make cycle_next thread safe

Author: eendebakpt

Lib/test/test_free_threading/test_itertools_batched.py renamed to Lib/test/test_free_threading/test_itertools.py

@@ -1,15 +1,15 @@
 import unittest
 from threading import Thread, Barrier
-from itertools import batched
+from itertools import batched, cycle
 from test.support import threading_helper
 
 
 threading_helper.requires_working_threading(module=True)
 
-class EnumerateThreading(unittest.TestCase):
+class ItertoolsThreading(unittest.TestCase):
 
     @threading_helper.reap_threads
-    def test_threading(self):
+    def test_batched(self):
         number_of_threads = 10
         number_of_iterations = 20
         barrier = Barrier(number_of_threads)
@@ -34,5 +34,31 @@ def work(it):
 
             barrier.reset()
 
+    @threading_helper.reap_threads
+    def test_cycle(self):
+        number_of_threads = 6
+        number_of_iterations = 10
+        number_of_cycles = 400
+
+        barrier = Barrier(number_of_threads)
+        def work(it):
+            barrier.wait()
+            for _ in range(number_of_cycles):
+                _ = next(it)
+
+        data = (1, 2, 3, 4)
+        for it in range(number_of_iterations):
+            cycle_iterator = cycle(data)
+            worker_threads = []
+            for ii in range(number_of_threads):
+                worker_threads.append(
+                    Thread(target=work, args=[cycle_iterator]))
+
+            with threading_helper.start_threads(worker_threads):
+                pass
+
+            barrier.reset()
+
+
 if __name__ == "__main__":
     unittest.main()

Modules/itertoolsmodule.c

@@ -1166,7 +1166,7 @@ itertools_cycle_impl(PyTypeObject *type, PyObject *iterable)
     }
     lz->it = it;
     lz->saved = saved;
-    lz->index = 0;
+    lz->index = -1;
 
     return (PyObject *)lz;
 }
@@ -1199,7 +1199,9 @@ cycle_next(PyObject *op)
     cycleobject *lz = cycleobject_CAST(op);
     PyObject *item;
 
-    if (lz->it != NULL) {
+    Py_ssize_t index = FT_ATOMIC_LOAD_SSIZE_RELAXED(lz->index);
+
+    if (index < 0) {
         item = PyIter_Next(lz->it);
         if (item != NULL) {
             if (PyList_Append(lz->saved, item)) {
@@ -1211,14 +1213,19 @@ cycle_next(PyObject *op)
         /* Note:  StopIteration is already cleared by PyIter_Next() */
         if (PyErr_Occurred())
             return NULL;
+        index = 0;
+        FT_ATOMIC_STORE_SSIZE_RELAXED(lz->index, 0);
+#ifndef Py_GIL_DISABLED
         Py_CLEAR(lz->it);
+#endif
     }
     if (PyList_GET_SIZE(lz->saved) == 0)
         return NULL;
-    item = PyList_GET_ITEM(lz->saved, lz->index);
-    lz->index++;
-    if (lz->index >= PyList_GET_SIZE(lz->saved))
-        lz->index = 0;
+    item = PyList_GET_ITEM(lz->saved, index);
+    index++;
+    if (index >= PyList_GET_SIZE(lz->saved)) {
+        FT_ATOMIC_STORE_SSIZE_RELAXED(lz->index, 0);
+    }
     return Py_NewRef(item);
 }