I think the race condition analysis here may be slightly off. Looking at _terminate_broken(), bpe.__cause__ is assigned on line 493 before set_exception() on line 498, so the test thread shouldn't be able to see __cause__ as None due to ordering.

The actual issue seems to be that when cause is None (the os._exit path), p.exitcode can still return None at line 487 because waitpid() hasn't collected the exit status yet — even though the process sentinel has already fired. This means errors stays empty, cause_str stays None, and bpe.__cause__ is never set at all.

A sleeping_retry in the test won't help here because nobody goes back to set __cause__ after the fact — once _terminate_broken() finishes, the exception is final.

A fix in _terminate_broken() itself might work better — calling p.join(timeout=3) before reading p.exitcode would ensure waitpid() has collected the exit status:

for p in self.processes.values():
    p.join(timeout=3)
    if p.exitcode is not None and p.exitcode != 0:
        errors.append(...)

The sentinel has already fired at this point so join() should return almost immediately. This way p.exitcode is reliably available when we read it.

This PR is stale because it has been open for 30 days with no activity.