sync: add `Notify::notify_waiters` (#3098)

This PR makes `Notify::notify_waiters` public. The method
already exists, but it changes the way `notify_waiters`,
is used. Previously in order for the consumer to
register interest, in a notification triggered by
`notify_waiters`, the `Notified` future had to be
polled. This introduced friction when using the api
as the future had to be pinned before polled.

This change introduces a counter that tracks how many
times `notified_waiters` has been called. Upon creation of
the future the number of times is loaded. When first
polled the future compares this number with the count
state of the `Notify` type. This avoids the need for
registering the waiter upfront.

Fixes: #3066

1 files changed, 21 insertions, 0 deletions

diff --git a/tokio/src/sync/tests/loom_notify.rs b/tokio/src/sync/tests/loom_notify.rs
index 79a5bf89..4be949a3 100644
--- a/tokio/src/sync/tests/loom_notify.rs
+++ b/tokio/src/sync/tests/loom_notify.rs
@@ -22,6 +22,27 @@ fn notify_one() {
 }
 
 #[test]
+fn notify_waiters() {
+    loom::model(|| {
+        let notify = Arc::new(Notify::new());
+        let tx = notify.clone();
+        let notified1 = notify.notified();
+        let notified2 = notify.notified();
+
+        let th = thread::spawn(move || {
+            tx.notify_waiters();
+        });
+
+        th.join().unwrap();
+
+        block_on(async {
+            notified1.await;
+            notified2.await;
+        });
+    });
+}
+
+#[test]
 fn notify_multi() {
     loom::model(|| {
         let notify = Arc::new(Notify::new());