@vstinner

> I'm not sure that I understand the bug.

The bug is, if a user makes an error and passes a Queue from context 'fork' to a child that is spawned using 'spawn', the passed Queue is, for obvious reasons, broken. 

The 'print("Child r_q: %r, %r, %r" % (r_q._rlock, r_q._wlock, r_q._sem), flush=True)' is simply a demonstration of a broken state of the SemLock observed in the child. 

The expected fix would be to stop the mixed context use of MP objects on the API level (ValueError?) or at least prevent a segfault.