code: func (d *Downloader) stateFetcher() {
  	for {
  		select {
  		case s := <-d.stateSyncStart:
  			for next := s; next != nil; {
  				next = d.runStateSync(next)
  			}
  		case <-d.stateCh:
  			// Ignore state responses while no sync is running.
  		case <-d.quitCh:
  			return
  		}
  	}
  }
docs: // stateFetcher manages the active state sync and accepts requests
  // on its behalf.