前言

这篇文章从区块传播策略入手，介绍新区块是如何传播到远端节点，以及新区块加入到远端节点本地链的过程，同时会介绍fetcher模块，fetcher的功能是处理Peer通知的区块信息。在介绍过程中，还会涉及到p2p，eth等模块，不会专门介绍，而是专注区块的传播和加入区块链的过程。

当前代码是以太坊Release 1.8，如果版本不同，代码上可能存在差异。

总体过程和传播策略

本节从宏观角度介绍，节点产生区块后，为了传播给远端节点做了啥，远端节点收到区块后又做了什么，每个节点都连接了很多Peer，它传播的策略是什么样的？

总体流程和策略可以总结为，传播给远端Peer节点，Peer验证区块无误后，加入到本地区块链，继续传播新区块信息。具体过程如下。

先看总体过程。产生区块后，miner模块会发布一个事件NewMinedBlockEvent，订阅事件的协程收到事件后，就会把新区块的消息，广播给它所有的peer，peer收到消息后，会交给自己的fetcher模块处理，fetcher进行基本的验证后，区块没问题，发现这个区块就是本地链需要的下一个区块，则交给blockChain进一步进行完整的验证，这个过程会执行区块所有的交易，无误后把区块加入到本地链，写入数据库，这个过程就是下面的流程图，图1。

总体流程图，能看到有个分叉，是因为节点传播新区块是有策略的。它的传播策略为：

1. 假如节点连接了N个Peer，它只向Peer列表的sqrt(N)个Peer广播完整的区块消息。
2. 向所有的Peer广播只包含区块Hash的消息。

策略图的效果如图2，红色节点将区块传播给黄色节点：

收到区块Hash的节点，需要从发送给它消息的Peer那里获取对应的完整区块，获取区块后就会按照图1的流程，加入到fetcher队列，最终插入本地区块链后，将区块的Hash值广播给和它相连，但还不知道这个区块的Peer。非产生区块节点的策略图，如图3，黄色节点将区块Hash传播给青色节点：

至此，可以看出以太坊采用以石击水的方式，像水纹一样，层层扩散新产生的区块。

Fetcher模块是干啥的

fetcher模块的功能，就是收集其他Peer通知它的区块信息：1）完整的区块2）区块Hash消息。根据通知的消息，获取完整的区块，然后传递给eth模块把区块插入区块链。

如果是完整区块，就可以传递给eth插入区块，如果只有区块Hash，则需要从其他的Peer获取此完整的区块，然后再传递给eth插入区块。

源码解读

本节介绍区块传播和处理的细节东西，方式仍然是先用图解释流程，再是代码流程。

产块节点的传播新区块

节点产生区块后，广播的流程可以表示为图4：

1. 发布事件
2. 事件处理函数选择要广播完整的Peer，然后将区块加入到它们的队列
3. 事件处理函数把区块Hash添加到所有Peer的另外一个通知队列
4. 每个Peer的广播处理函数，会遍历它的待广播区块队列和通知队列，把数据封装成消息，调用P2P接口发送出去

再看下代码上的细节。

1. worker.wait()函数发布事件NewMinedBlockEvent。
2. ProtocolManager.minedBroadcastLoop()是事件处理函数。它调用了2次pm.BroadcastBlock()。

// Mined broadcast loop
func (pm *ProtocolManager) minedBroadcastLoop() {
    // automatically stops if unsubscribe
    for obj := range pm.minedBlockSub.Chan() {
        switch ev := obj.Data.(type) {
        case core.NewMinedBlockEvent:
            pm.BroadcastBlock(ev.Block, true)  // First propagate block to peers
            pm.BroadcastBlock(ev.Block, false) // Only then announce to the rest
        }
    }
}

1. pm.BroadcastBlock()的入参propagate为真时，向部分Peer广播完整的区块，调用peer.AsyncSendNewBlock()，否则向所有Peer广播区块头，调用peer.AsyncSendNewBlockHash()，这2个函数就是把数据放入队列，此处不再放代码。

// BroadcastBlock will either propagate a block to a subset of it's peers, or
// will only announce it's availability (depending what's requested).
func (pm *ProtocolManager) BroadcastBlock(block *types.Block, propagate bool) {
    hash := block.Hash()
    peers := pm.peers.PeersWithoutBlock(hash)

    // If propagation is requested, send to a subset of the peer
    // 这种情况，要把区块广播给部分peer
    if propagate {
        // Calculate the TD of the block (it's not imported yet, so block.Td is not valid)
        // 计算新的总难度
        var td *big.Int
        if parent := pm.blockchain.GetBlock(block.ParentHash(), block.NumberU64()-1); parent != nil {
            td = new(big.Int).Add(block.Difficulty(), pm.blockchain.GetTd(block.ParentHash(), block.NumberU64()-1))
        } else {
            log.Error("Propagating dangling block", "number", block.Number(), "hash", hash)
            return
        }
        // Send the block to a subset of our peers
        // 广播区块给部分peer
        transfer := peers[:int(math.Sqrt(float64(len(peers))))]
        for _, peer := range transfer {
            peer.AsyncSendNewBlock(block, td)
        }
        log.Trace("Propagated block", "hash", hash, "recipients", len(transfer), "duration", common.PrettyDuration(time.Since(block.ReceivedAt)))
        return
    }
    // Otherwise if the block is indeed in out own chain, announce it
    // 把区块hash值广播给所有peer
    if pm.blockchain.HasBlock(hash, block.NumberU64()) {
        for _, peer := range peers {
            peer.AsyncSendNewBlockHash(block)
        }
        log.Trace("Announced block", "hash", hash, "recipients", len(peers), "duration", common.PrettyDuration(time.Since(block.ReceivedAt)))
    }
}

1. peer.broadcase()是每个Peer连接的广播函数，它只广播3种消息：交易、完整的区块、区块的Hash，这样表明了节点只会主动广播这3中类型的数据，剩余的数据同步，都是通过请求-响应的方式。

