[SR-14841] Concurrency: Resuming a stored continuation from an actor does not work

[mickeyl]

Previous ID	SR-14841
Radar	rdar://problem/79926026
Original Reporter	@mickeyl
Type	Bug
Status	Closed
Resolution	Done

Environment

• Xcode 13 beta 4, iOS 15 simulator on macOS 11.4 on Mac6,1 (2013).

• Xcode 13 beta 4, iOS 15 simulator on macOS 12b4 on M1-Mac (2020).

• Xcode 13 beta 4, macOS/Catalyst on macOS 12b4 on M1-Mac (2020).

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 42907e573028b4f88cb3e358fca70d55

relates to:

• SR-14875 Resuming continuations from actor contexts can hang

Issue Description:

Please consider the following example program which seems to expose an incompatibility when storing a continuation somewhere and resuming it later from within an actor.

• Changing the `actor` to a `class` makes the program work.

import Foundation
typealias Continuation = CheckedContinuation<String, Error>

public enum StreamError: Error {
    case invalidEncoding
}

public class StreamCommand {
    let continuation: Continuation

    init(continuation: Continuation) {
        self.continuation = continuation
    }

    func resumeContinuation() {
        let response = "fooBar"
        print("<triggering continuation resume> \(self.continuation)")
        self.continuation.resume(returning: response)
        print("</triggering continuation resume>")
    }
}

public actor StreamCommandQueue: NSObject {
    var activeCommand: StreamCommand?

    func send(string: String, timeout: TimeInterval) async throws -> String {
        print("awaiting...")
        let response: String = try await withCheckedThrowingContinuation { continuation in
            print("continuation: \(continuation)")
            self.activeCommand = StreamCommand(continuation: continuation)
            self.outputActiveCommand()
        }
        print("came back after awaiting")
        return response
    }

    func outputActiveCommand() {
        async {
            self.activeCommand?.resumeContinuation()
        }
    }

    func inputActiveCommand() { }
}

func doIt() {
    async {
        let streamQueue = StreamCommandQueue()
        do {
            let identification = try await streamQueue.send(string: "ATI\r", timeout: 1)
            print("identification: \(identification)")
        } catch {
            print("can't get identification: \(error)")
        }
    }
}

[typesanitizer]

@swift-ci create

[DougGregor]

This is working for me with the main and 5.5 branches. However, I note that this is missing the code to run doIt() anywhere. If you only had:

doIt()

then the program will likely exit before the async task completes. Put a silly

Thread.sleep(forTimeInterval: 1)

after it and things should be fine. Or, better yet, use `@main` with an async main function

[mickeyl]

@DougGregor Thanks for taking a look at it.

Obviously the example file was not self-contained, but a stripped down part of a larger program, my bad.

It's still not working for me with the latest 5.5 snapshot. Please use the example project @ https://github.com/mickeyl/swift-bugs and run it in iOS 15 simulator.

With the `StreamCommandQueue being an `actor`, the output is:

awaiting... continuation: CheckedContinuation<String, Error>(canary: Swift.CheckedContinuationCanary)
<triggering continuation resume> CheckedContinuation<String, Error>(canary: Swift.CheckedContinuationCanary)
</triggering continuation resume>

The continuation hangs.

With the `StreamCommandQueue` being a `class`, the output is:

awaiting... continuation: CheckedContinuation<String, Error>(canary: Swift.CheckedContinuationCanary) 
<triggering continuation resume> CheckedContinuation<String, Error>(canary: Swift.CheckedContinuationCanary)
</triggering continuation resume>
came back after awaiting

The continuation continues (sic!).

[mickeyl]

No changes in Xcode 12b3.

[mickeyl]

No changes in Xcode 12b4. It would be nice if this issue would either be confirmed or If I'm "holding it wrong", please tell me how actors and continuations are supposed to be used.

[typesanitizer]

Hi, this is confirmed in Xcode 13 beta 4 release notes (https://developer.apple.com/documentation/xcode-release-notes/xcode-13-beta-release-notes) under Known Issues

Resuming a stored continuation in a Task.init(priority:operation:) context (previously async(priority:operation:)). (SR-14802, SR-14841, SR-14875) 

with a potential workaround of using a detached task instead of Task.init/async { }.

[mickeyl]

This seems to work now with Xcode13b5

Swift version 5.5 (swiftlang-1300.0.29.102 clang-1300.0.28.1)

on macOS 12 beta.