gh-123471: Make itertools.chain thread-safe (#135689)

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 @@ class ItertoolsThreading(unittest.TestCase):
             barrier.wait()
             while True:
                 try:
-                    _ = next(it)
+                    next(it)
                 except StopIteration:
                     break
 
@@ -62,6 +62,34 @@ class ItertoolsThreading(unittest.TestCase):
 
             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\