Support nesting and add test

Lib/multiprocessing/synchronize.py

@@ -103,7 +103,7 @@ def __getstate__(self):
         if sys.platform == 'win32':
             h = context.get_spawning_popen().duplicate_for_child(sl.handle)
         else:
-            if self.is_fork_ctx:
+            if getattr(self, "is_fork_ctx", False):
                 raise RuntimeError('A SemLock created in a fork context is being '
                                    'shared with a process in a spawn context. This is '
                                    'not supported. Please use the same context to create '

Lib/test/_test_multiprocessing.py

@@ -5342,6 +5342,16 @@ def test_ignore_listener(self):
         finally:
             conn.close()
 
+# Utility functions used as target for spawn Process
+def put_one_in_queue(queue):
+    queue.put(1)
+
+def put_two_and_nest_once(queue):
+    queue.put(2)
+    process = multiprocessing.Process(target=put_one_in_queue, args=(queue,))
+    process.start()
+    process.join()
+
 class TestStartMethod(unittest.TestCase):
     @classmethod
     def _check_context(cls, conn):
@@ -5443,6 +5453,19 @@ def test_mixed_startmethod(self):
                 p.start()
                 p.join()
 
+    def test_nested_startmethod(self):
+        queue = multiprocessing.Queue()
+
+        process = multiprocessing.Process(target=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")