Attached PR 3523 is a temporary solution until we agree how to handle these threads and child processes in socketserver. I wrote the PR to fix "dangling threads" warnings to fix random buildbot failures.

The PR fixes warnings but also reenable test_logging tests which are currently skipped. These tests were skipped by bpo-30830 to prevent random buildbot failures.

See also the thread on the python-dev mailing list to discuss about bpo-31151 "socketserver.ForkingMixIn.server_close() leaks zombie processes":
https://mail.python.org/pipermail/python-dev/2017-August/148826.html

I tag this issue as release blocker as a remainder that we have to agree how to handle threads/processes before Python 3.7 feature freeze.