wgengine/tstun: make Close safe for concurrent use

wgengine/tstun/tun.go

@@ -63,6 +63,8 @@ type TUN struct {
 	// tdev is the underlying TUN device.
 	tdev tun.Device
 
+	closeOnce sync.Once
+
 	_                  [4]byte // force 64-bit alignment of following field on 32-bit
 	lastActivityAtomic int64   // unix seconds of last send or receive
 
@@ -140,15 +142,14 @@ func (t *TUN) SetDestIPActivityFuncs(m map[packet.IP]func()) {
 }
 
 func (t *TUN) Close() error {
-	select {
+	var err error
-	case <-t.closed:
+	t.closeOnce.Do(func() {
-		// continue
-	default:
 		// Other channels need not be closed: poll will exit gracefully after this.
 		close(t.closed)
-	}
 
-	return t.tdev.Close()
+		err = t.tdev.Close()
+	})
+	return err
 }
 
 func (t *TUN) Events() chan tun.Event {