[Concurrency] Continuations for interfacing async tasks with synchronous code

[ktoso (Konrad 'ktoso' Malawski 🐟🏴‍☠️)]

@gwendal.roue

@ktoso, do we have to expect developers to wrap many existing asynchronous APIs that support cancellation, such as completion blocks, Combine publishers, other reactive tools, DispatchQueue/thread/mutex/lock-based APIs, etc? Maybe those APIs will eventually be migrated to Swift's async model. But some will not. And this eventual migration will take time.

Meanwhile, do we really want to ignore cancellation?

Nothing is being ignored, it has nothing to do with this specific API, so there's no reason to cram in more complexity into it.

Through composition with APIs proposed in structured concurrency, it is able to handle cancelation when necessary, exactly like @jayton suggests (that's one of the ways):

@jayton

Based on the current structured concurrency pitch, I would expect this to be written as follows, no continuation-specific cancellation functionality needed:

func download(url: URL) async throws -> Data? {
    var urlSessionTask: URLSessionTask?

    return try await Task.withCancellationHandler { urlSessionTask?.cancel() } // was missing await here
    operation: {
        return try await withUnsafeThrowingContinuation { continuation in
            urlSessionTask = URLSession.shared.dataTask(with: url) { data, _, error in
                if let error = error {
                    // Ideally translate NSURLErrorCancelled to CancellationError here
                    continuation.resume(throwing: error)
                } else {
                    continuation.resume(returning: data)
                }
            }
            urlSessionTask?.resume()
        }
    }
}

Note that since you're in an async function already (in download), you can normally Task.isCanceled check before submitting any work as well.

Please refer to [Pitch #2] Structured Concurrency - #116 by ktoso for more details on structured concurrency and those APIs; If we made Task a value, then the insides of the continuation can check for it: task.isCanceled as well.