Ensure multiprocessing lock is valid for spawn Process before serializing it

Author: albanD

Lib/multiprocessing/synchronize.py

@@ -50,8 +50,8 @@ class SemLock(object):
     def __init__(self, kind, value, maxvalue, *, ctx):
         if ctx is None:
             ctx = context._default_context.get_context()
-        name = ctx.get_start_method()
-        unlink_now = sys.platform == 'win32' or name == 'fork'
+        self.is_fork_ctx = ctx.get_start_method() == 'fork'
+        unlink_now = sys.platform == 'win32' or self.is_fork_ctx
         for i in range(100):
             try:
                 sl = self._semlock = _multiprocessing.SemLock(
@@ -103,6 +103,11 @@ def __getstate__(self):
         if sys.platform == 'win32':
             h = context.get_spawning_popen().duplicate_for_child(sl.handle)
         else:
+            if self.is_fork_ctx:
+                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 '
+                                   'multiprocessing objects and Process.')
             h = sl.handle
         return (h, sl.kind, sl.maxvalue, sl.name)
 

Lib/test/_test_multiprocessing.py

@@ -6155,3 +6155,9 @@ class SemLock(_multiprocessing.SemLock):
         name = f'test_semlock_subclass-{os.getpid()}'
         s = SemLock(1, 0, 10, name, False)
         _multiprocessing.sem_unlink(name)
+
+    def test_semlock_mixed_context(self):
+        queue = multiprocessing.get_context("fork").Queue()
+        p = multiprocessing.get_context("spawn").Process(target=close_queue, args=(queue,))
+        with self.assertRaisesRegex(RuntimeError, "A SemLock created in a fork"):
+            p.start()