fix: critical section for islice_next

Author: KowalskiThomas

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\