gh-77377: Ensure multiprocessing SemLock is valid for spawn-based Process before serializing it

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

@@ -5421,6 +5421,24 @@ def test_preload_resources(self):
             print(err)
             self.fail("failed spawning forkserver or grandchild")
 
+    @unittest.skipIf(sys.platform == "win32", "Only Spawn on windows so no risk of mixing")
+    @only_run_in_spawn_testsuite("avoids redundant testing since we ignore the global context.")
+    def test_mixed_startmethod(self):
+        # Fork-based locks cannot be used with spawned process
+        for process_method in ["spawn", "forkserver"]:
+            queue = multiprocessing.get_context("fork").Queue()
+            p = multiprocessing.get_context(process_method).Process(target=close_queue, args=(queue,))
+            with self.assertRaisesRegex(RuntimeError, "A SemLock created in a fork"):
+                p.start()
+
+        # non-fork-based locks can be used with all other start methods
+        for queue_method in ["spawn", "forkserver"]:
+            for process_method in multiprocessing.get_all_start_methods():
+                queue = multiprocessing.get_context(queue_method).Queue()
+                p = multiprocessing.get_context(process_method).Process(target=close_queue, args=(queue,))
+                p.start()
+                p.join()
+
 
 @unittest.skipIf(sys.platform == "win32",
                  "test semantics don't make sense on Windows")

Misc/NEWS.d/next/Core and Builtins/2023-07-25-22-35-35.gh-issue-77377.EHAbXx.rst

@@ -0,0 +1 @@
+Ensure that multiprocessing synchronization objects created in a fork context are not sent to a different process created in a spawn context. This changes a segfault into an actionable RuntimeError in the parent process.