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OK, it looks like your particular primary failure mode is 
inevitable at times.  If I boost the sleep in _TriggerThread.run
(), it fails every time, in the way you describe, on my box.

The difficulty appears to be one of test design.  
_doBlockingTest believes that no block_func passed to it can 
possibly return while the queue remains blocked, but the 
failing test is passing it a block_func that returns via raising 
FailingQueueException regardless of whether the queue is 
blocked.  So what we see is just the result of a thread race 
in the test.

But as Freud said, insight does not imply a cure <wink>.