What's the best way to notify many waiting tasks?

[arthurprs]

Consider an use-case where a producer wants to notify consumers about a change, even if they were doing something else at the time of the notification.

At first Notify and Notify::notify_waiters seem like a fit, but it's not. Because notify_waiters doesn't make the permit available for waiters that are not waiting at the time of the notification.

C: subscribe to S by cloning the Notify
P: notifier.notify_waiters()
C: notifier.notified().await <<< waits even if the permit was provided above

One way to get the expected behavior is to use a broadcast channel with capacity of 1.

C: subscribe to S by calling sender.subscribe()
P: sender.send(())
C: receiver.recv().await <<< wakes up correctly, with either Err(Lagged) or Ok(())

Is there a better way of achieving this behavior?

I did find #3066 but it was closed with a partial(?) solution.

[Darksonn]

I usually recommend a watch channel.

[arthurprs]

A Watch works too 🤔 It's just a bit unergonomic to keep both Sender and Receivers around, as there's no equivalent to subscribe()

[taiki-e]

It's just a bit unergonomic to keep both Sender and Receivers around, as there's no equivalent to subscribe()

I think we can expose Sender::subscribe (it is currently implemented as a private method).

tokio/tokio/src/sync/watch.rs

Line 413 in a08ce0d

pub(crate) fn subscribe(&self) -> Receiver<T> {

[arthurprs]

I think that would create a close to ideal component for this pattern 👍

I still have the felling that Notifier should support such pattern. The current notify_waiters is very easy to misuse and waiters end up waiting forever.

[arthurprs]

Opened #3757 so this is more visible.