gh-123471: Make itertools.product and itertools.combinations thread-safe (#132814)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/test/test_free_threading/test_itertools_combinatoric.py

@@ -0,0 +1,51 @@
+import unittest
+from threading import Thread, Barrier
+from itertools import combinations, product
+from test.support import threading_helper
+
+
+threading_helper.requires_working_threading(module=True)
+
+def test_concurrent_iteration(iterator, number_of_threads):
+    barrier = Barrier(number_of_threads)
+    def iterator_worker(it):
+        barrier.wait()
+        while True:
+            try:
+                _ = next(it)
+            except StopIteration:
+                return
+
+    worker_threads = []
+    for ii in range(number_of_threads):
+        worker_threads.append(
+            Thread(target=iterator_worker, args=[iterator]))
+
+    with threading_helper.start_threads(worker_threads):
+        pass
+
+    barrier.reset()
+
+class ItertoolsThreading(unittest.TestCase):
+
+    @threading_helper.reap_threads
+    def test_combinations(self):
+        number_of_threads = 10
+        number_of_iterations = 24
+
+        for it in range(number_of_iterations):
+            iterator = combinations((1, 2, 3, 4, 5), 2)
+            test_concurrent_iteration(iterator, number_of_threads)
+
+    @threading_helper.reap_threads
+    def test_product(self):
+        number_of_threads = 10
+        number_of_iterations = 24
+
+        for it in range(number_of_iterations):
+            iterator = product((1, 2, 3, 4, 5), (10, 20, 30))
+            test_concurrent_iteration(iterator, number_of_threads)
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Library/2025-04-22-21-00-23.gh-issue-123471.asOLA2.rst

@@ -0,0 +1 @@
+Make concurrent iterations over :class:`itertools.combinations` and :class:`itertools.product` safe under free-threading.

Modules/itertoolsmodule.c

@@ -2096,7 +2096,7 @@ product_traverse(PyObject *op, visitproc visit, void *arg)
 }
 
 static PyObject *
-product_next(PyObject *op)
+product_next_lock_held(PyObject *op)
 {
     productobject *lz = productobject_CAST(op);
     PyObject *pool;
@@ -2182,6 +2182,16 @@ empty:
     return NULL;
 }
 
+static PyObject *
+product_next(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    result = product_next_lock_held(op);
+    Py_END_CRITICAL_SECTION()
+    return result;
+}
+
 static PyMethodDef product_methods[] = {
     {"__sizeof__", product_sizeof, METH_NOARGS, sizeof_doc},
     {NULL,              NULL}   /* sentinel */
@@ -2329,7 +2339,7 @@ combinations_traverse(PyObject *op, visitproc visit, void *arg)
 }
 
 static PyObject *
-combinations_next(PyObject *op)
+combinations_next_lock_held(PyObject *op)
 {
     combinationsobject *co = combinationsobject_CAST(op);
     PyObject *elem;
@@ -2414,6 +2424,16 @@ empty:
     return NULL;
 }
 
+static PyObject *
+combinations_next(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    result = combinations_next_lock_held(op);
+    Py_END_CRITICAL_SECTION()
+    return result;
+}
+
 static PyMethodDef combinations_methods[] = {
     {"__sizeof__", combinations_sizeof, METH_NOARGS, sizeof_doc},
     {NULL,              NULL}   /* sentinel */