https://github.com/python/cpython/commit/26a1cd4e8c0c9ea1a6683abd82547ddee656ff3d
commit: 26a1cd4e8c0c9ea1a6683abd82547ddee656ff3d
branch: main
author: Pieter Eendebak <pieter.eende...@gmail.com>
committer: kumaraditya303 <kumaradi...@python.org>
date: 2025-06-02T20:13:32+05:30
summary:
gh-123471: make concurrent iteration over `itertools.cycle` safe under
free-threading (#131212)
Co-authored-by: Kumar Aditya <kumaradi...@python.org>
files:
A Lib/test/test_free_threading/test_itertools.py
A Misc/NEWS.d/next/Library/2025-03-13-20-48-58.gh-issue-123471.cM4w4f.rst
D Lib/test/test_free_threading/test_itertools_batched.py
M Modules/itertoolsmodule.c
diff --git a/Lib/test/test_free_threading/test_itertools_batched.py
b/Lib/test/test_free_threading/test_itertools.py
similarity index 53%
rename from Lib/test/test_free_threading/test_itertools_batched.py
rename to Lib/test/test_free_threading/test_itertools.py
index a754b4f9ea9902..8360afbf78cadd 100644
--- a/Lib/test/test_free_threading/test_itertools_batched.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
new file mode 100644
index 00000000000000..cfc783900de70f
--- /dev/null
+++ 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
index 943c1e8607b38f..2003546ce84cef 100644
--- 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[] = {
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