help understanding when clock.sleep(for:) might throw

[jaredh159]

Clock.sleep(for:) is -> async throws. I'm wanting to use the pattern shown in the docs like this:

self.heartbeatTask = Task {
  while true {
    try await self.clock.sleep(for: .seconds(90))
    self.doImportantThing()
  }
}

But I'm struggling to understand when this might throw, and if I need to worry about it. For context, I want this repeated chunk of work to ALWAYS happen, it's "safety critical" in the sense that it's for a parental controls app, and there is important work I want to do periodically to keep my contract with the parents, so I don't want some unexpected error case I hadn't thought of to tear down the task without my knowledge.

But I was having trouble finding docs here or in Apple's frameworks about why and when this might throw. I thought from some things I read that it might be the case that it only throws if I cancel the task? Is that correct? If I never cancel this task, can I be confident that it will always keep looping? (I'm using a suspending variant, in case that helps). Or do I need to wrap it with a do/catch and restart as a safeguard?

If there's any documentation of the conditions for throwing that someone could point me to, I'd be obliged.

Thanks for the wonderful library! 🙏

[mbrandonw]

Hi @jaredh159, I don't think it's documented, but you can look through the source code of ContinuousClock and other clocks to see that the only error ever thrown is CancellationError. And so it only throws if the surrounding async context is cancelled, such as your heartbeatTask. This would have been a very good use of typed throws to show that sleep only throws one kind of error.

I don't see any reason for you to force unwrap though. You can never be sure that you will never cancel the async context, and so it is better to not crash if that does happen. You can simply ignore it if you want with a do/catch, or you can let your Task be a throwing task.

Additionally, I would recommend using the timer method on clocks for this kind of thing instead of a while with a sleep, which is susceptible to drifting due to small inaccuracies in sleep.

[jaredh159]

thanks so much for the reply, that helps a lot. maybe i'll switch to timer... although i don't really care at all about minor clock drifts, just that the work is run roughly periodically.

I'm not sure what you meant by force-unwrapping though, I didn't unwrap in the sample code... I didn't show that I was holding on to an optional task, like:

var heartbeatTask: Task<Void, Error>?

and then assigning it. am I missing something about what you meant?

[mbrandonw]

Ah no, I just misread your post. I thought you had mentioned force unwrapping.