From 4186e14befb4f7aa9955926d538873ce4e6bbebc Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Tue, 29 Oct 2024 11:46:13 +0300
Subject: [PATCH] gh-126105: Fix crash in `ast` module, when `._fields` is
 deleted

---
 Lib/test/test_ast/test_ast.py                  | 17 +++++++++++++++++
 ...4-10-29-11-45-44.gh-issue-126105.cOL-R6.rst |  1 +
 Python/Python-ast.c                            | 18 ++++++++++--------
 3 files changed, 28 insertions(+), 8 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2024-10-29-11-45-44.gh-issue-126105.cOL-R6.rst

diff --git a/Lib/test/test_ast/test_ast.py b/Lib/test/test_ast/test_ast.py
index dd1c6447fca337..61c9ef01b7ecb6 100644
--- a/Lib/test/test_ast/test_ast.py
+++ b/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
diff --git a/Misc/NEWS.d/next/Library/2024-10-29-11-45-44.gh-issue-126105.cOL-R6.rst b/Misc/NEWS.d/next/Library/2024-10-29-11-45-44.gh-issue-126105.cOL-R6.rst
new file mode 100644
index 00000000000000..de8702b3cf65be
--- /dev/null
+++ b/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.
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index 38d74b48d232f8..50aef75d08bee2 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -5086,16 +5086,18 @@ ast_type_init(PyObject *self, PyObject *args, PyObject *kw)
     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;
     }