pythongh-143377: fix crashes in _format_TracebackException

Author: picnixz

Lib/test/test_crossinterp.py

@@ -1,11 +1,13 @@
 import contextlib
 import itertools
 import sys
+import traceback
 import types
 import unittest
 import warnings
 
-from test.support import import_helper
+from test import support
+from test.support import import_helper, swap_attr
 
 _testinternalcapi = import_helper.import_module('_testinternalcapi')
 _interpreters = import_helper.import_module('_interpreters')
@@ -1491,5 +1493,21 @@ def test_builtin_objects(self):
         ])
 
 
+class ExceptionTests(unittest.TestCase):
+
+    def test_capture_exception(self):
+        # Prevent crashes with incompatible TracebackException.format().
+        # Regression test for https://github.com/python/cpython/issues/143377.
+        with swap_attr(traceback.TracebackException, "format", lambda e: []):
+            _interpreters.capture_exception(ValueError())
+
+        with swap_attr(traceback.TracebackException, "format", lambda e: 1):
+            with support.catch_unraisable_exception() as cm:
+                _interpreters.capture_exception(ValueError())
+                self.assertEqual(cm.unraisable.exc_type, TypeError)
+                self.assertEqual(str(cm.unraisable.exc_value),
+                                 "can only join an iterable")
+
+
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-01-04-16-56-17.gh-issue-143377.YJqMCa.rst

@@ -0,0 +1,3 @@
+Fix a crash when formatting exceptions for which
+:meth:`traceback.TracebackException.format` return an empty iterable. Patch
+by Bénédikt Tran.

Python/crossinterp.c

@@ -1137,10 +1137,12 @@ _format_TracebackException(PyObject *tbexc)
     if (formatted_obj == NULL) {
         return NULL;
     }
-
     Py_ssize_t size = -1;
     const char *formatted = _copy_string_obj_raw(formatted_obj, &size);
     Py_DECREF(formatted_obj);
+    if (formatted == NULL || size == 0) {
+        return formatted;
+    }
     // We remove trailing the newline added by TracebackException.format().
     assert(formatted[size-1] == '\n');
     ((char *)formatted)[size-1] = '\0';