gh-123471: make `itertools.batched` thread-safe (#129416)

3 files changed, 51 insertions, 2 deletions

diff --git a/Lib/test/test_free_threading/test_itertools_batched.py b/Lib/test/test_free_threading/test_itertools_batched.py
new file mode 100644
index 0000000..fa9e06b
--- /dev/null
+++ b/Lib/test/test_free_threading/test_itertools_batched.py
@@ -0,0 +1,39 @@
+import unittest
+import sys
+from threading import Thread, Barrier
+from itertools import batched
+from test.support import threading_helper
+
+
+threading_helper.requires_working_threading(module=True)
+
+class EnumerateThreading(unittest.TestCase):
+
+    @threading_helper.reap_threads
+    def test_threading(self):
+        number_of_threads = 10
+        number_of_iterations = 20
+        barrier = Barrier(number_of_threads)
+        def work(it):
+            barrier.wait()
+            while True:
+                try:
+                    _ = next(it)
+                except StopIteration:
+                    break
+
+        data = tuple(range(1000))
+        for it in range(number_of_iterations):
+            batch_iterator = batched(data, 2)
+            worker_threads = []
+            for ii in range(number_of_threads):
+                worker_threads.append(
+                    Thread(target=work, args=[batch_iterator]))
+
+            with threading_helper.start_threads(worker_threads):
+                pass
+
+            barrier.reset()
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2025-02-19-08-06-37.gh-issue-123471.br7uyR.rst b/Misc/NEWS.d/next/Library/2025-02-19-08-06-37.gh-issue-123471.br7uyR.rst
new file mode 100644
index 0000000..f34d0bc
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-02-19-08-06-37.gh-issue-123471.br7uyR.rst
@@ -0,0 +1 @@
+Make concurrent iterations over :class:`itertools.batched` safe under free-threading.
diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
index 3e425ee..40e436a6 100644
--- a/Modules/itertoolsmodule.c
+++ b/Modules/itertoolsmodule.c
@@ -191,12 +191,12 @@ batched_next(PyObject *op)
 {
     batchedobject *bo = batchedobject_CAST(op);
     Py_ssize_t i;
-    Py_ssize_t n = bo->batch_size;
+    Py_ssize_t n = FT_ATOMIC_LOAD_SSIZE_RELAXED(bo->batch_size);
     PyObject *it = bo->it;
     PyObject *item;
     PyObject *result;
 
-    if (it == NULL) {
+    if (n < 0) {
         return NULL;
     }
     result = PyTuple_New(n);
@@ -218,19 +218,28 @@ batched_next(PyObject *op)
     if (PyErr_Occurred()) {
         if (!PyErr_ExceptionMatches(PyExc_StopIteration)) {
             /* Input raised an exception other than StopIteration */
+            FT_ATOMIC_STORE_SSIZE_RELAXED(bo->batch_size, -1);
+#ifndef Py_GIL_DISABLED
             Py_CLEAR(bo->it);
+#endif
             Py_DECREF(result);
             return NULL;
         }
         PyErr_Clear();
     }
     if (i == 0) {
+        FT_ATOMIC_STORE_SSIZE_RELAXED(bo->batch_size, -1);
+#ifndef Py_GIL_DISABLED
         Py_CLEAR(bo->it);
+#endif
         Py_DECREF(result);
         return NULL;
     }
     if (bo->strict) {
+        FT_ATOMIC_STORE_SSIZE_RELAXED(bo->batch_size, -1);
+#ifndef Py_GIL_DISABLED
         Py_CLEAR(bo->it);
+#endif
         Py_DECREF(result);
         PyErr_SetString(PyExc_ValueError, "batched(): incomplete batch");
         return NULL;