android: add logging to track VPN interface creation

Updates tailscale/tailscale#15506

Signed-off-by: kari-ts <kari@tailscale.com>

libtailscale/backend.go

@@ -225,6 +225,8 @@ func (a *App) runBackend(ctx context.Context) error {
 				if err := b.updateTUN(cfg.rcfg, cfg.dcfg); err != nil {
 					a.closeVpnService(err, b)
 				}
+			} else {
+				b.logger.Logf("Update TUN not called")
 			}
 		case s := <-onDisconnect:
 			b.CloseTUNs()

libtailscale/interfaces.go

@@ -166,6 +166,7 @@ type InputStream interface {
 // of various state changes.
 
 func RequestVPN(service IPNService) {
+	log.Printf("VPN requested")
 	onVPNRequested <- service
 }
 

libtailscale/net.go

@@ -207,6 +207,7 @@ func (b *backend) updateTUN(rcfg *router.Config, dcfg *dns.OSConfig) error {
 	b.logger.Logf("updateTUN: established VPN")
 
 	if parcelFD == nil {
+		b.logger.Logf("updateTUN: could not establish VPN because builder.Establish returned a nil ParcelFileDescriptor")
 		return errVPNNotPrepared
 	}