gh-123471: make concurrent iteration over itertools.cycle safe under free-threading (#131212)

eendebakpt · kumaraditya303 · web-flow · commit 26a1cd4e8c0c · 2025-06-02T20:13:32.000+05:30
Co-authored-by: Kumar Aditya &lt;kumaraditya@python.org&gt;
diff --git a/Lib/test/test_free_threading/test_itertools.py b/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()
diff --git a/Misc/NEWS.d/next/Library/2025-03-13-20-48-58.gh-issue-123471.cM4w4f.rst b/Misc/NEWS.d/next/Library/2025-03-13-20-48-58.gh-issue-123471.cM4w4f.rst
@@ -0,0 +1 @@
+Make concurrent iterations over :class:`itertools.cycle` safe under free-threading.
diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
@@ -1124,7 +1124,6 @@ typedef struct {
     PyObject *it;
     PyObject *saved;
     Py_ssize_t index;
-    int firstpass;
 } cycleobject;
 
 #define cycleobject_CAST(op)    ((cycleobject *)(op))
@@ -1165,8 +1164,7 @@ itertools_cycle_impl(PyTypeObject *type, PyObject *iterable)
     }
     lz->it = it;
     lz->saved = saved;
-    lz->index = 0;
-    lz->firstpass = 0;
+    lz->index = -1;
 
     return (PyObject *)lz;
 }
@@ -1199,11 +1197,11 @@ 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 (lz->firstpass)
-                return item;
             if (PyList_Append(lz->saved, item)) {
                 Py_DECREF(item);
                 return NULL;
@@ -1213,15 +1211,22 @@ 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;
-    return Py_NewRef(item);
+    item = PyList_GetItemRef(lz->saved, index);
+    assert(item);
+    index++;
+    if (index >= PyList_GET_SIZE(lz->saved)) {
+        index = 0;
+    }
+    FT_ATOMIC_STORE_SSIZE_RELAXED(lz->index, index);
+    return item;
 }
 
 static PyType_Slot cycle_slots[] = {

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Make concurrent iterations over :class:`itertools.cycle` safe under free-threading.