Cooperative task cancellation bug

[Adrian M.]

Short description of the issue:

Today I started a discussion in the Swift forums regarding cooperative task cancellation and AsyncSequence types. Digging through the implementations of types like Async[Throwing]Stream I discovered that those were not cooperative at all and will straight cut off the buffer from the upstream during cancellation. With the current non-cooperative behavior there's no issue in RxSwift's values implementation. However if the Async[Throwing]Stream were cooperative then the implementation would get stuck and never terminate the stream via onDispose.

Here's a custom wrapper AsyncThrowingCooperativeStream type, which simply avoids the direct cancellation forwarding to the inner AsyncThrowingStream._Storage.cancel() method, which would cause the termination of the buffer by calling AsyncThrowingStream._Storage.finish(). It gives the custom logic such as a wrapped Task or an Observable to properly receive the dispose message and forward its decision through the onDispose back to the captured continuation.

public struct AsyncThrowingCooperativeStream<Element, Failure> where Failure: Error {
  let continuation: AsyncThrowingStream<Element, Failure>.Continuation
  let stream: AsyncThrowingStream<Element, Failure>

  public init(
    _ elementType: Element.Type = Element.self,
    bufferingPolicy limit:
      AsyncThrowingStream<Element, Failure>.Continuation.BufferingPolicy = .unbounded,
    _ build: (AsyncThrowingStream<Element, Failure>.Continuation) -> Void
  ) where Failure == any Error {
    var streamContinuation: AsyncThrowingStream<Element, Failure>.Continuation! = nil
    let stream = AsyncThrowingStream<Element, Failure>(
      elementType,
      bufferingPolicy: limit
    ) { continuation in
      build(continuation)
      streamContinuation = continuation
    }
    self.continuation = streamContinuation
    self.stream = stream
  }
}

extension AsyncThrowingCooperativeStream: AsyncSequence {
  public struct Iterator: AsyncIteratorProtocol {
    let continuation: AsyncThrowingStream<Element, Failure>.Continuation

    // NOTE: This is `@unchecked Sendable` because `AsyncThrowingStream._Context` just captures
    // `AsyncThrowingStream._Storage` which itself is `@unchecked Sendable`, so we're safe here.
    struct _Box: @unchecked Sendable {
      let iterator: AsyncThrowingStream<Element, Failure>.Iterator
    }
    let box: _Box

    public mutating func next() async throws -> Element? {
      let box = self.box
      return try await withTaskCancellationHandler {
        try await withCheckedThrowingContinuation { continuation in
          // Detach the `next` method from the current parent task.
          Task.detached {
            var mutableIterator = box.iterator
            do {
              let element = try await mutableIterator.next()
              continuation.resume(returning: element)
            } catch {
              continuation.resume(throwing: error)
            }
          }
        }
      } onCancel: { [continuation] in
        // Forward the cancellation manually to the termination handler, then remove it so that
        // during a subsequent `next` call we do not signal another cancellation.
        let handler = continuation.onTermination
        continuation.onTermination = nil
        handler?(.cancelled)
      }
    }
  }

  public func makeAsyncIterator() -> Iterator {
    Iterator(continuation: continuation, box: Iterator._Box(iterator: stream.makeAsyncIterator()))
  }
}

extension AsyncThrowingCooperativeStream: Sendable where Element: Sendable {}

Here's the wrap similar to the implementation in the RxSwift module:

public extension ObservableConvertibleType {
  // This is a copy from RxSwift's repo which mimics `Publisher.values`
  var values: AsyncThrowingCooperativeStream<Element, Error> {
    AsyncThrowingCooperativeStream<Element, Error> { continuation in
      let disposable = asObservable().subscribe(
        onNext: { value in
          continuation.yield(value)
        },
        onError: { error in
          continuation.finish(throwing: error)
        },
        onCompleted: {
          continuation.finish(throwing: nil)
        },
        onDisposed: {
          continuation.finish(throwing: CancellationError()) // THE FIX ✅
        }
      )
      continuation.onTermination = { @Sendable _ in
        disposable.dispose()
      }
    }
  }
}

As you can see, onDispose finishes the cooperative stream continuation by throwing a CancellationError. An infinite running Observable which gets deposed without an error will essentially call onDispose, but it will never reach correctly the continuation, at least not a cooperative one.

Note: continuation.finish will change the internal state in such way that a subsequent call to it would result into a no-op. That said, if an Observable completes via onCompleted first, a call to onDisposed will not actually throw CancellationError as terminal would already equal finished and onTermination closure would already be nil-ed.

Expected outcome:

Right now, it's not a bug, but if AsyncThrowingStream ever gets changed to be cooperative on task cancellation, the values implementation will get stuck.

What actually happens:

Nothing right now. The AsyncThrowingStream buffer gets immediately terminated on cancellation and anything coming the Observable subscription will be completely ignored. You can use the above AsyncThrowingCooperativeStream to actually reproduce the potential future issue.

RxSwift/RxCocoa/RxBlocking/RxTest version/commit

Compared with RxSwift 6.5.0

Platform/Environment

Any.

How easy is to reproduce? (chances of successful reproduce after running the self contained code)

• easy, 100% repro

Xcode version:

Version 14.1 (14B47b)

Installation method:

• Swift Package

I have multiple versions of Xcode installed:

• no

Level of RxSwift knowledge:

• I have a significant code base

[Shai Mishali]

Hey @DevAndArtist -
Thanks for the suggestion and interesting linked discussion :)

I appreciate the code sample - for the time being this change in Swift would be considered breaking behavior for the behavior of AsyncStream, so I'd want to make sure it's for real changing before making arbitrary changes.

I'll keep this thread open while the discussion is ongoing and until (if) a resolution is made.

Thanks again!