ipn/localapi,client/local: add debug watcher for bus events (#16239)

Updates: #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -102,6 +102,12 @@ func debugCmd() *ffcli.Command {
 					return fs
 				})(),
 			},
+			{
+				Name:       "daemon-bus-events",
+				ShortUsage: "tailscale debug daemon-bus-events",
+				Exec:       runDaemonBusEvents,
+				ShortHelp:  "Watch events on the tailscaled bus",
+			},
 			{
 				Name:       "metrics",
 				ShortUsage: "tailscale debug metrics",
@@ -784,6 +790,24 @@ func runDaemonLogs(ctx context.Context, args []string) error {
 	}
 }
 
+func runDaemonBusEvents(ctx context.Context, args []string) error {
+	logs, err := localClient.StreamBusEvents(ctx)
+	if err != nil {
+		return err
+	}
+	defer logs.Close()
+	d := json.NewDecoder(bufio.NewReader(logs))
+	for {
+		var line eventbus.DebugEvent
+		err := d.Decode(&line)
+		if err != nil {
+			return err
+		}
+		fmt.Printf("[%d][%q][from: %q][to: %q] %s\n", line.Count, line.Type,
+			line.From, line.To, line.Event)
+	}
+}
+
 var metricsArgs struct {
 	watch bool
 }

cmd/tailscaled/tailscaled.go

@@ -719,6 +719,7 @@ func tryEngine(logf logger.Logf, sys *tsd.System, name string) (onlyNetstack boo
 		Dialer:        sys.Dialer.Get(),
 		SetSubsystem:  sys.Set,
 		ControlKnobs:  sys.ControlKnobs(),
+		EventBus:      sys.Bus.Get(),
 		DriveForLocal: driveimpl.NewFileSystemForLocal(logf),
 	}