以太坊源碼分析：fetcher模塊和區塊傳播

前言

這篇文章從區塊傳播策略入手，介紹新區塊是如何傳播到遠端節點，以及新區塊加入到遠端節點本地鏈的過程，同時會介紹fetcher模塊，fetcher的功能是處理Peer通知的區塊信息。在介紹過程當中，還會涉及到p2p，eth等模塊，不會專門介紹，而是專一區塊的傳播和加入區塊鏈的過程。

當前代碼是以太坊Release 1.8，若是版本不一樣，代碼上可能存在差別。

整體過程和傳播策略

本節從宏觀角度介紹，節點產生區塊後，爲了傳播給遠端節點作了啥，遠端節點收到區塊後又作了什麼，每一個節點都鏈接了不少Peer，它傳播的策略是什麼樣的？

整體流程和策略能夠總結爲，傳播給遠端Peer節點，Peer驗證區塊無誤後，加入到本地區塊鏈，繼續傳播新區塊信息。具體過程以下。

先看整體過程。產生區塊後，miner模塊會發佈一個事件NewMinedBlockEvent，訂閱事件的協程收到事件後，就會把新區塊的消息，廣播給它全部的peer，peer收到消息後，會交給本身的fetcher模塊處理，fetcher進行基本的驗證後，區塊沒問題，發現這個區塊就是本地鏈須要的下一個區塊，則交給blockChain進一步進行完整的驗證，這個過程會執行區塊全部的交易，無誤後把區塊加入到本地鏈，寫入數據庫，這個過程就是下面的流程圖，圖1。

整體流程圖，能看到有個分叉，是由於節點傳播新區塊是有策略的。它的傳播策略爲：

1. 假如節點鏈接了N個Peer，它只向Peer列表的sqrt(N)個Peer廣播完整的區塊消息。
2. 向全部的Peer廣播只包含區塊Hash的消息。

策略圖的效果如圖2，紅色節點將區塊傳播給黃色節點：

收到區塊Hash的節點，須要從發送給它消息的Peer那裏獲取對應的完整區塊，獲取區塊後就會按照圖1的流程，加入到fetcher隊列，最終插入本地區塊鏈後，將區塊的Hash值廣播給和它相連，但還不知道這個區塊的Peer。非產生區塊節點的策略圖，如圖3，黃色節點將區塊Hash傳播給青色節點：

至此，能夠看出以太坊採用以石擊水的方式，像水紋同樣，層層擴散新產生的區塊。

Fetcher模塊是幹啥的

fetcher模塊的功能，就是收集其餘Peer通知它的區塊信息：1）完整的區塊2）區塊Hash消息。根據通知的消息，獲取完整的區塊，而後傳遞給eth模塊把區塊插入區塊鏈。

若是是完整區塊，就能夠傳遞給eth插入區塊，若是隻有區塊Hash，則須要從其餘的Peer獲取此完整的區塊，而後再傳遞給eth插入區塊。

源碼解讀

本節介紹區塊傳播和處理的細節東西，方式仍然是先用圖解釋流程，再是代碼流程。

產塊節點的傳播新區塊

節點產生區塊後，廣播的流程能夠表示爲圖4：

1. 發佈事件
2. 事件處理函數選擇要廣播完整的Peer，而後將區塊加入到它們的隊列
3. 事件處理函數把區塊Hash添加到全部Peer的另一個通知隊列
4. 每一個Peer的廣播處理函數，會遍歷它的待廣播區塊隊列和通知隊列，把數據封裝成消息，調用P2P接口發送出去

再看下代碼上的細節。

1. worker.wait()函數發佈事件NewMinedBlockEvent。
2. ProtocolManager.minedBroadcastLoop()是事件處理函數。它調用了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()的入參propagate爲真時，向部分Peer廣播完整的區塊，調用peer.AsyncSendNewBlock()，不然向全部Peer廣播區塊頭，調用peer.AsyncSendNewBlockHash()，這2個函數就是把數據放入隊列，此處再也不放代碼。

// 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
    // 這種狀況，要把區塊廣播給部分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
    // 把區塊hash值廣播給全部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()是每一個Peer鏈接的廣播函數，它只廣播3種消息：交易、完整的區塊、區塊的Hash，這樣代表了節點只會主動廣播這3中類型的數據，剩餘的數據同步，都是經過請求-響應的方式。

