摘要：當(dāng)前代碼是以太坊，如果版本不同，代碼上可能存在差異。非產(chǎn)生區(qū)塊節(jié)點的策略圖，如圖，黃色節(jié)點將區(qū)塊傳播給青色節(jié)點至此，可以看出以太坊采用以石擊水的方式，像水紋一樣，層層擴散新產(chǎn)生的區(qū)塊。

這篇文章從區(qū)塊傳播策略入手，介紹新區(qū)塊是如何傳播到遠(yuǎn)端節(jié)點，以及新區(qū)塊加入到遠(yuǎn)端節(jié)點本地鏈的過程，同時會介紹fetcher模塊，fetcher的功能是處理Peer通知的區(qū)塊信息。在介紹過程中，還會涉及到p2p，eth等模塊，不會專門介紹，而是專注區(qū)塊的傳播和加入?yún)^(qū)塊鏈的過程。

當(dāng)前代碼是以太坊Release 1.8，如果版本不同，代碼上可能存在差異。

本節(jié)從宏觀角度介紹，節(jié)點產(chǎn)生區(qū)塊后，為了傳播給遠(yuǎn)端節(jié)點做了啥，遠(yuǎn)端節(jié)點收到區(qū)塊后又做了什么，每個節(jié)點都連接了很多Peer，它傳播的策略是什么樣的？

總體流程和策略可以總結(jié)為，傳播給遠(yuǎn)端Peer節(jié)點，Peer驗證區(qū)塊無誤后，加入到本地區(qū)塊鏈，繼續(xù)傳播新區(qū)塊信息。具體過程如下。

先看總體過程。產(chǎn)生區(qū)塊后，miner模塊會發(fā)布一個事件NewMinedBlockEvent，訂閱事件的協(xié)程收到事件后，就會把新區(qū)塊的消息，廣播給它所有的peer，peer收到消息后，會交給自己的fetcher模塊處理，fetcher進(jìn)行基本的驗證后，區(qū)塊沒問題，發(fā)現(xiàn)這個區(qū)塊就是本地鏈需要的下一個區(qū)塊，則交給blockChain進(jìn)一步進(jìn)行完整的驗證，這個過程會執(zhí)行區(qū)塊所有的交易，無誤后把區(qū)塊加入到本地鏈，寫入數(shù)據(jù)庫，這個過程就是下面的流程圖，圖1。

總體流程圖，能看到有個分叉，是因為節(jié)點傳播新區(qū)塊是有策略的。它的傳播策略為：

假如節(jié)點連接了N個Peer，它只向Peer列表的sqrt(N)個Peer廣播完整的區(qū)塊消息。

向所有的Peer廣播只包含區(qū)塊Hash的消息。

策略圖的效果如圖2，紅色節(jié)點將區(qū)塊傳播給黃色節(jié)點：

收到區(qū)塊Hash的節(jié)點，需要從發(fā)送給它消息的Peer那里獲取對應(yīng)的完整區(qū)塊，獲取區(qū)塊后就會按照圖1的流程，加入到fetcher隊列，最終插入本地區(qū)塊鏈后，將區(qū)塊的Hash值廣播給和它相連，但還不知道這個區(qū)塊的Peer。非產(chǎn)生區(qū)塊節(jié)點的策略圖，如圖3，黃色節(jié)點將區(qū)塊Hash傳播給青色節(jié)點：

至此，可以看出以太坊采用以石擊水的方式，像水紋一樣，層層擴散新產(chǎn)生的區(qū)塊。

fetcher模塊的功能，就是收集其他Peer通知它的區(qū)塊信息：1）完整的區(qū)塊2）區(qū)塊Hash消息。根據(jù)通知的消息，獲取完整的區(qū)塊，然后傳遞給eth模塊把區(qū)塊插入?yún)^(qū)塊鏈。

