shed: Index.Offset - correct handling of nil start Item

Author: tzdybal

shed/index.go

@@ -458,12 +458,15 @@ func (f Index) Offset(start *Item, shift int64) (i Item, err error) {
 		if err != nil {
 			return i, err
 		}
+	} else {
+		startKey = f.prefix
 	}
+
 	it := f.db.NewIterator()
 	defer it.Release()
 
 	ok := it.Seek(startKey)
-	if !ok || bytes.Compare(it.Key(), startKey) != 0 {
+	if !ok || !bytes.HasPrefix(it.Key(), startKey) {
 		return i, errors.New("start Item not found in index")
 	}
 
@@ -473,7 +476,7 @@ func (f Index) Offset(start *Item, shift int64) (i Item, err error) {
 		shift *= -1
 	}
 
-	key := startKey
+	key := it.Key()
 	for shift != 0 && next() {
 		key = it.Key()
 		if key[0] != f.prefix[0] {

shed/index_test.go

@@ -1247,4 +1247,18 @@ func TestIndexOffset(t *testing.T) {
 			tt.Error("expected error")
 		}
 	})
+
+	t.Run("nil start Item", func(tt *testing.T) {
+		item, err := index1.Offset(nil, 0)
+		if err != nil {
+			t.Error(err)
+		}
+		checkItem(t, item, items[0])
+
+		item, err = index2.Offset(nil, 10)
+		if err != nil {
+			t.Error(err)
+		}
+		checkItem(t, item, items[10])
+	})
 }