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gh-158929: Fix subinterpreter thread safety issues in `syslog` by ZeroIntensity · Pull Request #159000 · python/cpython · GitHub
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gh-158929: Fix subinterpreter thread safety issues in syslog #159000

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Merged
ZeroIntensity merged 4 commits into from
Oct 10, 2026
Merged

gh-158929: Fix subinterpreter thread safety issues in syslog #159000

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b0d71c2
Fix subinterpreter thread-safety issues in syslog.
ZeroIntensity Oct 8, 2026
9b4519c
Add blurb.
ZeroIntensity Oct 8, 2026
a04c3e9
Add a test case.
ZeroIntensity Oct 8, 2026
9e058aa
Fix re-entrancy issue.
ZeroIntensity Oct 8, 2026
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Fix subinterpreter thread-safety issues in syslog.
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@ZeroIntensity
ZeroIntensity committed Oct 8, 2026
commit b0d71c2c19000aac3fc398cd0333e71fd8a71091
16 changes: 6 additions & 10 deletions Modules/syslogmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -69,7 +69,7 @@ module syslog
/* only one instance, only one syslog, so globals should be ok,
* these fields are writable from the main interpreter only. */
static PyObject *S_ident_o = NULL; // identifier, held by openlog()
static char S_log_open = 0;
static int S_log_open = 0;

static inline int
is_main_interpreter(void)
Expand Down Expand Up @@ -189,8 +189,8 @@ syslog_openlog_impl(PyObject *module, PyObject *ident, long logopt,
}

openlog(ident_str, logopt, facility);
S_log_open = 1;
Py_XSETREF(S_ident_o, ident);
_Py_atomic_store_int(&S_log_open, 1);

Py_RETURN_NONE;
}
Expand Down Expand Up @@ -222,7 +222,7 @@ syslog_syslog_impl(PyObject *module, int group_left_1, int priority,
}

/* if log is not opened, open it now */
if (!S_log_open) {
if (!_Py_atomic_load_int(&S_log_open)) {
if (!is_main_interpreter()) {
PyErr_SetString(PyExc_RuntimeError, "subinterpreter can't use syslog.syslog() "
"until the syslog is opened by the main interpreter");
Expand All @@ -235,10 +235,6 @@ syslog_syslog_impl(PyObject *module, int group_left_1, int priority,
Py_DECREF(openlog_ret);
}

/* Incref ident, because it can be decrefed if syslog.openlog() is
* called when the GIL is released.
*/
PyObject *ident = Py_XNewRef(S_ident_o);
#ifdef __APPLE__
// gh-98178: On macOS, libc syslog() is not thread-safe
syslog(priority, "%s", message);
Expand All @@ -247,7 +243,6 @@ syslog_syslog_impl(PyObject *module, int group_left_1, int priority,
syslog(priority, "%s", message);
Py_END_ALLOW_THREADS;
#endif
Py_XDECREF(ident);
Py_RETURN_NONE;
}

Expand All @@ -273,10 +268,11 @@ syslog_closelog_impl(PyObject *module)
if (PySys_Audit("syslog.closelog", NULL) < 0) {
return NULL;
}
if (S_log_open) {

int was_open = _Py_atomic_exchange_int(&S_log_open, 0);
if (was_open) {
closelog();
Py_CLEAR(S_ident_o);
S_log_open = 0;
}
Py_RETURN_NONE;
}
Expand Down