   // 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
           }
       }
   }

Peer節點處理新區塊

本節介紹遠端節點收到2種區塊同步消息的處理，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，從整體流程圖1上能看出來，它須要先從給他發送消息Peer那裏獲取到完整的區塊，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受，但只要抓住上面的主線，代碼看起來仍是很清晰的。經過圖5先看下總體流程。

消息處理的起點是ProtocolManager.handleMsg，NewBlockMsg的處理流程是藍色標記的區域，紅色區域是單獨的協程，是fetcher處理隊列中區塊的流程，若是從隊列中取出的區塊是當前鏈須要的，校驗後，調用blockchian.InsertChain()把區塊插入到區塊鏈，最後寫入數據庫，這是黃色部分。最後，綠色部分是NewBlockHashesMsg的處理流程，代碼流程上是比較複雜的，爲了能經過圖描述總體流程，我把它簡化掉了。

仔細看看這幅圖，掌握總體的流程後，接下來看每一個步驟的細節。

NewBlockMsg的處理

本節介紹節點收到完整區塊的處理，流程以下：

1. 首先進行RLP編解碼，而後標記發送消息的Peer已經知道這個區塊，這樣本節點最後廣播這個區塊的Hash時，不會再發送給該Peer。
2. 將區塊存入到fetcher的隊列，調用fetcher.Enqueue。
3. 更新Peer的Head位置，而後判斷本地鏈是否落後於Peer的鏈，若是是，則經過Peer更新本地鏈。

只看handle.Msg()的NewBlockMsg相關的部分。

case msg.Code == NewBlockMsg:
    // Retrieve and decode the propagated block
    // 收到新區塊，解碼，賦值接收數據
    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
    // 標記peer知道這個區塊
    p.MarkBlock(request.Block.Hash())
    // 爲啥要如隊列？已經獲得完整的區塊了
    // 答：存入fetcher的優先級隊列，fetcher會從隊列中選取當前高度須要的塊
    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
    // 若是收到的塊的難度大於peer以前的，以及本身本地的，就去和這個peer同步
    // 問題：就只用了一下塊裏的hash指，爲啥不直接使用這個塊呢，若是這個塊不能用，幹嗎很多發送些數據，減小網絡負載呢。
    // 答案：實際上，這個塊加入到了優先級隊列中，當fetcher的loop檢查到當前下一個區塊的高度，正是隊列中有的，則再也不向peer請求
    // 該區塊，而是直接使用該區塊，檢查無誤後交給block chain執行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.
// 發給inject通道，當前協程在handleMsg，經過通道發送給fetcher的協程處理
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
    }
}

//------------------------ 如下 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

// enqueue schedules a new future import operation, if the block to be imported
// has not yet been seen.
// 把導入的新區塊放進來
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：將來太遠的塊丟棄
    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
    // 塊先加入優先級隊列，加入鏈以前，還有不少要作
    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隊列處理

本節咱們看看，區塊加入隊列後，fetcher如何處理區塊，爲什麼不直接校驗區塊，插入到本地鏈？

因爲以太坊又Uncle的機制，節點可能收到老一點的一些區塊。另外，節點可能因爲網絡緣由，落後了幾個區塊，因此可能收到「將來」的一些區塊，這些區塊都不能直接插入到本地鏈。

區塊入的隊列是一個優先級隊列，高度低的區塊會被優先取出來。fetcher.loop是單獨協程，不斷運轉，清理fecther中的事務和事件。首先會清理正在fetching的區塊，但已經超時。而後處理優先級隊列中的區塊，判斷高度是不是下一個區塊，若是是則調用f.insert()函數，校驗後調用BlockChain.InsertChain()，成功插入後，廣播新區塊的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
        // 清理過時的區塊
        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
            // 塊不是鏈須要的下一個塊，再入優先級隊列，中止循環
            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
            // 高度正好是咱們想要的，而且鏈上也沒有這個塊
            if number+maxUncleDist < height || f.getBlock(hash) != nil {
                f.forgetBlock(hash)
                continue
            }
            // 那麼，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
        // 驗證區塊頭，成功後廣播區塊
        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
        // 調用回調函數，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)
        }
    }()
}

