peer模組包含了downloader使用的peer節點，封裝了吞吐量，是否空閒，並記錄了之前失敗的資訊。

peer

// peerConnection represents an active peer from which hashes and blocks are retrieved.
type peerConnection struct {
    id string // Unique identifier of the peer

    headerIdle  int32 // Current header activity state of the peer (idle = 0, active = 1) 當前的header獲取的工作狀態。
    blockIdle   int32 // Current block activity state of the peer (idle = 0, active = 1)    當前的區塊獲取的工作狀態
    receiptIdle int32 // Current receipt activity state of the peer (idle = 0, active = 1) 當前的收據獲取的工作狀態
    stateIdle   int32 // Current node data activity state of the peer (idle = 0, active = 1) 當前節點狀態的工作狀態

    headerThroughput  float64 // Number of headers measured to be retrievable per second    //記錄每秒能夠接收多少個區塊頭的度量值
    blockThroughput   float64 // Number of blocks (bodies) measured to be retrievable per second  //記錄每秒能夠接收多少個區塊的度量值
    receiptThroughput float64 // Number of receipts measured to be retrievable per second 記錄每秒能夠接收多少個收據的度量值
    stateThroughput   float64 // Number of node data pieces measured to be retrievable per second  記錄每秒能夠接收多少個賬戶狀態的度量值

    rtt time.Duration // Request round trip time to track responsiveness (QoS)  請求迴應時間

    headerStarted  time.Time // Time instance when the last header fetch was started    記錄最後一個header fetch的請求時間
    blockStarted   time.Time // Time instance when the last block (body) fetch was started
    receiptStarted time.Time // Time instance when the last receipt fetch was started
    stateStarted   time.Time // Time instance when the last node data fetch was started
    
    lacking map[common.Hash]struct{} // Set of hashes not to request (didn't have previously)  記錄的Hash值不會去請求，一般是因為之前的請求失敗

    peer Peer           // eth的peer

    version int        // Eth protocol version number to switch strategies
    log     log.Logger // Contextual logger to add extra infos to peer logs
    lock    sync.RWMutex
}
 

FetchXXX
FetchHeaders FetchBodies等函式 主要呼叫了eth.peer的功能來進行傳送資料請求。

// FetchHeaders sends a header retrieval request to the remote peer.
func (p *peerConnection) FetchHeaders(from uint64, count int) error {
    // Sanity check the protocol version
    if p.version < 62 {
        panic(fmt.Sprintf("header fetch [eth/62+] requested on eth/%d", p.version))
    }
    // Short circuit if the peer is already fetching
    if !atomic.CompareAndSwapInt32(&p.headerIdle, 0, 1) {
        return errAlreadyFetching
    }
    p.headerStarted = time.Now()

    // Issue the header retrieval request (absolut upwards without gaps)
    go p.peer.RequestHeadersByNumber(from, count, 0, false)

    return nil
}
 

SetXXXIdle函式
SetHeadersIdle, SetBlocksIdle 等函式 設定peer的狀態為空閒狀態，允許它執行新的請求。 同時還會通過本次傳輸的資料的多少來重新評估鏈路的吞吐量。

// SetHeadersIdle sets the peer to idle, allowing it to execute new header retrieval
// requests. Its estimated header retrieval throughput is updated with that measured
// just now.
func (p *peerConnection) SetHeadersIdle(delivered int) {
    p.setIdle(p.headerStarted, delivered, &p.headerThroughput, &p.headerIdle)
}
 

setIdle

// setIdle sets the peer to idle, allowing it to execute new retrieval requests.
// Its estimated retrieval throughput is updated with that measured just now.
func (p *peerConnection) setIdle(started time.Time, delivered int, throughput *float64, idle *int32) {
    // Irrelevant of the scaling, make sure the peer ends up idle
    defer atomic.StoreInt32(idle, 0)

    p.lock.Lock()
    defer p.lock.Unlock()

    // If nothing was delivered (hard timeout / unavailable data), reduce throughput to minimum
    if delivered == 0 {
        *throughput = 0
        return
    }
    // Otherwise update the throughput with a new measurement
    elapsed := time.Since(started) + 1 // +1 (ns) to ensure non-zero divisor
    measured := float64(delivered) / (float64(elapsed) / float64(time.Second))
    
    // measurementImpact = 0.1 , 新的吞吐量=老的吞吐量*0.9 + 這次的吞吐量*0.1
    *throughput = (1-measurementImpact)*(*throughput) + measurementImpact*measured  
    // 更新RTT
    p.rtt = time.Duration((1-measurementImpact)*float64(p.rtt) + measurementImpact*float64(elapsed))

    p.log.Trace("Peer throughput measurements updated",
        "hps", p.headerThroughput, "bps", p.blockThroughput,
        "rps", p.receiptThroughput, "sps", p.stateThroughput,
        "miss", len(p.lacking), "rtt", p.rtt)
}

