awsecs client: simplify list/describe services

by removing ability to stream results between them, since this is such a minor optimization
and greatly complicates the code.

probe/awsecs/client.go

@@ -129,85 +129,67 @@ func (c ecsClientImpl) getCachedService(serviceName string) (EcsService, bool) {
 	return EcsService{}, false
 }
 
-// Returns a channel from which service ARNs can be read.
+// Returns a list of service names.
 // Cannot fail as it will attempt to deliver partial results, though that may end up being no results.
-func (c ecsClientImpl) listServices() <-chan string {
+func (c ecsClientImpl) listServices() []string {
 	log.Debugf("Listing ECS services")
-	results := make(chan string)
-	go func() {
-		count := 0
-		err := c.client.ListServicesPages(
-			&ecs.ListServicesInput{Cluster: &c.cluster},
-			func(page *ecs.ListServicesOutput, lastPage bool) bool {
-				for _, arn := range page.ServiceArns {
-					count++
-					results <- *arn
-				}
-				return true
-			},
-		)
-		if err != nil {
-			log.Warnf("Error listing ECS services, ECS service report may be incomplete: %v", err)
-		}
-		log.Debugf("Listed %d services", count)
-		close(results)
-	}()
+	results := []string{}
+	err := c.client.ListServicesPages(
+		&ecs.ListServicesInput{Cluster: &c.cluster},
+		func(page *ecs.ListServicesOutput, lastPage bool) bool {
+			for _, name := range page.ServiceArns {
+				results = append(results, *name)
+			}
+			return true
+		},
+	)
+	if err != nil {
+		log.Warnf("Error listing ECS services, ECS service report may be incomplete: %v", err)
+	}
+	log.Debugf("Listed %d services", len(results))
 	return results
 }
 
-// Returns (input, done) channels. Service ARNs given to input are batched and details are fetched,
-// with full EcsService objects being put into the cache. Closes done when finished.
-func (c ecsClientImpl) describeServices() (chan<- string, <-chan struct{}) {
-	input := make(chan string)
-	done := make(chan struct{})
+// Service names given are batched and details are fetched,
+// with full EcsService objects being put into the cache.
+// Cannot fail as it will attempt to deliver partial results.
+func (c ecsClientImpl) describeServices(services []string) {
+	const maxServices = 10 // How many services we can put in one Describe command
+	group := sync.WaitGroup{}
 
 	log.Debugf("Describing ECS services")
 
-	go func() {
-		const maxServices = 10 // How many services we can put in one Describe command
-		group := sync.WaitGroup{}
+	// split into batches
+	batches := make([][]string, 0, len(services)/maxServices+1)
+	for len(services) > maxServices {
+		batch := services[:maxServices]
+		services = services[maxServices:]
+		batches = append(batches, batch)
+	}
+	if len(services) > 0 {
+		batches = append(batches, services)
+	}
 
-		// count and calls is just for logging
-		count := 0
-		calls := 0
+	for _, batch := range batches {
+		group.Add(1)
+		go func(names []string) {
+			defer group.Done()
+			c.describeServicesBatch(names)
+		}(batch)
+	}
 
-		batch := make([]string, 0, maxServices)
-		for arn := range input {
-			batch = append(batch, arn)
-			if len(batch) == maxServices {
-				group.Add(1)
-				go func(arns []string) {
-					defer group.Done()
-					c.describeServicesBatch(arns)
-				}(batch)
-				count += len(batch)
-				calls++
-				batch = make([]string, 0, maxServices)
-			}
-		}
-		if len(batch) > 0 {
-			c.describeServicesBatch(batch)
-			count += len(batch)
-			calls++
-		}
-
-		log.Debugf("Described %d services in %d calls", count, calls)
-		group.Wait()
-		close(done)
-	}()
-
-	return input, done
+	group.Wait()
 }
 
-func (c ecsClientImpl) describeServicesBatch(arns []string) {
-	arnPtrs := make([]*string, 0, len(arns))
-	for i := range arns {
-		arnPtrs = append(arnPtrs, &arns[i])
+func (c ecsClientImpl) describeServicesBatch(names []string) {
+	namePtrs := make([]*string, 0, len(names))
+	for i := range names {
+		namePtrs = append(namePtrs, &names[i])
 	}
 
 	resp, err := c.client.DescribeServices(&ecs.DescribeServicesInput{
 		Cluster:  &c.cluster,
-		Services: arnPtrs,
+		Services: namePtrs,
 	})
 	if err != nil {
 		log.Warnf("Error describing some ECS services, ECS service report may be incomplete: %v", err)
@@ -306,33 +288,28 @@ func (c ecsClientImpl) ensureTasksAreCached(taskARNs []string) {
 }
 
 func (c ecsClientImpl) refreshServices(taskServiceMap map[string]string) map[string]bool {
-	toDescribe, done := c.describeServices()
 	servicesRefreshed := map[string]bool{}
+	toDescribe := []string{}
 	for _, serviceName := range taskServiceMap {
 		if servicesRefreshed[serviceName] {
 			continue
 		}
-		toDescribe <- serviceName
+		toDescribe = append(toDescribe, serviceName)
 		servicesRefreshed[serviceName] = true
 	}
-	close(toDescribe)
-	<-done
+	c.describeServices(toDescribe)
 	return servicesRefreshed
 }
 
 func (c ecsClientImpl) describeAllServices(servicesRefreshed map[string]bool) {
-	serviceNamesChan := c.listServices()
-	toDescribe, done := c.describeServices()
-	go func() {
-		for serviceName := range serviceNamesChan {
-			if !servicesRefreshed[serviceName] {
-				toDescribe <- serviceName
-				servicesRefreshed[serviceName] = true
-			}
+	toDescribe := []string{}
+	for _, serviceName := range c.listServices() {
+		if !servicesRefreshed[serviceName] {
+			toDescribe = append(toDescribe, serviceName)
+			servicesRefreshed[serviceName] = true
 		}
-		close(toDescribe)
-	}()
-	<-done
+	}
+	c.describeServices(toDescribe)
 }
 
 func (c ecsClientImpl) makeECSInfo(taskARNs []string, taskServiceMap map[string]string) EcsInfo {