Event sync primitive

[wyfo]

In quite a few projects, I found myself repeating the same pattern of an Event primitive, implemented like this:

use std::sync::atomic::{AtomicBool, Ordering};

use tokio::sync::Notify;

#[derive(Debug, Default)]
pub struct Event {
    triggered: AtomicBool,
    notify: Notify,
}

impl Event {
    pub fn new() -> Self {
        Self::default()
    }

    pub const fn const_new() -> Self {
        Self {
            triggered: AtomicBool::new(false),
            notify: Notify::const_new(),
        }
    }

    pub fn is_set(&self) -> bool {
        self.triggered.load(Ordering::Acquire)
    }

    pub fn set(&self) {
        if !self.triggered.swap(true, Ordering::Release) {
            self.notify.notify_waiters();
        }
    }

    pub fn clear(&self) {
        self.triggered.store(false, Ordering::Release);
    }

    pub async fn wait(&self) {
        if !self.is_set() {
            let notified = self.notify.notified();
            if !self.is_set() {
                notified.await;
            }
        }
    }
}

This Event (sometimes named Signal) may indeed be considered as a basic synchronization primitive, and Python for example even include it in its standard library.
So I was wondering if it could be included directly in tokio::sync module — the methods may be renamed, I took the names from Python for this example implementation.

[Darksonn]

I have often wanted a variant of OnceCell that works like your Event except that you can set a value when you call set, which is made available to the caller of wait.

We wont add it to OnceCell itself to avoid making the type larger, but we could have a new cell type.

[wyfo]

Actually, I've think it possible to add a wait_initialized method directly to OnceCell, so I've opened a #7161 to do it. And as a side effect, it even makes the type smaller.

As written in the PR, the Event primitive could then be defined as struct Event(OnceCell<()>).

[wyfo]

I've pushed another branch wyfo@e4ecb2f to add the Event primitive on top of the new OnceCell if the idea is accepted.