gh-151409: add critical section for islice_next

Lib/test/test_itertools.py

@@ -2489,6 +2489,34 @@ def __eq__(self, other):
         for j in range(2):
             next(g, None)  # shouldn't crash
 
+    @threading_helper.requires_working_threading()
+    def test_islice_thread_safety(self):
+        # gh-151409: islice must not yield more items than its stop argument
+        # when consumed concurrently from multiple threads.
+        STOP = 100
+        NTHREADS = 8
+        data = iter(range(STOP + NTHREADS * 2))
+        sl = islice(data, STOP)
+        results: list[int] = []
+        lock = threading.Lock()
+
+        def consume() -> None:
+            while True:
+                v = next(sl, None)
+                if v is None:
+                    break
+                with lock:
+                    results.append(v)
+
+        threads = [threading.Thread(target=consume) for _ in range(NTHREADS)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        self.assertLessEqual(len(results), STOP)
+        self.assertEqual(sorted(results), list(range(len(results))))
+
 
 class SubclassWithKwargsTest(unittest.TestCase):
     def test_keywords_in_subclass(self):

Misc/NEWS.d/next/Library/2026-06-12-15-44-31.gh-issue-151409.QAx38E.rst

@@ -0,0 +1 @@
+Fix :class:`itertools.islice` to be thread-safe when shared across threads.

Modules/itertoolsmodule.c

@@ -1625,8 +1625,8 @@ islice_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
-static PyObject *
-islice_next(PyObject *op)
+static inline PyObject *
+islice_next_lock_held(PyObject *op)
 {
     isliceobject *lz = isliceobject_CAST(op);
     PyObject *item;
@@ -1665,6 +1665,16 @@ islice_next(PyObject *op)
     return NULL;
 }
 
+static PyObject *
+islice_next(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    result = islice_next_lock_held(op);
+    Py_END_CRITICAL_SECTION();
+    return result;
+}
+
 PyDoc_STRVAR(islice_doc,
 "islice(iterable, stop) --> islice object\n\
 islice(iterable, start, stop[, step]) --> islice object\n\