AsyncStream(unfolding:onCancel:) onCancel block is called twice

[BrentMifsud]

Description

When cancelling an AsyncStream(unfolding:onCancel:), the onCancel block is executed twice.

Reproduction

let task = Task {
    let stream = AsyncStream {
        try? await Task.sleep(for: .seconds(1))
        return Int.random(in: 1...100)
    } onCancel: {
        print("Stream cancelled") // this prints twice
    }

    for await value in stream {
        print("Received value: \(value)")
    }
}

Task {
    try? await Task.sleep(for: .seconds(5))
    task.cancel()
}

also attached a playground that reproduces the issue:

AsyncStreamCancellation.playground.zip

Expected behavior

the onCancel block should only be called once

Environment

swift-driver version: 1.127.8 Apple Swift version 6.2 (swiftlang-6.2.0.13.10 clang-1700.3.13.4)
Target: arm64-apple-macosx15.0

Also happens in Xcode 26 beta 3, so the issue is not fixed on the current swift 6.2 beta toolchain.

Additional information

No response

[jamieQ]

looked into this a bit, and i think what's happening in the example is:

1. an iterator is suspended awaiting a return from the unfolding closure (i.e. the client-supplied closure is executing)
2. the iterator's Task is canceled – this causes onCancel to be invoked
3. the iterator returns a non-nil value from the suspended unfolding closure after it resumes
4. the iterator invokes next() once more (since it hasn't yet returned nil), which calls withTaskCancellationHandler again, invoking onCancel another time

i think this could be prevented fairly straightforwardly by clearing the onCancel reference once it's been called, but that does require introducing some additional synchronization in some manner. opened a draft PR with a possible fix here: #83190