bpo-45383: Inherit MetaClass from bases in FromSpec API

Doc/c-api/type.rst

@@ -196,8 +196,9 @@ The following functions and structs are used to create
    (:const:`Py_TPFLAGS_HEAPTYPE`).
 
    The metaclass *metaclass* is used to construct the resulting type object.
-   When *metaclass* is ``NULL``, the default :c:type:`PyType_Type` is used
-   instead. Note that metaclasses that override
+   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.
 
    The *bases* argument can be used to specify base classes; it can either
@@ -228,17 +229,33 @@ The following functions and structs are used to create
       The function now accepts a single class as the *bases* argument and
       ``NULL`` as the ``tp_doc`` slot.
 
+   .. versionchanged:: 3.12
+
+      The function now finds and uses a metaclass corresponding to the provided
+      base classes.  Previously, only :class:`type` instances were returned.
+
 
 .. c:function:: PyObject* PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
 
    Equivalent to ``PyType_FromMetaclass(NULL, NULL, spec, bases)``.
 
    .. versionadded:: 3.3
 
+   .. versionchanged:: 3.12
+
+      The function now finds and uses a metaclass corresponding to the provided
+      base classes.  Previously, only :class:`type` instances were returned.
+
 .. c:function:: PyObject* PyType_FromSpec(PyType_Spec *spec)
 
    Equivalent to ``PyType_FromMetaclass(NULL, NULL, spec, NULL)``.
 
+   .. versionchanged:: 3.12
+
+      The function now finds and uses a metaclass corresponding to the
+      base classes provided in *Py_tp_base[s]* slots.
+      Previously, only :class:`type` instances were returned.
+
 .. c:type:: PyType_Spec
 
    Structure defining a type's behavior.

Misc/NEWS.d/next/C API/2021-10-05-21-59-43.bpo-45383.TVClgf.rst

@@ -0,0 +1,3 @@
+The `PyType_FromSpec` API will now find and use a metaclass
+based on the provided bases.
+An error will be raised if there is a metaclass conflict.

Modules/_testcapimodule.c

@@ -28,6 +28,9 @@
 #include <float.h>                // FLT_MAX
 #include <signal.h>
 
+#include <ffi.h>  /* required by ctypes.h */
+#include "_ctypes/ctypes.h"  /* To test metaclass inheritance */
+
 #ifdef MS_WINDOWS
 #  include <winsock2.h>           // struct timeval
 #endif
@@ -1208,6 +1211,142 @@ test_get_type_name(PyObject *self, PyObject *Py_UNUSED(ignored))
 }
 
