Fix /find empty response resulting in "null"

api/graphite.go

@@ -492,7 +492,7 @@ func findPickle(nodes []idx.Node, request models.GraphiteFind, fromUnix, toUnix
 var treejsonContext = make(map[string]int)
 
 func findTreejson(query string, nodes []idx.Node) models.SeriesTree {
-	tree := models.NewSeriesTree()
+	tree := models.SeriesTree{}
 	seen := make(map[string]struct{})
 
 	for _, g := range nodes {
@@ -530,7 +530,7 @@ func findTreejson(query string, nodes []idx.Node) models.SeriesTree {
 		}
 		tree.Add(&t)
 	}
-	return *tree
+	return tree
 }
 
 func (s *Server) metricsDelete(ctx *middleware.Context, req models.MetricsDelete) {

api/models/graphite.go

@@ -228,10 +228,6 @@ func NewSeriesPickleItem(path string, isLeaf bool, intervals [][]int64) SeriesPi
 
 type SeriesTree []SeriesTreeItem
 
-func NewSeriesTree() *SeriesTree {
-	return new(SeriesTree)
-}
-
 func (s *SeriesTree) Add(i *SeriesTreeItem) {
 	*s = append(*s, *i)
 }

api/models/graphite_test.go

@@ -7,6 +7,18 @@ import (
 	"github.com/grafana/metrictank/idx"
 )
 
+func TestEmptySeriesTree(t *testing.T) {
+	// mimic what findTreejson does
+	tree := SeriesTree{}
+	out, err := json.Marshal(tree)
+	if err != nil {
+		t.Error(err)
+	}
+	if string(out) != "[]" {
+		t.Errorf("expected output '[]'. got %q", out)
+	}
+}
+
 func TestGraphiteNames(t *testing.T) {
 
 	cases := []struct {