test-reactor-flux-mergeSequential

Test to demonstrate an apparent problem (or possibly a misunderstanding of mine) with Reactor's Flux.mergeSequential()

Building

./gradlew build

Running the tests

Unit Test

The unit test is run automatically with the build and may also be run via:

./gradlew test

About the Test

In the event that Flux.mergeSequential(Publisher<? extends I>... sources) is invoked with a group of Producers (mono1, mono2, ..., monoN, monoFinal) for which the earlier ones are waiting on a signal from the last one, it appears that the last one is never executed, and thus the first N never complete.

The test class demonstrating this is MergeSequentialUnitTest which has three test methods in it:

1. test_mergeSequential_failsWhenDelayed: Demonstrates the error case.
2. test_mergeSequential_succeedsWhenNotDelayed: Demonstrates that all Producers complete if the initial producers are not dependent on the last producer to complete.
3. test_mergeSequential_succeedsWhenDelayedWithDoOnSubscribeWorkaround: Demonstrates the workaround of applying Mono.doOnSubscribe(Consumer<? super Subscription> onSubscribe) to monoFinal. (Mono.log() works as well.)

The test code is currently configured to run with Reactor 3.6.5 which does not work. The [build.gradle.kts] also includes commented lines for Reactor 3.5.0 (first version I found that does not work) and Reactor 3.4.37 (last version that does work).