.poll_reserve(cx)) {
+            Ok(()) => {}
             Err(_) => return Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into())),
-            Ok(permit) => permit,
         };
 
+        let num_bytes = std::cmp::min(MAX_FRAME_SIZE, buf.len());
         let frame = buf[..num_bytes].to_vec();
-        permit.send(Event::Message(frame));
 
-        Poll::Ready(Ok(num_bytes))
+        match self.tx.send_item(Event::Message(frame)) {
+            Ok(()) => Poll::Ready(Ok(num_bytes)),
+            Err(_) => Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into())),
+        }
     }
 
     fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), std::io::Error>> {
         Poll::Ready(Ok(()))
     }
 
     fn poll_shutdown(
-        self: Pin<&mut Self>,
+        mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
     ) -> Poll<Result<(), std::io::Error>> {
-        let future = self.tx.reserve();
-        futures::pin_mut!(future);
-
-        let permit = match futures::ready!(future.poll(cx)) {
+        match futures::ready!(self.tx.poll_reserve(cx)) {
+            Ok(()) => {}
             Err(_) => return Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into())),
-            Ok(permit) => permit,
         };
-        permit.send(Event::Close);
 
-        Poll::Ready(Ok(()))
+        match self.tx.send_item(Event::Close) {
+            Ok(()) => Poll::Ready(Ok(())),
+            Err(_) => Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into())),
+        }
     }
 }
 
@@ -458,4 +457,47 @@ mod tests {
         )
         .await;
     }
+
+    #[tokio::test]
+    async fn backpressure_released_wakes_blocked_writer() {
+        use tokio::time::{sleep, timeout, Duration};
+
+        let (mut substream, mut handle) = Substream::new();
+
+        // Fill the channel to capacity, same pattern as `backpressure_works`.
+        for _ in 0..128 {
+            substream.write_all(&vec![0u8; 2 * MAX_FRAME_SIZE]).await.unwrap();
+        }
+
+        // Spawn a writer task that will try to write once more. This should initially block
+        // because the channel is full and rely on the AtomicWaker to be woken later.
+        let writer = tokio::spawn(async move {
+            substream
+                .write_all(&vec![1u8; MAX_FRAME_SIZE])
+                .await
+                .expect("write should eventually succeed");
+        });
+
+        // Give the writer a short moment to reach the blocked (Pending) state.
+        sleep(Duration::from_millis(10)).await;
+        assert!(
+            !writer.is_finished(),
+            "writer should be blocked by backpressure"
+        );
+
+        // Now consume a single message from the receiving side. This will:
+        //  - free capacity in the channel
+        //  - call `write_waker.wake()` from `poll_next`
+        //
+        // That wake must cause the blocked writer to be polled again and complete its write.
+        let _ = handle.next().await.expect("expected at least one outbound message");
+
+        // The writer should now complete in a timely fashion, proving that:
+        //  - registering the waker before `try_reserve` works (no lost wakeup)
+        //  - the wake from `poll_next` correctly unblocks the writer.
+        timeout(Duration::from_secs(1), writer)
+            .await
+            .expect("writer task did not complete after capacity was freed")
+            .expect("writer task panicked");
+    }
 }