gh-93649: Split vectorcall testing from _testcapimodule.c (GH-94549)

The `_testcapimodule.c` file is getting too large to work with effectively.
This PR lays out a general structure of how tests can be split up, with more splitting to come later if the structure is OK.

Vectorcall tests aren't the biggest issue -- it's just an area I want to work on next, so I'm starting here.
An issue specific to vectorcall tests is that it wasn't clear that e.g. `MethodDescriptor2` is related to testing vectorcall: the `/* Test PEP 590 */` section had an ambiguous end. Separate file should make things like this much clearer.
OTOH, for some pieces it might not be clear where they should be -- I left `meth_fastcall` with tests of the other calling conventions. IMO, even with the ambiguity it's still worth it to split the huge file up.

I'm not sure about the buildsystem changes, hopefully CI will tell me what's wrong.

@vstinner, @markshannon: Do you think this is a good idea?

Automerge-Triggered-By: GH:encukou

Modules/_testcapi/README.txt

@@ -0,0 +1,3 @@
+Tests in this directory are compiled into the _testcapi extension.
+The main file for the extension is Modules/_testcapimodule.c, which
+calls `_PyTestCapi_Init_*` from these functions.

Modules/_testcapi/parts.h

@@ -0,0 +1,3 @@
+#include "Python.h"
+
+PyAPI_FUNC(int) _PyTestCapi_Init_Vectorcall(PyObject *module);

