René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Free-threaded: concurrent syslog.openlog() and sys.argv[0] assignment crash the interpreter","articleBody":"On the free-threaded build, calling `syslog.openlog()` (without `ident`) in several threads while another thread assigns `sys.argv[0]` crashes the interpreter with SIGSEGV.\n\nI ran the script below as a separate process (`python3.14t repro.py`, 5 times, default environment) on macOS 27.0.1 arm64 with the python-build-standalone build `cpython-3.14.6+freethreaded-macos-aarch64-none` installed by uv. Every run was killed by a signal: SIGSEGV in 2 runs and SIGTRAP in 3 (shell exit codes 139 and 133). An earlier driver with the same structure (three `openlog()` threads, one writer, 4 s, `PYTHONMALLOC=debug`) crashed 3.14.0rc1, 3.14.6 and 3.15.0b4 in 5 of 5 runs each; only the last 200 characters of stderr were kept from those runs, so they give no crash-site evidence. I have not tested 3.14.8 or `main`.\n\nReading the source on `main` (4c73f50ca9, lines 109–110), `syslog_get_argv()` in `Modules/syslogmodule.c` takes `sys.argv[0]` with `PyList_GetItem()` (a borrowed reference) and increfs it only afterwards. This suggests a possible borrowed-reference lifetime race with a concurrent replacement of `sys.argv[0]`; I have not established that this race causes the observed SIGSEGV. `PyList_GetItemRef()` may address this particular window; I have not validated a patch.\n\nThe same pattern was listed as item 91 of #157197, which was closed for procedural reasons and whose linked gists are gone; I am not aware of another report specifically tracking it.\n\n\u003cdetails>\u003csummary>Reproducer (run as a separate process; repeat a few times)\u003c/summary>\n\n```python\nimport sys\nimport syslog\nimport threading\nimport time\n\nassert not sys._is_gil_enabled(), \"needs a free-threaded build with the GIL disabled\"\n\nstop = threading.Event()\n\ndef reader():\n    while not stop.is_set():\n        syslog.openlog()\n\ndef writer():\n    i = 0\n    while not stop.is_set():\n        i += 1\n        sys.argv[0] = f\"prog-{i}\" + \"z\" * 24\n\nthreads = [threading.Thread(target=reader) for _ in range(3)]\nthreads.append(threading.Thread(target=writer))\nfor t in threads:\n    t.start()\ntime.sleep(4)\nstop.set()\nfor t in threads:\n    t.join()\nsyslog.closelog()\nprint(\"no crash in 4 s\")\n```\n\n\u003c/details>\n\n\n\u003c!-- gh-linked-prs -->\n### Linked PRs\n* gh-159884\n* gh-159952\n* gh-159954\n\u003c!-- /gh-linked-prs -->\n","author":{"url":"https://github.com/espressolee","@type":"Person","name":"espressolee"},"datePublished":"2026-10-20T07:00:55Z","url":"https://github.com/python/cpython/issues/159864"}