Add GetFreePort in both TCP and UDP to avoid UDP listen fail

client.go

@@ -898,7 +898,11 @@ func (c *TunaSessionClient) startExits() error {
 	listeners := make([]net.Listener, c.config.NumTunaListeners)
 	var err error
 	for i := 0; i < len(listeners); i++ {
-		listeners[i], err = net.Listen("tcp", "127.0.0.1:")
+		port, err := GetFreePort(0)
+		if err != nil {
+			return err
+		}
+		listeners[i], err = net.Listen("tcp", fmt.Sprintf("127.0.0.1:%v", port))
 		if err != nil {
 			return err
 		}

udp.go

@@ -109,6 +109,10 @@ func (c *TunaSessionClient) DialUDPWithConfig(remoteAddr string, config *nkn.Dia
 }
 
 func (c *TunaSessionClient) handleUdpListenerTcp(tcpConn *Conn, remoteAddr string, sessionID []byte) {
+	if c.listenerUdpSess == nil { // Udp listener not started
+		tcpConn.Close()
+		return
+	}
 	sessKey := sessionKey(remoteAddr, sessionID)
 
 	c.listenerUdpSess.Lock()

util.go

@@ -2,7 +2,11 @@ package session
 
 import (
 	"encoding/hex"
+	"errors"
+	"fmt"
+	"net"
 	"strconv"
+	"sync"
 	"time"
 )
 
@@ -17,3 +21,40 @@ func sessionKey(remoteAddr string, sessionID []byte) string {
 func connID(i int) string {
 	return strconv.Itoa(i)
 }
+
+// Get free port start from parameter `port`
+// If paramenter `port` is 0, return system available port
+// The returned port is free in both TCP and UDP
+var lock sync.Mutex
+
+func GetFreePort(port int) (int, error) {
+	// to avoid race condition
+	lock.Lock()
+	defer lock.Unlock()
+
+	for i := 0; i < 100; i++ {
+		addr, err := net.ResolveTCPAddr("tcp", fmt.Sprintf("127.0.0.1:%d", port))
+		if err != nil {
+			return 0, err
+		}
+
+		l, err := net.ListenTCP("tcp", addr)
+		if err != nil {
+			return 0, err
+		}
+		defer l.Close()
+
+		port = l.Addr().(*net.TCPAddr).Port
+		u, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.ParseIP("127.0.0.1"), Port: port})
+		if err != nil {
+			l.Close()
+			port++
+			continue
+		}
+		u.Close()
+
+		return port, nil
+	}
+
+	return 0, errors.New("failed to find free port after 100 tries")
+}

util_test.go

@@ -0,0 +1,12 @@
+package session
+
+import "testing"
+
+// go test -v -run=TestGetFreePort
+func TestGetFreePort(t *testing.T) {
+	port, err := GetFreePort(0)
+	if err != nil {
+		t.Error(err)
+	}
+	t.Log(port)
+}