Add extra test with sneaky subscribe()

We have to re-open the channel if a new receiver has subscribed
after closing the channel.

tokio/src/sync/broadcast.rs

@@ -876,6 +876,11 @@ fn new_receiver<T>(shared: Arc<Shared<T>>) -> Receiver<T> {
     assert!(tail.rx_cnt != MAX_RECEIVERS, "max receivers");
 
     tail.rx_cnt = tail.rx_cnt.checked_add(1).expect("overflow");
+    if tail.closed {
+        // Potentially need to re-open the channel, if a new receiver has been added between calls
+        // to poll()
+        tail.closed = false;
+    }
 
     let next = tail.pos;
 

tokio/tests/sync_broadcast.rs

@@ -673,3 +673,37 @@ fn broadcast_sender_closed() {
     assert!(task.is_woken());
     assert_ready!(task.poll());
 }
+
+#[test]
+fn broadcast_sender_closed_with_extra_subscribe() {
+    let (tx, rx) = broadcast::channel::<()>(1);
+    let rx2 = tx.subscribe();
+
+    let mut task = task::spawn(tx.closed());
+    assert_pending!(task.poll());
+
+    drop(rx);
+    assert!(!task.is_woken());
+    assert_pending!(task.poll());
+
+    drop(rx2);
+    assert!(task.is_woken());
+
+    let rx3 = tx.subscribe();
+    assert_pending!(task.poll());
+
+    drop(rx3);
+    assert!(task.is_woken());
+    assert_ready!(task.poll());
+
+    let mut task2 = task::spawn(tx.closed());
+    assert_ready!(task2.poll());
+
+    let rx4 = tx.subscribe();
+    let mut task3 = task::spawn(tx.closed());
+    assert_pending!(task3.poll());
+
+    drop(rx4);
+    assert!(task3.is_woken());
+    assert_ready!(task3.poll());
+}