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()

This is locking pattern known to be easy to use wrong in subtle ways. It can be used safely, but is very easy to use wrong. The core problem is as implemented LegacyMutex actually isn't fully Sendable. Here's an example:

class NonSendable {}

struct UsesLegacyMutex: Sendable {
	private let locked = LegacyMutex(NonSendable())

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

@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