Modules/_testcapi/vectorcall.c

@@ -0,0 +1,270 @@
+#include "parts.h"
+#include <stddef.h>  // offsetof
+
+
+/* Test PEP 590 - Vectorcall */
+
+static int
+fastcall_args(PyObject *args, PyObject ***stack, Py_ssize_t *nargs)
+{
+    if (args == Py_None) {
+        *stack = NULL;
+        *nargs = 0;
+    }
+    else if (PyTuple_Check(args)) {
+        *stack = ((PyTupleObject *)args)->ob_item;
+        *nargs = PyTuple_GET_SIZE(args);
+    }
+    else {
+        PyErr_SetString(PyExc_TypeError, "args must be None or a tuple");
+        return -1;
+    }
+    return 0;
+}
+
+
+static PyObject *
+test_pyobject_fastcall(PyObject *self, PyObject *args)
+{
+    PyObject *func, *func_args;
+    PyObject **stack;
+    Py_ssize_t nargs;
+
+    if (!PyArg_ParseTuple(args, "OO", &func, &func_args)) {
+        return NULL;
+    }
+
+    if (fastcall_args(func_args, &stack, &nargs) < 0) {
+        return NULL;
+    }
+    return _PyObject_FastCall(func, stack, nargs);
+}
+
+static PyObject *
+test_pyobject_fastcalldict(PyObject *self, PyObject *args)
+{
+    PyObject *func, *func_args, *kwargs;
+    PyObject **stack;
+    Py_ssize_t nargs;
+
+    if (!PyArg_ParseTuple(args, "OOO", &func, &func_args, &kwargs)) {
+        return NULL;
+    }
+
+    if (fastcall_args(func_args, &stack, &nargs) < 0) {
+        return NULL;
+    }
+
+    if (kwargs == Py_None) {
+        kwargs = NULL;
+    }
+    else if (!PyDict_Check(kwargs)) {
+        PyErr_SetString(PyExc_TypeError, "kwnames must be None or a dict");
+        return NULL;
+    }
+
+    return PyObject_VectorcallDict(func, stack, nargs, kwargs);
+}
+
+static PyObject *
+test_pyobject_vectorcall(PyObject *self, PyObject *args)
+{
+    PyObject *func, *func_args, *kwnames = NULL;
+    PyObject **stack;
+    Py_ssize_t nargs, nkw;
+
+    if (!PyArg_ParseTuple(args, "OOO", &func, &func_args, &kwnames)) {
+        return NULL;
+    }
+
+    if (fastcall_args(func_args, &stack, &nargs) < 0) {
+        return NULL;
+    }
+
+    if (kwnames == Py_None) {
+        kwnames = NULL;
+    }
+    else if (PyTuple_Check(kwnames)) {
+        nkw = PyTuple_GET_SIZE(kwnames);
+        if (nargs < nkw) {
+            PyErr_SetString(PyExc_ValueError, "kwnames longer than args");
+            return NULL;
+        }
+        nargs -= nkw;
+    }
+    else {
+        PyErr_SetString(PyExc_TypeError, "kwnames must be None or a tuple");
+        return NULL;
+    }
+    return PyObject_Vectorcall(func, stack, nargs, kwnames);
+}
+
+static PyObject *
+test_pyvectorcall_call(PyObject *self, PyObject *args)
+{
+    PyObject *func;
+    PyObject *argstuple;
+    PyObject *kwargs = NULL;
+
+    if (!PyArg_ParseTuple(args, "OO|O", &func, &argstuple, &kwargs)) {
+        return NULL;
+    }
+
+    if (!PyTuple_Check(argstuple)) {
+        PyErr_SetString(PyExc_TypeError, "args must be a tuple");
+        return NULL;
+    }
+    if (kwargs != NULL && !PyDict_Check(kwargs)) {
+        PyErr_SetString(PyExc_TypeError, "kwargs must be a dict");
+        return NULL;
+    }
+
+    return PyVectorcall_Call(func, argstuple, kwargs);
+}
+
+static PyMethodDef TestMethods[] = {
+    {"pyobject_fastcall", test_pyobject_fastcall, METH_VARARGS},
+    {"pyobject_fastcalldict", test_pyobject_fastcalldict, METH_VARARGS},
+    {"pyobject_vectorcall", test_pyobject_vectorcall, METH_VARARGS},
+    {"pyvectorcall_call", test_pyvectorcall_call, METH_VARARGS},
+    {NULL},
+};
+
+
+typedef struct {
+    PyObject_HEAD
+    vectorcallfunc vectorcall;
+} MethodDescriptorObject;
+
+static PyObject *
+MethodDescriptor_vectorcall(PyObject *callable, PyObject *const *args,
+                            size_t nargsf, PyObject *kwnames)
+{
+    /* True if using the vectorcall function in MethodDescriptorObject
+     * but False for MethodDescriptor2Object */
+    MethodDescriptorObject *md = (MethodDescriptorObject *)callable;
+    return PyBool_FromLong(md->vectorcall != NULL);
+}
+
+static PyObject *
+MethodDescriptor_new(PyTypeObject* type, PyObject* args, PyObject *kw)
+{
+    MethodDescriptorObject *op = (MethodDescriptorObject *)type->tp_alloc(type, 0);
+    op->vectorcall = MethodDescriptor_vectorcall;
+    return (PyObject *)op;
+}
+
+static PyObject *
+func_descr_get(PyObject *func, PyObject *obj, PyObject *type)
+{
+    if (obj == Py_None || obj == NULL) {
+        Py_INCREF(func);
+        return func;
+    }
+    return PyMethod_New(func, obj);
+}
+
+static PyObject *
+nop_descr_get(PyObject *func, PyObject *obj, PyObject *type)
+{
+    Py_INCREF(func);
+    return func;
+}
+
+static PyObject *
+call_return_args(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    Py_INCREF(args);
+    return args;
+}
+
+static PyTypeObject MethodDescriptorBase_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "MethodDescriptorBase",
+    sizeof(MethodDescriptorObject),
+    .tp_new = MethodDescriptor_new,
+    .tp_call = PyVectorcall_Call,
+    .tp_vectorcall_offset = offsetof(MethodDescriptorObject, vectorcall),
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE |
+                Py_TPFLAGS_METHOD_DESCRIPTOR | Py_TPFLAGS_HAVE_VECTORCALL,
+    .tp_descr_get = func_descr_get,
+};
+
+static PyTypeObject MethodDescriptorDerived_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "MethodDescriptorDerived",
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+};
+
+static PyTypeObject MethodDescriptorNopGet_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "MethodDescriptorNopGet",
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .tp_call = call_return_args,
+    .tp_descr_get = nop_descr_get,
+};
+
+typedef struct {
+    MethodDescriptorObject base;
+    vectorcallfunc vectorcall;
+} MethodDescriptor2Object;
+
+static PyObject *
+MethodDescriptor2_new(PyTypeObject* type, PyObject* args, PyObject *kw)
+{
+    MethodDescriptor2Object *op = PyObject_New(MethodDescriptor2Object, type);
+    op->base.vectorcall = NULL;
+    op->vectorcall = MethodDescriptor_vectorcall;
+    return (PyObject *)op;
+}
+
+static PyTypeObject MethodDescriptor2_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "MethodDescriptor2",
+    sizeof(MethodDescriptor2Object),
+    .tp_new = MethodDescriptor2_new,
+    .tp_call = PyVectorcall_Call,
+    .tp_vectorcall_offset = offsetof(MethodDescriptor2Object, vectorcall),
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_VECTORCALL,
+};
+
+
+int
+_PyTestCapi_Init_Vectorcall(PyObject *m) {
+    if (PyModule_AddFunctions(m, TestMethods) < 0) {
+        return -1;
+    }
+
+    if (PyType_Ready(&MethodDescriptorBase_Type) < 0) {
+        return -1;
+    }
+    if (PyModule_AddType(m, &MethodDescriptorBase_Type) < 0) {
+        return -1;
+    }
+
+    MethodDescriptorDerived_Type.tp_base = &MethodDescriptorBase_Type;
+    if (PyType_Ready(&MethodDescriptorDerived_Type) < 0) {
+        return -1;
+    }
+    if (PyModule_AddType(m, &MethodDescriptorDerived_Type) < 0) {
+        return -1;
+    }
+
+    MethodDescriptorNopGet_Type.tp_base = &MethodDescriptorBase_Type;
+    if (PyType_Ready(&MethodDescriptorNopGet_Type) < 0) {
+        return -1;
+    }
+    if (PyModule_AddType(m, &MethodDescriptorNopGet_Type) < 0) {
+        return -1;
+    }
+
+    MethodDescriptor2_Type.tp_base = &MethodDescriptorBase_Type;
+    if (PyType_Ready(&MethodDescriptor2_Type) < 0) {
+        return -1;
+    }
+    if (PyModule_AddType(m, &MethodDescriptor2_Type) < 0) {
+        return -1;
+    }
+
+    return 0;
+}