以太坊源码深入分析(7)-- 以太坊Downloader源码分析

上一节分析到Fetcher用于同步网络节点的新区块和新的交易数据,如果新区块和本地最新的区块相隔距离较远,说明本地区块数据太旧,Fetcher就不会同步这些区块。这时候就要借助Downloader来同步完整的区块数据。

一,启动Downloader
ProtocolManager初始化的时候会进行Downloader的初始化:

func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, chain BlockChain, lightchain LightChain, dropPeer peerDropFn) *Downloader {
    if lightchain == nil {
        lightchain = chain
    }

    dl := &Downloader{
        mode:           mode,
        stateDB:        stateDb,
        mux:            mux,
        queue:          newQueue(),
        peers:          newPeerSet(),
        rttEstimate:    uint64(rttMaxEstimate),
        rttConfidence:  uint64(1000000),
        blockchain:     chain,
        lightchain:     lightchain,
        dropPeer:       dropPeer,
        headerCh:       make(chan dataPack, 1),
        bodyCh:         make(chan dataPack, 1),
        receiptCh:      make(chan dataPack, 1),
        bodyWakeCh:     make(chan bool, 1),
        receiptWakeCh:  make(chan bool, 1),
        headerProcCh:   make(chan []*types.Header, 1),
        quitCh:         make(chan struct{}),
        stateCh:        make(chan dataPack),
        stateSyncStart: make(chan *stateSync),
        trackStateReq:  make(chan *stateReq),
    }
    go dl.qosTuner()
    go dl.stateFetcher()
    return dl
}

首先初始化Downloader对象的成员,然后启动dl.qosTuner() goroutine计算请求回路时间,启动dl.stateFetcher() goroutine 开启Downloader状态监控。

ProtocolManager收到新的区块消息广播或者有新的P2P网络节点加入的时候会调用ProtocolManager的 synchronise(peer *peer)方法,这时候会调用Downloader的Synchronise(peer.id, pHead, pTd, mode)方法。

Synchronise方法,重置d.queue和d.peers,清空d.bodyWakeCh, d.receiptWakeCh,d.headerCh, d.bodyCh, d.receiptCh,d.headerProcCh。调用d.syncWithPeer(p, hash, td)方法:

func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td *big.Int) (err error) {
    d.mux.Post(StartEvent{})
    defer func() {
        // reset on error
        if err != nil {
            d.mux.Post(FailedEvent{err})
        } else {
            d.mux.Post(DoneEvent{})
        }
    }()
    if p.version < 62 {
        return errTooOld
    }

    log.Debug("Synchronising with the network", "peer", p.id, "eth", p.version, "head", hash, "td", td, "mode", d.mode)
    defer func(start time.Time) {
        log.Debug("Synchronisation terminated", "elapsed", time.Since(start))
    }(time.Now())

    // Look up the sync boundaries: the common ancestor and the target block
    latest, err := d.fetchHeight(p)
    if err != nil {
        return err
    }
    height := latest.Number.Uint64()

    origin, err := d.findAncestor(p, height)
    if err != nil {
        return err
    }
    d.syncStatsLock.Lock()
    if d.syncStatsChainHeight <= origin || d.syncStatsChainOrigin > origin {
        d.syncStatsChainOrigin = origin
    }
    d.syncStatsChainHeight = height
    d.syncStatsLock.Unlock()

    // Ensure our origin point is below any fast sync pivot point
    pivot := uint64(0)
    if d.mode == FastSync {
        if height <= uint64(fsMinFullBlocks) {
            origin = 0
        } else {
            pivot = height - uint64(fsMinFullBlocks)
            if pivot <= origin {
                origin = pivot - 1
            }
        }
    }
    d.committed = 1
    if d.mode == FastSync && pivot != 0 {
        d.committed = 0
    }
    // Initiate the sync using a concurrent header and content retrieval algorithm
    d.queue.Prepare(origin+1, d.mode)
    if d.syncInitHook != nil {
        d.syncInitHook(origin, height)
    }