   // broadcast is a write loop that multiplexes block propagations, announcements
   // and transaction broadcasts into the remote peer. The goal is to have an async
   // writer that does not lock up node internals.
   func (p *peer) broadcast() {
       for {
           select {
           // 广播交易
           case txs := <-p.queuedTxs:
               if err := p.SendTransactions(txs); err != nil {
                   return
               }
               p.Log().Trace("Broadcast transactions", "count", len(txs))
           // 广播完整的新区块
           case prop := <-p.queuedProps:
               if err := p.SendNewBlock(prop.block, prop.td); err != nil {
                   return
               }
               p.Log().Trace("Propagated block", "number", prop.block.Number(), "hash", prop.block.Hash(), "td", prop.td)
   
           // 广播区块Hash
           case block := <-p.queuedAnns:
               if err := p.SendNewBlockHashes([]common.Hash{block.Hash()}, []uint64{block.NumberU64()}); err != nil {
                   return
               }
               p.Log().Trace("Announced block", "number", block.Number(), "hash", block.Hash())
   
           case <-p.term:
               return
           }
       }
   }

Peer节点处理新区块

本节介绍远端节点收到2种区块同步消息的处理，其中NewBlockMsg的处理流程比较清晰，也简洁。NewBlockHashesMsg消息的处理就绕了2绕，从总体流程图1上能看出来，它需要先从给他发送消息Peer那里获取到完整的区块，剩下的流程和NewBlockMsg又一致了。

这部分涉及的模块多，画出来有种眼花缭乱的感觉，但只要抓住上面的主线，代码看起来还是很清晰的。通过图5先看下整体流程。

消息处理的起点是ProtocolManager.handleMsg，NewBlockMsg的处理流程是蓝色标记的区域，红色区域是单独的协程，是fetcher处理队列中区块的流程，如果从队列中取出的区块是当前链需要的，校验后，调用blockchian.InsertChain()把区块插入到区块链，最后写入数据库，这是黄色部分。最后，绿色部分是NewBlockHashesMsg的处理流程，代码流程上是比较复杂的，为了能通过图描述整体流程，我把它简化掉了。

仔细看看这幅图，掌握整体的流程后，接下来看每个步骤的细节。

NewBlockMsg的处理

本节介绍节点收到完整区块的处理，流程如下：

1. 首先进行RLP编解码，然后标记发送消息的Peer已经知道这个区块，这样本节点最后广播这个区块的Hash时，不会再发送给该Peer。
2. 将区块存入到fetcher的队列，调用fetcher.Enqueue。
3. 更新Peer的Head位置，然后判断本地链是否落后于Peer的链，如果是，则通过Peer更新本地链。

只看handle.Msg()的NewBlockMsg相关的部分。

case msg.Code == NewBlockMsg:
    // Retrieve and decode the propagated block
    // 收到新区块，解码，赋值接收数据
    var request newBlockData
    if err := msg.Decode(&request); err != nil {
        return errResp(ErrDecode, "%v: %v", msg, err)
    }
    request.Block.ReceivedAt = msg.ReceivedAt
    request.Block.ReceivedFrom = p

    // Mark the peer as owning the block and schedule it for import
    // 标记peer知道这个区块
    p.MarkBlock(request.Block.Hash())
    // 为啥要如队列？已经得到完整的区块了
    // 答：存入fetcher的优先级队列，fetcher会从队列中选取当前高度需要的块
    pm.fetcher.Enqueue(p.id, request.Block)

    // Assuming the block is importable by the peer, but possibly not yet done so,
    // calculate the head hash and TD that the peer truly must have.
    // 截止到parent区块的头和难度
    var (
        trueHead = request.Block.ParentHash()
        trueTD   = new(big.Int).Sub(request.TD, request.Block.Difficulty())
    )
    // Update the peers total difficulty if better than the previous
    // 如果收到的块的难度大于peer之前的，以及自己本地的，就去和这个peer同步
    // 问题：就只用了一下块里的hash指，为啥不直接使用这个块呢，如果这个块不能用，干嘛不少发送些数据，减少网络负载呢。
    // 答案：实际上，这个块加入到了优先级队列中，当fetcher的loop检查到当前下一个区块的高度，正是队列中有的，则不再向peer请求
    // 该区块，而是直接使用该区块，检查无误后交给block chain执行insertChain
    if _, td := p.Head(); trueTD.Cmp(td) > 0 {
        p.SetHead(trueHead, trueTD)

        // Schedule a sync if above ours. Note, this will not fire a sync for a gap of
        // a singe block (as the true TD is below the propagated block), however this
        // scenario should easily be covered by the fetcher.
        currentBlock := pm.blockchain.CurrentBlock()
        if trueTD.Cmp(pm.blockchain.GetTd(currentBlock.Hash(), currentBlock.NumberU64())) > 0 {
            go pm.synchronise(p)
        }
    }
//------------------------ 以上 handleMsg

// Enqueue tries to fill gaps the the fetcher's future import queue.
// 发给inject通道，当前协程在handleMsg，通过通道发送给fetcher的协程处理
func (f *Fetcher) Enqueue(peer string, block *types.Block) error {
    op := &inject{
        origin: peer,
        block:  block,
    }
    select {
    case f.inject <- op:
        return nil
    case <-f.quit:
        return errTerminated
    }
}

//------------------------ 以下 fetcher.loop处理inject部分
case op := <-f.inject:
    // A direct block insertion was requested, try and fill any pending gaps
    // 区块加入队列，首先也填入未决的间距
    propBroadcastInMeter.Mark(1)
    f.enqueue(op.origin, op.block)

//------------------------  如队列函数

// enqueue schedules a new future import operation, if the block to be imported
// has not yet been seen.
// 把导入的新区块放进来
func (f *Fetcher) enqueue(peer string, block *types.Block) {
    hash := block.Hash()

