Cancellelation hangs when awaiting and still calls the uni when subscribing about smallrye-mutiny HOT 2 CLOSED

What you see might be counter intuitive at first, but it's working as intended 😃

I hope the explanations below will help you.

Case 1

This is expected, see the implementation of a Uni from a known value: https://github.com/smallrye/smallrye-mutiny/blob/main/implementation/src/main/java/io/smallrye/mutiny/operators/uni/builders/UniCreateFromKnownItem.java#L35

If the cancellation arrives soon enough, then no item is ever emitted due to a cancellation check in the forward method.
The .await().indefinitely() operator never receives an item, hence it hangs forever.

Remember that cancellation is a signal sent to the operators above, not the operators below.

Case 2

.subscribe().with() is not blocking, so a subscription request is sent to the operators above. Since that's an immediate Uni there are only good reasons for it to reach the operators below.

Indeed a cancellation signal is an eventual cancellation, not an immediate one, see Rule 1.8 of the Reactive Streams protocol.

More generally for cancellation to work you need an initial publisher that's properly dealing with cancellation. Still, cancellation may not always work because of timing issues (e.g., the item is sent fast enough before the cancellation request has been propagated).

from smallrye-mutiny.

Thanks so much for your explanations and taking the time. As usual, super detailed and helpful.

Remember that cancellation is a signal sent to the operators above, not the operators below.

I don't know why I keep forgetting that. I don't know why but I still feel this is "somewhat" dangerous. I feel like a sort of checked exception in that case could make sense if you await and what you are awaiting on gets cancelled, but definitely does not play nice with the whole reactive universe.

For case 2 I do understand. I do understand that .subscribe().with() is non blocking, but there's something in me that thought that the onSubscribtion().invoke() would actually block the subscription. i.e. the operation would happen after the onSubscription event.

from smallrye-mutiny.