fix nilpointer dereference in server hopping mode, in case ports obvpn wants...

fix nilpointer dereference in server hopping mode, in case ports obvpn wants to listen to are already bound. Allow 10% failure rate for failing port bindings for now, clients should hop to next IP:port tuple if dialing fails.

server/udpserver.go

@@ -55,14 +55,28 @@ func (s *UDPServer) Start() error {
 	if s.cfg.HoppingEnabled {
 		listeners = make([]net.Listener, s.cfg.PortCount)
 		portHopRange := int(s.cfg.MaxHopPort - s.cfg.MinHopPort)
+		listenerCount := 0
 		for i := 0; i < int(s.cfg.PortCount); i++ {
 			portOffset := r.Intn(portHopRange)
 			addr := net.JoinHostPort(s.cfg.Obfs4ListenIP, fmt.Sprint(portOffset+int(s.cfg.MinHopPort)))
-			listeners[i], err = listenConfig.Listen(s.ctx, addr)
+			listener, err := listenConfig.Listen(s.ctx, addr)
 
 			if err != nil {
 				s.logger.Printf("Error binding to %s: %v", addr, err)
+				continue
 			}
+			listeners[listenerCount] = listener
+			listenerCount++
+		}
+
+		if listenerCount < int(float64(s.cfg.PortCount)*0.9) {
+			// if too many of the required ports are already occupied, clients will suffer from
+			// bad throughput due to many dialing errors
+			return fmt.Errorf("too many port binding errors: %d/%d", len(listeners), int(s.cfg.PortCount))
+		} else if listenerCount < int(s.cfg.PortCount) {
+			// adjust the listener slice size so that we avoid nil entries
+			listeners = listeners[:listenerCount]
+			s.debug.Printf("WARNING: listening on less ports then clients expect: %d / %d", len(listeners), int(s.cfg.PortCount))
 		}
 	} else {
 		listenAddr := net.JoinHostPort(s.cfg.Obfs4ListenIP, strconv.Itoa(s.cfg.Obfs4ListenPort))