Merge branch 'main' into 3willows-patch-1

Author: 3willows

Lib/test/_test_multiprocessing.py

@@ -6844,6 +6844,28 @@ def f(x): return x*x
         self.assertEqual("332833500", out.decode('utf-8').strip())
         self.assertFalse(err, msg=err.decode('utf-8'))
 
+    def test_forked_thread_not_started(self):
+        # gh-134381: Ensure that a thread that has not been started yet in
+        # the parent process can be started within a forked child process.
+
+        if multiprocessing.get_start_method() != "fork":
+            self.skipTest("fork specific test")
+
+        q = multiprocessing.Queue()
+        t = threading.Thread(target=lambda: q.put("done"), daemon=True)
+
+        def child():
+            t.start()
+            t.join()
+
+        p = multiprocessing.Process(target=child)
+        p.start()
+        p.join(support.SHORT_TIMEOUT)
+
+        self.assertEqual(p.exitcode, 0)
+        self.assertEqual(q.get_nowait(), "done")
+        close_queue(q)
+
 
 #
 # Mixins

Misc/NEWS.d/next/Core_and_Builtins/2025-05-22-14-48-19.gh-issue-134381.2BXhth.rst

@@ -0,0 +1 @@
+Fix :exc:`RuntimeError` when using a not-started :class:`threading.Thread` after calling :func:`os.fork`

Misc/NEWS.d/next/Library/2024-06-06-17-49-07.gh-issue-120170.DUxhmT.rst

@@ -0,0 +1,3 @@
+Fix an issue in the :mod:`!_pickle` extension module in which importing
+:mod:`multiprocessing` could change how pickle identifies which module an
+object belongs to, potentially breaking the unpickling of those objects.

Modules/_pickle.c

@@ -1879,6 +1879,10 @@ _checkmodule(PyObject *module_name, PyObject *module,
             _PyUnicode_EqualToASCIIString(module_name, "__main__")) {
         return -1;
     }
+    if (PyUnicode_Check(module_name) &&
+            _PyUnicode_EqualToASCIIString(module_name, "__mp_main__")) {
+        return -1;
+    }
 
     PyObject *candidate = getattribute(module, dotted_path, 0);
     if (candidate == NULL) {

Modules/_threadmodule.c

@@ -296,6 +296,12 @@ _PyThread_AfterFork(struct _pythread_runtime_state *state)
             continue;
         }
 
+        // Keep handles for threads that have not been started yet. They are
+        // safe to start in the child process.
+        if (handle->state == THREAD_HANDLE_NOT_STARTED) {
+            continue;
+        }
+
         // Mark all threads as done. Any attempts to join or detach the
         // underlying OS thread (if any) could crash. We are the only thread;
         // it's safe to set this non-atomically.