From b12e99261e656585ffbaa395af7c5dbaee5ad1ad Mon Sep 17 00:00:00 2001
From: Sam Gross <colesbury@gmail.com>
Date: Fri, 11 Oct 2024 03:56:01 -0400
Subject: [PATCH] gh-125221: Fix free-threading data race in
 `object.__reduce_ex__` (#125267)

---
 ...-10-10-14-47-13.gh-issue-125221.nfSQzT.rst |  2 ++
 Objects/object.c                              |  8 ++++++++
 Objects/typeobject.c                          | 20 +++++--------------
 3 files changed, 15 insertions(+), 15 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2024-10-10-14-47-13.gh-issue-125221.nfSQzT.rst

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-10-10-14-47-13.gh-issue-125221.nfSQzT.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-10-10-14-47-13.gh-issue-125221.nfSQzT.rst
new file mode 100644
index 00000000000..c79650c3a64
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2024-10-10-14-47-13.gh-issue-125221.nfSQzT.rst
@@ -0,0 +1,2 @@
+Fix possible race condition when calling :meth:`~object.__reduce_ex__` for the
+first time in the free threading build.
diff --git a/Objects/object.c b/Objects/object.c
index 27d06cc0812..4a4c5bf7d7f 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -2372,6 +2372,14 @@ _PyTypes_InitTypes(PyInterpreterState *interp)
         }
     }
 
+    // Cache __reduce__ from PyBaseObject_Type object
+    PyObject *baseobj_dict = _PyType_GetDict(&PyBaseObject_Type);
+    PyObject *baseobj_reduce = PyDict_GetItemWithError(baseobj_dict, &_Py_ID(__reduce__));
+    if (baseobj_reduce == NULL && PyErr_Occurred()) {
+        return _PyStatus_ERR("Can't get __reduce__ from base object");
+    }
+    _Py_INTERP_CACHED_OBJECT(interp, objreduce) = baseobj_reduce;
+
     // Must be after static types are initialized
     if (_Py_initialize_generic(interp) < 0) {
         return _PyStatus_ERR("Can't initialize generic types");
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index d90bb5825fd..6ca4406ec0e 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -7359,18 +7359,7 @@ static PyObject *
 object___reduce_ex___impl(PyObject *self, int protocol)
 /*[clinic end generated code: output=2e157766f6b50094 input=f326b43fb8a4c5ff]*/
 {
-#define objreduce \
-    (_Py_INTERP_CACHED_OBJECT(_PyInterpreterState_GET(), objreduce))
-    PyObject *reduce, *res;
-
-    if (objreduce == NULL) {
-        PyObject *dict = lookup_tp_dict(&PyBaseObject_Type);
-        objreduce = PyDict_GetItemWithError(dict, &_Py_ID(__reduce__));
-        if (objreduce == NULL && PyErr_Occurred()) {
-            return NULL;
-        }
-    }
-
+    PyObject *reduce;
     if (PyObject_GetOptionalAttr(self, &_Py_ID(__reduce__), &reduce) < 0) {
         return NULL;
     }
@@ -7384,10 +7373,12 @@ object___reduce_ex___impl(PyObject *self, int protocol)
             Py_DECREF(reduce);
             return NULL;
         }
-        override = (clsreduce != objreduce);
+
+        PyInterpreterState *interp = _PyInterpreterState_GET();
+        override = (clsreduce != _Py_INTERP_CACHED_OBJECT(interp, objreduce));
         Py_DECREF(clsreduce);
         if (override) {
-            res = _PyObject_CallNoArgs(reduce);
+            PyObject *res = _PyObject_CallNoArgs(reduce);
             Py_DECREF(reduce);
             return res;
         }
@@ -7396,7 +7387,6 @@ object___reduce_ex___impl(PyObject *self, int protocol)
     }
 
     return _common_reduce(self, protocol);
-#undef objreduce
 }
 
 static PyObject *