gh-77377: Ensure multiprocessing SemLock is valid for spawn-based Process before serializing it (#107275)

Ensure multiprocessing SemLock is valid for spawn Process before serializing it.

Creating a multiprocessing SemLock with a fork context, and then trying to pass it to a spawn-created Process, would segfault if not detected early.

---------

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

1 files changed, 22 insertions, 0 deletions

diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 1075496..a826e31 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -5421,6 +5421,28 @@ class TestStartMethod(unittest.TestCase):
             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.")
+    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()
+            process_ctx = multiprocessing.get_context(process_method)
+            p = process_ctx.Process(target=close_queue, args=(queue,))
+            err_msg = "A SemLock created in a fork"
+            with self.assertRaisesRegex(RuntimeError, err_msg):
+                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()
+                process_ctx = multiprocessing.get_context(process_method)
+                p = process_ctx.Process(target=close_queue, args=(queue,))
+                p.start()
+                p.join()
+
 
 @unittest.skipIf(sys.platform == "win32",
                  "test semantics don't make sense on Windows")