    // Ensure the peer isn't DOSing us
    // 防止peer的DOS攻击
    count := f.queues[peer] + 1
    if count > blockLimit {
        log.Debug("Discarded propagated block, exceeded allowance", "peer", peer, "number", block.Number(), "hash", hash, "limit", blockLimit)
        propBroadcastDOSMeter.Mark(1)
        f.forgetHash(hash)
        return
    }
    // Discard any past or too distant blocks
    // 高度检查：未来太远的块丢弃
    if dist := int64(block.NumberU64()) - int64(f.chainHeight()); dist < -maxUncleDist || dist > maxQueueDist {
        log.Debug("Discarded propagated block, too far away", "peer", peer, "number", block.Number(), "hash", hash, "distance", dist)
        propBroadcastDropMeter.Mark(1)
        f.forgetHash(hash)
        return
    }
    // Schedule the block for future importing
    // 块先加入优先级队列，加入链之前，还有很多要做
    if _, ok := f.queued[hash]; !ok {
        op := &inject{
            origin: peer,
            block:  block,
        }
        f.queues[peer] = count
        f.queued[hash] = op
        f.queue.Push(op, -float32(block.NumberU64()))
        if f.queueChangeHook != nil {
            f.queueChangeHook(op.block.Hash(), true)
        }
        log.Debug("Queued propagated block", "peer", peer, "number", block.Number(), "hash", hash, "queued", f.queue.Size())
    }
}

fetcher队列处理

本节我们看看，区块加入队列后，fetcher如何处理区块，为何不直接校验区块，插入到本地链？

由于以太坊又Uncle的机制，节点可能收到老一点的一些区块。另外，节点可能由于网络原因，落后了几个区块，所以可能收到“未来”的一些区块，这些区块都不能直接插入到本地链。

区块入的队列是一个优先级队列，高度低的区块会被优先取出来。fetcher.loop是单独协程，不断运转，清理fecther中的事务和事件。首先会清理正在fetching的区块，但已经超时。然后处理优先级队列中的区块，判断高度是否是下一个区块，如果是则调用f.insert()函数，校验后调用BlockChain.InsertChain()，成功插入后，广播新区块的Hash。

// Loop is the main fetcher loop, checking and processing various notification
// events.
func (f *Fetcher) loop() {
    // Iterate the block fetching until a quit is requested
    fetchTimer := time.NewTimer(0)
    completeTimer := time.NewTimer(0)

    for {
        // Clean up any expired block fetches
        // 清理过期的区块
        for hash, announce := range f.fetching {
            if time.Since(announce.time) > fetchTimeout {
                f.forgetHash(hash)
            }
        }
        // Import any queued blocks that could potentially fit
        // 导入队列中合适的块
        height := f.chainHeight()
        for !f.queue.Empty() {
            op := f.queue.PopItem().(*inject)
            hash := op.block.Hash()
            if f.queueChangeHook != nil {
                f.queueChangeHook(hash, false)
            }
            // If too high up the chain or phase, continue later
            // 块不是链需要的下一个块，再入优先级队列，停止循环
            number := op.block.NumberU64()
            if number > height+1 {
                f.queue.Push(op, -float32(number))
                if f.queueChangeHook != nil {
                    f.queueChangeHook(hash, true)
                }
                break
            }
            // Otherwise if fresh and still unknown, try and import
            // 高度正好是我们想要的，并且链上也没有这个块
            if number+maxUncleDist < height || f.getBlock(hash) != nil {
                f.forgetBlock(hash)
                continue
            }
            // 那么，块插入链
            f.insert(op.origin, op.block)
        }
        
        //省略
    }
}

func (f *Fetcher) insert(peer string, block *types.Block) {
    hash := block.Hash()

    // Run the import on a new thread
    log.Debug("Importing propagated block", "peer", peer, "number", block.Number(), "hash", hash)
    go func() {
        defer func() { f.done <- hash }()

        // If the parent's unknown, abort insertion
        parent := f.getBlock(block.ParentHash())
        if parent == nil {
            log.Debug("Unknown parent of propagated block", "peer", peer, "number", block.Number(), "hash", hash, "parent", block.ParentHash())
            return
        }
        // Quickly validate the header and propagate the block if it passes
        // 验证区块头，成功后广播区块
        switch err := f.verifyHeader(block.Header()); err {
        case nil:
            // All ok, quickly propagate to our peers
            propBroadcastOutTimer.UpdateSince(block.ReceivedAt)
            go f.broadcastBlock(block, true)

        case consensus.ErrFutureBlock:
            // Weird future block, don't fail, but neither propagate

        default:
            // Something went very wrong, drop the peer
            log.Debug("Propagated block verification failed", "peer", peer, "number", block.Number(), "hash", hash, "err", err)
            f.dropPeer(peer)
            return
        }
        // Run the actual import and log any issues
        // 调用回调函数，实际是blockChain.insertChain
        if _, err := f.insertChain(types.Blocks{block}); err != nil {
            log.Debug("Propagated block import failed", "peer", peer, "number", block.Number(), "hash", hash, "err", err)
            return
        }
        // If import succeeded, broadcast the block
        propAnnounceOutTimer.UpdateSince(block.ReceivedAt)
        go f.broadcastBlock(block, false)

        // Invoke the testing hook if needed
        if f.importedHook != nil {
            f.importedHook(block)
        }
    }()
}

NewBlockHashesMsg的处理

本节介绍NewBlockHashesMsg的处理，其实，消息处理是简单的，而复杂一点的是从Peer哪获取完整的区块，下节再看。

流程如下:

1. 对消息进行RLP解码，然后标记Peer已经知道此区块。
2. 寻找出本地区块链不存在的区块Hash值，把这些未知的Hash通知给fetcher。
3. fetcher.Notify记录好通知信息，塞入notify通道，以便交给fetcher的协程。
4. fetcher.loop()会对notify中的消息进行处理，确认区块并非DOS攻击，然后检查区块的高度，判断该区块是否已经在fetching或者comleting(代表已经下载区块头，在下载body)，如果都没有，则加入到announced中，触发0s定时器，进行处理。

