gh-103968: Deprecate creating heap types whose metaclass has custom tp_new. (GH-103972)

(That's a mouthful of an edge case!)

Co-authored-by: Barney Gale <[email protected]>

Author: encukou

Doc/c-api/type.rst

@@ -256,8 +256,13 @@ The following functions and structs are used to create
    The metaclass *metaclass* is used to construct the resulting type object.
    When *metaclass* is ``NULL``, the metaclass is derived from *bases*
    (or *Py_tp_base[s]* slots if *bases* is ``NULL``, see below).
-   Note that metaclasses that override
-   :c:member:`~PyTypeObject.tp_new` are not supported.
+
+   Metaclasses that override :c:member:`~PyTypeObject.tp_new` are not
+   supported.
+   (For backwards compatibility, other ``PyType_From*`` functions allow
+   such metaclasses. They ignore ``tp_new``, which may result in incomplete
+   initialization. This is deprecated and in Python 3.14+ such metaclasses will
+   not be supported.)
 
    The *bases* argument can be used to specify base classes; it can either
    be only one class or a tuple of classes.
@@ -305,6 +310,11 @@ The following functions and structs are used to create
       The function now finds and uses a metaclass corresponding to the provided
       base classes.  Previously, only :class:`type` instances were returned.
 
+      The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
+      which may result in incomplete initialization.
+      Creating classes whose metaclass overrides
+      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
+      will be no longer allowed.
 
 .. c:function:: PyObject* PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
 
@@ -317,6 +327,12 @@ The following functions and structs are used to create
       The function now finds and uses a metaclass corresponding to the provided
       base classes.  Previously, only :class:`type` instances were returned.
 
+      The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
+      which may result in incomplete initialization.
+      Creating classes whose metaclass overrides
+      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
+      will be no longer allowed.
+
 .. c:function:: PyObject* PyType_FromSpec(PyType_Spec *spec)
 
    Equivalent to ``PyType_FromMetaclass(NULL, NULL, spec, NULL)``.
@@ -327,6 +343,12 @@ The following functions and structs are used to create
       base classes provided in *Py_tp_base[s]* slots.
       Previously, only :class:`type` instances were returned.
 
+      The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
+      which may result in incomplete initialization.
+      Creating classes whose metaclass overrides
+      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
+      will be no longer allowed.
+
 .. c:type:: PyType_Spec
 
    Structure defining a type's behavior.

Doc/whatsnew/3.12.rst

@@ -1320,6 +1320,21 @@ Porting to Python 3.12
   available on debug builds.  If you happen to be using it then you'll
   need to start using ``_Py_GetGlobalRefTotal()``.
 
+* The following functions now select an appropriate metaclass for the newly
+  created type:
+
+  * :c:func:`PyType_FromSpec`
+  * :c:func:`PyType_FromSpecWithBases`
+  * :c:func:`PyType_FromModuleAndSpec`
+
+  Creating classes whose metaclass overrides :c:member:`~PyTypeObject.tp_new`
+  is deprecated, and in Python 3.14+ it will be disallowed.
+  Note that these functions ignore ``tp_new`` of the metaclass, possibly
+  allowing incomplete initialization.
+
+  Note that :c:func:`PyType_FromMetaclass` (added in Python 3.12)
+  already disallows creating classes whose metaclass overrides ``tp_new``.
+
 Deprecated
 ----------
 
@@ -1396,6 +1411,11 @@ Deprecated
 * ``_PyErr_ChainExceptions`` is deprecated. Use ``_PyErr_ChainExceptions1``
   instead. (Contributed by Irit Katriel in :gh:`102192`.)
 
+* Using :c:func:`PyType_FromSpec`, :c:func:`PyType_FromSpecWithBases`
+  or :c:func:`PyType_FromModuleAndSpec` to create a class whose metaclass
+  overrides :c:member:`~PyTypeObject.tp_new` is deprecated.
+  Call the metaclass instead.
+
 Removed
 -------
 

Lib/test/test_capi/test_misc.py

@@ -681,6 +681,20 @@ def test_heaptype_with_custom_metaclass(self):
         with self.assertRaisesRegex(TypeError, msg):
             t = _testcapi.pytype_fromspec_meta(_testcapi.HeapCTypeMetaclassCustomNew)
 
+    def test_heaptype_with_custom_metaclass_deprecation(self):
+        # gh-103968: a metaclass with custom tp_new is deprecated, but still
+        # allowed for functions that existed in 3.11
+        # (PyType_FromSpecWithBases is used here).
+        class Base(metaclass=_testcapi.HeapCTypeMetaclassCustomNew):
+            pass
+
+        with warnings_helper.check_warnings(
+                ('.*custom tp_new.*in Python 3.14.*', DeprecationWarning),
+                ):
+            sub = _testcapi.make_type_with_base(Base)
+        self.assertTrue(issubclass(sub, Base))
+        self.assertIsInstance(sub, _testcapi.HeapCTypeMetaclassCustomNew)
+
     def test_multiple_inheritance_ctypes_with_weakref_or_dict(self):
 
         with self.assertRaises(TypeError):

Misc/NEWS.d/next/C API/2023-04-28-18-04-38.gh-issue-103968.EnVvOx.rst

@@ -0,0 +1,4 @@
+:c:func:`PyType_FromSpec` and its variants now allow creating classes whose
+metaclass overrides :c:member:`~PyTypeObject.tp_new`. The ``tp_new`` is
+ignored. This behavior is deprecated and will be disallowed in 3.14+. The
+new :c:func:`PyType_FromMetaclass` already disallows it.

