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).

4 files changed, 34 insertions, 20 deletions

diff --git a/Lib/test/test_ast/test_ast.py b/Lib/test/test_ast/test_ast.py
index 739a020..67ab8cf 100644
--- a/Lib/test/test_ast/test_ast.py
+++ b/Lib/test/test_ast/test_ast.py
@@ -84,6 +84,23 @@ 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
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 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;
     }
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index 38d74b4..41299b2 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -5083,19 +5083,17 @@ 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;
     }