sync: fix missing notification during mpsc close (#2854)

When the mpsc channel receiver closes the channel, receiving should
return `None` once all in-progress sends have completed. When a sender
reserves capacity, this prevents the receiver from fully shutting down.
Previously, when the sender, after reserving capacity, dropped without
sending a message, the receiver was not notified. This results in
blocking the shutdown process until all sender handles drop.

This patch adds a receiver notification when the channel is both closed
and all outstanding sends have completed.

tokio/tests/sync_mpsc.rs

@@ -534,3 +534,25 @@ async fn blocking_send_async() {
     let (mut tx, _rx) = mpsc::channel::<()>(1);
     let _ = tx.blocking_send(());
 }
+
+#[test]
+fn ready_close_cancel_bounded() {
+    use futures::future::poll_fn;
+
+    let (mut tx, mut rx) = mpsc::channel::<()>(100);
+    let _tx2 = tx.clone();
+
+    {
+        let mut ready = task::spawn(async { poll_fn(|cx| tx.poll_ready(cx)).await });
+        assert_ready_ok!(ready.poll());
+    }
+
+    rx.close();
+
+    let mut recv = task::spawn(async { rx.recv().await });
+    assert_pending!(recv.poll());
+
+    drop(tx);
+
+    assert!(recv.is_woken());
+}