gh-126105: Fix crash in `ast` module, when `._fields` is deleted (#126115)

Previously, if the `ast.AST._fields` attribute was deleted, attempts to create a new `as`t node would crash due to the assumption that `_fields` always had a non-NULL value. Now it has been fixed by adding an extra check to ensure that `_fields` does not have a NULL value (this can happen when you manually remove `_fields` attribute).

1 files changed, 8 insertions, 10 deletions

diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index 32eac3a..853a3e9 100755
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -884,19 +884,17 @@ ast_type_init(PyObject *self, PyObject *args, PyObject *kw)
     Py_ssize_t i, numfields = 0;
     int res = -1;
     PyObject *key, *value, *fields, *attributes = NULL, *remaining_fields = NULL;
-    if (PyObject_GetOptionalAttr((PyObject*)Py_TYPE(self), state->_fields, &fields) < 0) {
+
+    fields = PyObject_GetAttr((PyObject*)Py_TYPE(self), state->_fields);
+    if (fields == NULL) {
         goto cleanup;
     }
-    if (fields) {
-        numfields = PySequence_Size(fields);
-        if (numfields == -1) {
-            goto cleanup;
-        }
-        remaining_fields = PySet_New(fields);
-    }
-    else {
-        remaining_fields = PySet_New(NULL);
+
+    numfields = PySequence_Size(fields);
+    if (numfields == -1) {
+        goto cleanup;
     }
+    remaining_fields = PySet_New(fields);
     if (remaining_fields == NULL) {
         goto cleanup;
     }