code: func (p *peerConnection) MarkLacking(hash common.Hash) {
  	p.lock.Lock()
  	defer p.lock.Unlock()
  
  	for len(p.lacking) >= maxLackingHashes {
  		for drop := range p.lacking {
  			delete(p.lacking, drop)
  			break
  		}
  	}
  	p.lacking[hash] = struct{}{}
  }
docs: // MarkLacking appends a new entity to the set of items (blocks, receipts, states)
  // that a peer is known not to have (i.e. have been requested before). If the
  // set reaches its maximum allowed capacity, items are randomly dropped off.