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.

8 files changed, 22 insertions, 16 deletions

diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index c59ad24..c723a9c 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -3186,8 +3186,8 @@ bytearrayiter_next(bytesiterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index 51d0871..495c3eb 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -3628,8 +3628,8 @@ striter_next(striterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index e4dff98..d774586 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -2985,8 +2985,8 @@ static PyObject *dictiter_iternextkey(dictiterobject *di)
     return key;
 
 fail:
-    Py_DECREF(d);
     di->di_dict = NULL;
+    Py_DECREF(d);
     return NULL;
 }
 
@@ -3066,8 +3066,8 @@ static PyObject *dictiter_iternextvalue(dictiterobject *di)
     return value;
 
 fail:
-    Py_DECREF(d);
     di->di_dict = NULL;
+    Py_DECREF(d);
     return NULL;
 }
 
@@ -3161,8 +3161,8 @@ static PyObject *dictiter_iternextitem(dictiterobject *di)
     return result;
 
 fail:
-    Py_DECREF(d);
     di->di_dict = NULL;
+    Py_DECREF(d);
     return NULL;
 }
 
diff --git a/Objects/iterobject.c b/Objects/iterobject.c
index 2fb0c88..ab29ff8 100644
--- a/Objects/iterobject.c
+++ b/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;
 }
diff --git a/Objects/listobject.c b/Objects/listobject.c
index eee7c68..d688179 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -2782,8 +2782,8 @@ listiter_next(listiterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 
@@ -2912,9 +2912,17 @@ 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--;
@@ -2922,10 +2930,8 @@ 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;
 }
 
diff --git a/Objects/setobject.c b/Objects/setobject.c
index 582f280..4ef692d 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -839,8 +839,8 @@ static PyObject *setiter_iternext(setiterobject *si)
     return key;
 
 fail:
-    Py_DECREF(so);
     si->si_set = NULL;
+    Py_DECREF(so);
     return NULL;
 }
 
diff --git a/Objects/tupleobject.c b/Objects/tupleobject.c
index 7efa1a6..7920fec 100644
--- a/Objects/tupleobject.c
+++ b/Objects/tupleobject.c
@@ -964,8 +964,8 @@ tupleiter_next(tupleiterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index adc4615..230125b 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -15149,8 +15149,8 @@ unicodeiter_next(unicodeiterobject *it)
         return item;
     }
 
-    Py_DECREF(seq);
     it->it_seq = NULL;
+    Py_DECREF(seq);
     return NULL;
 }