Exceptions thrown inside of SubFlow don't cancel entire stream when using SupervisionStrategy.Stop

[mdedetrich]

What happened?

Whenever an exception is thrown in a SubFlow (i.e. as a result of using splitAfter for creating sub flows based on time slice) it does not respect the configured actor supervision strategy which means instead of the stream being terminated (which is what you would expect), it just repeats itself indefinitely.

What did you expect to happen?

If an exception is thrown inside a SubFlow it should cancel the parent stream which should propagate and cancel all substreams.

What else do we need to know?

Upstream issue/comment is here akka/akka#23066 (comment)

[mdedetrich]

So it turns out it is possible to cancel the parent stream when an exception is thrown, its just that you need to use a custom SubstreamCancelStrategy (it defaults to .drain where as we want .propagate). Its kind of weird that for this specific Flow operator it ignores the SupervisionStrategy so an upstream ticket was created at akka/akka#31394.