[3.6] bpo-26762, bpo-31019: Backport multiprocessing fixes from master to 3.6 (#2879)

* bpo-26762: Avoid daemon process in _test_multiprocessing (#2842)

test_level() of _test_multiprocessing._TestLogging now uses regular
processes rather than daemon processes to prevent zombi processes
(to not "leak" processes).
(cherry picked from commit 06634950c5)

* test_multiprocessing: Fix dangling process/thread (#2850)

bpo-26762: Fix more dangling processes and threads in
test_multiprocessing:

* Queue: call close() followed by join_thread()
* Process: call join() or self.addCleanup(p.join)
(cherry picked from commit d7e64d9934)

* test_multiprocessing detects dangling per test case (#2841)

bpo-26762: test_multiprocessing now detects dangling processes and
threads per test case classes:

* setUpClass()/tearDownClass() of mixin classes now check if
  multiprocessing.process._dangling or threading._dangling was
  modified to detect "dangling" processses and threads.
* ManagerMixin.tearDownClass() now also emits a warning if it still
  has more than one active child process after 5 seconds.
* tearDownModule() now checks for dangling processes and threads
  before sleep 500 ms. And it now only sleeps if there is a least one
  dangling process or thread.
(cherry picked from commit ffb49408f0)

* bpo-26762: test_multiprocessing close more queues (#2855)

* Close explicitly queues to make sure that we don't leave dangling
  threads
* test_queue_in_process(): remove unused queue
* test_access() joins also the process to fix a random warning
(cherry picked from commit b4c52966c8)

* bpo-31019: Fix multiprocessing.Process.is_alive() (#2875)

multiprocessing.Process.is_alive() now removes the process from the
_children set if the process completed.

The change prevents leaking "dangling" processes.
(cherry picked from commit 2db64823c2)

Lib/multiprocessing/process.py

@@ -132,10 +132,16 @@ class BaseProcess(object):
         if self is _current_process:
             return True
         assert self._parent_pid == os.getpid(), 'can only test a child process'
+
         if self._popen is None:
             return False
-        self._popen.poll()
-        return self._popen.returncode is None
+
+        returncode = self._popen.poll()
+        if returncode is None:
+            return True
+        else:
+            _children.discard(self)
+            return False
 
     @property
     def name(self):