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.

Lib/test/_test_multiprocessing.py

@@ -4303,7 +4303,32 @@ class TestSimpleQueue(unittest.TestCase):
 # Mixins
 #
 
-class ProcessesMixin(object):
+class BaseMixin(object):
+    @classmethod
+    def setUpClass(cls):
+        cls.dangling = (multiprocessing.process._dangling.copy(),
+                        threading._dangling.copy())
+
+    @classmethod
+    def tearDownClass(cls):
+        # bpo-26762: Some multiprocessing objects like Pool create reference
+        # cycles. Trigger a garbage collection to break these cycles.
+        test.support.gc_collect()
+
+        processes = set(multiprocessing.process._dangling) - set(cls.dangling[0])
+        if processes:
+            print('Warning -- Dangling processes: %s' % processes,
+                  file=sys.stderr)
+        processes = None
+
+        threads = set(threading._dangling) - set(cls.dangling[1])
+        if threads:
+            print('Warning -- Dangling threads: %s' % threads,
+                  file=sys.stderr)
+        threads = None
+
+
+class ProcessesMixin(BaseMixin):
     TYPE = 'processes'
     Process = multiprocessing.Process
     connection = multiprocessing.connection
@@ -4326,7 +4351,7 @@ class ProcessesMixin(object):
     RawArray = staticmethod(multiprocessing.RawArray)
 
 
-class ManagerMixin(object):
+class ManagerMixin(BaseMixin):
     TYPE = 'manager'
     Process = multiprocessing.Process
     Queue = property(operator.attrgetter('manager.Queue'))
@@ -4350,6 +4375,7 @@ class ManagerMixin(object):
 
     @classmethod
     def setUpClass(cls):
+        super().setUpClass()
         cls.manager = multiprocessing.Manager()
 
     @classmethod
@@ -4357,23 +4383,35 @@ class ManagerMixin(object):
         # only the manager process should be returned by active_children()
         # but this can take a bit on slow machines, so wait a few seconds
         # if there are other children too (see #17395)
+        start_time = time.monotonic()
         t = 0.01
-        while len(multiprocessing.active_children()) > 1 and t < 5:
+        while len(multiprocessing.active_children()) > 1:
             time.sleep(t)
             t *= 2
+            dt = time.monotonic() - start_time
+            if dt >= 5.0:
+                print("Warning -- multiprocessing.Manager still has %s active "
+                      "children after %s seconds"
+                      % (multiprocessing.active_children(), dt),
+                      file=sys.stderr)
+                break
+
         gc.collect()                       # do garbage collection
         if cls.manager._number_of_objects() != 0:
             # This is not really an error since some tests do not
             # ensure that all processes which hold a reference to a
             # managed object have been joined.
-            print('Shared objects which still exist at manager shutdown:')
+            print('Warning -- Shared objects which still exist at manager '
+                  'shutdown:')
             print(cls.manager._debug_info())
         cls.manager.shutdown()
         cls.manager.join()
         cls.manager = None
 
+        super().tearDownClass()
+
 
-class ThreadsMixin(object):
+class ThreadsMixin(BaseMixin):
     TYPE = 'threads'
     Process = multiprocessing.dummy.Process
     connection = multiprocessing.dummy.connection
@@ -4450,18 +4488,33 @@ def install_tests_in_module_dict(remote_globs, start_method):
         multiprocessing.get_logger().setLevel(LOG_LEVEL)
 
     def tearDownModule():
+        need_sleep = False
+
+        # bpo-26762: Some multiprocessing objects like Pool create reference
+        # cycles. Trigger a garbage collection to break these cycles.
+        test.support.gc_collect()
+
         multiprocessing.set_start_method(old_start_method[0], force=True)
         # pause a bit so we don't get warning about dangling threads/processes
-        time.sleep(0.5)
+        processes = set(multiprocessing.process._dangling) - set(dangling[0])
+        if processes:
+            need_sleep = True
+            print('Warning -- Dangling processes: %s' % processes,
+                  file=sys.stderr)
+        processes = None
+
+        threads = set(threading._dangling) - set(dangling[1])
+        if threads:
+            need_sleep = True
+            print('Warning -- Dangling threads: %s' % threads,
+                  file=sys.stderr)
+        threads = None
+
+        # Sleep 500 ms to give time to child processes to complete.
+        if need_sleep:
+            time.sleep(0.5)
         multiprocessing.process._cleanup()
-        gc.collect()
-        tmp = set(multiprocessing.process._dangling) - set(dangling[0])
-        if tmp:
-            print('Dangling processes:', tmp, file=sys.stderr)
-        del tmp
-        tmp = set(threading._dangling) - set(dangling[1])
-        if tmp:
-            print('Dangling threads:', tmp, file=sys.stderr)
+        test.support.gc_collect()
 
     remote_globs['setUpModule'] = setUpModule
     remote_globs['tearDownModule'] = tearDownModule