cli: handle panic caused by `linkerd metrics` port-forward failure (#4007)

* cli: handle `linkerd metrics` port-forward gracefully

- add return for routine in func `Init()` in case of error
- add return from func `getMetrics()` if error from `portforward.Init()`

* Remove select block at pkg/k8s/portforward.go

- It is now the caller's responsibility to call pf.Stop()

Signed-off-by: Mayank Shah <mayankshah1614@gmail.com>

cli/cmd/metrics.go

@@ -165,6 +165,7 @@ func getMetrics(
 	defer portforward.Stop()
 	if err = portforward.Init(); err != nil {
 		fmt.Fprintf(os.Stderr, "Error running port-forward: %s", err)
+		return nil, err
 	}
 
 	metricsURL := portforward.URLFor("/metrics")

pkg/k8s/portforward.go

@@ -177,14 +177,7 @@ func (pf *PortForward) Init() error {
 	go func() {
 		if err := pf.run(); err != nil {
 			failure <- err
-		}
-
-		select {
-		case <-pf.GetStop():
-			// stopCh was closed, do nothing
-		default:
-			// pf.run() returned for some other reason, close stopCh
-			pf.Stop()
+			return
 		}
 	}()
 
@@ -203,6 +196,7 @@ func (pf *PortForward) Init() error {
 }
 
 // Stop terminates the port-forward connection.
+// It is the caller's responsibility to call Stop even in case of errors
 func (pf *PortForward) Stop() {
 	close(pf.stopCh)
 }