gh-88110: Clear concurrent.futures.thread._threads_queues after fork to avoid joining parent process' threads (GH-126098)

Threads are gone after fork, so clear the queues too. Otherwise the
child process (here created via multiprocessing.Process) crashes on
interpreter exit.

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/test_concurrent_futures/test_thread_pool.py

@@ -66,6 +66,25 @@ class ThreadPoolExecutorTest(ThreadPoolMixin, ExecutorTest, BaseTestCase):
                 with futures.ProcessPoolExecutor(1, mp_context=mp.get_context('fork')) as workers:
                     workers.submit(tuple)
 
+    @support.requires_fork()
+    @unittest.skipUnless(hasattr(os, 'register_at_fork'), 'need os.register_at_fork')
+    def test_process_fork_from_a_threadpool(self):
+        # bpo-43944: clear concurrent.futures.thread._threads_queues after fork,
+        # otherwise child process will try to join parent thread
+        def fork_process_and_return_exitcode():
+            # Ignore the warning about fork with threads.
+            with self.assertWarnsRegex(DeprecationWarning,
+                                       r"use of fork\(\) may lead to deadlocks in the child"):
+                p = mp.get_context('fork').Process(target=lambda: 1)
+                p.start()
+            p.join()
+            return p.exitcode
+
+        with futures.ThreadPoolExecutor(1) as pool:
+            process_exitcode = pool.submit(fork_process_and_return_exitcode).result()
+
+        self.assertEqual(process_exitcode, 0)
+
     def test_executor_map_current_future_cancel(self):
         stop_event = threading.Event()
         log = []