如果是完整區(qū)塊，就可以傳遞給eth插入?yún)^(qū)塊，如果只有區(qū)塊Hash，則需要從其他的Peer獲取此完整的區(qū)塊，然后再傳遞給eth插入?yún)^(qū)塊。

本節(jié)介紹區(qū)塊傳播和處理的細(xì)節(jié)東西，方式仍然是先用圖解釋流程，再是代碼流程。

節(jié)點產(chǎn)生區(qū)塊后，廣播的流程可以表示為圖4：

發(fā)布事件

事件處理函數(shù)選擇要廣播完整的Peer，然后將區(qū)塊加入到它們的隊列

事件處理函數(shù)把區(qū)塊Hash添加到所有Peer的另外一個通知隊列

每個Peer的廣播處理函數(shù)，會遍歷它的待廣播區(qū)塊隊列和通知隊列，把數(shù)據(jù)封裝成消息，調(diào)用P2P接口發(fā)送出去

再看下代碼上的細(xì)節(jié)。

worker.wait()函數(shù)發(fā)布事件NewMinedBlockEvent。

ProtocolManager.minedBroadcastLoop()是事件處理函數(shù)。它調(diào)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
        }
    }
}

pm.BroadcastBlock()的入?yún)?b>propagate為真時，向部分Peer廣播完整的區(qū)塊，調(diào)用peer.AsyncSendNewBlock()，否則向所有Peer廣播區(qū)塊頭，調(diào)用peer.AsyncSendNewBlockHash()，這2個函數(shù)就是把數(shù)據(jù)放入隊列，此處不再放代碼。

// 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
    // 這種情況，要把區(qū)塊廣播給部分peer
    if propagate {
        // Calculate the TD of the block (it"s not imported yet, so block.Td is not valid)
        // 計算新的總難度
        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
        // 廣播區(qū)塊給部分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
    // 把區(qū)塊hash值廣播給所有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)))
    }
}

peer.broadcase()是每個Peer連接的廣播函數(shù)，它只廣播3種消息：交易、完整的區(qū)塊、區(qū)塊的Hash，這樣表明了節(jié)點只會主動廣播這3中類型的數(shù)據(jù)，剩余的數(shù)據(jù)同步，都是通過請求-響應(yīng)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 {
        // 廣播交易
        case txs := <-p.queuedTxs:
            if err := p.SendTransactions(txs); err != nil {
                return
            }
            p.Log().Trace("Broadcast transactions", "count", len(txs))
        // 廣播完整的新區(qū)塊
        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)

        // 廣播區(qū)塊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
        }
    }
}

本節(jié)介紹遠(yuǎn)端節(jié)點收到2種區(qū)塊同步消息的處理，其中NewBlockMsg的處理流程比較清晰，也簡潔。NewBlockHashesMsg消息的處理就繞了2繞，從總體流程圖1上能看出來，它需要先從給他發(fā)送消息Peer那里獲取到完整的區(qū)塊，剩下的流程和NewBlockMsg又一致了。

這部分涉及的模塊多，畫出來有種眼花繚亂的感覺，但只要抓住上面的主線，代碼看起來還是很清晰的。通過圖5先看下整體流程。

消息處理的起點是ProtocolManager.handleMsg，NewBlockMsg的處理流程是藍(lán)色標(biāo)記的區(qū)域，紅色區(qū)域是多帶帶的協(xié)程，是fetcher處理隊列中區(qū)塊的流程，如果從隊列中取出的區(qū)塊是當(dāng)前鏈需要的，校驗后，調(diào)用blockchian.InsertChain()把區(qū)塊插入到區(qū)塊鏈，最后寫入數(shù)據(jù)庫，這是黃色部分。最后，綠色部分是NewBlockHashesMsg的處理流程，代碼流程上是比較復(fù)雜的，為了能通過圖描述整體流程，我把它簡化掉了。

