Futures-timer: reusing a fused timer with reset in loop?

1

Hi

I want to wait for messages on a future's channel with variable timeout and have something like this:

async fn process(mut recv: UnboundedReceiver<Event>) {
    let mut timer = Delay::new(Duration::from_millis(10)).fuse();
    loop {
        select! {
            ev = recv.next() => {
		// handle event
            }
            _ = timer => {
		// handle timeout
            }
        }
        // want to do this now but can't since timer is fused
        timer.reset(Instant::now() + new_calculated_timeout);
        // have to do it this (wasteful) way instead
        timer = Delay::new(new_calculated_timeout).fuse();
    }
}

I'm thinking there must be some way to do this without having to create a new timer each time.. Am I trying to solve this with the wrong timer api?

2

Why do you need to fuse it?

3

select! requires it

4

Can you not use the select function instead?

5

Wrap Delay and implement FusedFuture. (and Future)
In what you show can probably get away with is_terminated just return false.

1 Like
6

Thanks! I'll give it a try and post my solution/results here after.

7

I'm also considering wrapping future's mpsc receiver to add a next_deadline(deadline: Instant) function...

8

Hmm, I don't see how I can implement future for Delay without modifying the source code. How would I forward the poll call to Delay (which is private)? For my purpose I'm ok with just implementing FusedFuture for Delay directly and keep a custom crate for futures-timer.

9

I think your not understanding what I mean be wrapping. You seem to be describing an extension trait. I am referring to a new structure composed of the original with similar methods/traits.

struct FusedDelay(Delay);
10

Yes this is what I have but struct FusedDelay { delay: Delay }. Implementing FusedFuture is easy, however I also need to implement the Future trait for FusedDelay. From FusedDelay I can't access any of Delay's private functions or fields so there's no easy way of forwarding the poll() call from FusedDelay to Delay.

11

This should be good:

impl Future for FusedDelay {
    type Output = Instant;
    fn poll(self: Pin<&mut Self>, ctx: &mut Context<'_>) -> Poll<Instant> {
        unsafe {
            self.map_unchecked_mut(|fd| &mut fd.delay).poll(ctx)
        }
    }
}

You can avoid unsafe with a crate such as pin-project.

1 Like
12

That's beautiful. Thanks!

13

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