[3.12] gh-108520: Fix bad fork detection in nested multiprocessing use case (GH-108568) (#108691)

gh-108520: Fix bad fork detection in nested multiprocessing use case (GH-108568)

gh-107275 introduced a regression where a SemLock would fail being passed along nested child processes, as the `is_fork_ctx` attribute would be left missing after the first deserialization.

---------

(cherry picked from commit add8d45cbe)

Co-authored-by: albanD <desmaison.alban@gmail.com>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>

Lib/test/_test_multiprocessing.py

@@ -5405,6 +5405,32 @@ class TestStartMethod(unittest.TestCase):
                 p.start()
                 p.join()
 
+    @classmethod
+    def _put_one_in_queue(cls, queue):
+        queue.put(1)
+
+    @classmethod
+    def _put_two_and_nest_once(cls, queue):
+        queue.put(2)
+        process = multiprocessing.Process(target=cls._put_one_in_queue, args=(queue,))
+        process.start()
+        process.join()
+
+    def test_nested_startmethod(self):
+        # gh-108520: Regression test to ensure that child process can send its
+        # arguments to another process
+        queue = multiprocessing.Queue()
+
+        process = multiprocessing.Process(target=self._put_two_and_nest_once, args=(queue,))
+        process.start()
+        process.join()
+
+        results = []
+        while not queue.empty():
+            results.append(queue.get())
+
+        self.assertEqual(results, [2, 1])
+
 
 @unittest.skipIf(sys.platform == "win32",
                  "test semantics don't make sense on Windows")