仔細(xì)看看這幅圖，掌握整體的流程后，接下來看每個步驟的細(xì)節(jié)。

本節(jié)介紹節(jié)點收到完整區(qū)塊的處理，流程如下：

首先進(jìn)行RLP編解碼，然后標(biāo)記發(fā)送消息的Peer已經(jīng)知道這個區(qū)塊，這樣本節(jié)點最后廣播這個區(qū)塊的Hash時，不會再發(fā)送給該Peer。

將區(qū)塊存入到fetcher的隊列，調(diào)用fetcher.Enqueue。

更新Peer的Head位置，然后判斷本地鏈?zhǔn)欠衤浜笥赑eer的鏈，如果是，則通過Peer更新本地鏈。

只看handle.Msg()的NewBlockMsg相關(guān)的部分。

case msg.Code == NewBlockMsg:
    // Retrieve and decode the propagated block
    // 收到新區(qū)塊，解碼，賦值接收數(shù)據(jù)
    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
    // 標(biāo)記peer知道這個區(qū)塊
    p.MarkBlock(request.Block.Hash())
    // 為啥要如隊列？已經(jīng)得到完整的區(qū)塊了
    // 答：存入fetcher的優(yōu)先級隊列，fetcher會從隊列中選取當(dāng)前高度需要的塊
    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區(qū)塊的頭和難度
    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
    // 如果收到的塊的難度大于peer之前的，以及自己本地的，就去和這個peer同步
    // 問題：就只用了一下塊里的hash指，為啥不直接使用這個塊呢，如果這個塊不能用，干嘛不少發(fā)送些數(shù)據(jù)，減少網(wǎng)絡(luò)負(fù)載呢。
    // 答案：實際上，這個塊加入到了優(yōu)先級隊列中，當(dāng)fetcher的loop檢查到當(dāng)前下一個區(qū)塊的高度，正是隊列中有的，則不再向peer請求
    // 該區(qū)塊，而是直接使用該區(qū)塊，檢查無誤后交給block chain執(zhí)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.
// 發(fā)給inject通道，當(dāng)前協(xié)程在handleMsg，通過通道發(fā)送給fetcher的協(xié)程處理
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處理inject部分
case op := <-f.inject:
    // A direct block insertion was requested, try and fill any pending gaps
    // 區(qū)塊加入隊列，首先也填入未決的間距
    propBroadcastInMeter.Mark(1)
    f.enqueue(op.origin, op.block)

//------------------------  如隊列函數(shù)

// enqueue schedules a new future import operation, if the block to be imported
// has not yet been seen.
// 把導(dǎo)入的新區(qū)塊放進(jìn)來
func (f *Fetcher) enqueue(peer string, block *types.Block) {
    hash := block.Hash()

    // Ensure the peer isn"t DOSing us
    // 防止peer的DOS攻擊
    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
    // 高度檢查：未來太遠(yuǎn)的塊丟棄
    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
    // 塊先加入優(yōu)先級隊列，加入鏈之前，還有很多要做
    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())
    }
}

本節(jié)我們看看，區(qū)塊加入隊列后，fetcher如何處理區(qū)塊，為何不直接校驗區(qū)塊，插入到本地鏈？

由于以太坊又Uncle的機制，節(jié)點可能收到老一點的一些區(qū)塊。另外，節(jié)點可能由于網(wǎng)絡(luò)原因，落后了幾個區(qū)塊，所以可能收到“未來”的一些區(qū)塊，這些區(qū)塊都不能直接插入到本地鏈。