关于announced下节再介绍。

// handleMsg()部分
case msg.Code == NewBlockHashesMsg:
    var announces newBlockHashesData
    if err := msg.Decode(&announces); err != nil {
        return errResp(ErrDecode, "%v: %v", msg, err)
    }
    // Mark the hashes as present at the remote node
    for _, block := range announces {
        p.MarkBlock(block.Hash)
    }
    // Schedule all the unknown hashes for retrieval
    // 把本地链没有的块hash找出来，交给fetcher去下载
    unknown := make(newBlockHashesData, 0, len(announces))
    for _, block := range announces {
        if !pm.blockchain.HasBlock(block.Hash, block.Number) {
            unknown = append(unknown, block)
        }
    }
    for _, block := range unknown {
        pm.fetcher.Notify(p.id, block.Hash, block.Number, time.Now(), p.RequestOneHeader, p.RequestBodies)
    }

// Notify announces the fetcher of the potential availability of a new block in
// the network.
// 通知fetcher（自己）有新块产生，没有块实体，有hash、高度等信息
func (f *Fetcher) Notify(peer string, hash common.Hash, number uint64, time time.Time,
    headerFetcher headerRequesterFn, bodyFetcher bodyRequesterFn) error {
    block := &announce{
        hash:        hash,
        number:      number,
        time:        time,
        origin:      peer,
        fetchHeader: headerFetcher,
        fetchBodies: bodyFetcher,
    }
    select {
    case f.notify <- block:
        return nil
    case <-f.quit:
        return errTerminated
    }
}

// fetcher.loop()的notify通道消息处理
case notification := <-f.notify:
    // A block was announced, make sure the peer isn't DOSing us
    propAnnounceInMeter.Mark(1)
    count := f.announces[notification.origin] + 1
    if count > hashLimit {
        log.Debug("Peer exceeded outstanding announces", "peer", notification.origin, "limit", hashLimit)
        propAnnounceDOSMeter.Mark(1)
        break
    }

    // If we have a valid block number, check that it's potentially useful
    // 高度检查
    if notification.number > 0 {
        if dist := int64(notification.number) - int64(f.chainHeight()); dist < -maxUncleDist || dist > maxQueueDist {
            log.Debug("Peer discarded announcement", "peer", notification.origin, "number", notification.number, "hash", notification.hash, "distance", dist)
            propAnnounceDropMeter.Mark(1)
            break
        }
    }

    // All is well, schedule the announce if block's not yet downloading
    // 检查是否已经在下载，已下载则忽略
    if _, ok := f.fetching[notification.hash]; ok {
        break
    }
    if _, ok := f.completing[notification.hash]; ok {
        break
    }
    // 更新peer已经通知给我们的区块数量
    f.announces[notification.origin] = count
    // 把通知信息加入到announced，供调度
    f.announced[notification.hash] = append(f.announced[notification.hash], notification)
    if f.announceChangeHook != nil && len(f.announced[notification.hash]) == 1 {
        f.announceChangeHook(notification.hash, true)
    }
    if len(f.announced) == 1 {
        // 有通知放入到announced，则重设0s定时器，loop的另外一个分支会处理这些通知
        f.rescheduleFetch(fetchTimer)
    }

fetcher获取完整区块

本节介绍fetcher获取完整区块的过程，这也是fetcher最重要的功能，会涉及到fetcher至少80%的代码。单独拉放一大节吧。

Fetcher的大头

Fetcher最主要的功能就是获取完整的区块，然后在合适的实际交给InsertChain去验证和插入到本地区块链。我们还是从宏观入手，看Fetcher是如何工作的，一定要先掌握好宏观，因为代码层面上没有这么清晰。

宏观

首先，看两个节点是如何交互，获取完整区块，使用时序图的方式看一下，见图6，流程很清晰不再文字介绍。

再看下获取区块过程中，fetcher内部的状态转移，它使用状态来记录，要获取的区块在什么阶段，见图7。我稍微解释一下：

1. 收到NewBlockHashesMsg后，相关信息会记录到announced，进入announced状态，代表了本节点接收了消息。
2. announced由fetcher协程处理，经过校验后，会向给他发送消息的Peer发送请求，请求该区块的区块头，然后进入fetching状态。
3. 获取区块头后，如果区块头表示没有交易和uncle，则转移到completing状态，并且使用区块头合成完整的区块，加入到queued优先级队列。
4. 获取区块头后，如果区块头表示该区块有交易和uncle，则转移到fetched状态，然后发送请求，请求交易和uncle，然后转移到completing状态。
5. 收到交易和uncle后，使用头、交易、uncle这3个信息，生成完整的区块，加入到队列queued。

微观

接下来就是从代码角度看如何获取完整区块的流程了，有点多，看不懂的时候，再回顾下上面宏观的介绍图。

首先看Fetcher的定义，它存放了通信数据和状态管理，捡加注释的看，上文提到的状态，里面都有。

// Fetcher is responsible for accumulating block announcements from various peers
// and scheduling them for retrieval.
// 积累块通知，然后调度获取这些块
type Fetcher struct {
    // Various event channels
    // 收到区块hash值的通道
    notify chan *announce
    // 收到完整区块的通道
    inject chan *inject

    blockFilter chan chan []*types.Block
    // 过滤header的通道的通道
    headerFilter chan chan *headerFilterTask
    // 过滤body的通道的通道
    bodyFilter chan chan *bodyFilterTask

    done chan common.Hash
    quit chan struct{}

    // Announce states
    // Peer已经给了本节点多少区块头通知
    announces map[string]int // Per peer announce counts to prevent memory exhaustion
    // 已经announced的区块列表
    announced map[common.Hash][]*announce // Announced blocks, scheduled for fetching
    // 正在fetching区块头的请求
    fetching map[common.Hash]*announce // Announced blocks, currently fetching
    // 已经fetch到区块头，还差body的请求，用来获取body
    fetched map[common.Hash][]*announce // Blocks with headers fetched, scheduled for body retrieval
    // 已经得到区块头的
    completing map[common.Hash]*announce // Blocks with headers, currently body-completing

