linkerd mc link|gateways small improvements

When issuing `linkerd mc gateways` with no links, the command waited
till it timed out (30s by default) before showing an empty table. This
change refactors the concurrency code in `getGatewayMetrics()` to avoid
that.

Also the `linkerd mc link -h` help text for the `--gateway` flag was
showing a dupe default value:

```
If false, allows a link to be created against a cluster that does not have a gateway service (default true) (default true)
```

multicluster/cmd/gateways.go

@@ -8,7 +8,7 @@ import (
 	"io"
 	"os"
 	"strings"
-	"sync/atomic"
+	"sync"
 	"time"
 
 	"github.com/linkerd/linkerd2/cli/table"
@@ -198,15 +198,15 @@ func newGatewaysCommand() *cobra.Command {
 func getGatewayMetrics(k8sAPI *k8s.KubernetesAPI, pods []corev1.Pod, leaders map[string]struct{}, wait time.Duration) []gatewayMetrics {
 	var metrics []gatewayMetrics
 	metricsChan := make(chan gatewayMetrics)
-	var activeRoutines int32
+	var wg sync.WaitGroup
 	for _, pod := range pods {
 		if _, found := leaders[pod.Name]; !found {
 			continue
 		}
 
-		atomic.AddInt32(&activeRoutines, 1)
+		wg.Add(1)
 		go func(p corev1.Pod) {
-			defer atomic.AddInt32(&activeRoutines, -1)
+			defer wg.Done()
 			name := p.Labels[k8s.RemoteClusterNameLabel]
 			container, err := getServiceMirrorContainer(p)
 			if err != nil {
@@ -224,20 +224,29 @@ func getGatewayMetrics(k8sAPI *k8s.KubernetesAPI, pods []corev1.Pod, leaders map
 			}
 		}(pod)
 	}
+
+	go func() {
+		wg.Wait()
+		close(metricsChan)
+	}()
+
 	timeout := time.NewTimer(wait)
 	defer timeout.Stop()
+
 wait:
 	for {
 		select {
 		case metric := <-metricsChan:
+			if metric.clusterName == "" {
+				// channel closed
+				break wait
+			}
 			metrics = append(metrics, metric)
 		case <-timeout.C:
 			break wait
 		}
-		if atomic.LoadInt32(&activeRoutines) == 0 {
-			break
-		}
 	}
+
 	return metrics
 }
 

multicluster/cmd/link.go

@@ -351,7 +351,7 @@ A full list of configurable values can be found at https://github.com/linkerd/li
 	cmd.Flags().StringVar(&opts.gatewayAddresses, "gateway-addresses", opts.gatewayAddresses, "If specified, overwrites gateway addresses when gateway service is not type LoadBalancer (comma separated list)")
 	cmd.Flags().Uint32Var(&opts.gatewayPort, "gateway-port", opts.gatewayPort, "If specified, overwrites gateway port when gateway service is not type LoadBalancer")
 	cmd.Flags().BoolVar(&opts.ha, "ha", opts.ha, "Enable HA configuration for the service-mirror deployment (default false)")
-	cmd.Flags().BoolVar(&opts.enableGateway, "gateway", opts.enableGateway, "If false, allows a link to be created against a cluster that does not have a gateway service (default true)")
+	cmd.Flags().BoolVar(&opts.enableGateway, "gateway", opts.enableGateway, "If false, allows a link to be created against a cluster that does not have a gateway service")
 
 	pkgcmd.ConfigureNamespaceFlagCompletion(
 		cmd, []string{"namespace", "gateway-namespace"},