 
+static PyType_Slot empty_type_slots[] = {
+    {0, 0},
+};
+
+static PyType_Spec MinimalMetaclass_spec = {
+    .name = "_testcapi.MinimalMetaclass",
+    .basicsize = sizeof(PyHeapTypeObject),
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .slots = empty_type_slots,
+};
+
+static PyType_Spec MinimalType_spec = {
+    .name = "_testcapi.MinimalSpecType",
+    .basicsize = sizeof(PyObject),
+    .flags = Py_TPFLAGS_DEFAULT,
+    .slots = empty_type_slots,
+};
+
+static PyObject *
+test_from_spec_metatype_inheritance(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    PyObject *metaclass = NULL;
+    PyObject *class = NULL;
+    PyObject *new = NULL;
+    PyObject *result = NULL;
+
+    metaclass = PyType_FromSpecWithBases(&MinimalMetaclass_spec, (PyObject*)&PyType_Type);
+    if (metaclass == NULL) {
+        goto finally;
+    }
+    class = PyObject_CallFunction(metaclass, "s(){}", "TestClass");
+    if (class == NULL) {
+        goto finally;
+    }
+
+    new = PyType_FromSpecWithBases(&MinimalType_spec, class);
+    if (new == NULL) {
+        goto finally;
+    }
+    if (Py_TYPE(new) != (PyTypeObject*)metaclass) {
+        PyErr_SetString(PyExc_AssertionError,
+                "Metaclass not set properly!");
+        goto finally;
+    }
+    result = Py_NewRef(Py_None);
+
+finally:
+    Py_XDECREF(metaclass);
+    Py_XDECREF(class);
+    Py_XDECREF(new);
+    return result;
+}
+
+
+static PyObject *
+test_from_spec_invalid_metatype_inheritance(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    PyObject *metaclass_a = NULL;
+    PyObject *metaclass_b = NULL;
+    PyObject *class_a = NULL;
+    PyObject *class_b = NULL;
+    PyObject *bases = NULL;
+    PyObject *new = NULL;
+    PyObject *meta_error_string = NULL;
+    PyObject *exc_type = NULL;
+    PyObject *exc_value = NULL;
+    PyObject *exc_traceback = NULL;
+    PyObject *result = NULL;
+
+    metaclass_a = PyType_FromSpecWithBases(&MinimalMetaclass_spec, (PyObject*)&PyType_Type);
+    if (metaclass_a == NULL) {
+        goto finally;
+    }
+    metaclass_b = PyType_FromSpecWithBases(&MinimalMetaclass_spec, (PyObject*)&PyType_Type);
+    if (metaclass_b == NULL) {
+        goto finally;
+    }
+    class_a = PyObject_CallFunction(metaclass_a, "s(){}", "TestClassA");
+    if (class_a == NULL) {
+        goto finally;
+    }
+
+    class_b = PyObject_CallFunction(metaclass_b, "s(){}", "TestClassB");
+    if (class_b == NULL) {
+        goto finally;
+    }
+
+    bases = PyTuple_Pack(2, class_a, class_b);
+    if (bases == NULL) {
+        goto finally;
+    }
+
+    /*
+     * The following should raise a TypeError due to a MetaClass conflict.
+     */
+    new = PyType_FromSpecWithBases(&MinimalType_spec, bases);
+    if (new != NULL) {
+        PyErr_SetString(PyExc_AssertionError,
+                "MetaType conflict not recognized by PyType_FromSpecWithBases");
+            goto finally;
+    }
+
+    // Assert that the correct exception was raised
+    if (PyErr_ExceptionMatches(PyExc_TypeError)) {
+        PyErr_Fetch(&exc_type, &exc_value, &exc_traceback);
+
+        meta_error_string = PyUnicode_FromString("metaclass conflict:");
+        if (meta_error_string == NULL) {
+            goto finally;
+        }
+        int res = PyUnicode_Contains(exc_value, meta_error_string);
+        if (res < 0) {
+            goto finally;
+        }
+        if (res == 0) {
+            PyErr_SetString(PyExc_AssertionError,
+                    "TypeError did not inlclude expected message.");
+            goto finally;
+        }
+        result = Py_NewRef(Py_None);
+    }
+finally:
+    Py_XDECREF(metaclass_a);
+    Py_XDECREF(metaclass_b);
+    Py_XDECREF(bases);
+    Py_XDECREF(new);
+    Py_XDECREF(meta_error_string);
+    Py_XDECREF(exc_type);
+    Py_XDECREF(exc_value);
+    Py_XDECREF(exc_traceback);
+    Py_XDECREF(class_a);
+    Py_XDECREF(class_b);
+    return result;
+}
+
+
 static PyObject *
 simple_str(PyObject *self) {
     return PyUnicode_FromString("<test>");
@@ -5938,6 +6077,11 @@ static PyMethodDef TestMethods[] = {
     {"test_get_type_name",        test_get_type_name,            METH_NOARGS},
     {"test_get_type_qualname",    test_get_type_qualname,        METH_NOARGS},
     {"test_type_from_ephemeral_spec", test_type_from_ephemeral_spec, METH_NOARGS},
+    {"test_from_spec_metatype_inheritance", test_from_spec_metatype_inheritance,
+     METH_NOARGS},
+    {"test_from_spec_invalid_metatype_inheritance",
+     test_from_spec_invalid_metatype_inheritance,
+     METH_NOARGS},
     {"get_kwargs", _PyCFunction_CAST(get_kwargs),
       METH_VARARGS|METH_KEYWORDS},
     {"getargs_tuple",           getargs_tuple,                   METH_VARARGS},