cmd/tailscale: wait for previous tunnel(s) to close before establishing new

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

cmd/tailscale/multitun.go

@@ -29,6 +29,7 @@ type multiTUN struct {
 	mtus         chan chan mtuReply
 	names        chan chan nameReply
 	shutdowns    chan struct{}
+	shutdownDone chan struct{}
 }
 
 // tunDevice wraps and drives a single run.Device.
@@ -74,6 +75,7 @@ func newTUNDevices() *multiTUN {
 		mtus:         make(chan chan mtuReply),
 		names:        make(chan chan nameReply),
 		shutdowns:    make(chan struct{}),
+		shutdownDone: make(chan struct{}),
 	}
 	go d.run()
 	return d
@@ -112,6 +114,7 @@ func (d *multiTUN) run() {
 				<-dev.readDone
 			}
 			devices = nil
+			d.shutdownDone <- struct{}{}
 		case <-d.close:
 			var derr error
 			for _, dev := range devices {
@@ -279,6 +282,7 @@ func (d *multiTUN) Events() chan tun.Event {
 
 func (d *multiTUN) Shutdown() {
 	d.shutdowns <- struct{}{}
+	<-d.shutdownDone
 }
 
 func (d *multiTUN) Close() error {