Fix percent_diff calculation when points are nulls

pkg/services/alerting/conditions/reducer.go

@@ -131,6 +131,8 @@ func (s *SimpleReducer) Reduce(series *tsdb.TimeSeries) null.Float {
 				break
 			}
 		}
+
+		if i >= 1 {
 			// get the oldest point
 			points = points[0:i]
 			for i := 0; i < len(points); i++ {
@@ -141,6 +143,7 @@ func (s *SimpleReducer) Reduce(series *tsdb.TimeSeries) null.Float {
 					break
 				}
 			}
+		}
 	case "count_non_null":
 		for _, v := range series.Points {
 			if v[0].Valid {

pkg/services/alerting/conditions/reducer_test.go

@@ -157,6 +157,18 @@ func TestSimpleReducer(t *testing.T) {
 			result := testReducer("percent_diff", 30, 40, 40)
 			So(result, ShouldEqual, float64(33.33333333333333))
 		})
+
+		Convey("percent_diff with only nulls", func() {
+			reducer := NewSimpleReducer("percent_diff")
+			series := &tsdb.TimeSeries{
+				Name: "test time serie",
+			}
+
+			series.Points = append(series.Points, tsdb.NewTimePoint(null.FloatFromPtr(nil), 1))
+			series.Points = append(series.Points, tsdb.NewTimePoint(null.FloatFromPtr(nil), 2))
+
+			So(reducer.Reduce(series).Valid, ShouldEqual, false)
+		})
 	})
 }