Combines withTaskCancellationHandler with a CheckedContinuation.

TaskCancellationHandler.swift

import Synchronization

func untilCancelled() async {
	print("1")
	let mutex: Mutex<CheckedContinuation<Void, Never>?> = .init(nil)
	
	await withTaskCancellationHandler {
		print("2")
		await withCheckedContinuation { continuation in
			print("3")
			mutex.withLock {
				if Task.isCancelled {
					print("abort")
					continuation.resume()
					return
				}
				
				print("4")
				$0 = continuation
			}
		}
	} onCancel: {
		let continuation = mutex.withLock {
			print("5")
			return $0.take()
		}
		continuation?.resume()
	}
	
	print("6")
}

func main() async throws {
	let hangsForever = Task {
		await untilCancelled()
	}
	
	// Comment out this line to test early cancellation.
	try await Task.sleep(for: .milliseconds(10))
	
	print("will cancel")
	hangsForever.cancel()
	print("did cancel")
	
	try await Task.sleep(for: .milliseconds(10))
}

try await main()

@EngOmarElsayed we’ve been using this package for a legacy mutex: https://github.com/swhitty/swift-mutex

@Noobish1 About your example… I'm wondering what the perform() would be doing. Because for anything "heavy" you'd probably want an asynchronous context, so you'd need to make a task there. And then, you could leave out the continuation handling entirely and just wrap what you're doing in a cancellation handler.

@EngOmarElsayed I'm not entirely sure what you mean in the reference to the forum post you liked. But if I understand you correctly, then in those cases where you have a Task, you don't need any of the dance here. I talked about this in my post under The simple example.

@macguru perform would be some API that uses closure callbacks that can't be easily converted to async/await.

	var thisIsNotActuallySafe: NonSendable {
		locked.withLock { $0 }
	}

But why this isn't safe, I think there is something I am missing but what is it 😅 ?

I'm not entirely sure what you mean in the reference to the forum post you liked. But if I understand you correctly, then in those cases where you have a Task, you don't need any of the dance here. I talked about this in my post under The simple example.

You are right, I was sharing something I found with you 😅

But why this isn't safe, I think there is something I am missing but what is it 😅 ?

This type, as implemented, makes it possible to send non-Sendable data. That doesn't mean it actually will happen, just that the compiler will not be able to prevent it.

But why this isn't safe, I think there is something I am missing but what is it 😅 ?

This type, as implemented, makes it possible to send non-Sendable data. That doesn't mean it actually will happen, just that the compiler will not be able to prevent it.

Got you, thanks for the explanation