sync: implement Clone for broadcast::Receiver (#2933)

tokio/src/sync/broadcast.rs

@@ -405,7 +405,8 @@ const MAX_RECEIVERS: usize = usize::MAX >> 2;
 ///
 /// The `Sender` can be cloned to `send` to the same channel from multiple
 /// points in the process or it can be used concurrently from an `Arc`. New
-/// `Receiver` handles are created by calling [`Sender::subscribe`].
+/// `Receiver` handles can be cloned from an existing `Receiver` or created by
+/// calling [`Sender::subscribe`].
 ///
 /// If all [`Receiver`] handles are dropped, the `send` method will return a
 /// [`SendError`]. Similarly, if all [`Sender`] handles are dropped, the [`recv`]
@@ -569,19 +570,7 @@ impl<T> Sender<T> {
     /// ```
     pub fn subscribe(&self) -> Receiver<T> {
         let shared = self.shared.clone();
-
+        new_receiver(shared)
-        let mut tail = shared.tail.lock();
-
-        if tail.rx_cnt == MAX_RECEIVERS {
-            panic!("max receivers");
-        }
-
-        tail.rx_cnt = tail.rx_cnt.checked_add(1).expect("overflow");
-        let next = tail.pos;
-
-        drop(tail);
-
-        Receiver { shared, next }
     }
 
     /// Returns the number of active receivers
@@ -671,6 +660,22 @@ impl<T> Sender<T> {
     }
 }
 
+fn new_receiver<T>(shared: Arc<Shared<T>>) -> Receiver<T> {
+    let mut tail = shared.tail.lock();
+
+    if tail.rx_cnt == MAX_RECEIVERS {
+        panic!("max receivers");
+    }
+
+    tail.rx_cnt = tail.rx_cnt.checked_add(1).expect("overflow");
+
+    let next = tail.pos;
+
+    drop(tail);
+
+    Receiver { shared, next }
+}
+
 impl Tail {
     fn notify_rx(&mut self) {
         while let Some(mut waiter) = self.waiters.pop_back() {
@@ -980,6 +985,13 @@ impl<T: Clone> Receiver<T> {
     }
 }
 
+impl<T> Clone for Receiver<T> {
+    fn clone(&self) -> Self {
+        let shared = self.shared.clone();
+        new_receiver(shared)
+    }
+}
+
 impl<T> Drop for Receiver<T> {
     fn drop(&mut self) {
         let mut tail = self.shared.tail.lock();

tokio/src/sync/tests/loom_broadcast.rs

@@ -92,11 +92,12 @@ fn broadcast_two() {
     });
 }
 
+// Exercise the Receiver Clone impl as well
 #[test]
 fn broadcast_wrap() {
     loom::model(|| {
         let (tx, mut rx1) = broadcast::channel(2);
-        let mut rx2 = tx.subscribe();
+        let mut rx2 = rx1.clone();
 
         let th1 = thread::spawn(move || {
             block_on(async {