Free-threaded: concurrent syslog.openlog() and sys.argv[0] assignment crash the interpreter #159864

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.

I 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.

Reading the source on main (4c73f50, 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.

The 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.

import sys
import syslog
import threading
import time

assert not sys._is_gil_enabled(), "needs a free-threaded build with the GIL disabled"

stop = threading.Event()

def reader():
    while not stop.is_set():
        syslog.openlog()

def writer():
    i = 0
    while not stop.is_set():
        i += 1
        sys.argv[0] = f"prog-{i}" + "z" * 24

threads = [threading.Thread(target=reader) for _ in range(3)]
threads.append(threading.Thread(target=writer))
for t in threads:
    t.start()
time.sleep(4)
stop.set()
for t in threads:
    t.join()
syslog.closelog()
print("no crash in 4 s")

Linked PRs

• gh-159864: Fix syslog.openlog() crash with concurrent sys.argv changes #159884
• [3.15] gh-159864: Fix syslog.openlog() crash with concurrent sys.argv changes (GH-159884) #159952
• [3.14] gh-159864: Fix syslog.openlog() crash with concurrent sys.argv changes (GH-159884) #159954