Modules/_testcapi/heaptype.c

@@ -22,7 +22,7 @@ static PyObject *pytype_fromspec_meta(PyObject* self, PyObject *meta)
         "_testcapi.HeapCTypeViaMetaclass",
         sizeof(PyObject),
         0,
-        Py_TPFLAGS_DEFAULT,
+        Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
         HeapCTypeViaMetaclass_slots
     };
 
@@ -385,6 +385,19 @@ make_immutable_type_with_base(PyObject *self, PyObject *base)
     return PyType_FromSpecWithBases(&ImmutableSubclass_spec, base);
 }
 
+static PyObject *
+make_type_with_base(PyObject *self, PyObject *base)
+{
+    assert(PyType_Check(base));
+    PyType_Spec ImmutableSubclass_spec = {
+        .name = "_testcapi.Subclass",
+        .basicsize = (int)((PyTypeObject*)base)->tp_basicsize,
+        .slots = empty_type_slots,
+        .flags = Py_TPFLAGS_DEFAULT,
+    };
+    return PyType_FromSpecWithBases(&ImmutableSubclass_spec, base);
+}
+
 
 static PyMethodDef TestMethods[] = {
     {"pytype_fromspec_meta",    pytype_fromspec_meta,            METH_O},
@@ -397,6 +410,7 @@ static PyMethodDef TestMethods[] = {
      test_from_spec_invalid_metatype_inheritance,
      METH_NOARGS},
     {"make_immutable_type_with_base", make_immutable_type_with_base, METH_O},
+    {"make_type_with_base", make_type_with_base, METH_O},
     {NULL},
 };
 

Objects/typeobject.c

@@ -3950,9 +3950,10 @@ check_basicsize_includes_size_and_offsets(PyTypeObject* type)
     return 1;
 }
 
-PyObject *
-PyType_FromMetaclass(PyTypeObject *metaclass, PyObject *module,
-                     PyType_Spec *spec, PyObject *bases_in)
+static PyObject *
+_PyType_FromMetaclass_impl(
+    PyTypeObject *metaclass, PyObject *module,
+    PyType_Spec *spec, PyObject *bases_in, int _allow_tp_new)
 {
     /* Invariant: A non-NULL value in one of these means this function holds
      * a strong reference or owns allocated memory.
@@ -4127,9 +4128,21 @@ PyType_FromMetaclass(PyTypeObject *metaclass, PyObject *module,
         goto finally;
     }
     if (metaclass->tp_new != PyType_Type.tp_new) {
-        PyErr_SetString(PyExc_TypeError,
-                        "Metaclasses with custom tp_new are not supported.");
-        goto finally;
+        if (_allow_tp_new) {
+            if (PyErr_WarnFormat(
+                    PyExc_DeprecationWarning, 1,
+                    "Using PyType_Spec with metaclasses that have custom "
+                    "tp_new is deprecated and will no longer be allowed in "
+                    "Python 3.14.") < 0) {
+                goto finally;
+            }
+        }
+        else {
+            PyErr_SetString(
+                PyExc_TypeError,
+                "Metaclasses with custom tp_new are not supported.");
+            goto finally;
+        }
     }
 
     /* Calculate best base, and check that all bases are type objects */
@@ -4316,22 +4329,29 @@ PyType_FromMetaclass(PyTypeObject *metaclass, PyObject *module,
     return (PyObject*)res;
 }
 
+PyObject *
+PyType_FromMetaclass(PyTypeObject *metaclass, PyObject *module,
+                     PyType_Spec *spec, PyObject *bases_in)
+{
+    return _PyType_FromMetaclass_impl(metaclass, module, spec, bases_in, 0);
+}
+
 PyObject *
 PyType_FromModuleAndSpec(PyObject *module, PyType_Spec *spec, PyObject *bases)
 {
-    return PyType_FromMetaclass(NULL, module, spec, bases);
+    return _PyType_FromMetaclass_impl(NULL, module, spec, bases, 1);
 }
 
 PyObject *
 PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
 {
-    return PyType_FromMetaclass(NULL, NULL, spec, bases);
+    return _PyType_FromMetaclass_impl(NULL, NULL, spec, bases, 1);
 }
 
 PyObject *
 PyType_FromSpec(PyType_Spec *spec)
 {
-    return PyType_FromMetaclass(NULL, NULL, spec, NULL);
+    return _PyType_FromMetaclass_impl(NULL, NULL, spec, NULL, 1);
 }
 
 PyObject *