NewBlockHashesMsg的處理

本節介紹NewBlockHashesMsg的處理，其實，消息處理是簡單的，而複雜一點的是從Peer哪獲取完整的區塊，下節再看。

流程以下:

1. 對消息進行RLP解碼，而後標記Peer已經知道此區塊。
2. 尋找出本地區塊鏈不存在的區塊Hash值，把這些未知的Hash通知給fetcher。
3. fetcher.Notify記錄好通知信息，塞入notify通道，以便交給fetcher的協程。
4. fetcher.loop()會對notify中的消息進行處理，確認區塊並不是DOS攻擊，而後檢查區塊的高度，判斷該區塊是否已經在fetching或者comleting(表明已經下載區塊頭，在下載body)，若是都沒有，則加入到announced中，觸發0s定時器，進行處理。

關於announced下節再介紹。

// 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（本身）有新塊產生，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已經通知給咱們的區塊數量
    f.announces[notification.origin] = count
    // 把通知信息加入到announced，供調度
    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0s定時器，loop的另一個分支會處理這些通知
        f.rescheduleFetch(fetchTimer)
    }

fetcher獲取完整區塊

本節介紹fetcher獲取完整區塊的過程，這也是fetcher最重要的功能，會涉及到fetcher至少80%的代碼。單獨拉放一大節吧。

Fetcher的大頭

Fetcher最主要的功能就是獲取完整的區塊，而後在合適的實際交給InsertChain去驗證和插入到本地區塊鏈。咱們仍是從宏觀入手，看Fetcher是如何工做的，必定要先掌握好宏觀，由於代碼層面上沒有這麼清晰。

宏觀

首先，看兩個節點是如何交互，獲取完整區塊，使用時序圖的方式看一下，見圖6，流程很清晰再也不文字介紹。

再看下獲取區塊過程當中，fetcher內部的狀態轉移，它使用狀態來記錄，要獲取的區塊在什麼階段，見圖7。我稍微解釋一下：

1. 收到NewBlockHashesMsg後，相關信息會記錄到announced，進入announced狀態，表明了本節點接收了消息。
2. announced由fetcher協程處理，通過校驗後，會向給他發送消息的Peer發送請求，請求該區塊的區塊頭，而後進入fetching狀態。
3. 獲取區塊頭後，若是區塊頭表示沒有交易和uncle，則轉移到completing狀態，而且使用區塊頭合成完整的區塊，加入到queued優先級隊列。
4. 獲取區塊頭後，若是區塊頭表示該區塊有交易和uncle，則轉移到fetched狀態，而後發送請求，請求交易和uncle，而後轉移到completing狀態。
5. 收到交易和uncle後，使用頭、交易、uncle這3個信息，生成完整的區塊，加入到隊列queued。

微觀

接下來就是從代碼角度看如何獲取完整區塊的流程了，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，它存放了通訊數據和狀態管理，撿加註釋的看，上文提到的狀態，裏面都有。

// Fetcher is responsible for accumulating block announcements from various peers
// and scheduling them for retrieval.
// 積累塊通知，而後調度獲取這些塊
type Fetcher struct {
    // Various event channels
    // 收到區塊hash值的通道
    notify chan *announce
    // 收到完整區塊的通道
    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已經給了本節點多少區塊頭通知
    announces map[string]int // Per peer announce counts to prevent memory exhaustion
    // 已經announced的區塊列表
    announced map[common.Hash][]*announce // Announced blocks, scheduled for fetching
    // 正在fetching區塊頭的請求
    fetching map[common.Hash]*announce // Announced blocks, currently fetching
    // 已經fetch到區塊頭，還差body的請求，用來獲取body
    fetched map[common.Hash][]*announce // Blocks with headers fetched, scheduled for body retrieval
    // 已經獲得區塊頭的
    completing map[common.Hash]*announce // Blocks with headers, currently body-completing