    // Block cache
    // queue，优先级队列，高度做优先级
    // queues，统计peer通告了多少块
    // queued，代表这个块如队列了，
    queue  *prque.Prque            // Queue containing the import operations (block number sorted)
    queues map[string]int          // Per peer block counts to prevent memory exhaustion
    queued map[common.Hash]*inject // Set of already queued blocks (to dedupe imports)

    // Callbacks
    getBlock       blockRetrievalFn   // Retrieves a block from the local chain
    verifyHeader   headerVerifierFn   // Checks if a block's headers have a valid proof of work，验证区块头，包含了PoW验证
    broadcastBlock blockBroadcasterFn // Broadcasts a block to connected peers，广播给peer
    chainHeight    chainHeightFn      // Retrieves the current chain's height
    insertChain    chainInsertFn      // Injects a batch of blocks into the chain，插入区块到链的函数
    dropPeer       peerDropFn         // Drops a peer for misbehaving

    // Testing hooks
    announceChangeHook func(common.Hash, bool) // Method to call upon adding or deleting a hash from the announce list
    queueChangeHook    func(common.Hash, bool) // Method to call upon adding or deleting a block from the import queue
    fetchingHook       func([]common.Hash)     // Method to call upon starting a block (eth/61) or header (eth/62) fetch
    completingHook     func([]common.Hash)     // Method to call upon starting a block body fetch (eth/62)
    importedHook       func(*types.Block)      // Method to call upon successful block import (both eth/61 and eth/62)
}

NewBlockHashesMsg消息的处理前面的小节已经讲过了，不记得可向前翻看。这里从announced的状态处理说起。loop()中，fetchTimer超时后，代表了收到了消息通知，需要处理，会从announced中选择出需要处理的通知，然后创建请求，请求区块头，由于可能有很多节点都通知了它某个区块的Hash，所以随机的从这些发送消息的Peer中选择一个Peer，发送请求的时候，为每个Peer都创建了单独的协程。

case <-fetchTimer.C:
    // At least one block's timer ran out, check for needing retrieval
    // 有区块通知，去处理
    request := make(map[string][]common.Hash)

    for hash, announces := range f.announced {
        if time.Since(announces[0].time) > arriveTimeout-gatherSlack {
            // Pick a random peer to retrieve from, reset all others
            // 可能有很多peer都发送了这个区块的hash值，随机选择一个peer
            announce := announces[rand.Intn(len(announces))]
            f.forgetHash(hash)

            // If the block still didn't arrive, queue for fetching
            // 本地还没有这个区块，创建获取区块的请求
            if f.getBlock(hash) == nil {
                request[announce.origin] = append(request[announce.origin], hash)
                f.fetching[hash] = announce
            }
        }
    }
    // Send out all block header requests
    // 把所有的request发送出去
    // 为每一个peer都创建一个协程，然后请求所有需要从该peer获取的请求
    for peer, hashes := range request {
        log.Trace("Fetching scheduled headers", "peer", peer, "list", hashes)

        // Create a closure of the fetch and schedule in on a new thread
        fetchHeader, hashes := f.fetching[hashes[0]].fetchHeader, hashes
        go func() {
            if f.fetchingHook != nil {
                f.fetchingHook(hashes)
            }
            for _, hash := range hashes {
                headerFetchMeter.Mark(1)
                fetchHeader(hash) // Suboptimal, but protocol doesn't allow batch header retrievals
            }
        }()
    }
    // Schedule the next fetch if blocks are still pending
    f.rescheduleFetch(fetchTimer)

从Notify的调用中，可以看出，fetcherHeader()的实际函数是RequestOneHeader()，该函数使用的消息是GetBlockHeadersMsg，可以用来请求多个区块头，不过fetcher只请求一个。

pm.fetcher.Notify(p.id, block.Hash, block.Number, time.Now(), p.RequestOneHeader, p.RequestBodies)

// RequestOneHeader is a wrapper around the header query functions to fetch a
// single header. It is used solely by the fetcher.
func (p *peer) RequestOneHeader(hash common.Hash) error {
    p.Log().Debug("Fetching single header", "hash", hash)
    return p2p.Send(p.rw, GetBlockHeadersMsg, &getBlockHeadersData{Origin: hashOrNumber{Hash: hash}, Amount: uint64(1), Skip: uint64(0), Reverse: false})
}

GetBlockHeadersMsg的处理如下：因为它是获取多个区块头的，所以处理起来比较“麻烦”，还好，fetcher只获取一个区块头，其处理在20行~33行，获取下一个区块头的处理逻辑，这里就不看了，最后调用SendBlockHeaders()将区块头发送给请求的节点，消息是BlockHeadersMsg。

// handleMsg()
// Block header query, collect the requested headers and reply
case msg.Code == GetBlockHeadersMsg:
    // Decode the complex header query
    var query getBlockHeadersData
    if err := msg.Decode(&query); err != nil {
        return errResp(ErrDecode, "%v: %v", msg, err)
    }
    hashMode := query.Origin.Hash != (common.Hash{})

    // Gather headers until the fetch or network limits is reached
    // 收集区块头，直到达到限制
    var (
        bytes   common.StorageSize
        headers []*types.Header
        unknown bool
    )
    // 自己已知区块 && 少于查询的数量 && 大小小于2MB && 小于能下载的最大数量
    for !unknown && len(headers) < int(query.Amount) && bytes < softResponseLimit && len(headers) < downloader.MaxHeaderFetch {
        // Retrieve the next header satisfying the query
        // 获取区块头
        var origin *types.Header
        if hashMode {
            // fetcher 使用的模式
            origin = pm.blockchain.GetHeaderByHash(query.Origin.Hash)
        } else {
            origin = pm.blockchain.GetHeaderByNumber(query.Origin.Number)
        }
        if origin == nil {
            break
        }
        number := origin.Number.Uint64()
        headers = append(headers, origin)
        bytes += estHeaderRlpSize

