bpo-39573: Convert Py_TYPE() and Py_SIZE() back to macros (GH-23366)

This change partically reverts
commit ad3252bad905d41635bcbb4b76db30d570cf0087
and the commit fe2978b3b940fe2478335e3a2ca5ad22338cdf9c.

Many third party C extension modules rely on the ability of using
Py_TYPE() to set an object type: "Py_TYPE(obj) = type;" or to set an
object type using: "Py_SIZE(obj) = size;".

1 files changed, 25 insertions, 0 deletions

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 7b6da1e..a1d4c92 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -5612,6 +5612,30 @@ pynumber_tobase(PyObject *module, PyObject *args)
 
 static PyObject *test_buildvalue_issue38913(PyObject *, PyObject *);
 
+
+static PyObject*
+test_set_type_size(PyObject* self, PyObject* ignored)
+{
+    PyObject *obj = PyList_New(0);
+    if (obj == NULL) {
+        return NULL;
+    }
+
+    // Ensure that following tests don't modify the object,
+    // to ensure that Py_DECREF() will not crash.
+    assert(Py_TYPE(obj) == &PyList_Type);
+    assert(Py_SIZE(obj) == 0);
+
+    // bpo-39573: Check that Py_TYPE() and Py_SIZE() can be used
+    // as l-values to set an object type and size.
+    Py_TYPE(obj) = &PyList_Type;
+    Py_SIZE(obj) = 0;
+
+    Py_DECREF(obj);
+    Py_RETURN_NONE;
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
     {"raise_memoryerror",       raise_memoryerror,               METH_NOARGS},
@@ -5883,6 +5907,7 @@ static PyMethodDef TestMethods[] = {
     {"meth_fastcall_keywords", (PyCFunction)(void(*)(void))meth_fastcall_keywords, METH_FASTCALL|METH_KEYWORDS},
     {"pynumber_tobase", pynumber_tobase, METH_VARARGS},
     {"without_gc", without_gc, METH_O},
+    {"test_set_type_size", test_set_type_size, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };