core: add Backwards option for storage iterators

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

pkg/compiler/syscall_test.go

@@ -44,6 +44,7 @@ func TestFindFlags(t *testing.T) {
 	require.EqualValues(t, storage.DeserializeValues, istorage.FindDeserialize)
 	require.EqualValues(t, storage.PickField0, istorage.FindPick0)
 	require.EqualValues(t, storage.PickField1, istorage.FindPick1)
+	require.EqualValues(t, storage.Backwards, istorage.FindBackwards)
 }
 
 type syscallTestCase struct {

pkg/core/interop/storage/find.go

@@ -19,9 +19,10 @@ const (
 	FindDeserialize  = 1 << 3
 	FindPick0        = 1 << 4
 	FindPick1        = 1 << 5
+	FindBackwards    = 1 << 7
 
 	FindAll = FindDefault | FindKeysOnly | FindRemovePrefix | FindValuesOnly |
-		FindDeserialize | FindPick0 | FindPick1
+		FindDeserialize | FindPick0 | FindPick1 | FindBackwards
 )
 
 // Iterator is an iterator state representation.
@@ -111,8 +112,9 @@ func Find(ic *interop.Context) error {
 	if opts&FindDeserialize == 0 && (opts&FindPick0 != 0 || opts&FindPick1 != 0) {
 		return fmt.Errorf("%w: PickN is specified without Deserialize", errFindInvalidOptions)
 	}
+	bkwrds := opts&FindBackwards != 0
 	ctx, cancel := context.WithCancel(context.Background())
-	seekres := ic.DAO.SeekAsync(ctx, stc.ID, storage.SeekRange{Prefix: prefix})
+	seekres := ic.DAO.SeekAsync(ctx, stc.ID, storage.SeekRange{Prefix: prefix, Backwards: bkwrds})
 	item := NewIterator(seekres, prefix, opts)
 	ic.VM.Estack().PushItem(stackitem.NewInterop(item))
 	ic.RegisterCancelFunc(func() {

pkg/core/interop/storage/storage_test.go

@@ -197,7 +197,18 @@ func TestFind(t *testing.T) {
 			}),
 		})
 	})
-
+	t.Run("normal invocation, backwards", func(t *testing.T) {
+		testFind(t, []byte{0x01}, istorage.FindBackwards, []stackitem.Item{
+			stackitem.NewStruct([]stackitem.Item{
+				stackitem.NewByteArray(skeys[0]),
+				stackitem.NewByteArray(items[0]),
+			}),
+			stackitem.NewStruct([]stackitem.Item{
+				stackitem.NewByteArray(skeys[2]),
+				stackitem.NewByteArray(items[2]),
+			}),
+		})
+	})
 	t.Run("keys only", func(t *testing.T) {
 		testFind(t, []byte{0x01}, istorage.FindKeysOnly, []stackitem.Item{
 			stackitem.NewByteArray(skeys[2]),

pkg/interop/storage/storage.go

@@ -36,6 +36,8 @@ const (
 	PickField0 FindFlags = 1 << 4
 	// PickField1 is used to get second field in a serialized struct or array.
 	PickField1 FindFlags = 1 << 5
+	// Backwards is used to iterate over elements in reversed (descending) order.
+	Backwards FindFlags = 1 << 7
 )
 
 // ConvertContextToReadOnly returns new context from the given one, but with