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sync: Remove readiness assertion in `watch::Receiver::changed() #2839

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merged 3 commits into from
Sep 22, 2020
Merged

sync: Remove readiness assertion in `watch::Receiver::changed() #2839

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22a73ab
sync: Remove readiness assertion in `watch::Receiver::changed()
zaharidichev Sep 17, 2020
2d0e3f6
Do not transfer notifications when trigerred by notify_waiters
zaharidichev Sep 18, 2020
3cbc546
Address nits
zaharidichev Sep 22, 2020
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22 changes: 19 additions & 3 deletions tokio/src/sync/tests/loom_watch.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,14 +6,30 @@ use loom::thread;
#[test]
fn smoke() {
loom::model(|| {
let (tx, mut rx) = watch::channel(1);
let (tx, mut rx1) = watch::channel(1);
let mut rx2 = rx1.clone();
let mut rx3 = rx1.clone();
let mut rx4 = rx1.clone();
let mut rx5 = rx1.clone();

let th = thread::spawn(move || {
tx.send(2).unwrap();
});

block_on(rx.changed()).unwrap();
assert_eq!(*rx.borrow(), 2);
block_on(rx1.changed()).unwrap();
assert_eq!(*rx1.borrow(), 2);

block_on(rx2.changed()).unwrap();
assert_eq!(*rx2.borrow(), 2);

block_on(rx3.changed()).unwrap();
assert_eq!(*rx3.borrow(), 2);

block_on(rx4.changed()).unwrap();
assert_eq!(*rx4.borrow(), 2);

block_on(rx5.changed()).unwrap();
assert_eq!(*rx5.borrow(), 2);

th.join().unwrap();
})
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24 changes: 16 additions & 8 deletions tokio/src/sync/watch.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -251,14 +251,22 @@ impl<T> Receiver<T> {
let notified = self.shared.notify_rx.notified();
pin!(notified);

// Polling the future once is guaranteed to return `Pending` as `watch`
// only notifies using `notify_waiters`.
crate::future::poll_fn(|cx| {
let res = Pin::new(&mut notified).poll(cx);
assert!(!res.is_ready());
Poll::Ready(())
})
.await;
// Polling the future here has dual purpose. The first one is to register
// the waiter so when `notify_waiters` is called it is notified. The second
// is to cover the case where another instance of `Notiified` has been dropped
// without receiving its notification. If that was the case polling the
// future for the first time will use this "lost" notification and return
// `Ready` immediatelly without registering any waiter
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a couple typos and grammar nits:

Suggested change
// Polling the future here has dual purpose. The first one is to register
// the waiter so when `notify_waiters` is called it is notified. The second
// is to cover the case where another instance of `Notiified` has been dropped
// without receiving its notification. If that was the case polling the
// future for the first time will use this "lost" notification and return
// `Ready` immediatelly without registering any waiter
// Polling the future here has a dual purpose. The first one is to register
// the waiter so that it is notified when `notify_waiters` is called. The second
// is to cover the case where another instance of `Notified` has been dropped
// without receiving its notification. If this has happened, polling the future
// for the first time will use this "lost" notification and return `Ready`
// immediately without registering any waiter.

let aquired_lost_notification =
crate::future::poll_fn(|cx| match Pin::new(&mut notified).poll(cx) {
Poll::Ready(()) => Poll::Ready(true),
Poll::Pending => Poll::Ready(false),
})
.await;

if aquired_lost_notification {
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nit, take it or leave it: "acquired_lost_notification" is kind of a mouthful, what about something like

Suggested change
let aquired_lost_notification =
crate::future::poll_fn(|cx| match Pin::new(&mut notified).poll(cx) {
Poll::Ready(()) => Poll::Ready(true),
Poll::Pending => Poll::Ready(false),
})
.await;
if aquired_lost_notification {
let already_notified =
crate::future::poll_fn(|cx| match Pin::new(&mut notified).poll(cx) {
Poll::Ready(()) => Poll::Ready(true),
Poll::Pending => Poll::Ready(false),
})
.await;
if already_notified {

(also, "acquired" is spelled wrong in the code, so we should fix that even if we don't change the name :) )

return Ok(());
}

if let Some(ret) = maybe_changed(&self.shared, &mut self.version) {
return ret;
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