bpo-42350: Fix Thread._reset_internal_locks() (GH-23268)

Fix the threading.Thread class at fork: do nothing if the thread is
already stopped (ex: fork called at Python exit). Previously, an
error was logged in the child process.

Lib/test/test_threading.py

@@ -469,6 +469,35 @@ class ThreadTests(BaseTestCase):
         t = threading.Thread(daemon=True)
         self.assertTrue(t.daemon)
 
+    @unittest.skipUnless(hasattr(os, 'fork'), 'needs os.fork()')
+    def test_fork_at_exit(self):
+        # bpo-42350: Calling os.fork() after threading._shutdown() must
+        # not log an error.
+        code = textwrap.dedent("""
+            import atexit
+            import os
+            import sys
+            from test.support import wait_process
+
+            # Import the threading module to register its "at fork" callback
+            import threading
+
+            def exit_handler():
+                pid = os.fork()
+                if not pid:
+                    print("child process ok", file=sys.stderr, flush=True)
+                    # child process
+                    sys.exit()
+                else:
+                    wait_process(pid, exitcode=0)
+
+            # exit_handler() will be called after threading._shutdown()
+            atexit.register(exit_handler)
+        """)
+        _, out, err = assert_python_ok("-c", code)
+        self.assertEqual(out, b'')
+        self.assertEqual(err.rstrip(), b'child process ok')
+
     @unittest.skipUnless(hasattr(os, 'fork'), 'test needs fork()')
     def test_dummy_thread_after_fork(self):
         # Issue #14308: a dummy thread in the active list doesn't mess up