Possible Deadlock in BehaviorSubject.Subscribe

[Joost-Jens-Luminis]

Possible Deadlock in BehaviorSubject.Subscribe

[akarnokd]

BehaviorSubject is very special in that it promises to emit the stored value to the incoming observer before it relays future values. The trouble is, a concurrent BehaviorSubject::OnNext call can happen while a BehaviourSubject::Subscribe is ongoing. OnNext mustn't be called concurrently so the two method calls must be serialized, hence the emission under the lock.

This happened in our case as the subscription was cancelled before the Subscribe was even called

The IDisposable returned by the BehaviorSubject::Subscribe can't be disposed upfront as it does not exist until the method returns it. There must have been something else causing the deadlock. Could you please share more details about the relevant code of yours?

[Joost-Jens-Luminis]

Thank you for your quick reply, it's much appreciated.

Thinking about it, I agree that the Dispose couldn't cause the deadlock, except for when a subscribe causes a dispose of a previous subscription (but why should it?). I must have been confused as several dispose calls were waiting to get the lock and in the stacktrace that was always as a result of an Subscribe call. This was on Android though which may have caused issues in the stacktrace.

To be honest, there were a bunch of issues happening at the same time divided over several threads (up to 15 according to the parallel stacks window) which caused problems, making it difficult to separate which calls exactly caused the deadlock. (It took me a week and half to figure it all out)
I do know that the dispose was involved and making this change solved the deadlock for us. :)

As for what emits when; if the promise that the stored value is first emitted before any OnNext calls is sacred, then I agree my suggested change can't be made, even though it is unlikely an incoming OnNext call is emiting before the Subscribe completes. But if you believe it can happen (and I do think it can in theory), then by extension, there is no guarantee that all OnNext calls are emitted in the order they are received, which sort of makes the promise a bit of a weird one to make (what is the point of it).

I just wanted to put my issue and solution out here, as for us it works (and it may help other people). I do think it's clear a deadlock "could" happen there. But if my solution doesn't fit in the context of what the BehaviorSubject promises (or should promise) then feel free to close this issue.

Once again, I appreciate your quick response. Thanks again.