eendebakpt · May 29, 2026 · Aug 30, 2026 · Aug 31, 2026
diff --git a/Lib/test/test_free_threading/test_itertools.py b/Lib/test/test_free_threading/test_itertools.py
@@ -71,6 +71,13 @@ def test_combinations_with_replacement(self):
                 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

@@ -968,76 +968,49 @@ def test_pairwise(self):
             pairwise(None)                                  # non-iterable argument
 
     def test_pairwise_reenter(self):
-        def check(reenter_at, expected):
+        # gh-149557: re-entering pairwise.__next__ from within the input
+        # iterator's __next__ now raises RuntimeError, matching the behavior
+        # of itertools.tee and of generators.
+        def check(reenter_at):
             class I:
                 count = 0
                 def __iter__(self):
                     return self
                 def __next__(self):
-                    self.count +=1
+                    self.count += 1
                     if self.count in 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], ([1], [3])),
-            (([1], [3]), [4]),
-            ([4], [5]),
-        ])
-        check({3}, [
-            ([1], [2]),
-            ([2], ([2], [4])),
-            (([2], [4]), [5]),
-            ([5], [6]),
-        ])
-        check({1, 2}, [
-            ((([3], [4]), [5]), [6]),
-            ([6], [7]),
-        ])
-        check({1, 3}, [
-            (([2], ([2], [4])), [5]),
-            ([5], [6]),
-        ])
-        check({1, 4}, [
-            (([2], [3]), (([2], [3]), [5])),
-            ((([2], [3]), [5]), [6]),
-            ([6], [7]),
-        ])
-        check({2, 3}, [
-            ([1], ([1], ([1], [4]))),
-            (([1], ([1], [4])), [5]),
-            ([5], [6]),
-        ])
+            with self.assertRaisesRegex(RuntimeError, "pairwise"):
+                list(it)
 
-    def test_pairwise_reenter2(self):
-        def check(maxcount, expected):
-            class I:
-                count = 0
-                def __iter__(self):
-                    return self
-                def __next__(self):
-                    if self.count >= maxcount:
-                        raise StopIteration
-                    self.count +=1
-                    if self.count == 1:
-                        return next(it, None)
-                    return [self.count]  # new object
+        # Re-entry on any of the first few __next__ calls must raise.
+        for reenter_at in [{1}, {2}, {3}, {1, 2}, {1, 3}, {1, 4}, {2, 3}]:
+            with self.subTest(reenter_at=reenter_at):
+                check(reenter_at)
 
-            it = pairwise(I())
-            self.assertEqual(list(it), expected)
-
-        check(1, [])
-        check(2, [])
-        check(3, [])
-        check(4, [(([2], [3]), [4])])
+    def test_pairwise_reenter2(self):
+        # gh-149557: variant of test_pairwise_reenter where the re-entrant
+        # call uses next(it, None) (i.e. supplies a default).  Even with a
+        # default that suppresses StopIteration, the re-entry itself must
+        # raise RuntimeError, propagating out of the outer pairwise call.
+        class I:
+            count = 0
+            def __iter__(self):
+                return self
+            def __next__(self):
+                if self.count >= 4:
+                    raise StopIteration
+                self.count += 1
+                if self.count == 1:
+                    return next(it, None)
+                return [self.count]  # new object
+
+        it = pairwise(I())
+        with self.assertRaisesRegex(RuntimeError, "pairwise"):
+            list(it)
 
     def test_product(self):
         for args, result in [

diff --git a/Misc/NEWS.d/next/Library/2026-05-28-21-02-32.gh-issue-149557.GtPxZK.rst b/Misc/NEWS.d/next/Library/2026-05-28-21-02-32.gh-issue-149557.GtPxZK.rst
@@ -0,0 +1,8 @@
+:class:`itertools.pairwise` iterators are now safe to share between
+threads in the :term:`free threaded <free threading>` build: concurrent
+calls to ``__next__`` are serialized via a critical section
+(:gh:`123471`). Re-entrant calls (e.g. when the input iterator's
+``__next__`` calls back into the :class:`!pairwise` object) now raise
+:exc:`RuntimeError`, matching the behavior of generators and
+:class:`itertools.tee` and fixing a use-after-free crash
+(:gh:`149557`). Patch by Pieter Eendebak.
@@ -278,6 +278,7 @@ typedef struct {
     PyObject *it;
     PyObject *old;
     PyObject *result;
+    uint8_t running;
 } pairwiseobject;
 
 #define pairwiseobject_CAST(op) ((pairwiseobject *)(op))
@@ -344,36 +345,44 @@ 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);
+
+    // Re-entrancy guard. The enclosing critical section serializes calls
+    // from different threads; this flag catches same-thread re-entry (the
+    // input iterator's __next__ calling back into pairwise.__next__) and
+    // raises, matching itertools.tee and generators.
+    if (po->running) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "pairwise() iterator already executing");
+        return NULL;
+    }
+    po->running = 1;
+
     PyObject *it = po->it;
     PyObject *old = po->old;
     PyObject *new, *result;
 
     if (it == NULL) {
-        return NULL;
+        result = NULL;
+        goto done;
     }
     if (old == NULL) {
         old = (*Py_TYPE(it)->tp_iternext)(it);
-        Py_XSETREF(po->old, old);
         if (old == NULL) {
             Py_CLEAR(po->it);
-            return NULL;
-        }
-        it = po->it;
-        if (it == NULL) {
-            Py_CLEAR(po->old);
-            return NULL;
+            result = NULL;
+            goto done;
         }
+        po->old = old;  // po->old was NULL; no decref needed
     }
-    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;
+        result = NULL;
+        goto done;
     }
 
     result = po->result;
@@ -393,8 +402,20 @@ pairwise_next(PyObject *op)
         result = _PyTuple_FromPair(old, new);
     }
 
-    Py_XSETREF(po->old, new);
-    Py_DECREF(old);
+    Py_SETREF(po->old, new);  // po->old == old, known non-NULL
+
+done:
+    po->running = 0;
+    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;
 }