bpo-30775: Fix refleaks in test_multiprocessing (#2467)

Forgetting to call Process.join() can keep some resources alive.

Lib/test/_test_multiprocessing.py

@@ -1253,10 +1253,19 @@ def __init__(self, namespace, f, args, n, wait_before_exit=False):
         self._can_exit = namespace.Event()
         if not wait_before_exit:
             self._can_exit.set()
+
+        threads = []
         for i in range(n):
             p = namespace.Process(target=self.task)
             p.daemon = True
             p.start()
+            threads.append(p)
+
+        def finalize(threads):
+            for p in threads:
+                p.join()
+
+        self._finalizer = weakref.finalize(self, finalize, threads)
 
     def task(self):
         pid = os.getpid()
@@ -1279,6 +1288,9 @@ def wait_for_finished(self):
     def do_finish(self):
         self._can_exit.set()
 
+    def close(self):
+        self._finalizer()
+
 
 class AppendTrue(object):
     def __init__(self, obj):
@@ -1311,8 +1323,11 @@ def DummyList(self):
 
     def run_threads(self, f, args):
         b = Bunch(self, f, args, self.N-1)
-        f(*args)
-        b.wait_for_finished()
+        try:
+            f(*args)
+            b.wait_for_finished()
+        finally:
+            b.close()
 
     @classmethod
     def multipass(cls, barrier, results, n):