gh-123471: Make concurrent iteration over itertools.cycle safe under free-threading

Lib/test/test_free_threading/test_itertools_batched.py → 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()

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.

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[] ={