[3.13] gh-130230: Fix crash in pow() with only Decimal third argument (GH-130237)

Include/internal/pycore_typeobject.h

@@ -199,6 +199,7 @@ extern PyObject * _PyType_GetMRO(PyTypeObject *type);
 extern PyObject* _PyType_GetSubclasses(PyTypeObject *);
 extern int _PyType_HasSubclasses(PyTypeObject *);
 PyAPI_FUNC(PyObject *) _PyType_GetModuleByDef2(PyTypeObject *, PyTypeObject *, PyModuleDef *);
+PyAPI_FUNC(PyObject *) _PyType_GetModuleByDef3(PyTypeObject *, PyTypeObject *, PyTypeObject *, PyModuleDef *);
 
 // PyType_Ready() must be called if _PyType_IsReady() is false.
 // See also the Py_TPFLAGS_READY flag.

Lib/test/test_decimal.py

@@ -4458,6 +4458,15 @@ def test_implicit_context(self):
             self.assertIs(Decimal("NaN").fma(7, 1).is_nan(), True)
             # three arg power
             self.assertEqual(pow(Decimal(10), 2, 7), 2)
+            if self.decimal == C:
+                self.assertEqual(pow(10, Decimal(2), 7), 2)
+                self.assertEqual(pow(10, 2, Decimal(7)), 2)
+            else:
+                # XXX: Three-arg power doesn't use __rpow__.
+                self.assertRaises(TypeError, pow, 10, Decimal(2), 7)
+                # XXX: There is no special method to dispatch on the
+                # third arg of three-arg power.
+                self.assertRaises(TypeError, pow, 10, 2, Decimal(7))
             # exp
             self.assertEqual(Decimal("1.01").exp(), 3)
             # is_normal

Misc/NEWS.d/next/Library/2025-02-17-21-16-51.gh-issue-130230.9ta9P9.rst

@@ -0,0 +1 @@
+Fix crash in :func:`pow` with only :class:`~decimal.Decimal` third argument.

Modules/_decimal/_decimal.c

@@ -140,6 +140,15 @@ find_state_left_or_right(PyObject *left, PyObject *right)
     return get_module_state(mod);
 }
 
+static inline decimal_state *
+find_state_ternary(PyObject *left, PyObject *right, PyObject *modulus)
+{
+    PyObject *mod = _PyType_GetModuleByDef3(Py_TYPE(left), Py_TYPE(right), Py_TYPE(modulus),
+                                            &_decimal_module);
+    assert(mod != NULL);
+    return get_module_state(mod);
+}
+
 
 #if !defined(MPD_VERSION_HEX) || MPD_VERSION_HEX < 0x02050000
   #error "libmpdec version >= 2.5.0 required"
@@ -4305,7 +4314,7 @@ nm_mpd_qpow(PyObject *base, PyObject *exp, PyObject *mod)
     PyObject *context;
     uint32_t status = 0;
 
-    decimal_state *state = find_state_left_or_right(base, exp);
+    decimal_state *state = find_state_ternary(base, exp, mod);
     CURRENT_CONTEXT(state, context);
     CONVERT_BINOP(&a, &b, base, exp, context);
 

Objects/typeobject.c

@@ -5038,6 +5038,26 @@ _PyType_GetModuleByDef2(PyTypeObject *left, PyTypeObject *right,
     return module;
 }
 
+PyObject *
+_PyType_GetModuleByDef3(PyTypeObject *left, PyTypeObject *right, PyTypeObject *third,
+                        PyModuleDef *def)
+{
+    PyObject *module = get_module_by_def(left, def);
+    if (module == NULL) {
+        module = get_module_by_def(right, def);
+        if (module == NULL) {
+            module = get_module_by_def(third, def);
+            if (module == NULL) {
+                PyErr_Format(
+                    PyExc_TypeError,
+                    "PyType_GetModuleByDef: No superclass of '%s', '%s' nor '%s' has "
+                    "the given module", left->tp_name, right->tp_name, third->tp_name);
+            }
+        }
+    }
+    return module;
+}
+
 void *
 PyObject_GetTypeData(PyObject *obj, PyTypeObject *cls)
 {