Issue #11004: Repair edge case in deque.count().

(Reviewed by Georg Brandl.)

Also made similar changes to deque.reverse() though this wasn't
strictly necessary (the edge case cannot occur with two pointers
moving to meet in the middle).  Making the change in reverse()
was more a matter of future-proofing.

1 files changed, 7 insertions, 4 deletions

diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index f4a2c8b..2391c0d 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -485,7 +485,8 @@ deque_reverse(dequeobject *deque, PyObject *unused)
         /* Advance left block/index pair */
         leftindex++;
         if (leftindex == BLOCKLEN) {
-            assert (leftblock->rightlink != NULL);
+            if (leftblock->rightlink == NULL)
+                break;
             leftblock = leftblock->rightlink;
             leftindex = 0;
         }
@@ -493,7 +494,8 @@ deque_reverse(dequeobject *deque, PyObject *unused)
         /* Step backwards with the right block/index pair */
         rightindex--;
         if (rightindex == -1) {
-            assert (rightblock->leftlink != NULL);
+            if (rightblock->leftlink == NULL)
+                break;
             rightblock = rightblock->leftlink;
             rightindex = BLOCKLEN - 1;
         }
@@ -509,7 +511,7 @@ deque_count(dequeobject *deque, PyObject *v)
 {
     block *leftblock = deque->leftblock;
     Py_ssize_t leftindex = deque->leftindex;
-    Py_ssize_t n = (deque->len);
+    Py_ssize_t n = deque->len;
     Py_ssize_t i;
     Py_ssize_t count = 0;
     PyObject *item;
@@ -533,7 +535,8 @@ deque_count(dequeobject *deque, PyObject *v)
         /* Advance left block/index pair */
         leftindex++;
         if (leftindex == BLOCKLEN) {
-            assert (leftblock->rightlink != NULL);
+            if (leftblock->rightlink == NULL)  /* can occur when i==n-1 */
+                break;
             leftblock = leftblock->rightlink;
             leftindex = 0;
         }