René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`RemoteUnwinder.get_stack_trace` is missing exceptions on some error code paths","articleBody":"# Bug report\n\nOn a [PR](https://github.com/python/cpython/actions/runs/21453460793/job/61787996150?pr=144312) recently, I saw the following crash in `test_external_inspection`. I don't think the crash is related to the PR:\n\n```\nTest that frame cache is per-thread and cache invalidation works independently. ... Assertion failed: (res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL), file C:\\a\\cpython\\cpython\\Python\\generated_cases.c.h, line 3938\nFatal Python error: Aborted\n\n\u003cCannot show all threads while the GIL is disabled\u003e\nStack (most recent call first):\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\test_external_inspection.py\", line 2702 in _get_frames_with_retry\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\test_external_inspection.py\", line 3268 in test_cache_per_thread_isolation\n  File \"C:\\a\\cpython\\cpython\\Lib\\unittest\\case.py\", line 613 in _callTestMethod\n  File \"C:\\a\\cpython\\cpython\\Lib\\unittest\\case.py\", line 667 in run\n  File \"C:\\a\\cpython\\cpython\\Lib\\unittest\\case.py\", line 723 in __call__\n  File \"C:\\a\\cpython\\cpython\\Lib\\unittest\\suite.py\", line 122 in run\n  File \"C:\\a\\cpython\\cpython\\Lib\\unittest\\suite.py\", line 84 in __call__\n  File \"C:\\a\\cpython\\cpython\\Lib\\unittest\\suite.py\", line 122 in run\n  File \"C:\\a\\cpython\\cpython\\Lib\\unittest\\suite.py\", line 84 in __call__\n  File \"C:\\a\\cpython\\cpython\\Lib\\unittest\\runner.py\", line 257 in run\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\single.py\", line 84 in _run_suite\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\single.py\", line 42 in run_unittest\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\single.py\", line 162 in test_func\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\single.py\", line 118 in regrtest_runner\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\single.py\", line 165 in _load_run_test\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\single.py\", line 210 in _runtest_env_changed_exc\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\single.py\", line 319 in _runtest\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\single.py\", line 348 in run_single_test\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\worker.py\", line 99 in worker_process\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\worker.py\", line 134 in main\n  File \"C:\\a\\cpython\\cpython\\Lib\\test\\libregrtest\\worker.py\", line 138 in \u003cmodule\u003e\n  File \"C:\\a\\cpython\\cpython\\Lib\\runpy.py\", line 87 in _run_code\n  File \"C:\\a\\cpython\\cpython\\Lib\\runpy.py\", line 196 in _run_module_as_main\n```\n\nI think that's in the call to `_remote_debugging.RemoteUnwinder.get_stack_trace`. Either it's returning NULL without an exception set or it's returning non-NULL with an exception. \n\nI asked Claude to find the missing exceptions and it came up with the following:\n\n```\n  Missing Exception Error Paths\n\n  1. asyncio.c:119-122 - Set object validation\n  if (mask \u003c 0 || mask \u003e= MAX_SET_TABLE_SIZE || num_els \u003c 0 || num_els \u003e mask + 1) {\n      set_exception_cause(unwinder, PyExc_RuntimeError,\n          \"Invalid set object (corrupted remote memory)\");\n      return -1;\n  }\n  2. object_reading.c:198-201 - PyLong size validation\n  if (size \u003c 0 || size \u003e MAX_LONG_DIGITS) {\n      set_exception_cause(unwinder, PyExc_RuntimeError,\n          \"Invalid PyLong size (corrupted remote memory)\");\n      return -1;\n  }\n  3. frames.c:50-54 - Stack chunk size validation\n  if (actual_size \u003c= offsetof(_PyStackChunk, data) || actual_size \u003e MAX_STACK_CHUNK_SIZE) {\n      PyMem_RawFree(this_chunk);\n      set_exception_cause(unwinder, PyExc_RuntimeError,\n          \"Invalid stack chunk size (corrupted remote memory)\");\n      return -1;\n  }\n  4. code_objects.c:448-451 - TLBC index validation\n  if (ctx-\u003etlbc_index \u003c 0 || ctx-\u003etlbc_index \u003e= tlbc_entry-\u003etlbc_array_size) {\n      set_exception_cause(unwinder, PyExc_RuntimeError,\n          \"Invalid tlbc_index (corrupted remote memory)\");\n      goto error;\n  }\n  5. module.c:596-601 - Thread list cycle detection\n  if (current_tstate == prev_tstate) {\n      Py_DECREF(interpreter_threads);\n      set_exception_cause(self, PyExc_RuntimeError,\n          \"Thread list cycle detected (corrupted remote memory)\");\n      Py_CLEAR(result);\n      goto exit;\n  }\n```\n\nThe `set_exception_cause` macro only sets an exception if `unwinder-\u003edebug` is set.\n\ncc @pablogsal \n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-144442\n* gh-145280\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/colesbury","@type":"Person","name":"colesbury"},"datePublished":"2026-01-28T22:21:15.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/144316/cpython/issues/144316"}