Test `caf.flow.generation` unstable on CI

[Neverlord]

I've seen this test fail on a couple of runs recently. We need to figure out whether it's an issue with the test or an actual bug in CAF.

[riemass]

Tried recreating the fail locally and couldn't bring it to fail. I think the test fails on fedora-39:no-exceptions, I'll try setting up the runner locally.

[riemass]

The test fails with this check (link):

Scenario: asynchronous buffers can generate flow items
  Given: a background thread writing into an async buffer
    When: canceling the subscription to the buffer
      Then: the producer receives a cancel event
        error: check failed
            loc: /actors/libcaf_core/caf/flow/generation.test.cpp:248
          check: should be true
        pass /actors/libcaf_core/caf/flow/generation.test.cpp:250

I was able to reproduce this using GCC 11.4 and GCC 13.2 running the test infinitely for a minute or two. Had no success with clang.

[riemass]

I've recorded and analyzed a test run that fails and one that succeeds. The vector res witch is a sink from the observable has 128 values in the failing run (expected empty). The test setup starts a new thread with a producer, and immediately calls dispose on the observable. In both runs the disposable is valid and schedules an action that calls from_steps::on_dispose. Since the default demand for from_steps is 128, my guess is that the producer thread will manage to squeeze in one step from the flow first and then call the scheduled action (from_steps::on_dispose). I've added a small delay to the producer thread and let the test run in an infinite loop for 15 minutes and I can't reproduce the issue anymore. FYI: @Neverlord