區(qū)塊入的隊列是一個優(yōu)先級隊列，高度低的區(qū)塊會被優(yōu)先取出來。fetcher.loop是多帶帶協(xié)程，不斷運轉(zhuǎn)，清理fecther中的事務(wù)和事件。首先會清理正在fetching的區(qū)塊，但已經(jīng)超時。然后處理優(yōu)先級隊列中的區(qū)塊，判斷高度是否是下一個區(qū)塊，如果是則調(diào)用f.insert()函數(shù)，校驗后調(diào)用BlockChain.InsertChain()，成功插入后，廣播新區(qū)塊的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
        // 清理過期的區(qū)塊
        for hash, announce := range f.fetching {
            if time.Since(announce.time) > fetchTimeout {
                f.forgetHash(hash)
            }
        }
        // Import any queued blocks that could potentially fit
        // 導(dǎo)入隊列中合適的塊
        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
            // 塊不是鏈需要的下一個塊，再入優(yōu)先級隊列，停止循環(huán)
            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
            // 高度正好是我們想要的，并且鏈上也沒有這個塊
            if number+maxUncleDist < height || f.getBlock(hash) != nil {
                f.forgetBlock(hash)
                continue
            }
            // 那么，塊插入鏈
            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
        // 驗證區(qū)塊頭，成功后廣播區(qū)塊
        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
        // 調(diào)用回調(diào)函數(shù)，實際是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)
        }
    }()
}

本節(jié)介紹NewBlockHashesMsg的處理，其實，消息處理是簡單的，而復(fù)雜一點的是從Peer哪獲取完整的區(qū)塊，下節(jié)再看。

流程如下:

對消息進(jìn)行RLP解碼，然后標(biāo)記Peer已經(jīng)知道此區(qū)塊。

尋找出本地區(qū)塊鏈不存在的區(qū)塊Hash值，把這些未知的Hash通知給fetcher。

fetcher.Notify記錄好通知信息，塞入notify通道，以便交給fetcher的協(xié)程。

fetcher.loop()會對notify中的消息進(jìn)行處理，確認(rèn)區(qū)塊并非DOS攻擊，然后檢查區(qū)塊的高度，判斷該區(qū)塊是否已經(jīng)在fetching或者comleting(代表已經(jīng)下載區(qū)塊頭，在下載body)，如果都沒有，則加入到announced中，觸發(fā)0s定時器，進(jìn)行處理。

關(guān)于announced下節(jié)再介紹。

// 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
    // 把本地鏈沒有的塊hash找出來，交給fetcher去下載
    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）有新塊產(chǎn)生，沒有塊實體，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通道消息處理
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
    // 高度檢查
    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
    // 檢查是否已經(jīng)在下載，已下載則忽略
    if _, ok := f.fetching[notification.hash]; ok {
        break
    }
    if _, ok := f.completing[notification.hash]; ok {
        break
    }
    // 更新peer已經(jīng)通知給我們的區(qū)塊數(shù)量
    f.announces[notification.origin] = count
    // 把通知信息加入到announced，供調(diào)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，則重設(shè)0s定時器，loop的另外一個分支會處理這些通知
        f.rescheduleFetch(fetchTimer)
    }

本節(jié)介紹fetcher獲取完整區(qū)塊的過程，這也是fetcher最重要的功能，會涉及到fetcher至少80%的代碼。多帶帶拉放一大節(jié)吧。

Fetcher最主要的功能就是獲取完整的區(qū)塊，然后在合適的實際交給InsertChain去驗證和插入到本地區(qū)塊鏈。我們還是從宏觀入手，看Fetcher是如何工作的，一定要先掌握好宏觀，因為代碼層面上沒有這么清晰。

首先，看兩個節(jié)點是如何交互，獲取完整區(qū)塊，使用時序圖的方式看一下，見圖6，流程很清晰不再文字介紹。

再看下獲取區(qū)塊過程中，fetcher內(nèi)部的狀態(tài)轉(zhuǎn)移，它使用狀態(tài)來記錄，要獲取的區(qū)塊在什么階段，見圖7。我稍微解釋一下：

收到NewBlockHashesMsg后，相關(guān)信息會記錄到announced，進(jìn)入announced狀態(tài)，代表了本節(jié)點接收了消息。

announced由fetcher協(xié)程處理，經(jīng)過校驗后，會向給他發(fā)送消息的Peer發(fā)送請求，請求該區(qū)塊的區(qū)塊頭，然后進(jìn)入fetching狀態(tài)。

