fix a race condition in quic.Server

server.go

@@ -24,7 +24,9 @@ type packetHandler interface {
 // A Server of QUIC
 type Server struct {
 	addr *net.UDPAddr
-	conn *net.UDPConn
+
+	conn      *net.UDPConn
+	connMutex sync.Mutex
 
 	signer crypto.Signer
 	scfg   *handshake.ServerConfig
@@ -74,7 +76,9 @@ func (s *Server) ListenAndServe() error {
 	if err != nil {
 		return err
 	}
+	s.connMutex.Lock()
 	s.conn = conn
+	s.connMutex.Unlock()
 
 	for {
 		data := make([]byte, protocol.MaxPacketSize)
@@ -103,6 +107,10 @@ func (s *Server) Close() error {
 		}
 	}
 	s.sessionsMutex.Unlock()
+
+	s.connMutex.Lock()
+	defer s.connMutex.Unlock()
+
 	if s.conn == nil {
 		return nil
 	}