bpo-1596321: Fix threading._shutdown() for the main thread (GH-28549) (GH-28589)

Fix the threading._shutdown() function when the threading module was
imported first from a thread different than the main thread: no
longer log an error at Python exit.

(cherry picked from commit 95d3137082)

Lib/test/test_threading.py

@@ -814,6 +814,39 @@ def noop(): pass
             threading.Thread(target=noop).start()
             # Thread.join() is not called
 
+    def test_import_from_another_thread(self):
+        # bpo-1596321: If the threading module is first import from a thread
+        # different than the main thread, threading._shutdown() must handle
+        # this case without logging an error at Python exit.
+        code = textwrap.dedent('''
+            import _thread
+            import sys
+
+            event = _thread.allocate_lock()
+            event.acquire()
+
+            def import_threading():
+                import threading
+                event.release()
+
+            if 'threading' in sys.modules:
+                raise Exception('threading is already imported')
+
+            _thread.start_new_thread(import_threading, ())
+
+            # wait until the threading module is imported
+            event.acquire()
+            event.release()
+
+            if 'threading' not in sys.modules:
+                raise Exception('threading is not imported')
+
+            # don't wait until the thread completes
+        ''')
+        rc, out, err = assert_python_ok("-c", code)
+        self.assertEqual(out, b'')
+        self.assertEqual(err, b'')
+
 
 class ThreadJoinOnShutdown(BaseTestCase):