[3.12] gh-126105: Fix crash in `ast` module, when `._fields` is delet… (#126132)

[3.12] gh-126105: Fix crash in `ast` module, when `._fields` is deleted (GH-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).
(cherry picked from commit b2eaa75b176e07730215d76d8dce4d63fb493391)

Co-authored-by: sobolevn <mail@sobolevn.me>

1 files changed, 7 insertions, 6 deletions

diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index ecaff20..3d5aeff 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -863,14 +863,15 @@ ast_type_init(PyObject *self, PyObject *args, PyObject *kw)
     Py_ssize_t i, numfields = 0;
     int res = -1;
     PyObject *key, *value, *fields;
-    if (_PyObject_LookupAttr((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;
-        }
+
+    numfields = PySequence_Size(fields);
+    if (numfields == -1) {
+        goto cleanup;
     }
 
     res = 0; /* if no error occurs, this stays 0 to the end */