Fix reverse iteration from a given key

Now when you reverse iterate past the last key, it goes backwards from the end.
When you reverse iterate before the first key, you get nothing out.

Note: This is a breaking change if users have come to depend on the old
behavior.

src/db.rs

@@ -466,10 +466,13 @@ impl DBIterator {
                 self.raw.seek_to_last();
                 self.direction = Direction::Reverse;
             }
-            IteratorMode::From(key, dir) => {
+            IteratorMode::From(key, Direction::Forward) => {
-                // TODO: Should use seek_for_prev when reversing
                 self.raw.seek(key);
-                self.direction = dir;
+                self.direction = Direction::Forward;
+            }
+            IteratorMode::From(key, Direction::Reverse) => {
+                self.raw.seek_for_prev(key);
+                self.direction = Direction::Reverse;
             }
         };
 

tests/test_iterator.rs

@@ -122,6 +122,11 @@ pub fn test_iterator() {
             let expected = vec![(cba(&k2), cba(&v2)), (cba(&k1), cba(&v1))];
             assert_eq!(iterator1.collect::<Vec<_>>(), expected);
         }
+        {
+            let iterator1 = db.iterator(IteratorMode::From(b"zz", Direction::Reverse));
+            let expected = vec![(cba(&k4), cba(&v4)), (cba(&k3), cba(&v3))];
+            assert_eq!(iterator1.take(2).collect::<Vec<_>>(), expected);
+        }
         {
             let iterator1 = db.iterator(IteratorMode::From(b"k0", Direction::Forward));
             assert!(iterator1.valid());
@@ -132,13 +137,13 @@ pub fn test_iterator() {
             let iterator4 = db.iterator(IteratorMode::From(b"k5", Direction::Forward));
             assert!(!iterator4.valid());
             let iterator5 = db.iterator(IteratorMode::From(b"k0", Direction::Reverse));
-            assert!(iterator5.valid());
+            assert!(!iterator5.valid());
             let iterator6 = db.iterator(IteratorMode::From(b"k1", Direction::Reverse));
             assert!(iterator6.valid());
             let iterator7 = db.iterator(IteratorMode::From(b"k11", Direction::Reverse));
             assert!(iterator7.valid());
             let iterator8 = db.iterator(IteratorMode::From(b"k5", Direction::Reverse));
-            assert!(!iterator8.valid());
+            assert!(iterator8.valid());
         }
         {
             let mut iterator1 = db.iterator(IteratorMode::From(b"k4", Direction::Forward));