gh-88110: clear concurrent.futures.thread._threads_queues after fork to avoid joining parent process' threads

Lib/concurrent/futures/thread.py

@@ -41,6 +41,7 @@ def _python_exit():
     os.register_at_fork(before=_global_shutdown_lock.acquire,
                         after_in_child=_global_shutdown_lock._at_fork_reinit,
                         after_in_parent=_global_shutdown_lock.release)
+    os.register_at_fork(after_in_child=_threads_queues.clear)
 
 
 class WorkerContext:

Lib/test/test_concurrent_futures/test_thread_pool.py

@@ -4,6 +4,7 @@
 import multiprocessing.util
 import os
 import threading
+import warnings
 import unittest
 from concurrent import futures
 from test import support
@@ -66,6 +67,25 @@ def submit(pool):
                 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")
+    @support.requires_resource("cpu")
+    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 warnings.catch_warnings(category=DeprecationWarning, action="ignore"):
+                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 = []

Misc/NEWS.d/next/Library/2023-02-15-23-54-42.gh-issue-88110.KU6erv.rst

@@ -0,0 +1,2 @@
+Fix :class:`multiprocessing.Process` exiting with code 1 even on successful run when
+threads are used in the same application too.