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

Lib/test/test_free_threading/test_syslog.py

@@ -1,6 +1,8 @@
+import sys
 import unittest
 import threading
 
+from test import support
 from test.support import import_helper, threading_helper
 from test.support.threading_helper import run_concurrently
 
@@ -39,6 +41,27 @@ def worker():
             nthreads=NTHREADS,
         )
 
+    def test_racing_openlog_argv(self):
+        # gh-159864: openlog() without an ident reads sys.argv[0] while
+        # another thread replaces it.
+        argv = ["prog"]
+
+        def opener():
+            for _ in range(1000):
+                syslog.openlog()
+
+        def writer():
+            for i in range(1000):
+                argv[0] = f"prog-{i}"
+
+        with support.swap_attr(sys, "argv", argv):
+            try:
+                run_concurrently(
+                    worker_func=[opener] * (NTHREADS - 1) + [writer],
+                )
+            finally:
+                syslog.closelog()
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2026-10-20-12-30-00.gh-issue-159864.4FZ6MR.rst

@@ -0,0 +1,2 @@
+Fix a crash in :func:`syslog.openlog` under the free-threaded build when
+:data:`sys.argv` is modified concurrently.

Modules/syslogmodule.c

@@ -106,8 +106,7 @@ syslog_get_argv(void)
         return NULL;
     }
 
-    scriptobj = PyList_GetItem(argv, 0);
-    Py_XINCREF(scriptobj);
+    scriptobj = PyList_GetItemRef(argv, 0);
     Py_DECREF(argv);
     if (scriptobj == NULL) {
         PyErr_Clear();