fix deadlock when closing the server and the connection at the same time

packet_handler_map.go

@@ -238,6 +238,10 @@ func (h *packetHandlerMap) SetServer(s unknownPacketHandler) {
 
 func (h *packetHandlerMap) CloseServer() {
 	h.mutex.Lock()
+	if h.server == nil {
+		h.mutex.Unlock()
+		return
+	}
 	h.server = nil
 	var wg sync.WaitGroup
 	for _, handler := range h.handlers {

packet_handler_map_test.go

@@ -251,6 +251,7 @@ var _ = Describe("Packet Handler Map", func() {
 			})
 
 			It("closes all server sessions", func() {
+				handler.SetServer(NewMockUnknownPacketHandler(mockCtrl))
 				clientSess := NewMockPacketHandler(mockCtrl)
 				clientSess.EXPECT().getPerspective().Return(protocol.PerspectiveClient)
 				serverSess := NewMockPacketHandler(mockCtrl)

server.go

@@ -274,12 +274,12 @@ func (s *baseServer) accept(ctx context.Context) (quicSession, error) {
 
 // Close the server
 func (s *baseServer) Close() error {
+	s.sessionHandler.CloseServer()
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 	if s.closed {
 		return nil
 	}
-	s.sessionHandler.CloseServer()
 	if s.serverError == nil {
 		s.serverError = errors.New("server closed")
 	}