Async, Rethrows, and Defer

[bbrk24]

@jrose

await Task {

In Swift, unlike e.g. C#, it’s not the task itself that’s awaited. I’m pretty sure this would have to be await Task { … }.value.

[FranzBusch]

@jrose

async let _cleanup = ({
  await Task.sleep(nanoseconds: .max)
  await Task {
    await actualCleanup()
  }
})()

Ignoring the missing .value this does work and is the current way to have an enforced clean-up, but we have to resort to unstructured Concurrency which indicates that we are lacking a feature here. We ought to be able to express everything with structured Concurrency so that the compiler can reason about this.

Having a defer where we can opt-out of cancellation would be great.

[pyrtsa (Pyry Jahkola)]

@jrose

await Task.sleep(nanoseconds: .max)

Fun gotcha: In my experience, Task.sleep(nanoseconds:) returns immediately if you do that. Sleeping the current task next-to-forever seems to work with .max / 2 nanoseconds however. Maybe it's internally reinterpreting the UInt64 as signed?

(Also you probably mean to try? await that so it handles and ignores CancellationError.)

[bbrk24]

@pyrtsa

Maybe it's internally reinterpreting the UInt64 as signed?

This would check out: setting only the MSb (i.e. (.max >> 1) + 1 or ~UInt64(Int64.max)) returns just as immediately. That should either be fixed or documented.

[rauhul (Rauhul Varma)]

The source of that bug is described here: Fix bug in Dispatch executor preventing long sleeps by rauhul · Pull Request #64304 · apple/swift · GitHub but I haven't found the time to follow up on my patch.