sync: tweak `watch` API (#2814)

Decouples getting the latest `watch` value from receiving the change
notification. The `Receiver` async method becomes
`Receiver::changed()`. The latest value is obtained from
`Receiver::borrow()`.

The implementation is updated to use `Notify`. This requires adding
`Notify::notify_waiters`. This method is generally useful but is kept
private for now.

1 files changed, 20 insertions, 0 deletions

diff --git a/tokio/src/sync/tests/loom_watch.rs b/tokio/src/sync/tests/loom_watch.rs
new file mode 100644
index 00000000..7944cab8
--- /dev/null
+++ b/tokio/src/sync/tests/loom_watch.rs
@@ -0,0 +1,20 @@
+use crate::sync::watch;
+
+use loom::future::block_on;
+use loom::thread;
+
+#[test]
+fn smoke() {
+    loom::model(|| {
+        let (tx, mut rx) = watch::channel(1);
+
+        let th = thread::spawn(move || {
+            tx.send(2).unwrap();
+        });
+
+        block_on(rx.changed()).unwrap();
+        assert_eq!(*rx.borrow(), 2);
+
+        th.join().unwrap();
+    })
+}