code: func (f *lightFetcher) registerPeer(p *peer) {
  	p.lock.Lock()
  	p.hasBlock = func(hash common.Hash, number uint64, hasState bool) bool {
  		return f.peerHasBlock(p, hash, number, hasState)
  	}
  	p.lock.Unlock()
  
  	f.lock.Lock()
  	defer f.lock.Unlock()
  	f.peers[p] = &fetcherPeerInfo{nodeByHash: make(map[common.Hash]*fetcherTreeNode)}
  }
docs: // registerPeer adds a new peer to the fetcher's peer set