獲取區(qū)塊頭后，如果區(qū)塊頭表示沒有交易和uncle，則轉(zhuǎn)移到completing狀態(tài)，并且使用區(qū)塊頭合成完整的區(qū)塊，加入到queued優(yōu)先級隊列。

獲取區(qū)塊頭后，如果區(qū)塊頭表示該區(qū)塊有交易和uncle，則轉(zhuǎn)移到fetched狀態(tài)，然后發(fā)送請求，請求交易和uncle，然后轉(zhuǎn)移到completing狀態(tài)。

收到交易和uncle后，使用頭、交易、uncle這3個信息，生成完整的區(qū)塊，加入到隊列queued。

接下來就是從代碼角度看如何獲取完整區(qū)塊的流程了，有點多，看不懂的時候，再回顧下上面宏觀的介紹圖。

首先看Fetcher的定義，它存放了通信數(shù)據(jù)和狀態(tài)管理，撿加注釋的看，上文提到的狀態(tài)，里面都有。

// Fetcher is responsible for accumulating block announcements from various peers
// and scheduling them for retrieval.
// 積累塊通知，然后調(diào)度獲取這些塊
type Fetcher struct {
    // Various event channels
    // 收到區(qū)塊hash值的通道
    notify chan *announce
    // 收到完整區(qū)塊的通道
    inject chan *inject

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

    done chan common.Hash
    quit chan struct{}

    // Announce states
    // Peer已經(jīng)給了本節(jié)點多少區(qū)塊頭通知
    announces map[string]int // Per peer announce counts to prevent memory exhaustion
    // 已經(jīng)announced的區(qū)塊列表
    announced map[common.Hash][]*announce // Announced blocks, scheduled for fetching
    // 正在fetching區(qū)塊頭的請求
    fetching map[common.Hash]*announce // Announced blocks, currently fetching
    // 已經(jīng)fetch到區(qū)塊頭，還差body的請求，用來獲取body
    fetched map[common.Hash][]*announce // Blocks with headers fetched, scheduled for body retrieval
    // 已經(jīng)得到區(qū)塊頭的
    completing map[common.Hash]*announce // Blocks with headers, currently body-completing

    // Block cache
    // queue，優(yōu)先級隊列，高度做優(yōu)先級
    // queues，統(tǒng)計peer通告了多少塊
    // queued，代表這個塊如隊列了，
    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，驗證區(qū)塊頭，包含了PoW驗證
    broadcastBlock blockBroadcasterFn // Broadcasts a block to connected peers，廣播給peer
    chainHeight    chainHeightFn      // Retrieves the current chain"s height
    insertChain    chainInsertFn      // Injects a batch of blocks into the chain，插入?yún)^(qū)塊到鏈的函數(shù)
    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消息的處理前面的小節(jié)已經(jīng)講過了，不記得可向前翻看。這里從announced的狀態(tài)處理說起。loop()中，fetchTimer超時后，代表了收到了消息通知，需要處理，會從announced中選擇出需要處理的通知，然后創(chuàng)建請求，請求區(qū)塊頭，由于可能有很多節(jié)點都通知了它某個區(qū)塊的Hash，所以隨機的從這些發(fā)送消息的Peer中選擇一個Peer，發(fā)送請求的時候，為每個Peer都創(chuàng)建了多帶帶的協(xié)程。

case <-fetchTimer.C:
    // At least one block"s timer ran out, check for needing retrieval
    // 有區(qū)塊通知，去處理
    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都發(fā)送了這個區(qū)塊的hash值，隨機選擇一個peer
            announce := announces[rand.Intn(len(announces))]
            f.forgetHash(hash)

            // If the block still didn"t arrive, queue for fetching
            // 本地還沒有這個區(qū)塊，創(chuàng)建獲取區(qū)塊的請求
            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發(fā)送出去
    // 為每一個peer都創(chuàng)建一個協(xié)程，然后請求所有需要從該peer獲取的請求
    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)

