embassy-time: Timer with_cancellation

[kelko]

TL;DR:

I want to able to configure a Timer::after(time).await call to return early once I need to (similar to .NET Task.Delay with additional cancellation token as parameter).

I can combine Timer::after already with with_timeout or with_deadline if I see this correctly. A with_cancellation would be nice as well for situations I don't know up front when I need to cancel a timer (or generally a Future), similar to the CancellationToken in .NET.

background

I have a task that waits a certain amount of time to set a flag (AtomicBool) used in other places to switch some mode. This way I switch through modes in an automatic fashion to control what I display.

static CANCEL_PAGE: AtomicBool = AtomicBool::new(false);

pub async fn rotate(&self, spawner: Spawner) -> ! {
  // ...
  loop {
    // state handling for the loops current iteration
    let duration = // calculate duration for next cycle
    spawner.must_spawn(next_pane_counter(duration));
    // main logic stuff. In some internal places I check CANCEL_PAGE and break to the main loop if necessary
  }
}

#[embassy_executor::task]
async fn next_pane_counter(time: Duration) {
    Timer::after(time).await;
    CANCEL_PAGE.store(true, Ordering::Release);
}

That works fine. Maybe not best way to solve it, but it works.

Only now I want to be able to react on external interrupts (like button presses). I made a different function which also can set CANCEL_PAGE, and that works so far. But once the main loop continues a new next_pane_counter is spawned - and will crash once the pool_size is overstepped. But I don't know at compile time what the pool size needs to be, as the interrupts can come quickly.

Cancelling the previously spawned task is requested (see #3197) but not yet possible.
Being able to return early from the timer inside next_pane_counter would allow me to handle that situation on my own. But I don't see any way to return early from the timer.

[swork]

Could next_pane_counter take a embassy-sync's Channel::Receiver and select() on it and the timer? A message would serve as your timer-cancellation. Something like this:

static CANCEL_PAGE: AtomicBool = AtomicBool::new(false);

pub async fn rotate(&self, spawner: Spawner) -> ! {
  loop {
    let duration = whatever;
    let channel: Channel<NoopRawMutex, u8, 1> = Channel::new();
    spawner.must_spawn(next_pane_counter(duration, channel.receiver()));
    // at some point before the end of the loop:
    channel.send(b'X');
    // An await point is probably necessary to give next_pane_counter a chance to run.
  }
}

#[embassy_executor::task]
async fn next_pane_counter(time: Duration, rec: Receiver) {
    select(Timer::after(time), rec.receive()).await;
    CANCEL_PAGE.store(true, Ordering::Release);
}

[kelko]

Interesting. I wasn't aware of select.
I have to check that out. Thanks.

[kelko]

@swork Where does the select method in your example come from? Which crate?