Multicast onBackpressureBuffer Sink does not accept new Subs after dispose

Question

When I unsubscribe the last Sub and sub again, I expect the Sink/Flux to work or throw an Exception.

See my Testcode:

void playground() {
        Sinks.Many<String> sink = Sinks.many().multicast().onBackpressureBuffer();
        var disposer = sink.asFlux().subscribe(s -> System.out.println("1: "+s));
        System.out.println("CurrentSubs:" + sink.currentSubscriberCount());
        sink.tryEmitNext("Test1");
        var disposer2 = sink.asFlux().subscribe(s -> System.out.println("2: "+s));
        System.out.println("CurrentSubs:" + sink.currentSubscriberCount());
        sink.tryEmitNext("Test2");
        disposer.dispose();
        System.out.println("CurrentSubs:" + sink.currentSubscriberCount());
        sink.tryEmitNext("Test3");
        disposer2.dispose();
        System.out.println("CurrentSubs:" + sink.currentSubscriberCount());
        var disposer3 = sink.asFlux().subscribe(s -> System.out.println("3: "+s));
        System.out.println("CurrentSubs:" + sink.currentSubscriberCount());
        sink.tryEmitNext("Test4");
        disposer3.dispose();
        System.out.println("CurrentSubs:" + sink.currentSubscriberCount());
    }

And the Output:

1: Test1
CurrentSubs:2
1: Test2
2: Test2
CurrentSubs:1
2: Test3
CurrentSubs:0
CurrentSubs:0
CurrentSubs:0

This behaviour seems weird to me, I would at least expect an Exception when I try to sub again and fail.

Is there any reason behind this behaviour.

score 0 · Answer 1 · answered Jun 07 '23 at 13:41

I think this is kind of a gray area and not documented well. The onBackpressureBuffer() Sinks.Many does by default autoCancel, so once the subscriber count drops to zero, the operator stops emitting values to new subscribers. Since it is now in cancelled state, all new subscriptions will immediately complete without emitting a value right upon subscription. Therefore the following is true

assert disposer3.isDisposed();

In reactor, cancellation per se is not an error. Therefore, it is kind of OKish when you don't get an exception when subscribing to an autoCancelled Sinks.Many.

What bothers me more is that even though the Sink was autoCanceled, it still accepts emissions via tryEmitNext. So following is true even after the autoCancellation took place.

assert sink.tryEmitNext("Test4").isSuccess();

So the API will let you keep emitting values even though the Sink is pretty much dead. Only after you fill the internal queue in the Sink you get EmitResult.FAIL_OVERFLOW result. The Sink "knew" it has been autoCanceled and will still let you emit values which the new subscribers will never see... I can imagine a scenario where this could lead to an intermittent memory leak or such.

Anyway, if you want your example to work more intuitively just disable the autoCancel behavior of onBackpressureBuffer() like so

Sinks.Many<String> sink = Sinks
    .many()
    .multicast()
    .onBackpressureBuffer(Queues.SMALL_BUFFER_SIZE, false);

Multicast onBackpressureBuffer Sink does not accept new Subs after dispose

1 Answers1