從Notify的調(diào)用中，可以看出，fetcherHeader()的實際函數(shù)是RequestOneHeader()，該函數(shù)使用的消息是GetBlockHeadersMsg，可以用來請求多個區(qū)塊頭，不過fetcher只請求一個。

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的處理如下：因為它是獲取多個區(qū)塊頭的，所以處理起來比較“麻煩”，還好，fetcher只獲取一個區(qū)塊頭，其處理在20行~33行，獲取下一個區(qū)塊頭的處理邏輯，這里就不看了，最后調(diào)用SendBlockHeaders()將區(qū)塊頭發(fā)送給請求的節(jié)點，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
    // 收集區(qū)塊頭，直到達(dá)到限制
    var (
        bytes   common.StorageSize
        headers []*types.Header
        unknown bool
    )
    // 自己已知區(qū)塊 && 少于查詢的數(shù)量 && 大小小于2MB && 小于能下載的最大數(shù)量
    for !unknown && len(headers) < int(query.Amount) && bytes < softResponseLimit && len(headers) < downloader.MaxHeaderFetch {
        // Retrieve the next header satisfying the query
        // 獲取區(qū)塊頭
        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
        // 下一個區(qū)塊頭的獲取，不同策略，方式不同
        switch {
        case query.Origin.Hash != (common.Hash{}) && query.Reverse:
            // ...
        }
    }
    return p.SendBlockHeaders(headers)

BlockHeadersMsg的處理很有意思，因為GetBlockHeadersMsg并不是fetcher獨占的消息，downloader也可以調(diào)用，所以，響應(yīng)消息的處理需要分辨出是fetcher請求的，還是downloader請求的。它的處理邏輯是：fetcher先過濾收到的區(qū)塊頭，如果fetcher不要的，那就是downloader的，在調(diào)用fetcher.FilterHeaders的時候，fetcher就將自己要的區(qū)塊頭拿走了。

// 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
    // 檢查是不是當(dāng)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
    // 過濾是不是fetcher請求的區(qū)塊頭，去掉fetcher請求的區(qū)塊頭再交給downloader
    filter := len(headers) == 1
    if filter {
        // If it"s a potential DAO fork check, validate against the rules
        // 檢查是否硬分叉
        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過濾區(qū)塊頭
        headers = pm.fetcher.FilterHeaders(p.id, headers, time.Now())
    }
    // 剩下的區(qū)塊頭交給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()是一個很有大智慧的函數(shù)，看起來耐人尋味，但實在妙。它要把所有的區(qū)塊頭，都傳遞給fetcher協(xié)程，還要獲取fetcher協(xié)程處理后的結(jié)果。fetcher.headerFilter是存放通道的通道，而filter是存放包含區(qū)塊頭過濾任務(wù)的通道。它先把filter傳遞給了headerFilter，這樣fetcher協(xié)程就在另外一段等待了，而后將headerFilterTask傳入filter，fetcher就能讀到數(shù)據(jù)了，處理后，再將數(shù)據(jù)寫回filter而剛好被FilterHeaders函數(shù)處理了，該函數(shù)實際運行在handleMsg()的協(xié)程中。

每個Peer都會分配一個ProtocolManager然后處理該Peer的消息，但fetcher只有一個事件處理協(xié)程，如果不創(chuàng)建一個filter，fetcher哪知道是誰發(fā)給它的區(qū)塊頭呢？過濾之后，該如何發(fā)回去呢？

// FilterHeaders extracts all the headers that were explicitly requested by the fetcher,
// returning those that should be handled differently.
// 尋找出fetcher請求的區(qū)塊頭
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
    // 任務(wù)通道
    filter := make(chan *headerFilterTask)

    select {
    // 任務(wù)通道發(fā)送到這個通道
    case f.headerFilter <- filter:
    case <-f.quit:
        return nil
    }
    // Request the filtering of the header list
    // 創(chuàng)建過濾任務(wù)，發(fā)送到任務(wù)通道
    select {
    case filter <- &headerFilterTask{peer: peer, headers: headers, time: time}:
    case <-f.quit:
        return nil
    }
    // Retrieve the headers remaining after filtering
    // 從任務(wù)通道，獲取過濾的結(jié)果并返回
    select {
    case task := <-filter:
        return task.headers
    case <-f.quit:
        return nil
    }
}