        // Advance to the next header of the query
        // 下一个区块头的获取，不同策略，方式不同
        switch {
        case query.Origin.Hash != (common.Hash{}) && query.Reverse:
            // ...
        }
    }
    return p.SendBlockHeaders(headers)

BlockHeadersMsg的处理很有意思，因为GetBlockHeadersMsg并不是fetcher独占的消息，downloader也可以调用，所以，响应消息的处理需要分辨出是fetcher请求的，还是downloader请求的。它的处理逻辑是：fetcher先过滤收到的区块头，如果fetcher不要的，那就是downloader的，在调用fetcher.FilterHeaders的时候，fetcher就将自己要的区块头拿走了。

// handleMsg()
case msg.Code == BlockHeadersMsg:
    // A batch of headers arrived to one of our previous requests
    var headers []*types.Header
    if err := msg.Decode(&headers); err != nil {
        return errResp(ErrDecode, "msg %v: %v", msg, err)
    }
    // If no headers were received, but we're expending a DAO fork check, maybe it's that
    // 检查是不是当前DAO的硬分叉
    if len(headers) == 0 && p.forkDrop != nil {
        // Possibly an empty reply to the fork header checks, sanity check TDs
        verifyDAO := true

        // If we already have a DAO header, we can check the peer's TD against it. If
        // the peer's ahead of this, it too must have a reply to the DAO check
        if daoHeader := pm.blockchain.GetHeaderByNumber(pm.chainconfig.DAOForkBlock.Uint64()); daoHeader != nil {
            if _, td := p.Head(); td.Cmp(pm.blockchain.GetTd(daoHeader.Hash(), daoHeader.Number.Uint64())) >= 0 {
                verifyDAO = false
            }
        }
        // If we're seemingly on the same chain, disable the drop timer
        if verifyDAO {
            p.Log().Debug("Seems to be on the same side of the DAO fork")
            p.forkDrop.Stop()
            p.forkDrop = nil
            return nil
        }
    }
    // Filter out any explicitly requested headers, deliver the rest to the downloader
    // 过滤是不是fetcher请求的区块头，去掉fetcher请求的区块头再交给downloader
    filter := len(headers) == 1
    if filter {
        // If it's a potential DAO fork check, validate against the rules
        // 检查是否硬分叉
        if p.forkDrop != nil && pm.chainconfig.DAOForkBlock.Cmp(headers[0].Number) == 0 {
            // Disable the fork drop timer
            p.forkDrop.Stop()
            p.forkDrop = nil

            // Validate the header and either drop the peer or continue
            if err := misc.VerifyDAOHeaderExtraData(pm.chainconfig, headers[0]); err != nil {
                p.Log().Debug("Verified to be on the other side of the DAO fork, dropping")
                return err
            }
            p.Log().Debug("Verified to be on the same side of the DAO fork")
            return nil
        }
        // Irrelevant of the fork checks, send the header to the fetcher just in case
        // 使用fetcher过滤区块头
        headers = pm.fetcher.FilterHeaders(p.id, headers, time.Now())
    }
    // 剩下的区块头交给downloader
    if len(headers) > 0 || !filter {
        err := pm.downloader.DeliverHeaders(p.id, headers)
        if err != nil {
            log.Debug("Failed to deliver headers", "err", err)
        }
    }

FilterHeaders()是一个很有大智慧的函数，看起来耐人寻味，但实在妙。它要把所有的区块头，都传递给fetcher协程，还要获取fetcher协程处理后的结果。fetcher.headerFilter是存放通道的通道，而filter是存放包含区块头过滤任务的通道。它先把filter传递给了headerFilter，这样fetcher协程就在另外一段等待了，而后将headerFilterTask传入filter，fetcher就能读到数据了，处理后，再将数据写回filter而刚好被FilterHeaders函数处理了，该函数实际运行在handleMsg()的协程中。

每个Peer都会分配一个ProtocolManager然后处理该Peer的消息，但fetcher只有一个事件处理协程，如果不创建一个filter，fetcher哪知道是谁发给它的区块头呢？过滤之后，该如何发回去呢？

// FilterHeaders extracts all the headers that were explicitly requested by the fetcher,
// returning those that should be handled differently.
// 寻找出fetcher请求的区块头
func (f *Fetcher) FilterHeaders(peer string, headers []*types.Header, time time.Time) []*types.Header {
    log.Trace("Filtering headers", "peer", peer, "headers", len(headers))

    // Send the filter channel to the fetcher
    // 任务通道
    filter := make(chan *headerFilterTask)

    select {
    // 任务通道发送到这个通道
    case f.headerFilter <- filter:
    case <-f.quit:
        return nil
    }
    // Request the filtering of the header list
    // 创建过滤任务，发送到任务通道
    select {
    case filter <- &headerFilterTask{peer: peer, headers: headers, time: time}:
    case <-f.quit:
        return nil
    }
    // Retrieve the headers remaining after filtering
    // 从任务通道，获取过滤的结果并返回
    select {
    case task := <-filter:
        return task.headers
    case <-f.quit:
        return nil
    }
}

接下来要看f.headerFilter的处理，这段代码有90行，它做了一下几件事：

1. 从f.headerFilter取出filter，然后取出过滤任务task。
2. 它把区块头分成3类：unknown这不是分是要返回给调用者的，即handleMsg(), incomplete存放还需要获取body的区块头，complete存放只包含区块头的区块。遍历所有的区块头，填到到对应的分类中，具体的判断可看18行的注释，记住宏观中将的状态转移图。
3. 把unknonw中的区块返回给handleMsg()。
4. 把 incomplete的区块头获取状态移动到fetched状态，然后触发定时器，以便去处理complete的区块。
5. 把compelete的区块加入到queued。

// fetcher.loop()
case filter := <-f.headerFilter:
    // Headers arrived from a remote peer. Extract those that were explicitly
    // requested by the fetcher, and return everything else so it's delivered
    // to other parts of the system.
    // 收到从远端节点发送的区块头，过滤出fetcher请求的
    // 从任务通道获取过滤任务
    var task *headerFilterTask
    select {
    case task = <-filter:
    case <-f.quit:
        return
    }
    headerFilterInMeter.Mark(int64(len(task.headers)))

