From ae2ea4a62f60c72dae81ca6642944ca28cf59889 Mon Sep 17 00:00:00 2001
From: Jae Kwon <jae@tendermint.com>
Date: Mon, 22 Jan 2018 05:32:54 -0800
Subject: [PATCH] VersionedTree.SaveVersion() succeeds on idempontent

---
 nodedb.go         |  7 ++++---
 versioned_tree.go | 11 ++++++++++-
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/nodedb.go b/nodedb.go
index 06bd45e28..55f2bfff3 100644
--- a/nodedb.go
+++ b/nodedb.go
@@ -129,9 +129,10 @@ func (ndb *nodeDB) Has(hash []byte) bool {
 	return ndb.db.Get(key) != nil
 }
 
-// SaveBranch saves the given node and all of its descendants.  NOTE: This
-// function clears leftNode/rigthNode recursively and calls hashWithCount on
-// the given node.
+// SaveBranch saves the given node and all of its descendants.
+// NOTE: This function clears leftNode/rigthNode recursively and
+// calls _hash() on the given node.
+// TODO refactor, maybe use hashWithCount() but provide a callback.
 func (ndb *nodeDB) SaveBranch(node *Node) []byte {
 	if node.persisted {
 		return node.hash
diff --git a/versioned_tree.go b/versioned_tree.go
index ac72e5d6d..191d9700e 100644
--- a/versioned_tree.go
+++ b/versioned_tree.go
@@ -1,6 +1,7 @@
 package iavl
 
 import (
+	"bytes"
 	"fmt"
 
 	"github.com/pkg/errors"
@@ -149,7 +150,15 @@ func (tree *VersionedTree) SaveVersion() ([]byte, int64, error) {
 	version := tree.version + 1
 
 	if _, ok := tree.versions[version]; ok {
-		return nil, version, errors.Errorf("version %d was already saved", version)
+		// Same hash means idempotent.  Return success.
+		var existingHash = tree.versions[version].Hash()
+		var newHash = tree.orphaningTree.Hash()
+		if bytes.Equal(existingHash, newHash) {
+			tree.orphaningTree = newOrphaningTree(tree.versions[version].clone())
+			return existingHash, version, nil
+		}
+		return nil, version, errors.Errorf("version %d was already saved to different hash %X (existing hash %X)",
+			version, newHash, existingHash)
 	}
 
 	// Persist version and stash to .versions.