[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>

diff --git a/Lib/test/test_ast/test_ast.py b/Lib/test/test_ast/test_ast.py
index 7510995b20e9d490596adc1cb64fc60f6bcbd4a3..f07e2d027b1624b7a758f189d1bba65e56bd2abd 100644 (file)
--- a/Lib/test/test_ast/test_ast.py
+++ b/Lib/test/test_ast/test_ast.py
@@ -66,6 +66,23 @@ class AST_Tests(unittest.TestCase):
             # "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 = "type object 'ast.AST' has no attribute '_fields'"
+        # Both examples used to crash:
+        with self.assertRaisesRegex(AttributeError, msg):
+            ast.AST(arg1=123)
+        with self.assertRaisesRegex(AttributeError, 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 (file)
index 0000000..547eb3a
--- /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 the :attr:`ast.AST._fields` attribute is deleted.

diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index 2c34f5c1bc26758ddf3aa950ede9abc871186683..6f5a772dccbd3ffab333dbd15d0bce5b300c3eb3 100755 (executable)
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -813,14 +813,15 @@ 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 */

diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index ecaff2041f9565ff2c1129f3e6c1c0b15f22bd38..3d5aeff837953b66fe842f58e7fa22c21710e380 100644 (file)
--- 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 */