bpo-31234: Add test.support.wait_threads_exit() (#3578)

Use _thread.count() to wait until threads exit. The new context
manager prevents the "dangling thread" warning.

Lib/test/test_thread.py

@@ -59,12 +59,13 @@ class ThreadRunningTests(BasicThreadTest):
                 self.done_mutex.release()
 
     def test_starting_threads(self):
-        # Basic test for thread creation.
-        for i in range(NUMTASKS):
-            self.newtask()
-        verbose_print("waiting for tasks to complete...")
-        self.done_mutex.acquire()
-        verbose_print("all tasks done")
+        with support.wait_threads_exit():
+            # Basic test for thread creation.
+            for i in range(NUMTASKS):
+                self.newtask()
+            verbose_print("waiting for tasks to complete...")
+            self.done_mutex.acquire()
+            verbose_print("all tasks done")
 
     def test_stack_size(self):
         # Various stack size tests.
@@ -94,12 +95,13 @@ class ThreadRunningTests(BasicThreadTest):
             verbose_print("trying stack_size = (%d)" % tss)
             self.next_ident = 0
             self.created = 0
-            for i in range(NUMTASKS):
-                self.newtask()
+            with support.wait_threads_exit():
+                for i in range(NUMTASKS):
+                    self.newtask()
 
-            verbose_print("waiting for all tasks to complete")
-            self.done_mutex.acquire()
-            verbose_print("all tasks done")
+                verbose_print("waiting for all tasks to complete")
+                self.done_mutex.acquire()
+                verbose_print("all tasks done")
 
         thread.stack_size(0)
 
@@ -109,25 +111,28 @@ class ThreadRunningTests(BasicThreadTest):
         mut = thread.allocate_lock()
         mut.acquire()
         started = []
+
         def task():
             started.append(None)
             mut.acquire()
             mut.release()
-        thread.start_new_thread(task, ())
-        while not started:
-            time.sleep(POLL_SLEEP)
-        self.assertEqual(thread._count(), orig + 1)
-        # Allow the task to finish.
-        mut.release()
-        # The only reliable way to be sure that the thread ended from the
-        # interpreter's point of view is to wait for the function object to be
-        # destroyed.
-        done = []
-        wr = weakref.ref(task, lambda _: done.append(None))
-        del task
-        while not done:
-            time.sleep(POLL_SLEEP)
-        self.assertEqual(thread._count(), orig)
+
+        with support.wait_threads_exit():
+            thread.start_new_thread(task, ())
+            while not started:
+                time.sleep(POLL_SLEEP)
+            self.assertEqual(thread._count(), orig + 1)
+            # Allow the task to finish.
+            mut.release()
+            # The only reliable way to be sure that the thread ended from the
+            # interpreter's point of view is to wait for the function object to be
+            # destroyed.
+            done = []
+            wr = weakref.ref(task, lambda _: done.append(None))
+            del task
+            while not done:
+                time.sleep(POLL_SLEEP)
+            self.assertEqual(thread._count(), orig)
 
     def test_save_exception_state_on_error(self):
         # See issue #14474
@@ -140,16 +145,14 @@ class ThreadRunningTests(BasicThreadTest):
             except ValueError:
                 pass
             real_write(self, *args)
-        c = thread._count()
         started = thread.allocate_lock()
         with support.captured_output("stderr") as stderr:
             real_write = stderr.write
             stderr.write = mywrite
             started.acquire()
-            thread.start_new_thread(task, ())
-            started.acquire()
-            while thread._count() > c:
-                time.sleep(POLL_SLEEP)
+            with support.wait_threads_exit():
+                thread.start_new_thread(task, ())
+                started.acquire()
         self.assertIn("Traceback", stderr.getvalue())
 
 
@@ -181,13 +184,14 @@ class Barrier:
 class BarrierTest(BasicThreadTest):
 
     def test_barrier(self):
-        self.bar = Barrier(NUMTASKS)
-        self.running = NUMTASKS
-        for i in range(NUMTASKS):
-            thread.start_new_thread(self.task2, (i,))
-        verbose_print("waiting for tasks to end")
-        self.done_mutex.acquire()
-        verbose_print("tasks done")
+        with support.wait_threads_exit():
+            self.bar = Barrier(NUMTASKS)
+            self.running = NUMTASKS
+            for i in range(NUMTASKS):
+                thread.start_new_thread(self.task2, (i,))
+            verbose_print("waiting for tasks to end")
+            self.done_mutex.acquire()
+            verbose_print("tasks done")
 
     def task2(self, ident):
         for i in range(NUMTRIPS):
@@ -225,11 +229,10 @@ class TestForkInThread(unittest.TestCase):
     @unittest.skipUnless(hasattr(os, 'fork'), 'need os.fork')
     @support.reap_threads
     def test_forkinthread(self):
-        running = True
         status = "not set"
 
         def thread1():
-            nonlocal running, status
+            nonlocal status
 
             # fork in a thread
             pid = os.fork()
@@ -244,13 +247,11 @@ class TestForkInThread(unittest.TestCase):
                 # parent
                 os.close(self.write_fd)
                 pid, status = os.waitpid(pid, 0)
-                running = False
 
-        thread.start_new_thread(thread1, ())
-        self.assertEqual(os.read(self.read_fd, 2), b"OK",
-                         "Unable to fork() in thread")
-        while running:
-            time.sleep(POLL_SLEEP)
+        with support.wait_threads_exit():
+            thread.start_new_thread(thread1, ())
+            self.assertEqual(os.read(self.read_fd, 2), b"OK",
+                             "Unable to fork() in thread")
         self.assertEqual(status, 0)
 
     def tearDown(self):