code: func (d *Downloader) syncState(root common.Hash) *stateSync {
  	s := newStateSync(d, root)
  	select {
  	case d.stateSyncStart <- s:
  	case <-d.quitCh:
  		s.err = errCancelStateFetch
  		close(s.done)
  	}
  	return s
  }
docs: // syncState starts downloading state with the given root hash.