pythongh-109786: Fix leaks when re-enter itertools.pairwise.__next__()

Lib/test/test_itertools.py

@@ -1152,6 +1152,38 @@ def test_pairwise(self):
         with self.assertRaises(TypeError):
             pairwise(None)                                  # non-iterable argument
 
+    def test_pairwise_reenter(self):
+        def check(reenter_at, expected):
+            class I:
+                count = 0
+                def __iter__(self):
+                    return self
+                def __next__(self):
+                    self.count +=1
+                    if self.count == reenter_at:
+                        return next(it)
+                    return [self.count]  # new object
+
+            it = pairwise(I())
+            for item in expected:
+                self.assertEqual(next(it), item)
+
+        check(1, [
+            (([2], [3]), [4]),
+            ([4], [5]),
+        ])
+        check(2, [
+            ([1], ([3], [4])),
+            (([3], [4]), [5]),
+            ([5], [6]),
+        ])
+        check(3, [
+            ([1], [2]),
+            ([2], ([2], [4])),
+            (([2], [4]), [5]),
+            ([5], [6]),
+        ])
+
     def test_product(self):
         for args, result in [
             ([], [()]),                     # zero iterables

Misc/NEWS.d/next/Library/2023-09-23-14-40-51.gh-issue-109786.UX3pKv.rst

@@ -0,0 +1,2 @@
+Fix reference leaks when re-enter the ``__next__()`` method of
+:class:`itertools.pairwise`.

Modules/itertoolsmodule.c

@@ -330,21 +330,27 @@ pairwise_next(pairwiseobject *po)
         return NULL;
     }
     if (old == NULL) {
-        po->old = old = (*Py_TYPE(it)->tp_iternext)(it);
+        old = (*Py_TYPE(it)->tp_iternext)(it);
         if (old == NULL) {
             Py_CLEAR(po->it);
+            Py_CLEAR(po->old);
             return NULL;
         }
     }
+    else {
+        Py_INCREF(old);
+    }
     new = (*Py_TYPE(it)->tp_iternext)(it);
     if (new == NULL) {
         Py_CLEAR(po->it);
         Py_CLEAR(po->old);
+        Py_DECREF(old);
         return NULL;
     }
     /* Future optimization: Reuse the result tuple as we do in enumerate() */
     result = PyTuple_Pack(2, old, new);
-    Py_SETREF(po->old, new);
+    Py_XSETREF(po->old, new);
+    Py_DECREF(old);
     return result;
 }