gh-95196: Fixed instances of types.ClassMethodDescriptorType not being pickled correctly

Doc/whatsnew/3.12.rst

@@ -83,6 +83,9 @@ Other Language Changes
   mapping is hashable.
   (Contributed by Serhiy Storchaka in :gh:`87995`.)
 
+* :class:`types.ClassMethodDescriptorType` instances are now pickled correctly.
+  (Contributed by Nir Schulman in :gh:`95196`.)
+
 
 New Modules
 ===========

Include/internal/pycore_global_strings.h

@@ -341,6 +341,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(endpos)
         STRUCT_FOR_ID(env)
         STRUCT_FOR_ID(errors)
+        STRUCT_FOR_ID(eval)
         STRUCT_FOR_ID(event)
         STRUCT_FOR_ID(eventmask)
         STRUCT_FOR_ID(exc_type)

Lib/test/pickletester.py

@@ -2795,8 +2795,10 @@ class Nested(str):
             ({1, 2}.__contains__, (2,)),
             # unbound "coexist" method
             (set.__contains__, ({1, 2}, 2)),
-            # built-in class method
+            # built-in bound class method
             (dict.fromkeys, (("a", 1), ("b", 2))),
+            # built-in unbound class method
+            (dict.__dict__["fromkeys"], (dict, ("a", 1), ("b", 2))),
             # built-in static method
             (bytearray.maketrans, (b"abc", b"xyz")),
             # subclass methods

Misc/NEWS.d/next/Core and Builtins/2022-08-21-22-00-51.gh-issue-95196.MwXQ1l.rst

@@ -0,0 +1 @@
+Fixed :class:`types.ClassMethodDescriptorType` being pickled incorrectly.

Objects/descrobject.c

@@ -633,11 +633,29 @@ descr_reduce(PyDescrObject *descr, PyObject *Py_UNUSED(ignored))
                          PyDescr_TYPE(descr), PyDescr_NAME(descr));
 }
 
+
+static PyObject*
+classmethoddescr_reduce(PyDescrObject* descr, PyObject* Py_UNUSED(ignored))
+{
+    /* Ideally, we would want to use a different callable than eval in
+       order to get the descriptor. However, we need to ensure the pickled
+       object will not cause errors upon unpickling in older versions. */
+    return Py_BuildValue("N(s, N, {s:O, s:O})",
+        _PyEval_GetBuiltin(&_Py_ID(eval)), "cls.__dict__[name]", Py_None,
+        "cls", PyDescr_TYPE(descr), "name", PyDescr_NAME(descr)
+    );
+}
+
 static PyMethodDef descr_methods[] = {
     {"__reduce__", (PyCFunction)descr_reduce, METH_NOARGS, NULL},
     {NULL, NULL}
 };
 
+static PyMethodDef classmethoddescr_methods[] = {
+    {"__reduce__", (PyCFunction)classmethoddescr_reduce, METH_NOARGS, NULL},
+    {NULL, NULL}
+};
+
 static PyMemberDef descr_members[] = {
     {"__objclass__", T_OBJECT, offsetof(PyDescrObject, d_type), READONLY},
     {"__name__", T_OBJECT, offsetof(PyDescrObject, d_name), READONLY},
@@ -776,7 +794,7 @@ PyTypeObject PyClassMethodDescr_Type = {
     0,                                          /* tp_weaklistoffset */
     0,                                          /* tp_iter */
     0,                                          /* tp_iternext */
-    descr_methods,                              /* tp_methods */
+    classmethoddescr_methods,                   /* tp_methods */
     descr_members,                              /* tp_members */
     method_getset,                              /* tp_getset */
     0,                                          /* tp_base */