    fetchers := []func() error{
        func() error { return d.fetchHeaders(p, origin+1, pivot) }, // Headers are always retrieved
        func() error { return d.fetchBodies(origin + 1) },          // Bodies are retrieved during normal and fast sync
        func() error { return d.fetchReceipts(origin + 1) },        // Receipts are retrieved during fast sync
        func() error { return d.processHeaders(origin+1, pivot, td) },
    }
    if d.mode == FastSync {
        fetchers = append(fetchers, func() error { return d.processFastSyncContent(latest) })
    } else if d.mode == FullSync {
        fetchers = append(fetchers, d.processFullSyncContent)
    }
    return d.spawnSync(fetchers)
}

首先调用latest, err := d.fetchHeight(p)获取到peer节点最新的区块头,这个方法有点绕,我们来分析一下:

func (d *Downloader) fetchHeight(p *peerConnection) (*types.Header, error) {
    p.log.Debug("Retrieving remote chain height")

    // Request the advertised remote head block and wait for the response
    head, _ := p.peer.Head()
    go p.peer.RequestHeadersByHash(head, 1, 0, false)

    ttl := d.requestTTL()
    timeout := time.After(ttl)
    for {
        select {
        case <-d.cancelCh:
            return nil, errCancelBlockFetch

        case packet := <-d.headerCh:
            // Discard anything not from the origin peer
            if packet.PeerId() != p.id {
                log.Debug("Received headers from incorrect peer", "peer", packet.PeerId())
                break
            }
            // Make sure the peer actually gave something valid
            headers := packet.(*headerPack).headers
            if len(headers) != 1 {
                p.log.Debug("Multiple headers for single request", "headers", len(headers))
                return nil, errBadPeer
            }
            head := headers[0]
            p.log.Debug("Remote head header identified", "number", head.Number, "hash", head.Hash())
            return head, nil

        case <-timeout:
            p.log.Debug("Waiting for head header timed out", "elapsed", ttl)
            return nil, errTimeout

        case <-d.bodyCh:
        case <-d.receiptCh:
            // Out of bounds delivery, ignore
        }
    }
}

1,调用peer.RequestHeadersByHash(head, 1, 0, false),给网络节点发送一个GetBlockHeadersMsg的消息
2,然后阻塞住线程,直到收到d.headerCh或者timeout
3,本地节点会收到网络节点的BlockHeadersMsg的消息返回
4,调用downloader.DeliverHeaders(p.id, headers)
5,这时候会把p.id和headers打包发送给d.headerCh
6,这时候select收到d.headerCh,阻塞打开,并返回header内容

syncWithPeer() 方法接着调用 d.findAncestor(p, height)来获取本地节点和网络节点共同的祖先:

func (d *Downloader) findAncestor(p *peerConnection, height uint64) (uint64, error) {
    // Figure out the valid ancestor range to prevent rewrite attacks
    floor, ceil := int64(-1), d.lightchain.CurrentHeader().Number.Uint64()

    if d.mode == FullSync {
        ceil = d.blockchain.CurrentBlock().NumberU64()
    } else if d.mode == FastSync {
        ceil = d.blockchain.CurrentFastBlock().NumberU64()
    }
    if ceil >= MaxForkAncestry {
        floor = int64(ceil - MaxForkAncestry)
    }
    p.log.Debug("Looking for common ancestor", "local", ceil, "remote", height)

    // Request the topmost blocks to short circuit binary ancestor lookup
    head := ceil
    if head > height {
        head = height
    }
    from := int64(head) - int64(MaxHeaderFetch)
    if from < 0 {
        from = 0
    }
    // Span out with 15 block gaps into the future to catch bad head reports
    limit := 2 * MaxHeaderFetch / 16
    count := 1 + int((int64(ceil)-from)/16)
    if count > limit {
        count = limit
    }
    go p.peer.RequestHeadersByNumber(uint64(from), count, 15, false)

