bpo-45826: Fix a crash in suggestions.c by checking for `traceback is None` (GH-29590)

Lib/test/test_exceptions.py

@@ -1885,6 +1885,37 @@ class NameErrorTests(unittest.TestCase):
 
         self.assertNotIn("something", err.getvalue())
 
+    def test_issue45826(self):
+        # regression test for bpo-45826
+        def f():
+            with self.assertRaisesRegex(NameError, 'aaa'):
+                aab
+
+        try:
+            f()
+        except self.failureException:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertIn("aab", err.getvalue())
+
+    def test_issue45826_focused(self):
+        def f():
+            try:
+                nonsense
+            except BaseException as E:
+                E.with_traceback(None)
+                raise ZeroDivisionError()
+
+        try:
+            f()
+        except ZeroDivisionError:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertIn("nonsense", err.getvalue())
+        self.assertIn("ZeroDivisionError", err.getvalue())
+
 
 class AttributeErrorTests(unittest.TestCase):
     def test_attributes(self):

Misc/NEWS.d/next/Core and Builtins/2021-11-17-08-05-27.bpo-45826.OERoTm.rst

@@ -0,0 +1 @@
+Fixed a crash when calling ``.with_traceback(None)`` on ``NameError``. This occurs internally in ``unittest.TestCase.assertRaises()``.

Python/suggestions.c

@@ -202,13 +202,21 @@ offer_suggestions_for_name_error(PyNameErrorObject *exc)
     PyTracebackObject *traceback = (PyTracebackObject *) exc->traceback; // borrowed reference
     // Abort if we don't have a variable name or we have an invalid one
     // or if we don't have a traceback to work with
-    if (name == NULL || traceback == NULL || !PyUnicode_CheckExact(name)) {
+    if (name == NULL || !PyUnicode_CheckExact(name) ||
+        traceback == NULL || !Py_IS_TYPE(traceback, &PyTraceBack_Type)
+    ) {
         return NULL;
     }
 
     // Move to the traceback of the exception
-    while (traceback->tb_next != NULL) {
-        traceback = traceback->tb_next;
+    while (1) {
+        PyTracebackObject *next = traceback->tb_next;
+        if (next == NULL || !Py_IS_TYPE(next, &PyTraceBack_Type)) {
+            break;
+        }
+        else {
+            traceback = next;
+        }
     }
 
     PyFrameObject *frame = traceback->tb_frame;