gh-123471: Make itertools.chain thread-safe

Lib/test/test_free_threading/test_itertools.py

@@ -1,6 +1,6 @@
 import unittest
 from threading import Thread, Barrier
-from itertools import batched, cycle
+from itertools import batched, chain, cycle
 from test.support import threading_helper
 
 
@@ -17,7 +17,7 @@ def work(it):
  barrier.wait()
  while True:
  try:
-_ = next(it)
+ next(it)
  except StopIteration:
  break
 
@@ -62,6 +62,34 @@ def work(it):
 
  barrier.reset()
 
+ @threading_helper.reap_threads
+ def test_chain(self):
+ number_of_threads = 6
+ number_of_iterations = 20
+
+ barrier = Barrier(number_of_threads)
+ def work(it):
+ barrier.wait()
+ while True:
+ try:
+ next(it)
+ except StopIteration:
+ break
+
+ data = [(1, )] * 200
+ for it in range(number_of_iterations):
+ chain_iterator = chain(*data)
+ worker_threads = []
+ for ii in range(number_of_threads):
+ worker_threads.append(
+ Thread(target=work, args=[chain_iterator]))
+
+ with threading_helper.start_threads(worker_threads):
+ pass
+
+ barrier.reset()
+
+
 
 if __name__ == "__main__":
  unittest.main()

Misc/NEWS.d/next/Library/2025-06-18-19-25-32.gh-issue-123471.lx1Xbt.rst

@@ -0,0 +1 @@
+Make concurrent iterations over :class:`itertools.chain` safe under :term:`free threading`.

Modules/itertoolsmodule.c

@@ -1880,8 +1880,8 @@ chain_traverse(PyObject *op, visitproc visit, void *arg)
  return 0;
 }
 
-static PyObject *
-chain_next(PyObject *op)
+static inline PyObject *
+chain_next_lock_held(PyObject *op)
 {
  chainobject *lz = chainobject_CAST(op);
  PyObject *item;
@@ -1919,6 +1919,16 @@ chain_next(PyObject *op)
  return NULL;
 }
 
+static PyObject *
+chain_next(PyObject *op)
+{
+ PyObject *result;
+ Py_BEGIN_CRITICAL_SECTION(op);
+ result = chain_next_lock_held(op);
+ Py_END_CRITICAL_SECTION()
+ return result;
+}
+
 PyDoc_STRVAR(chain_doc,
 "chain(*iterables)\n\
 --\n\