    // Wait for the remote response to the head fetch
    number, hash := uint64(0), common.Hash{}

    ttl := d.requestTTL()
    timeout := time.After(ttl)

    for finished := false; !finished; {
        select {
        case <-d.cancelCh:
            return 0, errCancelHeaderFetch

        case packet := <-d.headerCh:
            // Discard anything not from the origin peer
            if packet.PeerId() != p.id {
                log.Debug("Received headers from incorrect peer", "peer", packet.PeerId())
                break
            }
            // Make sure the peer actually gave something valid
            headers := packet.(*headerPack).headers
            if len(headers) == 0 {
                p.log.Warn("Empty head header set")
                return 0, errEmptyHeaderSet
            }
            // Make sure the peer's reply conforms to the request
            for i := 0; i < len(headers); i++ {
                if number := headers[i].Number.Int64(); number != from+int64(i)*16 {
                    p.log.Warn("Head headers broke chain ordering", "index", i, "requested", from+int64(i)*16, "received", number)
                    return 0, errInvalidChain
                }
            }
            // Check if a common ancestor was found
            finished = true
            for i := len(headers) - 1; i >= 0; i-- {
                // Skip any headers that underflow/overflow our requested set
                if headers[i].Number.Int64() < from || headers[i].Number.Uint64() > ceil {
                    continue
                }
                // Otherwise check if we already know the header or not
                if (d.mode == FullSync && d.blockchain.HasBlock(headers[i].Hash(), headers[i].Number.Uint64())) || (d.mode != FullSync && d.lightchain.HasHeader(headers[i].Hash(), headers[i].Number.Uint64())) {
                    number, hash = headers[i].Number.Uint64(), headers[i].Hash()

                    // If every header is known, even future ones, the peer straight out lied about its head
                    if number > height && i == limit-1 {
                        p.log.Warn("Lied about chain head", "reported", height, "found", number)
                        return 0, errStallingPeer
                    }
                    break
                }
            }

        case <-timeout:
            p.log.Debug("Waiting for head header timed out", "elapsed", ttl)
            return 0, errTimeout

        case <-d.bodyCh:
        case <-d.receiptCh:
            // Out of bounds delivery, ignore
        }
    }
    // If the head fetch already found an ancestor, return
    if !common.EmptyHash(hash) {
        if int64(number) <= floor {
            p.log.Warn("Ancestor below allowance", "number", number, "hash", hash, "allowance", floor)
            return 0, errInvalidAncestor
        }
        p.log.Debug("Found common ancestor", "number", number, "hash", hash)
        return number, nil
    }
    // Ancestor not found, we need to binary search over our chain
    start, end := uint64(0), head
    if floor > 0 {
        start = uint64(floor)
    }
    for start+1 < end {
        // Split our chain interval in two, and request the hash to cross check
        check := (start + end) / 2

        ttl := d.requestTTL()
        timeout := time.After(ttl)

        go p.peer.RequestHeadersByNumber(check, 1, 0, false)

        // Wait until a reply arrives to this request
        for arrived := false; !arrived; {
            select {
            case <-d.cancelCh:
                return 0, errCancelHeaderFetch

            case packer := <-d.headerCh:
                // Discard anything not from the origin peer
                if packer.PeerId() != p.id {
                    log.Debug("Received headers from incorrect peer", "peer", packer.PeerId())
                    break
                }
                // Make sure the peer actually gave something valid
                headers := packer.(*headerPack).headers
                if len(headers) != 1 {
                    p.log.Debug("Multiple headers for single request", "headers", len(headers))
                    return 0, errBadPeer
                }
                arrived = true

