Check ASG min OD rules before launching new spot instance (#291)

In the event an ASG has just enough OD instances to meet the minimum
number of OD instances, the code should not launch a new spot instance.

core/autoscaling.go

@@ -342,6 +342,12 @@ func (a *autoScalingGroup) process() {
 				"No running on-demand instances were found, nothing to do here...")
 			return
 		}
+
+		if !a.needReplaceOnDemandInstances() {
+			logger.Println("Not allowed to replace any of the running OD instances in ", a.name)
+			return
+		}
+
 		a.loadLaunchConfiguration()
 		err := onDemandInstance.launchSpotReplacement()
 		if err != nil {

core/autoscaling_test.go

@@ -1379,6 +1379,29 @@ func TestNeedReplaceOnDemandInstances(t *testing.T) {
 			desiredCapacity: aws.Int64(4),
 			expectedRun:     true,
 		},
+		{name: "ASG has on-demand instances equal to the min on-demand number",
+			asgInstances: makeInstancesWithCatalog(
+				map[string]*instance{
+					"id-1": {
+						Instance: &ec2.Instance{
+							State:             &ec2.InstanceState{Name: aws.String(ec2.InstanceStateNameRunning)},
+							Placement:         &ec2.Placement{AvailabilityZone: aws.String("eu-west-1a")},
+							InstanceLifecycle: aws.String("on-demand"),
+						},
+					},
+					"id-2": {
+						Instance: &ec2.Instance{
+							State:             &ec2.InstanceState{Name: aws.String(ec2.InstanceStateNameRunning)},
+							Placement:         &ec2.Placement{AvailabilityZone: aws.String("eu-west-1b")},
+							InstanceLifecycle: aws.String("spot"),
+						},
+					},
+				},
+			),
+			minOnDemand:     1,
+			desiredCapacity: aws.Int64(2),
+			expectedRun:     false,
+		},
 	}
 
 	for _, tt := range tests {