code: func (sm *SyncManager) QueueHeaders(headers *wire.MsgHeaders, peer *peerpkg.Peer) {
  	// No channel handling here because peers do not need to block on
  	// headers messages.
  	if atomic.LoadInt32(&sm.shutdown) != 0 {
  		return
  	}
  
  	sm.msgChan <- &headersMsg{headers: headers, peer: peer}
  }
docs: // QueueHeaders adds the passed headers message and peer to the block handling
  // queue.