gh-123471: Make concurrent iteration over itertools.islice thread-safe using a critical section

Lib/test/test_free_threading/test_itertools.py

@@ -1,5 +1,5 @@
 import unittest
-from itertools import accumulate, batched, chain, combinations_with_replacement, cycle, permutations, zip_longest
+from itertools import accumulate, batched, chain, combinations_with_replacement, cycle, islice, permutations, zip_longest
 from test.support import threading_helper
 
 
@@ -55,6 +55,13 @@ def test_combinations_with_replacement(self):
             it = combinations_with_replacement(tuple(range(2)), 2)
             threading_helper.run_concurrently(work_iterator, nthreads=6, args=[it])
 
+    @threading_helper.reap_threads
+    def test_islice(self):
+        number_of_iterations = 6
+        for _ in range(number_of_iterations):
+            it = islice(tuple(range(10)), 1, 8, 2)
+            threading_helper.run_concurrently(work_iterator, nthreads=10, args=[it])
+
     @threading_helper.reap_threads
     def test_permutations(self):
         number_of_iterations = 6

Misc/NEWS.d/next/Library/2026-04-10-00-00-00.gh-issue-123471.islice_ft.rst

@@ -0,0 +1,2 @@
+Make concurrent iteration over :class:`itertools.islice` safe under
+free-threading.

Modules/itertoolsmodule.c

@@ -1624,7 +1624,7 @@ islice_traverse(PyObject *op, visitproc visit, void *arg)
 }
 
 static PyObject *
-islice_next(PyObject *op)
+islice_next_lock_held(PyObject *op)
 {
     isliceobject *lz = isliceobject_CAST(op);
     PyObject *item;
@@ -1663,6 +1663,16 @@ 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\