code: func (s *tikvStore) StartGCWorker() error {
  	if !s.enableGC || NewGCHandlerFunc == nil {
  		return nil
  	}
  
  	gcWorker, err := NewGCHandlerFunc(s, s.pdClient)
  	if err != nil {
  		return errors.Trace(err)
  	}
  	gcWorker.Start()
  	s.gcWorker = gcWorker
  	return nil
  }
docs: // StartGCWorker starts GC worker, it's called in BootstrapSession, don't call this function more than once.