接下來要看f.headerFilter的處理，這段代碼有90行，它做了一下幾件事：

從f.headerFilter取出filter，然后取出過濾任務(wù)task。

它把區(qū)塊頭分成3類：unknown這不是分是要返回給調(diào)用者的，即handleMsg(), incomplete存放還需要獲取body的區(qū)塊頭，complete存放只包含區(qū)塊頭的區(qū)塊。遍歷所有的區(qū)塊頭，填到到對應(yīng)的分類中，具體的判斷可看18行的注釋，記住宏觀中將的狀態(tài)轉(zhuǎn)移圖。

把unknonw中的區(qū)塊返回給handleMsg()。

把 incomplete的區(qū)塊頭獲取狀態(tài)移動到fetched狀態(tài)，然后觸發(fā)定時器，以便去處理complete的區(qū)塊。

把compelete的區(qū)塊加入到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.
    // 收到從遠(yuǎn)端節(jié)點發(fā)送的區(qū)塊頭，過濾出fetcher請求的
    // 從任務(wù)通道獲取過濾任務(wù)
    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請求的，complete放沒有交易和uncle的區(qū)塊，有頭就夠了，incomplete放
    // 還需要獲取uncle和交易的區(qū)塊
    unknown, incomplete, complete := []*types.Header{}, []*announce{}, []*types.Block{}
    // 遍歷所有收到的header
    for _, header := range task.headers {
        hash := header.Hash()

        // Filter fetcher-requested headers from other synchronisation algorithms
        // 是正在獲取的hash，并且對應(yīng)請求的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
            // 高度校驗，竟然不匹配，擾亂秩序，peer肯定是壞蛋。
            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
            // 本地鏈沒有當(dāng)前區(qū)塊
            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
                // 如果區(qū)塊沒有交易和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
                // 否則就是不完整的區(qū)塊
                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
            // 沒請求過的header
            unknown = append(unknown, header)
        }
    }
    // 把未知的區(qū)塊頭，再傳遞會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
    // 把未完整的區(qū)塊加入到fetched，跳過已經(jīng)在completeing中的，然后觸發(fā)completeTimer定時器
    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
    // 把只有頭的區(qū)塊入隊列
    for _, block := range complete {
        if announce := f.completing[block.Hash()]; announce != nil {
            f.enqueue(announce.origin, block)
        }
    }

跟隨狀態(tài)圖的轉(zhuǎn)義，剩下的工作是fetched轉(zhuǎn)移到completing，上面的流程已經(jīng)觸發(fā)了completeTimer定時器，超時后就會處理，流程與請求Header類似，不再贅述，此時發(fā)送的請求消息是GetBlockBodiesMsg，實際調(diào)的函數(shù)是RequestBodies。

// fetcher.loop()
case <-completeTimer.C:
    // At least one header"s timer ran out, retrieve everything
    // 至少有1個header已經(jīng)獲取完了
    request := make(map[string][]common.Hash)

    // 遍歷所有待獲取body的announce
    for hash, announces := range f.fetched {
        // Pick a random peer to retrieve from, reset all others
        // 隨機選一個Peer發(fā)送請求，因為可能已經(jīng)有很多Peer通知它這個區(qū)塊了
        announce := announces[rand.Intn(len(announces))]
        f.forgetHash(hash)

