> It looks like test_multiprocessing_fork doesn't clean up some of its subprocesses and then test_subprocess hangs on waitpid(0) forever.

Oh strange. This issue was supposed to be fixed. I spent a lot of time to fix such bug, but also make sure that not test leak any child process.

"test_subprocess hangs on waitpid(0) forever" I recall a similar bug that I fixed recently.

I spent time on test.support.reap_children() and test.support.threading_cleanup() to make sure that tests don't leak threads no child processes. It seems like I missed some tests.

In Lib/test/libregrtest/runtest.py, I added:

def post_test_cleanup():
    support.reap_children()

which is run after each test.

But currently, support.reap_children() calls os.waitpid(-1, os.WNOHANG) to check for child processes which already completed. It doesn't warn if there are child processes which are still running.

During my tests, I modified reap_children() to add a sleep(1).

Maybe on Linux we could use a cgroup to really check for all child processes?