gh-123471: Make concurrent iteration over itertools.pairwise thread-safe

Lib/test/test_free_threading/test_itertools.py

@@ -1,5 +1,5 @@
 import unittest
-from itertools import batched, chain, combinations_with_replacement, cycle, permutations
+from itertools import batched, chain, combinations_with_replacement, cycle, pairwise, permutations
 from test.support import threading_helper
 
 
@@ -48,6 +48,13 @@ def test_combinations_with_replacement(self):
             it = combinations_with_replacement(tuple(range(2)), 2)
             threading_helper.run_concurrently(work_iterator, nthreads=6, args=[it])
 
+    @threading_helper.reap_threads
+    def test_pairwise(self):
+        number_of_iterations = 10
+        for _ in range(number_of_iterations):
+            it = pairwise(tuple(range(100)))
+            threading_helper.run_concurrently(work_iterator, nthreads=10, args=[it])
+
     @threading_helper.reap_threads
     def test_permutations(self):
         number_of_iterations = 6

Misc/NEWS.d/next/Library/2026-02-04-21-17-43.gh-issue-123471.tVAWYF.rst

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

Modules/itertoolsmodule.c

@@ -343,7 +343,7 @@ pairwise_traverse(PyObject *op, visitproc visit, void *arg)
 }
 
 static PyObject *
-pairwise_next(PyObject *op)
+pairwise_next_lock_held(PyObject *op)
 {
     pairwiseobject *po = pairwiseobject_CAST(op);
     PyObject *it = po->it;
@@ -366,7 +366,7 @@ pairwise_next(PyObject *op)
             return NULL;
         }
     }
-    Py_INCREF(old);
+    Py_INCREF(old); // needed because of reentant calls via call to the iterator
     new = (*Py_TYPE(it)->tp_iternext)(it);
     if (new == NULL) {
         Py_CLEAR(po->it);
@@ -380,7 +380,7 @@ pairwise_next(PyObject *op)
         Py_INCREF(result);
         PyObject *last_old = PyTuple_GET_ITEM(result, 0);
         PyObject *last_new = PyTuple_GET_ITEM(result, 1);
-        PyTuple_SET_ITEM(result, 0, Py_NewRef(old));
+        PyTuple_SET_ITEM(result, 0, old); // consume the reference from old
         PyTuple_SET_ITEM(result, 1, Py_NewRef(new));
         Py_DECREF(last_old);
         Py_DECREF(last_new);
@@ -391,13 +391,25 @@ pairwise_next(PyObject *op)
     else {
         result = PyTuple_New(2);
         if (result != NULL) {
-            PyTuple_SET_ITEM(result, 0, Py_NewRef(old));
+            PyTuple_SET_ITEM(result, 0, old);
             PyTuple_SET_ITEM(result, 1, Py_NewRef(new));
         }
+        else {
+            Py_DECREF(old);
+        }
     }
 
     Py_XSETREF(po->old, new);
-    Py_DECREF(old);
+    return result;
+}
+
+static PyObject *
+pairwise_next(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    result = pairwise_next_lock_held(op);
+    Py_END_CRITICAL_SECTION()
     return result;
 }