gh-144356: Make set iterator __length_hint__ and iternext race-safe under no-gil

Lib/test/test_free_threading/test_set.py

@@ -148,6 +148,43 @@ def read_set():
         for t in threads:
             t.join()
 
+    @threading_helper.reap_threads
+    def test_length_hint_with_mutating_set(self):
+        NUM_ITERS = 10
+        NUM_THREADS = 10
+        NUM_LOOPS = 400
+
+        for _ in range(NUM_ITERS):
+            s = set(range(NUM_LOOPS ))
+            it = iter(s)
+
+            def worker():
+                for i in range(NUM_LOOPS):
+                    it.__length_hint__()
+                    s.add(i)
+                    s.discard(i - 1)
+
+            threading_helper.run_concurrently(worker, nthreads=NUM_THREADS)
+
+    @threading_helper.reap_threads
+    def test_length_hint_exhaust_race(self):
+        NUM_ITERS = 10
+        NUM_THREADS = 10
+
+        for _ in range(NUM_ITERS):
+            s = set(range(256))
+            it = iter(s)
+
+            def worker():
+                while True:
+                    it.__length_hint__()
+                    try:
+                        next(it)
+                    except StopIteration:
+                        break
+
+            threading_helper.run_concurrently(worker, nthreads=NUM_THREADS)
+
 
 @threading_helper.requires_working_threading()
 class SmallSetTest(RaceTestBase, unittest.TestCase):

Misc/NEWS.d/next/Core_and_Builtins/2026-01-31-03-40-28.gh-issue-144356.otfq_X.rst

@@ -0,0 +1 @@
+Fix potential races in set iterators (``__length_hint__`` and iteration) in free-threaded builds.

Objects/setobject.c

@@ -1056,8 +1056,13 @@ setiter_len(PyObject *op, PyObject *Py_UNUSED(ignored))
 {
     setiterobject *si = (setiterobject*)op;
     Py_ssize_t len = 0;
-    if (si->si_set != NULL && si->si_used == si->si_set->used)
-        len = si->len;
+    PySetObject *so = si->si_set;
+
+    Py_BEGIN_CRITICAL_SECTION(op);
+    if (si->si_pos >= 0 && si->si_used == FT_ATOMIC_LOAD_SSIZE_RELAXED(so->used)) {
+len = si->len;
+    }
+    Py_END_CRITICAL_SECTION();
     return PyLong_FromSsize_t(len);
 }
 
@@ -1089,17 +1094,23 @@ static PyMethodDef setiter_methods[] = {
     {NULL,              NULL}           /* sentinel */
 };
 
-static PyObject *setiter_iternext(PyObject *self)
+static PyObject *
+setiter_iternext(PyObject *self)
 {
     setiterobject *si = (setiterobject*)self;
     PyObject *key = NULL;
     Py_ssize_t i, mask;
     setentry *entry;
     PySetObject *so = si->si_set;
 
-    if (so == NULL)
+#ifdef Py_GIL_DISABLED
+    assert(so != NULL);
+#else
+    if (so == NULL) {
         return NULL;
-    assert (PyAnySet_Check(so));
+    }
+#endif
+    assert(PyAnySet_Check(so));
 
     Py_ssize_t so_used = FT_ATOMIC_LOAD_SSIZE_RELAXED(so->used);
     Py_ssize_t si_used = FT_ATOMIC_LOAD_SSIZE_RELAXED(si->si_used);
@@ -1110,26 +1121,52 @@ static PyObject *setiter_iternext(PyObject *self)
         return NULL;
     }
 
+#ifdef Py_GIL_DISABLED
+    Py_BEGIN_CRITICAL_SECTION2(self, so);
+#else
     Py_BEGIN_CRITICAL_SECTION(so);
+#endif
+
     i = si->si_pos;
-    assert(i>=0);
+#ifdef Py_GIL_DISABLED
+    if (i < 0) {
+        /* iterator already exhausted */
+        goto done;
+    }
+#endif
+
     entry = so->table;
     mask = so->mask;
     while (i <= mask && (entry[i].key == NULL || entry[i].key == dummy)) {
         i++;
     }
     if (i <= mask) {
         key = Py_NewRef(entry[i].key);
+        si->si_pos = i + 1;
+        si->len--;
     }
+    else {
+        /* exhausted */
+        si->si_pos = -1;
+#ifndef Py_GIL_DISABLED
+        si->si_set = NULL;
+#endif
+    }
+
+#ifdef Py_GIL_DISABLED
+done:
+    Py_END_CRITICAL_SECTION2();
+    return key;
+#else
     Py_END_CRITICAL_SECTION();
-    si->si_pos = i+1;
+
     if (key == NULL) {
-        si->si_set = NULL;
+        /* exhausted */
         Py_DECREF(so);
         return NULL;
     }
-    si->len--;
     return key;
+#endif
 }
 
 PyTypeObject PySetIter_Type = {