feat: add api to save ChangeSet

CHANGELOG.md

@@ -13,6 +13,7 @@
 - [#636](https://github.com/cosmos/iavl/pull/636) Speed up rollback method: `LoadVersionForOverwriting`.
 - [#654](https://github.com/cosmos/iavl/pull/654) Add API `TraverseStateChanges` to extract state changes from iavl versions.
 - [#638](https://github.com/cosmos/iavl/pull/638) Make LazyLoadVersion check the opts.InitialVersion, add API `LazyLoadVersionForOverwriting`.
+- [#695](https://github.com/cosmos/iavl/pull/695) Add API `SaveChangeSet` to save the changeset as a new version.
 
 ## 0.19.4 (October 28, 2022)
 

diff_test.go

@@ -21,19 +21,10 @@ func TestDiffRoundTrip(t *testing.T) {
 	db := db.NewMemDB()
 	tree, err := NewMutableTree(db, 0, true)
 	require.NoError(t, err)
-	for _, cs := range changeSets {
-		for _, pair := range cs.Pairs {
-			if pair.Delete {
-				_, removed, err := tree.Remove(pair.Key)
-				require.True(t, removed)
-				require.NoError(t, err)
-			} else {
-				_, err := tree.Set(pair.Key, pair.Value)
-				require.NoError(t, err)
-			}
-		}
-		_, _, err := tree.SaveVersion()
+	for i := range changeSets {
+		v, err := tree.SaveChangeSet(&changeSets[i])
 		require.NoError(t, err)
+		require.Equal(t, int64(i+1), v)
 	}
 
 	// extract change sets from db

mutable_tree.go

@@ -1012,3 +1012,29 @@ func (tree *MutableTree) balance(node *Node) (newSelf *Node, err error) {
 	// Nothing changed
 	return node, nil
 }
+
+// SaveChangeSet saves a ChangeSet to the tree.
+// It is used to replay a ChangeSet as a new version.
+func (tree *MutableTree) SaveChangeSet(cs *ChangeSet) (int64, error) {
+	// if the tree has uncommitted changes, return error
+	if tree.root != nil && !tree.root.persisted {
+		return 0, fmt.Errorf("cannot save changeset with uncommitted changes")
+	}
+	for _, pair := range cs.Pairs {
+		if pair.Delete {
+			_, removed, err := tree.Remove(pair.Key)
+			if !removed {
+				return 0, fmt.Errorf("attempted to remove non-existent key %s", pair.Key)
+			}
+			if err != nil {
+				return 0, err
+			}
+		} else {
+			if _, err := tree.Set(pair.Key, pair.Value); err != nil {
+				return 0, err
+			}
+		}
+	}
+	_, version, err := tree.SaveVersion()
+	return version, err
+}