                // Modify the search interval based on the response
                if (d.mode == FullSync && !d.blockchain.HasBlock(headers[0].Hash(), headers[0].Number.Uint64())) || (d.mode != FullSync && !d.lightchain.HasHeader(headers[0].Hash(), headers[0].Number.Uint64())) {
                    end = check
                    break
                }
                header := d.lightchain.GetHeaderByHash(headers[0].Hash()) // Independent of sync mode, header surely exists
                if header.Number.Uint64() != check {
                    p.log.Debug("Received non requested header", "number", header.Number, "hash", header.Hash(), "request", check)
                    return 0, errBadPeer
                }
                start = check

            case <-timeout:
                p.log.Debug("Waiting for search header timed out", "elapsed", ttl)
                return 0, errTimeout

            case <-d.bodyCh:
            case <-d.receiptCh:
                // Out of bounds delivery, ignore
            }
        }
    }
    // Ensure valid ancestry and return
    if int64(start) <= floor {
        p.log.Warn("Ancestor below allowance", "number", start, "hash", hash, "allowance", floor)
        return 0, errInvalidAncestor
    }
    p.log.Debug("Found common ancestor", "number", start, "hash", hash)
    return start, nil
}

1,调用peer.RequestHeadersByNumber(uint64(from), count, 15, false),获取header。这里传入 count和 15,指从本地最高的header往前数192个区块的头,每16个区块取一个区块头。为了后面select收到d.headerCh时加以验证。
2,select收到了headers,遍历header,看是否在本地是否存在这个header,如果有,并且不为空,就说明找到共同的祖先,返回祖先number
3,如果没有找到共同的祖先,再重新从本地的区块链MaxForkAncestry起的一半的位置开始取区块头,一一验证是否跟网络节点返回的header一致,如果有就说明有共同的祖先,并返回,没有的话就返回0.

继续syncWithPeer()方法,找到同步的轴心的pivot,最后把要同步的数据和同步的方法传给d.spawnSync(fetchers),并执行。d.spawnSync(fetchers)挨个执行传入的同步方法。

二,Downloader同步数据方法
fetchHeaders(),fetchBodies() , fetchReceipts()

func (d *Downloader) fetchHeaders(p *peerConnection, from uint64, pivot uint64) error {
    p.log.Debug("Directing header downloads", "origin", from)
    defer p.log.Debug("Header download terminated")

    // Create a timeout timer, and the associated header fetcher
    skeleton := true            // Skeleton assembly phase or finishing up
    request := time.Now()       // time of the last skeleton fetch request
    timeout := time.NewTimer(0) // timer to dump a non-responsive active peer
    <-timeout.C                 // timeout channel should be initially empty
    defer timeout.Stop()

    var ttl time.Duration
    getHeaders := func(from uint64) {
        request = time.Now()

        ttl = d.requestTTL()
        timeout.Reset(ttl)

        if skeleton {
            p.log.Trace("Fetching skeleton headers", "count", MaxHeaderFetch, "from", from)
            go p.peer.RequestHeadersByNumber(from+uint64(MaxHeaderFetch)-1, MaxSkeletonSize, MaxHeaderFetch-1, false)
        } else {
            p.log.Trace("Fetching full headers", "count", MaxHeaderFetch, "from", from)
            go p.peer.RequestHeadersByNumber(from, MaxHeaderFetch, 0, false)
        }
    }
    // Start pulling the header chain skeleton until all is done
    getHeaders(from)

    for {
        select {
        case <-d.cancelCh:
            return errCancelHeaderFetch

        case packet := <-d.headerCh:
            // Make sure the active peer is giving us the skeleton headers
            if packet.PeerId() != p.id {
                log.Debug("Received skeleton from incorrect peer", "peer", packet.PeerId())
                break
            }
            headerReqTimer.UpdateSince(request)
            timeout.Stop()

            // If the skeleton's finished, pull any remaining head headers directly from the origin
            if packet.Items() == 0 && skeleton {
                skeleton = false
                getHeaders(from)
                continue
            }
            // If no more headers are inbound, notify the content fetchers and return
            if packet.Items() == 0 {
                // Don't abort header fetches while the pivot is downloading
                if atomic.LoadInt32(&d.committed) == 0 && pivot <= from {
                    p.log.Debug("No headers, waiting for pivot commit")
                    select {
                    case <-time.After(fsHeaderContCheck):
                        getHeaders(from)
                        continue
                    case <-d.cancelCh:
                        return errCancelHeaderFetch
                    }
                }
                // Pivot done (or not in fast sync) and no more headers, terminate the process
                p.log.Debug("No more headers available")
                select {
                case d.headerProcCh <- nil:
                    return nil
                case <-d.cancelCh:
                    return errCancelHeaderFetch
                }
            }
            headers := packet.(*headerPack).headers