    // Split the batch of headers into unknown ones (to return to the caller),
    // known incomplete ones (requiring body retrievals) and completed blocks.
    // unknown的不是fetcher请求的，complete放没有交易和uncle的区块，有头就够了，incomplete放
    // 还需要获取uncle和交易的区块
    unknown, incomplete, complete := []*types.Header{}, []*announce{}, []*types.Block{}
    // 遍历所有收到的header
    for _, header := range task.headers {
        hash := header.Hash()

        // Filter fetcher-requested headers from other synchronisation algorithms
        // 是正在获取的hash，并且对应请求的peer，并且未fetched，未completing，未queued
        if announce := f.fetching[hash]; announce != nil && announce.origin == task.peer && f.fetched[hash] == nil && f.completing[hash] == nil && f.queued[hash] == nil {
            // If the delivered header does not match the promised number, drop the announcer
            // 高度校验，竟然不匹配，扰乱秩序，peer肯定是坏蛋。
            if header.Number.Uint64() != announce.number {
                log.Trace("Invalid block number fetched", "peer", announce.origin, "hash", header.Hash(), "announced", announce.number, "provided", header.Number)
                f.dropPeer(announce.origin)
                f.forgetHash(hash)
                continue
            }
            // Only keep if not imported by other means
            // 本地链没有当前区块
            if f.getBlock(hash) == nil {
                announce.header = header
                announce.time = task.time

                // If the block is empty (header only), short circuit into the final import queue
                // 如果区块没有交易和uncle，加入到complete
                if header.TxHash == types.DeriveSha(types.Transactions{}) && header.UncleHash == types.CalcUncleHash([]*types.Header{}) {
                    log.Trace("Block empty, skipping body retrieval", "peer", announce.origin, "number", header.Number, "hash", header.Hash())

                    block := types.NewBlockWithHeader(header)
                    block.ReceivedAt = task.time

                    complete = append(complete, block)
                    f.completing[hash] = announce
                    continue
                }
                // Otherwise add to the list of blocks needing completion
                // 否则就是不完整的区块
                incomplete = append(incomplete, announce)
            } else {
                log.Trace("Block already imported, discarding header", "peer", announce.origin, "number", header.Number, "hash", header.Hash())
                f.forgetHash(hash)
            }
        } else {
            // Fetcher doesn't know about it, add to the return list
            // 没请求过的header
            unknown = append(unknown, header)
        }
    }
    // 把未知的区块头，再传递会filter
    headerFilterOutMeter.Mark(int64(len(unknown)))
    select {
    case filter <- &headerFilterTask{headers: unknown, time: task.time}:
    case <-f.quit:
        return
    }
    // Schedule the retrieved headers for body completion
    // 把未完整的区块加入到fetched，跳过已经在completeing中的，然后触发completeTimer定时器
    for _, announce := range incomplete {
        hash := announce.header.Hash()
        if _, ok := f.completing[hash]; ok {
            continue
        }
        f.fetched[hash] = append(f.fetched[hash], announce)
        if len(f.fetched) == 1 {
            f.rescheduleComplete(completeTimer)
        }
    }
    // Schedule the header-only blocks for import
    // 把只有头的区块入队列
    for _, block := range complete {
        if announce := f.completing[block.Hash()]; announce != nil {
            f.enqueue(announce.origin, block)
        }
    }

跟随状态图的转义，剩下的工作是fetched转移到completing，上面的流程已经触发了completeTimer定时器，超时后就会处理，流程与请求Header类似，不再赘述，此时发送的请求消息是GetBlockBodiesMsg，实际调的函数是RequestBodies。

// fetcher.loop()
case <-completeTimer.C:
    // At least one header's timer ran out, retrieve everything
    // 至少有1个header已经获取完了
    request := make(map[string][]common.Hash)

    // 遍历所有待获取body的announce
    for hash, announces := range f.fetched {
        // Pick a random peer to retrieve from, reset all others
        // 随机选一个Peer发送请求，因为可能已经有很多Peer通知它这个区块了
        announce := announces[rand.Intn(len(announces))]
        f.forgetHash(hash)

        // If the block still didn't arrive, queue for completion
        // 如果本地没有这个区块，则放入到completing，创建请求
        if f.getBlock(hash) == nil {
            request[announce.origin] = append(request[announce.origin], hash)
            f.completing[hash] = announce
        }
    }
    // Send out all block body requests
    // 发送所有的请求，获取body，依然是每个peer一个单独协程
    for peer, hashes := range request {
        log.Trace("Fetching scheduled bodies", "peer", peer, "list", hashes)

        // Create a closure of the fetch and schedule in on a new thread
        if f.completingHook != nil {
            f.completingHook(hashes)
        }
        bodyFetchMeter.Mark(int64(len(hashes)))
        go f.completing[hashes[0]].fetchBodies(hashes)
    }
    // Schedule the next fetch if blocks are still pending
    f.rescheduleComplete(completeTimer)

handleMsg()处理该消息也是干净利落，直接获取RLP格式的body，然后发送响应消息。

// handleMsg()
case msg.Code == GetBlockBodiesMsg:
    // Decode the retrieval message
    msgStream := rlp.NewStream(msg.Payload, uint64(msg.Size))
    if _, err := msgStream.List(); err != nil {
        return err
    }
    // Gather blocks until the fetch or network limits is reached
    var (
        hash   common.Hash
        bytes  int
        bodies []rlp.RawValue
    )

    // 遍历所有请求
    for bytes < softResponseLimit && len(bodies) < downloader.MaxBlockFetch {
        // Retrieve the hash of the next block
        if err := msgStream.Decode(&hash); err == rlp.EOL {
            break
        } else if err != nil {
            return errResp(ErrDecode, "msg %v: %v", msg, err)
        }
        // Retrieve the requested block body, stopping if enough was found
        // 获取body，RLP格式
        if data := pm.blockchain.GetBodyRLP(hash); len(data) != 0 {
            bodies = append(bodies, data)
            bytes += len(data)
        }
    }
    return p.SendBlockBodiesRLP(bodies)

