以太坊节点广播block的时候一部分节点广播整个block内容，其余节点广播block的hash，
本篇分析一下节点收到block hash后的处理
收到NewBlockHash
eth/handler.go中收到NewBlockHashesMsg消息，看代码的处理：
[plain] view plain copy
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
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)
}
收到消息后：
1 遍历收到的block hashes，mark下对端节点拥有此block
2 再遍历收到的hashes，与本地对比看是否已有该block，没有的append到unknown
3 遍历unknown，然后调用fetcher的Notify，带hash，block num，request header函数和request body函数
[plain] view plain copy
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
}
}
Notify做的事情也很简单，new了一个announce结构，然后写到channel notify，实现了notify的作用，想必是有个地方一直在监听notify channel做事情，找到监听notify的地方
Fetcher获取header和body
（代码比较长，暂时先贴着了，要么分段帖也不好贴）
[plain] view plain copy
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
for hash, announce := range f.fetching {
if time.Since(announce.time) > fetchTimeout {
f.forgetHash(hash)
}
}
// Import any queued blocks that could potentially fit
height := f.chainHeight()
for !f.queue.Empty() {
op := f.queue.PopItem().(*inject)
if f.queueChangeHook != nil {
f.queueChangeHook(op.block.Hash(), false)
}
// If too high up the chain or phase, continue later
number := op.block.NumberU64()
if number > height+1 {
f.queue.Push(op, -float32(op.block.NumberU64()))
if f.queueChangeHook != nil {
f.queueChangeHook(op.block.Hash(), true)
}
break
}
// Otherwise if fresh and still unknown, try and import
hash := op.block.Hash()
if number+maxUncleDist < height || f.getBlock(hash) != nil {
f.forgetBlock(hash)
continue
}
f.insert(op.origin, op.block)
}
// Wait for an outside event to occur
select {
case <-f.quit:
// Fetcher terminating, abort all operations
return
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
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
if _, ok := f.fetching[notification.hash]; ok {
break
}
if _, ok := f.completing[notification.hash]; ok {
break
}
f.announces[notification.origin] = count
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 {
f.rescheduleFetch(fetchTimer)
}
case op := <-f.inject:
// A direct block insertion was requested, try and fill any pending gaps
propBroadcastInMeter.Mark(1)
f.enqueue(op.origin, op.block)
case hash := <-f.done:
// A pending import finished, remove all traces of the notification
f.forgetHash(hash)
f.forgetBlock(hash)
case <-fetchTimer.C:
// At least one block's timer ran out, check for needing retrieval
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
announce := announces[rand.Intn(len(announces))]
f.forgetHash(hash)
// If the block still didn't arrive, queue for fetching
if f.getBlock(hash) == nil {
request[announce.origin] = append(request[announce.origin], hash)
f.fetching[hash] = announce
}
}
}
// Send out all block header requests
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)
case <-completeTimer.C:
// At least one header's timer ran out, retrieve everything
request := make(map[string][]common.Hash)
for hash, announces := range f.fetched {
// Pick a random peer to retrieve from, reset all others
announce := announces[rand.Intn(len(announces))]
f.forgetHash(hash)
// If the block still didn't arrive, queue for completion
if f.getBlock(hash) == nil {
request[announce.origin] = append(request[announce.origin], hash)
f.completing[hash] = announce
}
}
// Send out all block body requests
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)
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.
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, incomplete, complete := []*types.Header{}, []*announce{}, []*types.Block{}
for _, header := range task.headers {
hash := header.Hash()
// Filter fetcher-requested headers from other synchronisation algorithms
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
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
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
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
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
unknown = append(unknown, header)
}
}
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
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
for _, block := range complete {
if announce := f.completing[block.Hash()]; announce != nil {
f.enqueue(announce.origin, block)
}
}
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{}
for i := 0; i < len(task.transactions) && i < len(task.uncles); i++ {
// Match up a body to any possible completion request
matched := false
for hash, announce := range f.completing {
if f.queued[hash] == nil {
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
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)
}
}
}
}
if matched {
task.transactions = append(task.transactions[:i], task.transactions[i+1:]...)
task.uncles = append(task.uncles[:i], task.uncles[i+1:]...)
