Introduce notified_owned for thread-safe notification futures.

[Sagebati]

Allows returning an owned notification future that can be moved across threads. Inspired by #7231

Motivation

A common pattern when using Notify is to launch tasks and then notify them at the same time using notify_waiters. In the current API it can be quite challenging because you don't know if the task got the time to register before notifying. A good way of doing it would be to register then launch the task. Problem to do that you would need the result future from notified() (aka Notified<'_>) to be Send.

Ex:

let n = Arc::new(Notify::new()); // We can't use notified here because we need to send it.

tokio::spawn({
  let n = n.clone();
  async move {
    // if we need the code to be correct we need to register the waiter here
    tokio::sleep(10).await;
    n.notified().await;
    println!("event received");
}})

n.notify_waiters();

Ex: Workaround using a oneshot and Shared<_>

let (sx, rx) = oneshot();
let rx = rx.shared();

for _ in 0..10 {
  let rx = rx.clone();
  tokio::spawn(async move { rx.clone().await; println!("event received"); });
}

sx.send(());

Solution

Added a new method notified_owned that returns Send Notified<'static>.

[ADD-SP]

I think #7231 is a reasonable use case , and this issue is similar to #1998.

Could you add some tests against this new feature? Also we'd better aligning the naming pattern of tokio::sync::OwnedSemaphorePermit instead of Notified<'static>, how about OwnedNotified?

[Darksonn]

I'm not sure if we want more future types, but if we do, it has to be a new type. An enum pays a cost for everyone using notified, and we don't want to go that route.

[Sagebati]

Fair, I didn't take into account the cost of the access, any workaround for not repeating the implementation between Notified and NotifiedOwned ? I suppose using an union is cursed, maybe a macro ?

[Darksonn]

Let's avoid unions or macros. How about extracting common logic into shared functions?

[ariaandika]

Hi, any update on this ? if you’re no longer working on it, id be happy to take it over.

[Sagebati]

Sorry, you can take over. I don't have time right now to work on it Le ven. 18 juil. 2025, 13:44, Aria Andika ***@***.***> a écrit :

…

*ariaandika* left a comment (tokio-rs/tokio#7391) <#7391 (comment)> Hi, any update on this ? if you’re no longer working on it, id be happy to take it over. — Reply to this email directly, view it on GitHub <#7391 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACQOJGDT6TW66SDQUKG3QJ33JDM3BAVCNFSM6AAAAAB6XSFJGCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTAOBZGIYDGMJSHE> . You are receiving this because you authored the thread.Message ID: ***@***.***>