app/vmselect/promql: properly return durations smaller than one second from duration_over_time() function

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1780

app/vmselect/promql/rollup.go

@@ -891,11 +891,10 @@ func newRollupDurationOverTime(args []interface{}) (rollupFunc, error) {
 	rf := func(rfa *rollupFuncArg) float64 {
 		// There is no need in handling NaNs here, since they must be cleaned up
 		// before calling rollup funcs.
-		values := rfa.values
+		timestamps := rfa.timestamps
-		if len(values) == 0 {
+		if len(timestamps) == 0 {
 			return nan
 		}
-		timestamps := rfa.timestamps
 		tPrev := timestamps[0]
 		dSum := int64(0)
 		dMax := int64(dMaxs[rfa.idx] * 1000)
@@ -906,7 +905,7 @@ func newRollupDurationOverTime(args []interface{}) (rollupFunc, error) {
 			}
 			tPrev = t
 		}
-		return float64(dSum / 1000)
+		return float64(dSum) / 1000
 	}
 	return rf, nil
 }

app/vmselect/promql/rollup_test.go

@@ -193,6 +193,27 @@ func testRollupFunc(t *testing.T, funcName string, args []interface{}, meExpecte
 	}
 }
 
+func TestRollupDurationOverTime(t *testing.T) {
+	f := func(maxInterval, dExpected float64) {
+		t.Helper()
+		maxIntervals := []*timeseries{{
+			Values:     []float64{maxInterval},
+			Timestamps: []int64{123},
+		}}
+		var me metricsql.MetricExpr
+		args := []interface{}{&metricsql.RollupExpr{Expr: &me}, maxIntervals}
+		testRollupFunc(t, "duration_over_time", args, &me, dExpected)
+	}
+	f(-123, 0)
+	f(0, 0)
+	f(0.001, 0)
+	f(0.005, 0.007)
+	f(0.01, 0.036)
+	f(0.02, 0.125)
+	f(1, 0.125)
+	f(100, 0.125)
+}
+
 func TestRollupShareLEOverTime(t *testing.T) {
 	f := func(le, vExpected float64) {
 		t.Helper()