gh-123471: Make itertools.zip_longest safe in the FT build

Lib/test/test_free_threading/test_itertools.py

@@ -1,5 +1,5 @@
 import unittest
-from itertools import accumulate, batched, chain, combinations_with_replacement, cycle, permutations
+from itertools import accumulate, batched, chain, combinations_with_replacement, cycle, permutations, zip_longest
 from test.support import threading_helper
 
 
@@ -62,6 +62,13 @@ def test_permutations(self):
             it = permutations(tuple(range(4)), 2)
             threading_helper.run_concurrently(work_iterator, nthreads=6, args=[it])
 
+    @threading_helper.reap_threads
+    def test_zip_longest(self):
+        number_of_iterations = 10
+        for _ in range(number_of_iterations):
+            it = zip_longest(list(range(4)), list(range(8)), fillvalue=0)
+            threading_helper.run_concurrently(work_iterator, nthreads=10, args=[it])
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2026-03-17-19-51-05.gh-issue-123471.oY4UR5.rst

@@ -0,0 +1 @@
+Make concurrent iteration over :class:`itertools.zip_longest` safe under free-threading.

Modules/itertoolsmodule.c

@@ -3876,7 +3876,7 @@ zip_longest_traverse(PyObject *op, visitproc visit, void *arg)
 }
 
 static PyObject *
-zip_longest_next(PyObject *op)
+zip_longest_next_lock_held(PyObject *op)
 {
     ziplongestobject *lz = ziplongestobject_CAST(op);
     Py_ssize_t i;
@@ -3947,6 +3947,16 @@ zip_longest_next(PyObject *op)
     return result;
 }
 
+static PyObject *
+zip_longest_next(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    result = zip_longest_next_lock_held(op);
+    Py_END_CRITICAL_SECTION()
+    return result;
+}
+
 PyDoc_STRVAR(zip_longest_doc,
 "zip_longest(*iterables, fillvalue=None)\n\
 --\n\