app/vmselect/promql: fix when the parameter of maxValue(), minValue() leading by NaN. it will cause {top,bottom}k_{max,min} return inappropriate result (#883)

app/vmselect/promql/aggr.go

@@ -740,12 +740,21 @@ func minValue(values []float64) float64 {
 	if len(values) == 0 {
 		return nan
 	}
-	min := values[0]
-	for _, v := range values[1:] {
-		if v < min {
+	aNumberFounded := false
+	var min float64
+	for _, v := range values {
+		if !math.IsNaN(v) && !aNumberFounded {
+			min = v
+			aNumberFounded = true
+			continue
+		}
+		if !math.IsNaN(v) && v < min {
 			min = v
 		}
 	}
+	if !aNumberFounded {
+		return nan
+	}
 	return min
 }
 
@@ -753,12 +762,21 @@ func maxValue(values []float64) float64 {
 	if len(values) == 0 {
 		return nan
 	}
-	max := values[0]
-	for _, v := range values[1:] {
-		if v > max {
+	aNumberFounded := false
+	var max float64
+	for _, v := range values {
+		if !math.IsNaN(v) && !aNumberFounded {
+			max = v
+			aNumberFounded = true
+			continue
+		}
+		if !math.IsNaN(v) && v > max {
 			max = v
 		}
 	}
+	if !aNumberFounded {
+		return nan
+	}
 	return max
 }