Add test code for non Py_TPFLAGS_PREHEADER case

Author: corona10

Modules/_testcapi/mem.c

@@ -342,6 +342,46 @@ test_pyobject_setallocators(PyObject *self, PyObject *Py_UNUSED(ignored))
     return test_setallocators(PYMEM_DOMAIN_OBJ);
 }
 
+static PyObject *
+test_pyobject_gc(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    PyTupleObject *obj = PyObject_GC_NewVar(PyTupleObject, &PyTuple_Type, 3);
+    obj->ob_item[0] = Py_NewRef(Py_None);
+    obj->ob_item[1] = Py_NewRef(Py_None);
+    obj->ob_item[2] = Py_NewRef(Py_None);
+
+    if (obj == NULL) {
+        goto alloc_failed;
+    }
+    if (Py_SIZE(obj) != 3) {
+        Py_DECREF(obj);
+        PyErr_SetString(PyExc_RuntimeError, "Invalid Py_SIZE(obj)");
+        return NULL;
+    }
+
+    PyTupleObject *new_obj = PyObject_GC_Resize(PyTupleObject, obj, 6);
+    if (new_obj == NULL) {
+        Py_DECREF(obj);
+        goto alloc_failed;
+    }
+
+    new_obj->ob_item[3] = Py_NewRef(Py_None);
+    new_obj->ob_item[4] = Py_NewRef(Py_None);
+    new_obj->ob_item[5] = Py_NewRef(Py_None);
+    if (Py_SIZE(new_obj) != 6) {
+        Py_DECREF(new_obj);
+        PyErr_SetString(PyExc_RuntimeError, "Invalid Py_SIZE(new_obj)");
+        return NULL;
+    }
+
+    Py_DECREF(new_obj);
+    Py_RETURN_NONE;
+
+alloc_failed:
+    PyErr_NoMemory();
+    return NULL;
+}
+
 static PyObject *
 test_pyobject_new(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
@@ -693,6 +733,7 @@ static PyMethodDef test_methods[] = {
     {"test_pymem_setrawallocators",   test_pymem_setrawallocators,   METH_NOARGS},
     {"test_pyobject_new",             test_pyobject_new,             METH_NOARGS},
     {"test_pyobject_setallocators",   test_pyobject_setallocators,   METH_NOARGS},
+    {"test_pyobject_gc",              test_pyobject_gc,              METH_NOARGS},
 
     // Tracemalloc tests
     {"tracemalloc_track",             tracemalloc_track,             METH_VARARGS},