gh-151364: Fix data race on kqueue file descriptor in free-threaded build

Lib/test/test_free_threading/test_select.py

@@ -0,0 +1,38 @@
+import unittest
+
+from test.support import import_helper, threading_helper
+from test.support.threading_helper import run_concurrently
+
+select = import_helper.import_module("select")
+
+NTHREADS = 4
+
+
+@threading_helper.requires_working_threading()
+@unittest.skipUnless(hasattr(select, "kqueue"), "test needs select.kqueue()")
+class KqueueTests(unittest.TestCase):
+    def test_close_while_reading(self):
+        # gh-151364: close() must not race with closed, fileno() and control()
+        def reader(kq):
+            for _ in range(100):
+                kq.closed
+                try:
+                    kq.fileno()
+                    kq.control(None, 0, 0)
+                except (ValueError, OSError):
+                    pass
+
+        def closer(kq):
+            kq.close()
+
+        for _ in range(20):
+            kq = select.kqueue()
+            run_concurrently(
+                worker_func=[closer] + [reader] * NTHREADS,
+                args=(kq,),
+            )
+            self.assertTrue(kq.closed)
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Library/2026-10-02-07-30-49.gh-issue-151364.is5v4P.rst

@@ -0,0 +1,2 @@
+Fix data races on the file descriptor of :func:`select.kqueue` objects in
+the :term:`free-threaded build`. Based on a patch by Victor Stinner.

Modules/selectmodule.c

@@ -14,6 +14,7 @@
 
 #include "Python.h"
 #include "pycore_fileutils.h"     // _Py_set_inheritable()
+#include "pycore_pyatomic_ft_wrappers.h" // FT_ATOMIC_LOAD_INT_RELAXED()
 #include "pycore_time.h"          // _PyTime_FromSecondsObject()
 #include "pycore_tuple.h"         // _PyTuple_FromPairSteal
 
@@ -2162,7 +2163,7 @@ kqueue_queue_internal_close(kqueue_queue_Object *self)
     int save_errno = 0;
     if (self->kqfd >= 0) {
         int kqfd = self->kqfd;
-        self->kqfd = -1;
+        FT_ATOMIC_STORE_INT_RELAXED(self->kqfd, -1);
         _selectstate *state = _selectstate_by_type(Py_TYPE(self));
         kqueue_tracking_remove(state, self);
         Py_BEGIN_ALLOW_THREADS
@@ -2276,7 +2277,7 @@ static PyObject *
 kqueue_queue_get_closed(PyObject *op, void *Py_UNUSED(closure))
 {
     kqueue_queue_Object *self = kqueue_queue_Object_CAST(op);
-    if (self->kqfd < 0) {
+    if (FT_ATOMIC_LOAD_INT_RELAXED(self->kqfd) < 0) {
         Py_RETURN_TRUE;
     }
     Py_RETURN_FALSE;
@@ -2292,9 +2293,10 @@ static PyObject *
 select_kqueue_fileno_impl(kqueue_queue_Object *self)
 /*[clinic end generated code: output=716f46112a4f6e5c input=41911c539ca2b0ca]*/
 {
-    if (self->kqfd < 0)
+    int kqfd = FT_ATOMIC_LOAD_INT_RELAXED(self->kqfd);
+    if (kqfd < 0)
         return kqueue_queue_err_closed();
-    return PyLong_FromLong(self->kqfd);
+    return PyLong_FromLong(kqfd);
 }
 
 /*[clinic input]
@@ -2349,7 +2351,7 @@ select_kqueue_control_impl(kqueue_queue_Object *self, PyObject *changelist,
     PyTime_t timeout, deadline = 0;
     _selectstate *state = _selectstate_by_type(Py_TYPE(self));
 
-    if (self->kqfd < 0)
+    if (FT_ATOMIC_LOAD_INT_RELAXED(self->kqfd) < 0)
         return kqueue_queue_err_closed();
 
     if (maxevents < 0) {
@@ -2431,8 +2433,8 @@ select_kqueue_control_impl(kqueue_queue_Object *self, PyObject *changelist,
     do {
         Py_BEGIN_ALLOW_THREADS
         errno = 0;
-        gotevents = kevent(self->kqfd, chl, nchanges,
-                           evl, maxevents, ptimeoutspec);
+        gotevents = kevent(FT_ATOMIC_LOAD_INT_RELAXED(self->kqfd),
+                           chl, nchanges, evl, maxevents, ptimeoutspec);
         Py_END_ALLOW_THREADS
 
         if (errno != EINTR)