import: set correct version on empty import (#230)

* import: set correct version for empty import

CHANGELOG.md

@@ -10,6 +10,8 @@ Special thanks to external contributors on this release:
 
 ### Bug Fixes
 
+- [import] [\#230](https://github.com/tendermint/iavl/pull/230) Set correct version when committing an empty import.
+
 ## 0.13.2 (March 18, 2020)
 
 ### Improvements

import.go

@@ -157,10 +157,12 @@ func (i *Importer) Commit() error {
 		return ErrNoImport
 	}
 
-	switch {
-	case len(i.stack) == 1:
+	switch len(i.stack) {
+	case 0:
+		i.batch.Set(i.tree.ndb.rootKey(i.version), []byte{})
+	case 1:
 		i.batch.Set(i.tree.ndb.rootKey(i.version), i.stack[0].hash)
-	case len(i.stack) > 2:
+	default:
 		return errors.Errorf("invalid node structure, found stack size %v when committing",
 			len(i.stack))
 	}

import_test.go

@@ -3,6 +3,7 @@ package iavl
 import (
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	db "github.com/tendermint/tm-db"
@@ -199,6 +200,18 @@ func TestImporter_Commit_Closed(t *testing.T) {
 	require.Equal(t, ErrNoImport, err)
 }
 
+func TestImporter_Commit_Empty(t *testing.T) {
+	tree, err := NewMutableTree(db.NewMemDB(), 0)
+	require.NoError(t, err)
+	importer, err := tree.Import(3)
+	require.NoError(t, err)
+	defer importer.Close()
+
+	err = importer.Commit()
+	require.NoError(t, err)
+	assert.EqualValues(t, 3, tree.Version())
+}
+
 func BenchmarkImport(b *testing.B) {
 	b.StopTimer()
 	tree := setupExportTreeSized(b, 4096)