gh-126105: Fix crash in ast module, when ._fields is deleted

Lib/test/test_ast/test_ast.py

@@ -84,6 +84,23 @@ def test_AST_objects(self):
             # "ast.AST constructor takes 0 positional arguments"
             ast.AST(2)
 
+    def test_AST_fields_NULL_check(self):
+        # See: https://github.com/python/cpython/issues/126105
+        old_value = ast.AST._fields
+
+        def cleanup():
+            ast.AST._fields = old_value
+        self.addCleanup(cleanup)
+
+        del ast.AST._fields
+
+        msg = 'AST has no fields'
+        # Both examples used to crash:
+        with self.assertRaisesRegex(TypeError, msg):
+            ast.AST(arg1=123)
+        with self.assertRaisesRegex(TypeError, msg):
+            ast.AST()
+
     def test_AST_garbage_collection(self):
         class X:
             pass

Misc/NEWS.d/next/Library/2024-10-29-11-45-44.gh-issue-126105.cOL-R6.rst

@@ -0,0 +1 @@
+Fix a crash in :mod:`ast` when ``_fields`` attribute is deleted.

Parser/asdl_c.py

@@ -887,16 +887,18 @@ def visitModule(self, mod):
     if (PyObject_GetOptionalAttr((PyObject*)Py_TYPE(self), state->_fields, &fields) < 0) {
         goto cleanup;
     }
-    if (fields) {
-        numfields = PySequence_Size(fields);
-        if (numfields == -1) {
-            goto cleanup;
-        }
-        remaining_fields = PySet_New(fields);
+    if (fields == NULL) {
+        PyErr_Format(PyExc_TypeError,
+                     "%.400s has no fields",
+                     _PyType_Name(Py_TYPE(self)));
+        goto cleanup;
     }
-    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;
     }