            // If we received a skeleton batch, resolve internals concurrently
            if skeleton {
                filled, proced, err := d.fillHeaderSkeleton(from, headers)
                if err != nil {
                    p.log.Debug("Skeleton chain invalid", "err", err)
                    return errInvalidChain
                }
                headers = filled[proced:]
                from += uint64(proced)
            }
            // Insert all the new headers and fetch the next batch
            if len(headers) > 0 {
                p.log.Trace("Scheduling new headers", "count", len(headers), "from", from)
                select {
                case d.headerProcCh <- headers:
                case <-d.cancelCh:
                    return errCancelHeaderFetch
                }
                from += uint64(len(headers))
            }
            getHeaders(from)

        case <-timeout.C:
            if d.dropPeer == nil {
                // The dropPeer method is nil when `--copydb` is used for a local copy.
                // Timeouts can occur if e.g. compaction hits at the wrong time, and can be ignored
                p.log.Warn("Downloader wants to drop peer, but peerdrop-function is not set", "peer", p.id)
                break
            }
            // Header retrieval timed out, consider the peer bad and drop
            p.log.Debug("Header request timed out", "elapsed", ttl)
            headerTimeoutMeter.Mark(1)
            d.dropPeer(p.id)

            // Finish the sync gracefully instead of dumping the gathered data though
            for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
                select {
                case ch <- false:
                case <-d.cancelCh:
                }
            }
            select {
            case d.headerProcCh <- nil:
            case <-d.cancelCh:
            }
            return errBadPeer
        }
    }
}

1,getHeaders()调用peer.RequestHeadersByNumber()方法 获取网络节点的headers。
2,有两种获取方式,首先走的是skeleton方式,从查找到的共同祖先区块+192个区块位置开始,每隔192个区块,获取128个区块头。非skeleton方式,从共同祖先区块开始,获取192个区块头。
3,如果第一种方式获取不到区块头,则执行第二种获取方式,如果第二种方式还是没有获取到区块头的话,直接返回
4,如果是skeleton获取到的,调用fillHeaderSkeleton()方法加入到skeleton header chain
5,然后调整from值,再递归调用getHeaders()方法

func (d *Downloader) fillHeaderSkeleton(from uint64, skeleton []*types.Header) ([]*types.Header, int, error) {
    log.Debug("Filling up skeleton", "from", from)
    d.queue.ScheduleSkeleton(from, skeleton)

    var (
        deliver = func(packet dataPack) (int, error) {
            pack := packet.(*headerPack)
            return d.queue.DeliverHeaders(pack.peerId, pack.headers, d.headerProcCh)
        }
        expire   = func() map[string]int { return d.queue.ExpireHeaders(d.requestTTL()) }
        throttle = func() bool { return false }
        reserve  = func(p *peerConnection, count int) (*fetchRequest, bool, error) {
            return d.queue.ReserveHeaders(p, count), false, nil
        }
        fetch    = func(p *peerConnection, req *fetchRequest) error { return p.FetchHeaders(req.From, MaxHeaderFetch) }
        capacity = func(p *peerConnection) int { return p.HeaderCapacity(d.requestRTT()) }
        setIdle  = func(p *peerConnection, accepted int) { p.SetHeadersIdle(accepted) }
    )
    err := d.fetchParts(errCancelHeaderFetch, d.headerCh, deliver, d.queue.headerContCh, expire,
        d.queue.PendingHeaders, d.queue.InFlightHeaders, throttle, reserve,
        nil, fetch, d.queue.CancelHeaders, capacity, d.peers.HeaderIdlePeers, setIdle, "headers")

    log.Debug("Skeleton fill terminated", "err", err)

