Prevent cases when db returned empty changes

commonspace/object/tree/objecttree/tree.go

@@ -53,6 +53,9 @@ func (t *Tree) Root() *Change {
 }
 
 func (t *Tree) AddFast(changes ...*Change) []*Change {
+	if len(changes) == 0 {
+		return nil
+	}
 	defer t.clearUnattached()
 	t.addedBuf = t.addedBuf[:0]
 	for _, c := range changes {

commonspace/object/tree/objecttree/tree_test.go

@@ -49,6 +49,10 @@ func TestTree_Add(t *testing.T) {
 		assert.Equal(t, tr.root.Id, "root")
 		assert.Equal(t, []string{"root"}, tr.Heads())
 	})
+	t.Run("empty tree add should not panic", func(t *testing.T) {
+		tr := &Tree{}
+		tr.AddFast()
+	})
 	t.Run("linear add", func(t *testing.T) {
 		tr := new(Tree)
 		res, _ := tr.Add(

commonspace/object/tree/objecttree/treebuilder.go

@@ -17,7 +17,7 @@ import (
 
 var (
 	log      = logger.NewNamedSugared("common.commonspace.objecttree")
-	ErrEmpty = errors.New("logs empty")
+	ErrEmpty = errors.New("database is empty")
 )
 
 type treeBuilder struct {
@@ -146,6 +146,9 @@ func (tb *treeBuilder) build(opts treeBuilderOpts) (tr *Tree, err error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to get changes after order: %w", err)
 	}
+	if len(changes) == 0 {
+		return nil, ErrEmpty
+	}
 	tr = &Tree{}
 	changes = append(changes, opts.newChanges...)
 	tr.AddFast(changes...)