GH-98363: Fix exception handling in batched() (GH-98523)

Lib/test/test_itertools.py

@@ -2012,6 +2012,20 @@ class E:
     def __next__(self):
         3 // 0
 
+class E2:
+    'Test propagation of exceptions after two iterations'
+    def __init__(self, seqn):
+        self.seqn = seqn
+        self.i = 0
+    def __iter__(self):
+        return self
+    def __next__(self):
+        if self.i == 2:
+            raise ZeroDivisionError
+        v = self.seqn[self.i]
+        self.i += 1
+        return v
+
 class S:
     'Test immediate stop'
     def __init__(self, seqn):
@@ -2050,6 +2064,7 @@ class TestVariousIteratorArgs(unittest.TestCase):
         self.assertRaises(TypeError, batched, X(s), 2)
         self.assertRaises(TypeError, batched, N(s), 2)
         self.assertRaises(ZeroDivisionError, list, batched(E(s), 2))
+        self.assertRaises(ZeroDivisionError, list, batched(E2(s), 4))
 
     def test_chain(self):
         for s in ("123", "", range(1000), ('do', 1.2), range(2000,2200,5)):

Modules/itertoolsmodule.c

@@ -154,23 +154,36 @@ batched_next(batchedobject *bo)
     if (result == NULL) {
         return NULL;
     }
+    iternextfunc iternext = *Py_TYPE(it)->tp_iternext;
+    PyObject **items = PySequence_Fast_ITEMS(result);
     for (i=0 ; i < n ; i++) {
-        item = PyIter_Next(it);
+        item = iternext(it);
         if (item == NULL) {
-            break;
+            goto null_item;
+        }
+        items[i] = item;
+    }
+    return result;
+
+ null_item:
+    if (PyErr_Occurred()) {
+        if (PyErr_ExceptionMatches(PyExc_StopIteration)) {
+            PyErr_Clear();
+        } else {
+            /* input raised an exception other than StopIteration */
+            Py_CLEAR(bo->it);
+            Py_DECREF(result);
+            return NULL;
         }
-        PyList_SET_ITEM(result, i, item);
     }
     if (i == 0) {
         Py_CLEAR(bo->it);
         Py_DECREF(result);
         return NULL;
     }
-    if (i < n) {
     PyObject *short_list = PyList_GetSlice(result, 0, i);
-        Py_SETREF(result, short_list);
+    Py_DECREF(result);
-    }
+    return short_list;
-    return result;
 }
 
 static PyTypeObject batched_type = {