BUG FIXED: RoundRobinCounter.Add will cause a out of range crash

when index >= len(rrc.Values)

go/stats/duration_counter.go

@@ -23,14 +23,11 @@ func NewRoundRobinCounter(slots int) *RoundRobinCounter {
 	return &RoundRobinCounter{LastIndex: -1, Values: make([]int64, slots), Counts: make([]int64, slots)}
 }
 func (rrc *RoundRobinCounter) Add(index int, val int64) {
+	if index >= len(rrc.Values) {
+		return
+	}
 	for rrc.LastIndex != index {
-		rrc.LastIndex++
-		if rrc.LastIndex >= len(rrc.Values) {
-			if index >= len(rrc.Values) {
-				break //just avoid endless loop
-			}
-			rrc.LastIndex = 0
-		}
+		rrc.LastIndex = (rrc.LastIndex + 1) % len(rrc.Values)
 		rrc.Values[rrc.LastIndex] = 0
 		rrc.Counts[rrc.LastIndex] = 0
 	}

go/stats/duration_counter_test.go

@@ -0,0 +1,19 @@
+package stats
+
+import "testing"
+
+func TestRobinCounter(t *testing.T) {
+	rrc := NewRoundRobinCounter(60)
+	rrc.Add(0, 1)
+	rrc.Add(50, 2)
+	if rrc.Count() != 2 {
+		t.Fatal()
+	}
+	if rrc.Sum() != 3 {
+		t.Fatal()
+	}
+	/*
+		index out of range
+	*/
+	rrc.Add(61, 1)
+}