    // Block cache
    // queue，優先級隊列，高度作優先級
    // queues，統計peer通告了多少塊
    // queued，表明這個塊如隊列了，
    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，插入區塊到鏈的函數
    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已經講過了，不記得可向前翻看。這裏從announced的狀態處理提及。loop()中，fetchTimer超時後，表明了收到了消息通知，須要處理，會從announced中選擇出須要處理的通知，而後建立請求，請求區塊頭，因爲可能有不少節點都通知了它某個區塊的Hash，因此隨機的從這些發送消息的Peer中選擇一個Peer，發送請求的時候，爲每一個Peer都建立了單獨的協程。

case <-fetchTimer.C:
    // At least one block's timer ran out, check for needing retrieval
    // 有區塊通知，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
            // 可能有不少peer都發送了這個區塊的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
            // 本地尚未這個區塊，建立獲取區塊的請求
            if f.getBlock(hash) == nil {
                request[announce.origin] = append(request[announce.origin], hash)
                f.fetching[hash] = announce
            }
        }
    }
    // Send out all block header requests
    // 把全部的request發送出去
    // 爲每個peer都建立一個協程，而後請求全部須要從該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的調用中，能夠看出，fetcherHeader()的實際函數是RequestOneHeader()，該函數使用的消息是GetBlockHeadersMsg，能夠用來請求多個區塊頭，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的處理以下：由於它是獲取多個區塊頭的，因此處理起來比較「麻煩」，還好，fetcher只獲取一個區塊頭，其處理在20行~33行，獲取下一個區塊頭的處理邏輯，這裏就不看了，最後調用SendBlockHeaders()將區塊頭髮送給請求的節點，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
    // 收集區塊頭，直到達到限制
    var (
        bytes   common.StorageSize
        headers []*types.Header
        unknown bool
    )
    // 本身已知區塊 && 少於查詢的數量 && 大小小於2MB && 小於能下載的最大數量
    for !unknown && len(headers) < int(query.Amount) && bytes < softResponseLimit && len(headers) < downloader.MaxHeaderFetch {
        // Retrieve the next header satisfying the query
        // 獲取區塊頭
        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
        // 下一個區塊頭的獲取，不一樣策略，方式不一樣
        switch {
        case query.Origin.Hash != (common.Hash{}) && query.Reverse:
            // ...
        }
    }
    return p.SendBlockHeaders(headers)

BlockHeadersMsg的處理頗有意思，由於GetBlockHeadersMsg並非fetcher獨佔的消息，downloader也能夠調用，因此，響應消息的處理須要分辨出是fetcher請求的，仍是downloader請求的。它的處理邏輯是：fetcher先過濾收到的區塊頭，若是fetcher不要的，那就是downloader的，在調用fetcher.FilterHeaders的時候，fetcher就將本身要的區塊頭拿走了。

// 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
    // 檢查是否是當前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
    // 過濾是否是fetcher請求的區塊頭，去掉fetcher請求的區塊頭再交給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過濾區塊頭
        headers = pm.fetcher.FilterHeaders(p.id, headers, time.Now())
    }
    // 剩下的區塊頭交給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()是一個頗有大智慧的函數，看起來回味無窮，但實在妙。它要把全部的區塊頭，都傳遞給fetcher協程，還要獲取fetcher協程處理後的結果。fetcher.headerFilter是存放通道的通道，而filter是存放包含區塊頭過濾任務的通道。它先把filter傳遞給了headerFilter，這樣fetcher協程就在另一段等待了，然後將headerFilterTask傳入filter，fetcher就能讀到數據了，處理後，再將數據寫回filter而恰好被FilterHeaders函數處理了，該函數實際運行在handleMsg()的協程中。

每一個Peer都會分配一個ProtocolManager而後處理該Peer的消息，但fetcher只有一個事件處理協程，若是不建立一個filter，fetcher哪知道是誰發給它的區塊頭呢？過濾以後，該如何發回去呢？

// 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
    // 任務通道
    filter := make(chan *headerFilterTask)

    select {
    // 任務通道發送到這個通道
    case f.headerFilter <- filter:
    case <-f.quit:
        return nil
    }
    // Request the filtering of the header list
    // 建立過濾任務，發送到任務通道
    select {
    case filter <- &headerFilterTask{peer: peer, headers: headers, time: time}:
    case <-f.quit:
        return nil
    }
    // Retrieve the headers remaining after filtering
    // 從任務通道，獲取過濾的結果並返回
    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，它作了一下幾件事：

