Update for scaling edge-case

- as reported on Slack and in issue #931 the gateway scaling code
was scaling to zero replicas as a result of the "proportional
scaling" added by @Templum's PR. This commit added a failing test
which was fixed by adding boundary checking - now if the scaling
amount is "0" we keep the current amount of replicas.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

gateway/handlers/alerthandler.go

@@ -5,12 +5,11 @@ package handlers
 
 import (
 	"encoding/json"
+	"fmt"
 	"io/ioutil"
 	"log"
 	"net/http"
 
-	"fmt"
-
 	"github.com/openfaas/faas/gateway/requests"
 )
 
@@ -119,7 +118,7 @@ func CalculateReplicas(status string, currentReplicas uint64, maxReplicas uint64
 	newReplicas := currentReplicas
 	step := uint64((float64(maxReplicas) / 100) * float64(scalingFactor))
 
-	if status == "firing" {
+	if status == "firing" && step > 0 {
 		if currentReplicas == 1 {
 			newReplicas = step
 		} else {

gateway/handlers/alerthandler_test.go

@@ -11,8 +11,8 @@ func TestDisabledScale(t *testing.T) {
 	minReplicas := uint64(1)
 	scalingFactor := uint64(0)
 	newReplicas := CalculateReplicas("firing", DefaultMinReplicas, DefaultMaxReplicas, minReplicas, scalingFactor)
-	if newReplicas != 0 {
-		t.Log("Expected not to scale")
+	if newReplicas != minReplicas {
+		t.Logf("Expected not to scale, but replicas were: %d", newReplicas)
 		t.Fail()
 	}
 }
@@ -27,6 +27,17 @@ func TestParameterEdge(t *testing.T) {
 	}
 }
 
+func TestScalingWithSameUpperLowerLimit(t *testing.T) {
+	minReplicas := uint64(1)
+	scalingFactor := uint64(20)
+	//	status string, currentReplicas uint64, maxReplicas uint64, minReplicas uint64, scalingFactor uint64)
+	newReplicas := CalculateReplicas("firing", minReplicas, minReplicas, minReplicas, scalingFactor)
+	if newReplicas != 1 {
+		t.Logf("Replicas - want: %d, got: %d", minReplicas, newReplicas)
+		t.Fail()
+	}
+}
+
 func TestMaxScale(t *testing.T) {
 	minReplicas := uint64(1)
 	scalingFactor := uint64(100)