cmd/tailscale: reset cached config when failing to set up VPN tunnel

Add a bit of logging while here.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

cmd/tailscale/backend.go

@@ -235,7 +235,7 @@ func (b *backend) updateTUN(service jni.Object, cfg *router.Config) error {
 		return nil
 	})
 	if err != nil {
-		b.lastCfg = cfg
+		b.lastCfg = nil
 		b.CloseTUNs()
 		return err
 	}

cmd/tailscale/main.go

@@ -193,6 +193,7 @@ func (a *App) runBackend() error {
 				if service != 0 {
 					if cfg != nil && state.State >= ipn.Starting {
 						if err := b.updateTUN(service, cfg); err != nil {
+							log.Printf("VPN update failed: %v", err)
 							a.notifyVPNClosed()
 						}
 					} else {
@@ -253,6 +254,7 @@ func (a *App) runBackend() error {
 			}
 			if cfg != nil && state.State >= ipn.Starting {
 				if err := b.updateTUN(service, cfg); err != nil {
+					log.Printf("VPN update failed: %v", err)
 					a.notifyVPNClosed()
 				}
 			}