    filled, proced := d.queue.RetrieveHeaders()
    return filled, proced, err
}

a) 把skeleton的headers加入queue.ScheduleSkeleton调度队列,
b) 然后执行d.fetchParts()方法。
d.fetchParts()方法主要做了这几件事情
1,对收到的headers执行d.queue.DeliverHeaders()方法。
2,如果d.queue.PendingHeaders有pending的headers,调用d.peers.HeaderIdlePeers获取到idle的peers
3,调用d.queue.ReserveHeaders把pending的headers储备到idle的peers里面
4,用idle的peers调用p.FetchHeaders(req.From, MaxHeaderFetch)去获取headers
c) 最后执行d.queue.RetrieveHeaders(),获取到filled进去的headers

其他同步区块数据的方法d.fetchBodies() , d.fetchReceipts() 和fetchHeaders()流程类似,还更简单一些。

三,Downloader同步数据过程
d.processHeaders(), d.processFastSyncContent(latest) , d.processFullSyncContent
1,d.processHeaders() 方法

func (d *Downloader) processHeaders(origin uint64, pivot uint64, td *big.Int) error {
    // Keep a count of uncertain headers to roll back
    rollback := []*types.Header{}
    defer func() {
        if len(rollback) > 0 {
            // Flatten the headers and roll them back
            hashes := make([]common.Hash, len(rollback))
            for i, header := range rollback {
                hashes[i] = header.Hash()
            }
            lastHeader, lastFastBlock, lastBlock := d.lightchain.CurrentHeader().Number, common.Big0, common.Big0
            if d.mode != LightSync {
                lastFastBlock = d.blockchain.CurrentFastBlock().Number()
                lastBlock = d.blockchain.CurrentBlock().Number()
            }
            d.lightchain.Rollback(hashes)
            curFastBlock, curBlock := common.Big0, common.Big0
            if d.mode != LightSync {
                curFastBlock = d.blockchain.CurrentFastBlock().Number()
                curBlock = d.blockchain.CurrentBlock().Number()
            }
            log.Warn("Rolled back headers", "count", len(hashes),
                "header", fmt.Sprintf("%d->%d", lastHeader, d.lightchain.CurrentHeader().Number),
                "fast", fmt.Sprintf("%d->%d", lastFastBlock, curFastBlock),
                "block", fmt.Sprintf("%d->%d", lastBlock, curBlock))
        }
    }()

    // Wait for batches of headers to process
    gotHeaders := false

    for {
        select {
        case <-d.cancelCh:
            return errCancelHeaderProcessing

        case headers := <-d.headerProcCh:
            // Terminate header processing if we synced up
            if len(headers) == 0 {
                // Notify everyone that headers are fully processed
                for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
                    select {
                    case ch <- false:
                    case <-d.cancelCh:
                    }
                }
                if d.mode != LightSync {
                    head := d.blockchain.CurrentBlock()
                    if !gotHeaders && td.Cmp(d.blockchain.GetTd(head.Hash(), head.NumberU64())) > 0 {
                        return errStallingPeer
                    }
                }
                if d.mode == FastSync || d.mode == LightSync {
                    head := d.lightchain.CurrentHeader()
                    if td.Cmp(d.lightchain.GetTd(head.Hash(), head.Number.Uint64())) > 0 {
                        return errStallingPeer
                    }
                }
                // Disable any rollback and return
                rollback = nil
                return nil
            }
            // Otherwise split the chunk of headers into batches and process them
            gotHeaders = true

            for len(headers) > 0 {
                // Terminate if something failed in between processing chunks
                select {
                case <-d.cancelCh:
                    return errCancelHeaderProcessing
                default:
                }
                // Select the next chunk of headers to import
                limit := maxHeadersProcess
                if limit > len(headers) {
                    limit = len(headers)
                }
                chunk := headers[:limit]

