Issue #26494: Fixed crash on iterating exhausting iterators.

Affected classes are generic sequence iterators, iterators of str, bytes,
bytearray, list, tuple, set, frozenset, dict, OrderedDict, corresponding
views and os.scandir() iterator.

Objects/bytearrayobject.c

@@ -3126,8 +3126,8 @@ bytearrayiter_next(bytesiterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 

Objects/bytesobject.c

@@ -3806,8 +3806,8 @@ striter_next(striterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 

Objects/dictobject.c

@@ -2988,8 +2988,8 @@ static PyObject *dictiter_iternextkey(dictiterobject *di)
     return key;
 
 fail:
-    Py_DECREF(d);
     di->di_dict = NULL;
+    Py_DECREF(d);
     return NULL;
 }
 
@@ -3069,8 +3069,8 @@ static PyObject *dictiter_iternextvalue(dictiterobject *di)
     return value;
 
 fail:
-    Py_DECREF(d);
     di->di_dict = NULL;
+    Py_DECREF(d);
     return NULL;
 }
 
@@ -3164,8 +3164,8 @@ static PyObject *dictiter_iternextitem(dictiterobject *di)
     return result;
 
 fail:
-    Py_DECREF(d);
     di->di_dict = NULL;
+    Py_DECREF(d);
     return NULL;
 }
 

Objects/iterobject.c

@@ -69,8 +69,8 @@ iter_iternext(PyObject *iterator)
         PyErr_ExceptionMatches(PyExc_StopIteration))
     {
         PyErr_Clear();
-        Py_DECREF(seq);
         it->it_seq = NULL;
+        Py_DECREF(seq);
     }
     return NULL;
 }

Objects/listobject.c

@@ -2776,8 +2776,8 @@ listiter_next(listiterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 
@@ -2906,9 +2906,17 @@ static PyObject *
 listreviter_next(listreviterobject *it)
 {
     PyObject *item;
-    Py_ssize_t index = it->it_index;
-    PyListObject *seq = it->it_seq;
+    Py_ssize_t index;
+    PyListObject *seq;
 
+    assert(it != NULL);
+    seq = it->it_seq;
+    if (seq == NULL) {
+        return NULL;
+    }
+    assert(PyList_Check(seq));
+
+    index = it->it_index;
     if (index>=0 && index < PyList_GET_SIZE(seq)) {
         item = PyList_GET_ITEM(seq, index);
         it->it_index--;
@@ -2916,10 +2924,8 @@ listreviter_next(listreviterobject *it)
         return item;
     }
     it->it_index = -1;
-    if (seq != NULL) {
-        it->it_seq = NULL;
-        Py_DECREF(seq);
-    }
+    it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 

Objects/setobject.c

@@ -916,8 +916,8 @@ static PyObject *setiter_iternext(setiterobject *si)
     return key;
 
 fail:
-    Py_DECREF(so);
     si->si_set = NULL;
+    Py_DECREF(so);
     return NULL;
 }
 

Objects/tupleobject.c

@@ -961,8 +961,8 @@ tupleiter_next(tupleiterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 

Objects/unicodeobject.c

@@ -15401,8 +15401,8 @@ unicodeiter_next(unicodeiterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }