SSEServer：The Shutdown method fails to exit normally

[uppercaveman]

func (s *SSEServer) Shutdown(ctx context.Context) error {
	s.mu.RLock()
	srv := s.srv
	s.mu.RUnlock()
	if srv != nil {
		s.sessions.Range(func(key, value interface{}) bool {
			if session, ok := value.(*sseSession); ok {
				close(session.done)
			}
			s.sessions.Delete(key)
			return true
		})
		return srv.Shutdown(ctx)
	}
	return nil
}

Why do we need to lock?