        // If the block still didn"t arrive, queue for completion
        // 如果本地沒有這個區(qū)塊，則放入到completing，創(chuàng)建請求
        if f.getBlock(hash) == nil {
            request[announce.origin] = append(request[announce.origin], hash)
            f.completing[hash] = announce
        }
    }
    // Send out all block body requests
    // 發(fā)送所有的請求，獲取body，依然是每個peer一個多帶帶協(xié)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()處理該消息也是干凈利落，直接獲取RLP格式的body，然后發(fā)送響應(yīng)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
    )

    // 遍歷所有請求
    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
        // 獲取body，RLP格式
        if data := pm.blockchain.GetBodyRLP(hash); len(data) != 0 {
            bodies = append(bodies, data)
            bytes += len(data)
        }
    }
    return p.SendBlockBodiesRLP(bodies)

響應(yīng)消息BlockBodiesMsg的處理與處理獲取header的處理原理相同，先交給fetcher過濾，然后剩下的才是downloader的。需要注意一點，響應(yīng)消息里只包含交易列表和叔塊列表。

// 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
    // 傳遞給downloader去處理
    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
    // 先讓fetcher過濾去fetcher請求的body，剩下的給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交給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)
        }
    }

過濾函數(shù)的原理也與Header相同。

// FilterBodies extracts all the block bodies that were explicitly requested by
// the fetcher, returning those that should be handled differently.
// 過去出fetcher請求的body，返回它沒有處理的，過程類型header的處理
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
    }
}

實際過濾body的處理瞧一下，這和Header的處理是不同的。直接看不點：

它要的區(qū)塊，多帶帶取出來存到blocks中，它不要的繼續(xù)留在task中。

判斷是不是fetcher請求的方法：如果交易列表和叔塊列表計算出的hash值與區(qū)塊頭中的一樣，并且消息來自請求的Peer，則就是fetcher請求的。

將blocks中的區(qū)塊加入到queued，終結(jié)。

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{}
    // 獲取的每個body的txs列表和uncle列表
    // 遍歷每個區(qū)塊的txs列表和uncle列表，計算hash后判斷是否是當(dāng)前fetcher請求的body
    for i := 0; i < len(task.transactions) && i < len(task.uncles); i++ {
        // Match up a body to any possible completion request
        matched := false

        // 遍歷所有保存的請求，因為tx和uncle，不知道它是屬于哪個區(qū)塊的，只能去遍歷所有的請求，通常量不大，所以遍歷沒有性能影響
        for hash, announce := range f.completing {
            if f.queued[hash] == nil {
                // 把傳入的每個塊的hash和unclehash和它請求出去的記錄進(jìn)行對比，匹配則說明是fetcher請求的區(qū)塊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

                    // 如果當(dāng)前鏈還沒有這個區(qū)塊，則收集這個區(qū)塊，合并成新區(qū)塊
                    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)
                    }
                }
            }
        }
        // 從task中移除fetcher請求的數(shù)據(jù)
        if matched {
            task.transactions = append(task.transactions[:i], task.transactions[i+1:]...)
            task.uncles = append(task.uncles[:i], task.uncles[i+1:]...)
            i--
            continue
        }
    }

    // 將剩余的數(shù)據(jù)返回
    bodyFilterOutMeter.Mark(int64(len(task.transactions)))
    select {
    case filter <- task:
    case <-f.quit:
        return
    }
    // Schedule the retrieved blocks for ordered import
    // 把收集的區(qū)塊加入到隊列
    for _, block := range blocks {
        if announce := f.completing[block.Hash()]; announce != nil {
            f.enqueue(announce.origin, block)
        }
    }
}

至此，fetcher獲取完整區(qū)塊的流程講完了，fetcher模塊中80%的代碼也都貼出來了，還有2個值得看看的函數(shù)：

forgetHash(hash common.Hash) ：用于清空指定hash指的記/狀態(tài)錄信息。

forgetBlock(hash common.Hash)：用于從隊列中移除一個區(qū)塊。

最后了，再回到開始看看fetcher模塊和新區(qū)塊的傳播流程，有沒有豁然開朗。