From 23a764aef034987075ffec561f5d69f40e30c09c Mon Sep 17 00:00:00 2001
From: Thomas Kowalski <thom.kowa@gmail.com>
Date: Mon, 1 Jun 2026 16:32:13 +0200
Subject: [PATCH] gh-150157: Fix critical section for PyDict_Next() in
 _pickle.c (GH-150158) (cherry picked from commit
 c5516e7e371f7b273eb37c7b65f14ef14ee81f11)

Co-authored-by: Thomas Kowalski <thom.kowa@gmail.com>
---
 ...5-21-20-47-45.gh-issue-150157.ZvmO-bQZ.rst |  3 +++
 Modules/_pickle.c                             | 20 +++++++++++++++----
 2 files changed, 19 insertions(+), 4 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-05-21-20-47-45.gh-issue-150157.ZvmO-bQZ.rst

diff --git a/Misc/NEWS.d/next/Library/2026-05-21-20-47-45.gh-issue-150157.ZvmO-bQZ.rst b/Misc/NEWS.d/next/Library/2026-05-21-20-47-45.gh-issue-150157.ZvmO-bQZ.rst
new file mode 100644
index 000000000000000..3a12e26cf736f79
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-05-21-20-47-45.gh-issue-150157.ZvmO-bQZ.rst
@@ -0,0 +1,3 @@
+Fix a crash in free-threaded builds that occurs when pickling by name
+objects without a ``__module__`` attribute while :data:`sys.modules`
+is concurrently being modified.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 75e1c4dea85e004..9a6f24ec8649630 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -1947,22 +1947,34 @@ whichmodule(PickleState *st, PyObject *global, PyObject *global_name, PyObject *
             return NULL;
         }
         if (PyDict_CheckExact(modules)) {
+            PyObject *found_name = NULL;
+            int error = 0;
             i = 0;
+            Py_BEGIN_CRITICAL_SECTION(modules);
             while (PyDict_Next(modules, &i, &module_name, &module)) {
                 Py_INCREF(module_name);
                 Py_INCREF(module);
                 if (_checkmodule(module_name, module, global, dotted_path) == 0) {
                     Py_DECREF(module);
-                    Py_DECREF(modules);
-                    return module_name;
+                    found_name = module_name;
+                    break;
                 }
                 Py_DECREF(module);
                 Py_DECREF(module_name);
                 if (PyErr_Occurred()) {
-                    Py_DECREF(modules);
-                    return NULL;
+                    error = 1;
+                    break;
                 }
             }
+            Py_END_CRITICAL_SECTION();
+            if (error) {
+                Py_DECREF(modules);
+                return NULL;
+            }
+            if (found_name != NULL) {
+                Py_DECREF(modules);
+                return found_name;
+            }
         }
         else {
             PyObject *iterator = PyObject_GetIter(modules);