[BUG] Possible deadlock in Async subjects #52
Description
Describe the bug
It looks like subject can deadlock if task, that is observing its values is cancelled at the same time as send is called on it
Thread 1:
AsyncBufferedChannel.sendacquiresos_unfair_lockAsyncBufferedChannel.sendattempts to resume continuation returning element- Resuming continuation attempts to acquire internal lock for task status
Thread 2:
- Task that awaits for values from subject is cancelled. This acquires internal lock for task state
unregister()is called and attempts to acquireos_unfair_lock
Thread 1 has os_unfair_lock, waits for internal lock
Thread 2 has internal lock, waits for os_unfair_lock
Situation can be reproduced with test like this, if run repeatedly for 100 times:
@Test
func send_deadlocks_when_cancelled_concurrently() throws {
let subject = AsyncPassthroughSubject<Int>()
let t1 = Task.detached {
for await _ in subject { }
}
let barrier = DispatchSemaphore(value: 0)
let done = DispatchSemaphore(value: 0)
DispatchQueue.global().async {
barrier.wait()
t1.cancel()
done.signal()
}
DispatchQueue.global().async {
barrier.wait()
subject.send(1)
done.signal()
}
barrier.signal()
barrier.signal()
done.wait()
done.wait()
}
It can also be reproduced using strict concurrency only, but setting up actual parallel execution is more complicated.
Additional context