code: func (sg *sessionPool) close() {
  	sg.mu.Lock()
  	defer sg.mu.Unlock()
  	// prevent closing resPool twice.
  	if sg.mu.closed || sg.resPool == nil {
  		return
  	}
  	logutil.Logger(ddlLogCtx).Info("[ddl] closing sessionPool")
  	sg.resPool.Close()
  	sg.mu.closed = true
  }
docs: // close clean up the sessionPool.