                // In case of header only syncing, validate the chunk immediately
                if d.mode == FastSync || d.mode == LightSync {
                    // Collect the yet unknown headers to mark them as uncertain
                    unknown := make([]*types.Header, 0, len(headers))
                    for _, header := range chunk {
                        if !d.lightchain.HasHeader(header.Hash(), header.Number.Uint64()) {
                            unknown = append(unknown, header)
                        }
                    }
                    // If we're importing pure headers, verify based on their recentness
                    frequency := fsHeaderCheckFrequency
                    if chunk[len(chunk)-1].Number.Uint64()+uint64(fsHeaderForceVerify) > pivot {
                        frequency = 1
                    }
                    if n, err := d.lightchain.InsertHeaderChain(chunk, frequency); err != nil {
                        // If some headers were inserted, add them too to the rollback list
                        if n > 0 {
                            rollback = append(rollback, chunk[:n]...)
                        }
                        log.Debug("Invalid header encountered", "number", chunk[n].Number, "hash", chunk[n].Hash(), "err", err)
                        return errInvalidChain
                    }
                    // All verifications passed, store newly found uncertain headers
                    rollback = append(rollback, unknown...)
                    if len(rollback) > fsHeaderSafetyNet {
                        rollback = append(rollback[:0], rollback[len(rollback)-fsHeaderSafetyNet:]...)
                    }
                }
                // Unless we're doing light chains, schedule the headers for associated content retrieval
                if d.mode == FullSync || d.mode == FastSync {
                    // If we've reached the allowed number of pending headers, stall a bit
                    for d.queue.PendingBlocks() >= maxQueuedHeaders || d.queue.PendingReceipts() >= maxQueuedHeaders {
                        select {
                        case <-d.cancelCh:
                            return errCancelHeaderProcessing
                        case <-time.After(time.Second):
                        }
                    }
                    // Otherwise insert the headers for content retrieval
                    inserts := d.queue.Schedule(chunk, origin)
                    if len(inserts) != len(chunk) {
                        log.Debug("Stale headers")
                        return errBadPeer
                    }
                }
                headers = headers[limit:]
                origin += uint64(limit)
            }
            // Signal the content downloaders of the availablility of new tasks
            for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
                select {
                case ch <- true:
                default:
                }
            }
        }
    }
}

1,收到从fetchHeaders()方法 中d.headerProcCh发送过来的headers
2,如果是FastSync或者LightSync模式,直接调用lightchain.InsertHeaderChain(chunk, frequency)插入到headerChain。
3,如果是FullSync或者FastSyn模式,调用d.queue.Schedule(chunk, origin),放入downloader.queue来调度

2,processFastSyncContent() 方法

func (d *Downloader) processFastSyncContent(latest *types.Header) error {
    // Start syncing state of the reported head block. This should get us most of
    // the state of the pivot block.
    stateSync := d.syncState(latest.Root)
    defer stateSync.Cancel()
    go func() {
        if err := stateSync.Wait(); err != nil && err != errCancelStateFetch {
            d.queue.Close() // wake up WaitResults
        }
    }()
    // Figure out the ideal pivot block. Note, that this goalpost may move if the
    // sync takes long enough for the chain head to move significantly.
    pivot := uint64(0)
    if height := latest.Number.Uint64(); height > uint64(fsMinFullBlocks) {
        pivot = height - uint64(fsMinFullBlocks)
    }
    // To cater for moving pivot points, track the pivot block and subsequently
    // accumulated download results separatey.
    var (
        oldPivot *fetchResult   // Locked in pivot block, might change eventually
        oldTail  []*fetchResult // Downloaded content after the pivot
    )
    for {
        // Wait for the next batch of downloaded data to be available, and if the pivot
        // block became stale, move the goalpost
        results := d.queue.Results(oldPivot == nil) // Block if we're not monitoring pivot staleness
        if len(results) == 0 {
            // If pivot sync is done, stop
            if oldPivot == nil {
                return stateSync.Cancel()
            }
            // If sync failed, stop
            select {
            case <-d.cancelCh:
                return stateSync.Cancel()
            default:
            }
        }
        if d.chainInsertHook != nil {
            d.chainInsertHook(results)
        }
        if oldPivot != nil {
            results = append(append([]*fetchResult{oldPivot}, oldTail...), results...)
        }
        // Split around the pivot block and process the two sides via fast/full sync
        if atomic.LoadInt32(&d.committed) == 0 {
            latest = results[len(results)-1].Header
            if height := latest.Number.Uint64(); height > pivot+2*uint64(fsMinFullBlocks) {
                log.Warn("Pivot became stale, moving", "old", pivot, "new", height-uint64(fsMinFullBlocks))
                pivot = height - uint64(fsMinFullBlocks)
            }
        }
        P, beforeP, afterP := splitAroundPivot(pivot, results)
        if err := d.commitFastSyncData(beforeP, stateSync); err != nil {
            return err
        }
        if P != nil {
            // If new pivot block found, cancel old state retrieval and restart
            if oldPivot != P {
                stateSync.Cancel()