i--
continue
}
}
bodyFilterOutMeter.Mark(int64(len(task.transactions)))
select {
case filter <- task:
case <-f.quit:
return
}
// Schedule the retrieved blocks for ordered import
for _, block := range blocks {
if announce := f.completing[block.Hash()]; announce != nil {
f.enqueue(announce.origin, block)
}
}
}
}
}
加入hash到announced
果然这里有个loop（），内部是一个for循环，在监听读取一些channel的数据
1 判断f.announces map内从对端peer发来的hash个数是否大于hashLimit（256），如果大于，那么对端peer可能在Dos攻击，break出去
2 判断block的num与自身chain高度的差值，如果小于-maxUncleDist（7）或者大于maxQueueDist（32），也break出去
3 判断是否在f.fetching，如果是break
4 判断是否在f.completing，如果是break
5 加入到f.announced, f.announces加1（此个数用来判断节点是否在Dos攻击）
6 一旦f.announced有元素了，就开始rescheduleFetch(fetchTimer)，内容是reset下fetchTimer，遍历announced中的条目，拿到最早的时间，然后reset fetchTimer为500ms减去最早的条目加入到announced后耗费的时间，目的是让fetchTimer尽快执行而又不浪费资源空转
获取headers
所以接下来看f.announced的处理在哪里
还在本函数中，case <- fetchTimer.C
这个timer一直在跑，每次跑完会reset一下，刚刚上面有讲过它的原理
1 遍历f.announced中的announce，对于同一个block的hash，可能有若干个节点广播过来
if time.Since(announces[0].time) > arriveTimeOut(500ms) - gatherSlack(100ms)
这里判断最早加入的hash的时间要大于400ms（500-100）才去处理，这个逻辑应该是为了等够一定的时间积攒一些节点广播同一个block然后随机挑选一个节点去同步，避免所有节点都向源节点同步造成源节点网络拥堵
把这些要请求的加入到request map内
同时把此hash从f.announced中移除并加入到f.fetching map内
2 遍历request map，启动goroutine去获取指定hash的block header，有几个header就几个goroutine
然后等待对端节点返回header
返回headers
还在本loop（）函数内，返回header是写入到channel f.headerFilter
1 遍历处理收到的headers，从f.fetching内拿到anounce（上一步放入到该map内），并且返回的peer也是获取的peer，不在fetched内，不在completing内，不在queued内才通过判断；否则加入unkonwn map内
2 如果不是要获取的高度的block header，continue
3 如果block是空的：既没有tx也没有uncle，那么直接组成block然后加入到complete map内并且加入f.completing，否则加入到incomplete map内
4 把unkonwn放入到filter处理一下（具体怎么处理再看一下）
5 遍历incomplete map，然后放入到f.fetched map内，表示拿到了header还需要继续拿body。同时rescheduleComplete（），同上面的rescheduleFetch，尽快取block，时间不大于100ms
6 遍历complete map，调用f.enqueue()，目的就是把完成的block加入到f.queued内，处理等后面拿到body一起说下
获取bodies
上面拿到header后把hash加入到f.fetched map内，接着拿body
还是在本loop（）内，等completeTimer到达：case <-completeTimer.C
1 遍历f.fetched，随机选一个去获取body
2 把hash加入到f.completing map内和request内
3 遍历request，启动gorountine去对端节点获取body内容
返回bodies
对端节点处理完返回对应的body后， 还是写入到本loop（）内的channel f.bodyFilter:
1 遍历收到的tx和uncle
2 如果hash不在f.queued（没有处理完成），并且判断收到的tx hash、unclue hash、peer与completing中的一致就加入到blocks map内
3遍历blocks，调用f.enqueue()加入到f.queue
处理完成
接着处理f.queue map，还是在本loop（）内循环处理的
如果f.queue不为空pop一个出来：f.queue.PopItem()，经过简单的check，f.insert（）加入到本地
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
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
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)
}
}()
}
启动gorountine插入到本地：
1 先VerifyHeader，通过后广播block到若干节点
2 f.insertChain()就是写入到本地的leveldb
3 插入成功后，广播block hash到其他节点
有人要问了，为什么广播hash放在最后呢，要做到尽快广播到全网是不是放在1也可以：
1是广播整个区块，3是广播hash，只有等2插入到本地后，3广播出去后，别的节点来获取自己才有东西给别人，这也是3放在2后面的原因
不过其实也是可以放在第1步，只是这么做的话需要做另外的处理，不利于代码的统一性
总结
自此就分析完了节点收到blockhash后的处理，大致如下：
1 交给fetcher处理，先去拿header
2 等拿到header后，如果body为空，直接组织成block；否则接着拿body
3 拿到body后跟header组织成block，然后插入到本地的leveldb
中间有一些小细节：
1 获取header或者body的时候会等一段时间（400ms、100ms）等收到若干个节点广播的hash，
然后随机选一个去获取，这也是网络负载均衡的设计；代价就是要等一段时间才能同步完成一个区块
2 防Dos攻击：会记录同一个节点同时有多少hash在处理，如果大于给定的256就认为是节点在Dos攻击
Fetcher和downloader的区别
看完了fetcher和downloader的代码就知道了二者的区别：
1 downloader是用来当本地与其他节点相差太多的时候同步到最新的区块
2 fetcher是收到其他节点广播的新区块hashes后去获取这些给定的区块的
随时随地看视频
