ECS service controls: Don't allow scale down below 1

as currently this would make it disappear (#2085).
See also #2197 (comment)

probe/awsecs/client.go

@@ -396,8 +396,8 @@ func (c ecsClientImpl) ScaleService(serviceName string, amount int) error {
 	}
 
 	newCount := service.DesiredCount + int64(amount)
-	if newCount < 0 {
-		return fmt.Errorf("Cannot reduce count below zero")
+	if newCount < 1 {
+		return fmt.Errorf("Cannot reduce count below one")
 	}
 	_, err := c.client.UpdateService(&ecs.UpdateServiceInput{
 		Cluster:      &c.cluster,

probe/awsecs/reporter.go

@@ -153,7 +153,12 @@ func (r Reporter) Tag(rpt report.Report) (report.Report, error) {
 				ServiceDesiredCount:   fmt.Sprintf("%d", service.DesiredCount),
 				ServiceRunningCount:   fmt.Sprintf("%d", service.RunningCount),
 				report.ControlProbeID: r.probeID,
-			}).WithLatestActiveControls(ScaleUp, ScaleDown))
+			}).WithLatestControls(map[string]report.NodeControlData{
+				ScaleUp: {Dead: false},
+				// We've decided for now to disable ScaleDown when only 1 task is desired,
+				// since scaling down to 0 would cause the service to disappear (#2085)
+				ScaleDown: {Dead: service.DesiredCount <= 1},
+			}))
 		}
 		log.Debugf("Created %v ECS service nodes", len(ecsInfo.Services))