Implement the feature

python · encukou · Sep 18, 2024 · Sep 17, 2024 · Sep 17, 2024 · Sep 17, 2024
commit 997e602b659db0aee2f649aed62bac8b4aab16f6
@@ -264,6 +264,24 @@ Type Objects
 
    .. versionadded:: 3.11
 
+.. c:function:: int PyType_GetBaseByToken(PyTypeObject *type, void *token, PyTypeObject **result)
+
+   Find the first superclass in *type*'s :term:`method resolution order` whose
+   :c:macro:`Py_tp_token` token is equal to the given one.
+
+   * If found, set *\*result* to a new :term:`strong reference`
+     to it and return ``1``.
+   * If not found, set *\*result* to ``NULL`` and return ``0``.
+   * On error, set *\*result* to ``NULL`` and return ``-1`` with an
+     exception set.
+
+   The *result* argument may be ``NULL``, in which case *\*result* is not set.
+   Use this if you need only the return value.
+
+   The *token* argument may not be ``NULL``.
+
+   .. versionadded:: 3.14
+
 .. c:function:: int PyUnstable_Type_AssignVersionTag(PyTypeObject *type)
 
    Attempt to assign a version tag to the given type.
@@ -488,6 +506,11 @@ The following functions and structs are used to create
       * ``Py_nb_add`` to set :c:member:`PyNumberMethods.nb_add`
       * ``Py_sq_length`` to set :c:member:`PySequenceMethods.sq_length`
 
+      An additional slot is supported that does not correspond to a
+      :c:type:`!PyTypeObject` struct field:
+
+      * :c:data:`Py_tp_token`
+
       The following “offset” fields cannot be set using :c:type:`PyType_Slot`:
 
          * :c:member:`~PyTypeObject.tp_weaklistoffset`
@@ -538,4 +561,47 @@ The following functions and structs are used to create
       The desired value of the slot. In most cases, this is a pointer
       to a function.
 
-      Slots other than ``Py_tp_doc`` may not be ``NULL``.
+      *pfunc* values may not be ``NULL``, except for the following slots:
+
+      * ``Py_tp_doc``
+      * :c:data:`Py_tp_token` (for clarity, prefer :c:data:`Py_TP_USE_SPEC`
+        rather than ``NULL``)
+
+.. c:macro:: Py_tp_token
+
+   A :c:member:`~PyType_Slot.slot` that records a static memory layout ID
+   for a class.
+
+   If the :c:type:`PyType_Spec` of the class is statically
+   allocated, the token can be set to the spec using the special value
+   :c:data:`Py_TP_USE_SPEC`:
+
+   .. code-block:: c
+
+      static PyType_Slot foo_slots[] = {
+         {Py_tp_token, Py_TP_USE_SPEC},
+
+   It can also be set to an arbitrary pointer, but you must ensure that:
+
+   * The pointer outlives the class, so it's not reused for something else
+     while the class exists.
+   * It "belongs" to the extension module where the class lives, so it will not
+     clash with other extensions.
+
+   Use :c:func:`PyType_GetBaseByToken` to check if a class's superclass has
+   a given token -- that is, check whether the memory layout is compatible.
+
+   To get the token for a given class (without considering superclasses),
+   use :c:func:`PyType_GetSlot` with ``Py_tp_token``.
+
+   .. versionadded:: 3.14
+
+   .. c:namespace:: NULL
+
+   .. c:macro:: Py_TP_USE_SPEC
+
+      Used as a value with :c:data:`Py_tp_token` to set the token to the
+      class's :c:type:`PyType_Spec`.
+      Expands to ``NULL``.
+
+      .. versionadded:: 3.14
diff --git a/Doc/data/stable_abi.dat b/Doc/data/stable_abi.dat
@@ -554,6 +554,11 @@ New Features
 
   (Contributed by Victor Stinner in :gh:`107954`.)
 
+* Add :c:func:`PyType_GetBaseByToken` and :c:data:`Py_tp_token` slot for easier
+  superclass identification, which attempts to resolve the `type checking issue
+  <https://peps.python.org/pep-0630/#type-checking>`__ mentioned in :pep:`630`
+  (:gh:`121079`).
+
 
 Porting to Python 3.14
 ----------------------

diff --git a/Include/cpython/object.h b/Include/cpython/object.h
@@ -269,6 +269,7 @@ typedef struct _heaptypeobject {
     struct _dictkeysobject *ht_cached_keys;
     PyObject *ht_module;
     char *_ht_tpname;  // Storage for "tp_name"; see PyType_FromModuleAndSpec
+    void *ht_token;  // Storage for the "Py_tp_token" slot
     struct _specialization_cache _spec_cache; // For use by the specializer.
 #ifdef Py_GIL_DISABLED
     Py_ssize_t unique_id;  // ID used for thread-local refcounting

diff --git a/Include/object.h b/Include/object.h
@@ -391,6 +391,10 @@ PyAPI_FUNC(PyObject *) PyType_FromMetaclass(PyTypeObject*, PyObject*, PyType_Spe
 PyAPI_FUNC(void *) PyObject_GetTypeData(PyObject *obj, PyTypeObject *cls);
 PyAPI_FUNC(Py_ssize_t) PyType_GetTypeDataSize(PyTypeObject *cls);
 #endif
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030E0000
+PyAPI_FUNC(int) PyType_GetBaseByToken(PyTypeObject *, void *, PyTypeObject **);
+#define Py_TP_USE_SPEC NULL
+#endif
 