1. 從f.headerFilter取出filter，而後取出過濾任務task。
2. 它把區塊頭分紅3類：unknown這不是分是要返回給調用者的，即handleMsg(), incomplete存放還須要獲取body的區塊頭，complete存放只包含區塊頭的區塊。遍歷全部的區塊頭，填到到對應的分類中，具體的判斷可看18行的註釋，記住宏觀中將的狀態轉移圖。
3. 把unknonw中的區塊返回給handleMsg()。
4. 把 incomplete的區塊頭獲取狀態移動到fetched狀態，而後觸發定時器，以便去處理complete的區塊。
5. 把compelete的區塊加入到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.
    // 收到從遠端節點發送的區塊頭，過濾出fetcher請求的
    // 從任務通道獲取過濾任務
    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的區塊，有頭就夠了，incomplete放
    // 還須要獲取uncle和交易的區塊
    unknown, incomplete, complete := []*types.Header{}, []*announce{}, []*types.Block{}
    // 遍歷全部收到的header
    for _, header := range task.headers {
        hash := header.Hash()

        // Filter fetcher-requested headers from other synchronisation algorithms
        // 是正在獲取的hash，而且對應請求的peer，而且未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，居然不匹配，擾亂秩序，peer確定是壞蛋。
            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
            // 本地鏈沒有當前區塊
            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
                // 若是區塊沒有交易和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
                // 不然就是不完整的區塊
                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
    // 把未完整的區塊加入到fetched，跳過已經在completeing中的，而後觸發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)
        }
    }

跟隨狀態圖的轉義，剩下的工做是fetched轉移到completing，上面的流程已經觸發了completeTimer定時器，超時後就會處理，流程與請求Header相似，再也不贅述，此時發送的請求消息是GetBlockBodiesMsg，實際調的函數是RequestBodies。

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

    // 遍歷全部待獲取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發送請求，由於可能已經有不少Peer通知它這個區塊了
        announce := announces[rand.Intn(len(announces))]
        f.forgetHash(hash)

        // If the block still didn't arrive, queue for completion
        // 若是本地沒有這個區塊，則放入到completing，建立請求
        if f.getBlock(hash) == nil {
            request[announce.origin] = append(request[announce.origin], hash)
            f.completing[hash] = announce
        }
    }
    // Send out all block body requests
    // 發送全部的請求，獲取body，依然是每一個peer一個單獨協程
    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()處理該消息也是乾淨利落，直接獲取RLP格式的body，而後發送響應消息。

// 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
    )

    // 遍歷全部請求
    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，而後剩下的纔是downloader的。須要注意一點，響應消息裏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的處理是不一樣的。直接看不點：

1. 它要的區塊，單獨取出來存到blocks中，它不要的繼續留在task中。
2. 判斷是否是fetcher請求的方法：若是交易列表和叔塊列表計算出的hash值與區塊頭中的同樣，而且消息來自請求的Peer，則就是fetcher請求的。
3. 將blocks中的區塊加入到queued，終結。

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{}
    // 獲取的每一個body的txs列表和uncle列表
    // 遍歷每一個區塊的txs列表和uncle列表，計算hash後判斷是不是當前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

        // 遍歷全部保存的請求，由於tx和uncle，不知道它是屬於哪一個區塊的，只能去遍歷全部的請求，一般量不大，因此遍歷沒有性能影響
        for hash, announce := range f.completing {
            if f.queued[hash] == nil {
                // 把傳入的每一個塊的hash和unclehash和它請求出去的記錄進行對比，匹配則說明是fetcher請求的區塊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

                    // 若是當前鏈尚未這個區塊，則收集這個區塊，合併成新區塊
                    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請求的數據
        if matched {
            task.transactions = append(task.transactions[:i], task.transactions[i+1:]...)
            task.uncles = append(task.uncles[:i], task.uncles[i+1:]...)
            i--
            continue
        }
    }

    // 將剩餘的數據返回
    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：

1. forgetHash(hash common.Hash) ：用於清空指定hash指的記/狀態錄信息。
2. forgetBlock(hash common.Hash)：用於從隊列中移除一個區塊。

最後了，再回到開始看看fetcher模塊和新區塊的傳播流程，有沒有豁然開朗。