http://buildbot.python.org/all/builders/AMD64%20FreeBSD%20CURRENT%20Non-Debug%203.5/builds/97/steps/test/logs/stdio

test_connection (test.test_multiprocessing_forkserver.WithThreadsTestConnection) ... FAIL

...

Dangling threads: {<DummyProcess(Thread-110, started daemon 34437426432)>}

======================================================================
FAIL: test_connection (test.test_multiprocessing_forkserver.WithThreadsTestConnection)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.5.koobs-freebsd-current.nondebug/build/Lib/test/_test_multiprocessing.py", line 2359, in test_connection
    self.assertEqual(poll(TIMEOUT1), True)
AssertionError: False != True

----------------------------------------------------------------------
Ran 272 tests in 124.653s

FAILED (failures=1, skipped=16)
Warning -- threading._dangling was modified by test_multiprocessing_forkserver
  Before: <_weakrefset.WeakSet object at 0x80493d080>
  After:  <_weakrefset.WeakSet object at 0x804956160> 
test test_multiprocessing_forkserver failed[105/398/1] test_multiprocessing_forkserver

Other test_multiprocessing_forkserver errors on AMD64 FreeBSD CURRENT Non-Debug 3.6.

http://buildbot.python.org/all/builders/AMD64%20FreeBSD%20CURRENT%20Non-Debug%203.6/builds/150/steps/test/logs/stdio

(...)
test_import (test.test_multiprocessing_forkserver._TestImportStar) ... ok
Dangling threads: {<Thread(Thread-97, started daemon 34452785920)>}

======================================================================
FAIL: test_active_children (test.test_multiprocessing_forkserver.WithProcessesTestProcess)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.6.koobs-freebsd-current.nondebug/build/Lib/test/_test_multiprocessing.py", line 345, in test_active_children
    self.assertIn(p, self.active_children())
AssertionError: <Process(Process-226, stopped daemon)> not found in []

======================================================================
FAIL: test_notify (test.test_multiprocessing_forkserver.WithThreadsTestCondition)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.6.koobs-freebsd-current.nondebug/build/Lib/test/_test_multiprocessing.py", line 886, in test_notify
    self.assertReturnsIfImplemented(1, get_value, woken)
  File "/usr/home/buildbot/python/3.6.koobs-freebsd-current.nondebug/build/Lib/test/_test_multiprocessing.py", line 165, in assertReturnsIfImplemented
    return self.assertEqual(value, res)
AssertionError: 1 != 0

----------------------------------------------------------------------
Ran 277 tests in 92.148s

FAILED (failures=2, skipped=16)
Warning -- threading._dangling was modified by test_multiprocessing_forkserver
  Before: <_weakrefset.WeakSet object at 0x805771518>
  After:  <_weakrefset.WeakSet object at 0x8009c6cf8> 
test test_multiprocessing_forkserver failed

I didn't see this bug recently. I fixed many bugs in the meanwhile, so maybe I fixed the bug?

See for example https://haypo.github.io/contrib-cpython-2017q3-part2.html