return error when loading version from an empty tree

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## Unreleased
+
+### Breaking Changes
+
+- [\#317](https://github.com/cosmos/iavl/pull/317) `LoadVersion()` and `LazyLoadVersion()` now error 
+  if called with a positive version number on an empty tree.
+
 ## 0.15.0-rc3 (August 27, 2020)
 
 ### Improvements

mutable_tree.go

@@ -284,7 +284,10 @@ func (tree *MutableTree) LazyLoadVersion(targetVersion int64) (int64, error) {
 
 	// no versions have been saved if the latest version is non-positive
 	if latestVersion <= 0 {
-		return 0, nil
+		if targetVersion <= 0 {
+			return 0, nil
+		}
+		return 0, fmt.Errorf("no versions found while trying to load %v", targetVersion)
 	}
 
 	// default to the latest version if the targeted version is non-positive
@@ -323,7 +326,10 @@ func (tree *MutableTree) LoadVersion(targetVersion int64) (int64, error) {
 	}
 
 	if len(roots) == 0 {
-		return 0, nil
+		if targetVersion <= 0 {
+			return 0, nil
+		}
+		return 0, fmt.Errorf("no versions found while trying to load %v", targetVersion)
 	}
 
 	firstVersion := int64(0)

mutable_tree_test.go

@@ -106,6 +106,40 @@ func TestMutableTree_DeleteVersions(t *testing.T) {
 	}
 }
 
+func TestMutableTree_LoadVersion_Empty(t *testing.T) {
+	memDB := db.NewMemDB()
+	tree, err := NewMutableTree(memDB, 0)
+	require.NoError(t, err)
+
+	version, err := tree.LoadVersion(0)
+	require.NoError(t, err)
+	assert.EqualValues(t, 0, version)
+
+	version, err = tree.LoadVersion(-1)
+	require.NoError(t, err)
+	assert.EqualValues(t, 0, version)
+
+	_, err = tree.LoadVersion(3)
+	require.Error(t, err)
+}
+
+func TestMutableTree_LazyLoadVersion_Empty(t *testing.T) {
+	memDB := db.NewMemDB()
+	tree, err := NewMutableTree(memDB, 0)
+	require.NoError(t, err)
+
+	version, err := tree.LazyLoadVersion(0)
+	require.NoError(t, err)
+	assert.EqualValues(t, 0, version)
+
+	version, err = tree.LazyLoadVersion(-1)
+	require.NoError(t, err)
+	assert.EqualValues(t, 0, version)
+
+	_, err = tree.LazyLoadVersion(3)
+	require.Error(t, err)
+}
+
 func TestMutableTree_InitialVersion(t *testing.T) {
 	memDB := db.NewMemDB()
 	tree, err := NewMutableTreeWithOpts(memDB, 0, &Options{InitialVersion: 9})