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() , 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:
				}
			}
		}
	}
}
主要做了这几件事:
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,带上交易和收据数据
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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