The comments in the test indicate that this possibility was considered and dealt with.  So that indicates it is a test bug in this particular test (test_close_when_done, for other readers), not in the fixture.  It looks like the race condition still exists, the window is just reduced.  Can you see any reason the assert shouldn't be moved to before the s.start_reserved_event.set()?  (Since I missed the clearing of the buffer, I'm not confident in my reading of the test intent :)