The problem is that socketserver.ThreadingMixIn spawns threads without waiting for their completion in server_close(). For example, the following two tests use socketserver.ThreadingMixIn and so can leak a running thread:

* test.test_logging.SocketHandlerTest.test_output()
* test.test_logging.DatagramHandlerTest.test_output()

Example:

10-STABLE-amd64% ./python -m test -v test_logging --fail-env-changed -F -m test.test_logging.DatagramHandlerTest.test_output -m test.test_logging.ConfigDictTest.test_listen_config_10_ok  --match=test.test_logging.SocketHandlerTest.test_output

(...)

0:00:09 load avg: 2.54 [  7] test_logging
test_output (test.test_logging.SocketHandlerTest) ... ok
test_output (test.test_logging.DatagramHandlerTest) ... ok
test_listen_config_10_ok (test.test_logging.ConfigDictTest) ... threading_setup (1, <_weakrefset.WeakSet object at 0x806243398>)