code: func (sm *SyncManager) Start() {
  	// Already started?
  	if atomic.AddInt32(&sm.started, 1) != 1 {
  		return
  	}
  
  	log.Trace("Starting sync manager")
  	sm.wg.Add(1)
  	go sm.blockHandler()
  }
docs: // Start begins the core block handler which processes block and inv messages.