btcd 源码分析系列：2 - addrmanager

参考：btcd

• addrmanager 主要提供了peer地址的管理功能，包括地址的增删查改
• 通过存储json序列化后的数据到本地文件实现持久化
• btc启动的时候，会读取该json文件，将保存的节点信息读取到内存中
• btc运行的过程中，每过一段时间（十分钟）就持久化一次，以备下次启动时使用
• btc退出时，会再次持久化一次

一、创建addrmanager对象

// New returns a new bitcoin address manager.
// Use Start to begin processing asynchronous address updates.
func New(dataDir string, lookupFunc func(string) ([]net.IP, error)) *AddrManager {
    am := AddrManager{
        peersFile:      filepath.Join(dataDir, "peers.json"),
        lookupFunc:     lookupFunc,
        rand:           rand.New(rand.NewSource(time.Now().UnixNano())),
        quit:           make(chan struct{}),
        localAddresses: make(map[string]*localAddress),
        version:        serialisationVersion,
    }
    am.reset()
    return &am
}

二、启动

• 通过started字段保证一个addrManager只能start一次

• loadPeers()就是读取硬盘上保存的peer信息到内存中

• addressHander 启动一个协程处理周期性的持久化和

  // Start begins the core address handler which manages a pool of known
  // addresses, timeouts, and interval based writes.
  func (a *AddrManager) Start() {
      // Already started?
      if atomic.AddInt32(&a.started, 1) != 1 {
          return
      }
  
      log.Trace("Starting address manager")
  
      // Load peers we already know about from file.
      a.loadPeers()
  
      // Start the address ticker to save addresses periodically.
      a.wg.Add(1)
      go a.addressHandler()
  }
  

三、停止

• stop的时候通过close channel 通知并等待子协程（处理持久化任务）的退出

  // Stop gracefully shuts down the address manager by stopping the main handler.
  func (a *AddrManager) Stop() error {
      if atomic.AddInt32(&a.shutdown, 1) != 1 {
          log.Warnf("Address manager is already in the process of " +
              "shutting down")
          return nil
      }
  
      log.Infof("Address manager shutting down")
      close(a.quit)
      a.wg.Wait()
      return nil
  }
  
  // addressHandler is the main handler for the address manager.  It must be run
  // as a goroutine.
  func (a *AddrManager) addressHandler() {
      dumpAddressTicker := time.NewTicker(dumpAddressInterval)
      defer dumpAddressTicker.Stop()
  out:
      for {
          select {
          case <-dumpAddressTicker.C:
              a.savePeers()
  
          case <-a.quit:
              break out
          }
      }
      a.savePeers()
      a.wg.Done()
      log.Trace("Address handler done")
  }
  

四、获取地址

• 用了自己的随机算法选取部分地址返回

• 这个随机算法简单的说就是，先根据百分比确定返回的数量max，然后选中第i个，并与i-n之间的随机一个交换，i从0依次递增到max-1，这样一共选出来max个

  // AddressCache returns the current address cache.  It must be treated as
  // read-only (but since it is a copy now, this is not as dangerous).
  func (a *AddrManager) AddressCache() []*wire.NetAddress {
      allAddr := a.getAddresses()
  
      numAddresses := len(allAddr) * getAddrPercent / 100
      if numAddresses > getAddrMax {
          numAddresses = getAddrMax
      }
  
      // Fisher-Yates shuffle the array. We only need to do the first
      // `numAddresses' since we are throwing the rest.
      for i := 0; i < numAddresses; i++ {
          // pick a number between current index and the end
          j := rand.Intn(len(allAddr)-i) + i
          allAddr[i], allAddr[j] = allAddr[j], allAddr[i]
      }
  
      // slice off the limit we are willing to share.
      return allAddr[0:numAddresses]
  }