Async - waiting for a condition

help
1

Hi,

I want to create a future that resolves when a certain condition becomes true.
I could be using a Condvar but I don't really need a mutex, ideally I would like to "set an event" and complete the future.

My approach so far

fn wait_popup<'a>(condition: &'a bool) -> impl Future<Output = ()> + 'a {
    WaitPopup{condition}
}

struct WaitPopup<'a> {
    condition: &'a bool
}

impl<'a> Future for WaitPopup<'a> {
    type Output = ();

    fn poll(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> std::task::Poll<Self::Output> {
        if *self.condition == true {
            Poll::Pending
        }
        else {
            Poll::Ready(())
        }
    }
}

This is working, but I don't know if I'm reinventing the wheel and there is already something I could use.
Any suggestion of how is this problem commonly solved?
Thanks!

2

Just having a condition you can check is not enough to create a future. You all need a way to notify when the check becomes true. When you return Pending, that is actually a promise that you will emit a wakeup to the waker in the Context argument once you want to be polled again — otherwise the runtime will never poll the future again.

Consider using a channel or tokio::sync::Notify or one of the other things in tokio::sync.

3

Expanding on alice's answer, you can use Context::waker to get a reference to a Waker you can use to signal to the executor that your Future is ready. Note that you can clone it and store/send it somewhere else where you'll know when the condition will become true.

On a sidenote, I don't see how your current code is working. If condition is false when WaitPopup is created then it will never be able to point to true because that would be UB.

4

Thanks for the suggestion @SkiFire13 . I need to explore the Context and Waker API's. For now I think I'll use Notify from Tokio as alice suggested.

And you're right, I tested my code on true/false condition state but I never got to set it because I need to change a few things to use RwMutex in order to be able to use different tasks. I supposed that when a type implemented a future and it was polled once, the runtime would take care of polling it again - which after reading a bit on what is this context argument that I'm not using I understand that I couldn't be more wrong by imagining it would be polled in a loop or something.

Thanks for pointing me on the right direction!

5

An important piece here is that "the runtime" is just more rust code. Nothing is going to happen without you telling it to.

you could write a runtime that polls every future once a second, and then your code would work, but that would also result in a lot of unnecessary work. The design of rusts async system is that executors shouldn't have to do any work unless a future wakes them.

6

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