响应消息BlockBodiesMsg的处理与处理获取header的处理原理相同，先交给fetcher过滤，然后剩下的才是downloader的。需要注意一点，响应消息里只包含交易列表和叔块列表。

// handleMsg()
case msg.Code == BlockBodiesMsg:
    // A batch of block bodies arrived to one of our previous requests
    var request blockBodiesData
    if err := msg.Decode(&request); err != nil {
        return errResp(ErrDecode, "msg %v: %v", msg, err)
    }
    // Deliver them all to the downloader for queuing
    // 传递给downloader去处理
    transactions := make([][]*types.Transaction, len(request))
    uncles := make([][]*types.Header, len(request))

    for i, body := range request {
        transactions[i] = body.Transactions
        uncles[i] = body.Uncles
    }
    // Filter out any explicitly requested bodies, deliver the rest to the downloader
    // 先让fetcher过滤去fetcher请求的body，剩下的给downloader
    filter := len(transactions) > 0 || len(uncles) > 0
    if filter {
        transactions, uncles = pm.fetcher.FilterBodies(p.id, transactions, uncles, time.Now())
    }

    // 剩下的body交给downloader
    if len(transactions) > 0 || len(uncles) > 0 || !filter {
        err := pm.downloader.DeliverBodies(p.id, transactions, uncles)
        if err != nil {
            log.Debug("Failed to deliver bodies", "err", err)
        }
    }

过滤函数的原理也与Header相同。

// FilterBodies extracts all the block bodies that were explicitly requested by
// the fetcher, returning those that should be handled differently.
// 过去出fetcher请求的body，返回它没有处理的，过程类型header的处理
func (f *Fetcher) FilterBodies(peer string, transactions [][]*types.Transaction, uncles [][]*types.Header, time time.Time) ([][]*types.Transaction, [][]*types.Header) {
    log.Trace("Filtering bodies", "peer", peer, "txs", len(transactions), "uncles", len(uncles))

    // Send the filter channel to the fetcher
    filter := make(chan *bodyFilterTask)

    select {
    case f.bodyFilter <- filter:
    case <-f.quit:
        return nil, nil
    }
    // Request the filtering of the body list
    select {
    case filter <- &bodyFilterTask{peer: peer, transactions: transactions, uncles: uncles, time: time}:
    case <-f.quit:
        return nil, nil
    }
    // Retrieve the bodies remaining after filtering
    select {
    case task := <-filter:
        return task.transactions, task.uncles
    case <-f.quit:
        return nil, nil
    }
}

实际过滤body的处理瞧一下，这和Header的处理是不同的。直接看不点：

1. 它要的区块，单独取出来存到blocks中，它不要的继续留在task中。
2. 判断是不是fetcher请求的方法：如果交易列表和叔块列表计算出的hash值与区块头中的一样，并且消息来自请求的Peer，则就是fetcher请求的。
3. 将blocks中的区块加入到queued，终结。

case filter := <-f.bodyFilter:
    // Block bodies arrived, extract any explicitly requested blocks, return the rest
    var task *bodyFilterTask
    select {
    case task = <-filter:
    case <-f.quit:
        return
    }
    bodyFilterInMeter.Mark(int64(len(task.transactions)))

    blocks := []*types.Block{}
    // 获取的每个body的txs列表和uncle列表
    // 遍历每个区块的txs列表和uncle列表，计算hash后判断是否是当前fetcher请求的body
    for i := 0; i < len(task.transactions) && i < len(task.uncles); i++ {
        // Match up a body to any possible completion request
        matched := false

        // 遍历所有保存的请求，因为tx和uncle，不知道它是属于哪个区块的，只能去遍历所有的请求，通常量不大，所以遍历没有性能影响
        for hash, announce := range f.completing {
            if f.queued[hash] == nil {
                // 把传入的每个块的hash和unclehash和它请求出去的记录进行对比，匹配则说明是fetcher请求的区块body
                txnHash := types.DeriveSha(types.Transactions(task.transactions[i]))
                uncleHash := types.CalcUncleHash(task.uncles[i])

                if txnHash == announce.header.TxHash && uncleHash == announce.header.UncleHash && announce.origin == task.peer {
                    // Mark the body matched, reassemble if still unknown
                    matched = true

                    // 如果当前链还没有这个区块，则收集这个区块，合并成新区块
                    if f.getBlock(hash) == nil {
                        block := types.NewBlockWithHeader(announce.header).WithBody(task.transactions[i], task.uncles[i])
                        block.ReceivedAt = task.time

                        blocks = append(blocks, block)
                    } else {
                        f.forgetHash(hash)
                    }
                }
            }
        }
        // 从task中移除fetcher请求的数据
        if matched {
            task.transactions = append(task.transactions[:i], task.transactions[i+1:]...)
            task.uncles = append(task.uncles[:i], task.uncles[i+1:]...)
            i--
            continue
        }
    }

    // 将剩余的数据返回
    bodyFilterOutMeter.Mark(int64(len(task.transactions)))
    select {
    case filter <- task:
    case <-f.quit:
        return
    }
    // Schedule the retrieved blocks for ordered import
    // 把收集的区块加入到队列
    for _, block := range blocks {
        if announce := f.completing[block.Hash()]; announce != nil {
            f.enqueue(announce.origin, block)
        }
    }
}

至此，fetcher获取完整区块的流程讲完了，fetcher模块中80%的代码也都贴出来了，还有2个值得看看的函数：

1. forgetHash(hash common.Hash) ：用于清空指定hash指的记/状态录信息。
2. forgetBlock(hash common.Hash)：用于从队列中移除一个区块。

最后了，再回到开始看看fetcher模块和新区块的传播流程，有没有豁然开朗。

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