                stateSync = d.syncState(P.Header.Root)
                defer stateSync.Cancel()
                go func() {
                    if err := stateSync.Wait(); err != nil && err != errCancelStateFetch {
                        d.queue.Close() // wake up WaitResults
                    }
                }()
                oldPivot = P
            }
            // Wait for completion, occasionally checking for pivot staleness
            select {
            case <-stateSync.done:
                if stateSync.err != nil {
                    return stateSync.err
                }
                if err := d.commitPivotBlock(P); err != nil {
                    return err
                }
                oldPivot = nil

            case <-time.After(time.Second):
                oldTail = afterP
                continue
            }
        }
        // Fast sync done, pivot commit done, full import
        if err := d.importBlockResults(afterP); err != nil {
            return err
        }
    }
}

1,同步最新的状态信息,的到最新的pivot值
2,不停的从d.queue 的result缓存中获取要处理的result数据
3,如果results数据为空,同时pivot也为空的时候,说明同步完成了,并返回
4,根据pivot值和results计算:pivot值对应的result,和pivot值之前的results和pivot值之后的results
5,调用commitFastSyncData把pivot值之前的results 插入本地区块链中,带上收据和交易数据
6,更新同步状态信息后,把pivot值对应的result 调用commitPivotBlock插入本地区块链中,并调用FastSyncCommitHead,记录这个pivot的hash值
7,调用d.importBlockResults把pivot值之后的results插入本地区块链中,这时候不插入区块交易收据数据。

3,processFullSyncContent()方法

func (d *Downloader) processFullSyncContent() error {
    for {
        results := d.queue.Results(true)
        if len(results) == 0 {
            return nil
        }
        if d.chainInsertHook != nil {
            d.chainInsertHook(results)
        }
        if err := d.importBlockResults(results); err != nil {
            return err
        }
    }
}

func (d *Downloader) importBlockResults(results []*fetchResult) error {
    // Check for any early termination requests
    if len(results) == 0 {
        return nil
    }
    select {
    case <-d.quitCh:
        return errCancelContentProcessing
    default:
    }
    // Retrieve the a batch of results to import
    first, last := results[0].Header, results[len(results)-1].Header
    log.Debug("Inserting downloaded chain", "items", len(results),
        "firstnum", first.Number, "firsthash", first.Hash(),
        "lastnum", last.Number, "lasthash", last.Hash(),
    )
    blocks := make([]*types.Block, len(results))
    for i, result := range results {
        blocks[i] = types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles)
    }
    if index, err := d.blockchain.InsertChain(blocks); err != nil {
        log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
        return errInvalidChain
    }
    return nil
}

processFullSyncContent方法比较简单:直接获取缓存的results数据,并插入到本地区块链中。

总结:
Downloader看似非常复杂,其实逻辑还好,如果没有light模式,读起来会好很多。其实light模式不太成熟,基本也没什么用。fast模式比full模式逻辑上面多了一个pivot,处理起来就复杂很多。但是fast模式在本地存储了收据数据,大大减少了区块交易验证的时间。如果要更清楚明白fast模式的原理,可以看看以太坊白皮书关于fast模式同步这一部分:https://github.com/ethereum/go-ethereum/pull/1889

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