broadcast: Stop notifying after we've woken all wakers

Unboundedly looping within `notify_rx` as long as there are still
available wakers causes a quadratic slowdown as receivers which are
looping receiving from the channel are added. Instead of continually
waiting for new wakers, this commit modifies `notify_rx` to stop trying
to wake wakers once we've notified a number of wakers greater than or
equal to whatever the number of active wakers was when we started
notifying.

Fixes #5923

tokio/src/sync/broadcast.rs

@@ -820,6 +820,8 @@ fn new_receiver<T>(shared: Arc<Shared<T>>) -> Receiver<T> {
 impl<T> Shared<T> {
     fn notify_rx<'a, 'b: 'a>(&'b self, mut tail: MutexGuard<'a, Tail>) {
         let mut wakers = WakeList::new();
+        let active_receivers = tail.rx_cnt;
+        let mut woken = 0usize;
         'outer: loop {
             while wakers.can_push() {
                 match tail.waiters.pop_back() {
@@ -832,6 +834,7 @@ impl<T> Shared<T> {
 
                         if let Some(waker) = waiter.waker.take() {
                             wakers.push(waker);
+                            woken += 1;
                         }
                     }
                     None => {
@@ -850,6 +853,10 @@ impl<T> Shared<T> {
 
             wakers.wake_all();
 
+            if woken >= active_receivers {
+                return;
+            }
+
             // Acquire the lock again.
             tail = self.tail.lock();
         }