gh-154570: Fix itertools.accumulate() silently corrupting its total on reentrancy

Lib/test/test_itertools.py

@@ -157,6 +157,31 @@ def test_accumulate(self):
         with self.assertRaises(TypeError):
             list(accumulate([10, 20], 100))
 
+    def test_accumulate_reenter(self):
+        class I:
+            count = 0
+            def __iter__(self):
+                return self
+            def __next__(self):
+                self.count += 1
+                if self.count == 2:
+                    return next(it)
+                return self.count
+
+        it = accumulate(I())
+        self.assertEqual(next(it), 1)
+        with self.assertRaisesRegex(RuntimeError, "accumulate"):
+            next(it)
+
+    def test_accumulate_reenter_func(self):
+        def reenter(a, b):
+            return next(it)
+
+        it = accumulate([1, 2, 3], reenter)
+        self.assertEqual(next(it), 1)
+        with self.assertRaisesRegex(RuntimeError, "accumulate"):
+            next(it)
+
     def test_batched(self):
         self.assertEqual(list(batched('ABCDEFG', 3)),
                              [('A', 'B', 'C'), ('D', 'E', 'F'), ('G',)])

Misc/NEWS.d/next/Library/2026-07-23-22-00-00.gh-issue-154570.accumulate-reentrancy.rst

@@ -0,0 +1,2 @@
+Fix :func:`itertools.accumulate` silently corrupting its running total
+on reentrancy; it now raises :exc:`RuntimeError` instead.

Modules/itertoolsmodule.c

@@ -3079,6 +3079,7 @@ typedef struct {
     PyObject *binop;
     PyObject *initial;
     itertools_state *state;
+    uint8_t running;
 } accumulateobject;
 
 #define accumulateobject_CAST(op)   ((accumulateobject *)(op))
@@ -3120,6 +3121,7 @@ itertools_accumulate_impl(PyTypeObject *type, PyObject *iterable,
     lz->it = it;
     lz->initial = Py_XNewRef(initial);
     lz->state = find_state_by_type(type);
+    lz->running = 0;
     return (PyObject *)lz;
 }
 
@@ -3160,12 +3162,21 @@ accumulate_next_lock_held(PyObject *op)
         lz->initial = Py_NewRef(Py_None);
         return Py_NewRef(lz->total);
     }
+    if (lz->running) {
+        PyErr_SetString(PyExc_RuntimeError,
+                         "cannot re-enter the accumulate iterator");
+        return NULL;
+    }
+    lz->running = 1;
     val = (*Py_TYPE(lz->it)->tp_iternext)(lz->it);
-    if (val == NULL)
+    if (val == NULL) {
+        lz->running = 0;
         return NULL;
+    }
 
     if (lz->total == NULL) {
         lz->total = Py_NewRef(val);
+        lz->running = 0;
         return lz->total;
     }
 
@@ -3174,6 +3185,7 @@ accumulate_next_lock_held(PyObject *op)
     else
         newtotal = PyObject_CallFunctionObjArgs(lz->binop, lz->total, val, NULL);
     Py_DECREF(val);
+    lz->running = 0;
     if (newtotal == NULL)
         return NULL;