 /* Generic type check */
 PyAPI_FUNC(int) PyType_IsSubtype(PyTypeObject *, PyTypeObject *);

diff --git a/Include/typeslots.h b/Include/typeslots.h
@@ -90,3 +90,7 @@
 /* New in 3.14 */
 #define Py_tp_vectorcall 82
 #endif
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030E0000
+/* New in 3.14 */
+#define Py_tp_token 83
+#endif
@@ -1144,6 +1144,77 @@ class MyType:
         MyType.__module__ = 123
         self.assertEqual(get_type_fullyqualname(MyType), 'my_qualname')
 
+    def test_get_base_by_token(self):
+        def get_base_by_token(src, key, comparable=True):
+            def run(use_mro):
+                find_first = _testcapi.pytype_getbasebytoken
+                ret1, result = find_first(src, key, use_mro, True)
+                ret2, no_result = find_first(src, key, use_mro, False)
+                self.assertIn(ret1, (0, 1))
+                self.assertEqual(ret1, result is not None)
+                self.assertEqual(ret1, ret2)
+                self.assertIsNone(no_result)
+                return result
+
+            found_in_mro = run(True)
+            found_in_bases = run(False)
+            if comparable:
+                self.assertIs(found_in_mro, found_in_bases)
+                return found_in_mro
+            return found_in_mro, found_in_bases
+
+        create_type = _testcapi.create_type_with_token
+        get_token = _testcapi.get_tp_token
+
+        Py_TP_USE_SPEC = _testcapi.Py_TP_USE_SPEC
+        self.assertEqual(Py_TP_USE_SPEC, 0)
+
+        A1 = create_type('_testcapi.A1', Py_TP_USE_SPEC)
+        self.assertTrue(get_token(A1) != Py_TP_USE_SPEC)
+
+        B1 = create_type('_testcapi.B1', id(self))
+        self.assertTrue(get_token(B1) == id(self))
+
+        tokenA1 = get_token(A1)
+        # find A1 from A1
+        found = get_base_by_token(A1, tokenA1)
+        self.assertIs(found, A1)
+
+        # no token in static types
+        STATIC = type(1)
+        self.assertEqual(get_token(STATIC), 0)
+        found = get_base_by_token(STATIC, tokenA1)
+        self.assertIs(found, None)
+
+        # no token in pure subtypes
+        class A2(A1): pass
+        self.assertEqual(get_token(A2), 0)
+        # find A1
+        class Z(STATIC, B1, A2): pass
+        found = get_base_by_token(Z, tokenA1)
+        self.assertIs(found, A1)
+
+        # searching for NULL token is an error
+        with self.assertRaises(SystemError):
+            get_base_by_token(Z, 0)
+        with self.assertRaises(SystemError):
+            get_base_by_token(STATIC, 0)
+
+        # share the token with A1
+        C1 = create_type('_testcapi.C1', tokenA1)
+        self.assertTrue(get_token(C1) == tokenA1)
+
+        # find C1 first by shared token
+        class Z(C1, A2): pass
+        found = get_base_by_token(Z, tokenA1)
+        self.assertIs(found, C1)
+        # B1 not found
+        found = get_base_by_token(Z, get_token(B1))
+        self.assertIs(found, None)
+
+        with self.assertRaises(TypeError):
+            _testcapi.pytype_getbasebytoken(
+                'not a type', id(self), True, False)
 
     def test_gen_get_code(self):
         def genf(): yield

diff --git a/Lib/test/test_stable_abi_ctypes.py b/Lib/test/test_stable_abi_ctypes.py
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
@@ -1718,7 +1718,7 @@ def delx(self): del self.__x
                   '3P'                  # PyMappingMethods
                   '10P'                 # PySequenceMethods
                   '2P'                  # PyBufferProcs
-                  '6P'
+                  '7P'
                   '1PIP'                # Specializer cache
                   + typeid              # heap type id (free-threaded only)
                   )

@@ -2527,4 +2527,10 @@
 [function.PyLong_AsUInt64]
     added = '3.14'
 [const.Py_tp_vectorcall]
-    added = '3.14'
+    added = '3.14'
+[function.PyType_GetBaseByToken]
+    added = '3.14'
+[const.Py_tp_token]
+    added = '3.14'
+[const.Py_TP_USE_SPEC]
+    added = '3.14'
diff --git a/Modules/_testcapi/heaptype.c b/Modules/_testcapi/heaptype.c
@@ -410,6 +410,118 @@ pyobject_getitemdata(PyObject *self, PyObject *o)
 }
 
 
+static PyObject *
+create_type_with_token(PyObject *module, PyObject *args)
+{
+    const char *name;
+    PyObject *py_token;
+    if (!PyArg_ParseTuple(args, "sO", &name, &py_token)) {
+        return NULL;
+    }
+    void *token = PyLong_AsVoidPtr(py_token);
+    if (token == Py_TP_USE_SPEC) {
+        // Py_TP_USE_SPEC requires the spec that at least outlives the class
+        static PyType_Slot slots[] = {
+            {Py_tp_token, Py_TP_USE_SPEC},
+            {0},
+        };
+        static PyType_Spec spec = {
+            .name = "_testcapi.DefaultTokenTest",
+            .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+            .slots = slots,
+        };
+        PyObject *type = PyType_FromMetaclass(NULL, NULL, &spec, NULL);
+        if (!type) {
+            return NULL;
+        }
+        token = PyType_GetSlot((PyTypeObject *)type, Py_tp_token);
+        assert(!PyErr_Occurred());
+        Py_DECREF(type);
+        if (token != &spec) {
+            PyErr_SetString(PyExc_AssertionError,
+                            "failed to convert token from Py_TP_USE_SPEC");
+            return NULL;
+        }
+    }
+    // Test non-NULL token that must also outlive the class
+    PyType_Slot slots[] = {
+        {Py_tp_token, token},
+        {0},
+    };
+    PyType_Spec spec = {
+        .name = name,
+        .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+        .slots = slots,
+    };
+    return PyType_FromMetaclass(NULL, module, &spec, NULL);
+}
+
+static PyObject *
+get_tp_token(PyObject *self, PyObject *type)
+{
+    void *token = PyType_GetSlot((PyTypeObject *)type, Py_tp_token);
+    if (PyErr_Occurred()) {
+        return NULL;
+    }
+    return PyLong_FromVoidPtr(token);
+}
+
+static PyObject *
+pytype_getbasebytoken(PyObject *self, PyObject *args)
+{
+    PyTypeObject *type;
+    PyObject *py_token, *use_mro, *need_result;
+    if (!PyArg_ParseTuple(args, "OOOO",
+                          &type, &py_token, &use_mro, &need_result)) {
+        return NULL;
+    }
+
+    PyObject *mro_save = NULL;
+    if (use_mro != Py_True) {
+        // Test internal detail: PyType_GetBaseByToken works even with
+        // types that are only partially initialized (or torn down):
+        // if tp_mro=NULL we fall back to tp_bases.
+        assert(PyType_Check(type));
+        mro_save = type->tp_mro;
+        type->tp_mro = NULL;
+    }
+
+    void *token = PyLong_AsVoidPtr(py_token);
+    PyObject *result;
+    int ret;
+    if (need_result == Py_True) {
+        ret = PyType_GetBaseByToken(type, token, (PyTypeObject **)&result);
+    }
+    else {
+        result = NULL;
+        ret = PyType_GetBaseByToken(type, token, NULL);
+    }
+
+    if (use_mro != Py_True) {
+        type->tp_mro = mro_save;
+    }
+    if (ret < 0) {
+        assert(result == NULL);
+        return NULL;
+    }
+    PyObject *py_ret = PyLong_FromLong(ret);
+    if (py_ret == NULL) {
+        goto error;
+    }
+    PyObject *tuple = PyTuple_New(2);
+    if (tuple == NULL) {
+        goto error;
+    }
+    PyTuple_SET_ITEM(tuple, 0, py_ret);
+    PyTuple_SET_ITEM(tuple, 1, result ? result : Py_None);
+    return tuple;
+error:
+    Py_XDECREF(py_ret);
+    Py_XDECREF(result);
+    return NULL;
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"pytype_fromspec_meta",    pytype_fromspec_meta,            METH_O},
     {"test_type_from_ephemeral_spec", test_type_from_ephemeral_spec, METH_NOARGS},
@@ -423,6 +535,9 @@ static PyMethodDef TestMethods[] = {
     {"make_immutable_type_with_base", make_immutable_type_with_base, METH_O},
     {"make_type_with_base", make_type_with_base, METH_O},
     {"pyobject_getitemdata", pyobject_getitemdata, METH_O},
+    {"create_type_with_token", create_type_with_token, METH_VARARGS},
+    {"get_tp_token", get_tp_token, METH_O},
+    {"pytype_getbasebytoken", pytype_getbasebytoken, METH_VARARGS},
     {NULL},
 };
 
@@ -1287,6 +1402,8 @@ _PyTestCapi_Init_Heaptype(PyObject *m) {
         &PyType_Type, m, &HeapCTypeMetaclassNullNew_spec, (PyObject *) &PyType_Type);
     ADD("HeapCTypeMetaclassNullNew", HeapCTypeMetaclassNullNew);
 
+    ADD("Py_TP_USE_SPEC", PyLong_FromVoidPtr(Py_TP_USE_SPEC));
+
     PyObject *HeapCCollection = PyType_FromMetaclass(
         NULL, m, &HeapCCollection_spec, NULL);
     if (HeapCCollection == NULL) {