XXXCapacity函式，用來返回當前的連結允許的吞吐量。

// HeaderCapacity retrieves the peers header download allowance based on its
// previously discovered throughput.
func (p *peerConnection) HeaderCapacity(targetRTT time.Duration) int {
    p.lock.RLock()
    defer p.lock.RUnlock()
    // 這裡有點奇怪，targetRTT越大，請求的數量就越多。
    return int(math.Min(1+math.Max(1, p.headerThroughput*float64(targetRTT)/float64(time.Second)), float64(MaxHeaderFetch)))
}

Lacks 用來標記上次是否失敗，以便下次同樣的請求不通過這個peer

    // MarkLacking appends a new entity to the set of items (blocks, receipts, states)
    // that a peer is known not to have (i.e. have been requested before). If the
    // set reaches its maximum allowed capacity, items are randomly dropped off.
    func (p *peerConnection) MarkLacking(hash common.Hash) {
        p.lock.Lock()
        defer p.lock.Unlock()
    
        for len(p.lacking) >= maxLackingHashes {
            for drop := range p.lacking {
                delete(p.lacking, drop)
                break
            }
        }
        p.lacking[hash] = struct{}{}
    }
    
    // Lacks retrieves whether the hash of a blockchain item is on the peers lacking
    // list (i.e. whether we know that the peer does not have it).
    func (p *peerConnection) Lacks(hash common.Hash) bool {
        p.lock.RLock()
        defer p.lock.RUnlock()
    
        _, ok := p.lacking[hash]
        return ok
    }

peerSet

// peerSet represents the collection of active peer participating in the chain
// download procedure.
type peerSet struct {
    peers        map[string]*peerConnection
    newPeerFeed  event.Feed
    peerDropFeed event.Feed
    lock         sync.RWMutex
}

Register 和 UnRegister

// Register injects a new peer into the working set, or returns an error if the
// peer is already known.
//
// The method also sets the starting throughput values of the new peer to the
// average of all existing peers, to give it a realistic chance of being used
// for data retrievals.
func (ps *peerSet) Register(p *peerConnection) error {
    // Retrieve the current median RTT as a sane default
    p.rtt = ps.medianRTT()

    // Register the new peer with some meaningful defaults
    ps.lock.Lock()
    if _, ok := ps.peers[p.id]; ok {
        ps.lock.Unlock()
        return errAlreadyRegistered
    }
    if len(ps.peers) > 0 {
        p.headerThroughput, p.blockThroughput, p.receiptThroughput, p.stateThroughput = 0, 0, 0, 0

        for _, peer := range ps.peers {
            peer.lock.RLock()
            p.headerThroughput += peer.headerThroughput
            p.blockThroughput += peer.blockThroughput
            p.receiptThroughput += peer.receiptThroughput
            p.stateThroughput += peer.stateThroughput
            peer.lock.RUnlock()
        }
        p.headerThroughput /= float64(len(ps.peers))
        p.blockThroughput /= float64(len(ps.peers))
        p.receiptThroughput /= float64(len(ps.peers))
        p.stateThroughput /= float64(len(ps.peers))
    }
    ps.peers[p.id] = p
    ps.lock.Unlock()

    ps.newPeerFeed.Send(p)
    return nil
}

// Unregister removes a remote peer from the active set, disabling any further
// actions to/from that particular entity.
func (ps *peerSet) Unregister(id string) error {
    ps.lock.Lock()
    p, ok := ps.peers[id]
    if !ok {
        defer ps.lock.Unlock()
        return errNotRegistered
    }
    delete(ps.peers, id)
    ps.lock.Unlock()

    ps.peerDropFeed.Send(p)
    return nil
}

XXXIdlePeers

// HeaderIdlePeers retrieves a flat list of all the currently header-idle peers
// within the active peer set, ordered by their reputation.
func (ps *peerSet) HeaderIdlePeers() ([]*peerConnection, int) {
    idle := func(p *peerConnection) bool {
        return atomic.LoadInt32(&p.headerIdle) == 0
    }
    throughput := func(p *peerConnection) float64 {
        p.lock.RLock()
        defer p.lock.RUnlock()
        return p.headerThroughput
    }
    return ps.idlePeers(62, 64, idle, throughput)
}

// idlePeers retrieves a flat list of all currently idle peers satisfying the
// protocol version constraints, using the provided function to check idleness.
// The resulting set of peers are sorted by their measure throughput.
func (ps *peerSet) idlePeers(minProtocol, maxProtocol int, idleCheck func(*peerConnection) bool, throughput func(*peerConnection) float64) ([]*peerConnection, int) {
    ps.lock.RLock()
    defer ps.lock.RUnlock()

    idle, total := make([]*peerConnection, 0, len(ps.peers)), 0
    for _, p := range ps.peers { //首先抽取idle的peer
        if p.version >= minProtocol && p.version <= maxProtocol {
            if idleCheck(p) {
                idle = append(idle, p)
            }
            total++
        }
    }
    for i := 0; i < len(idle); i++ { // 氣泡排序， 從吞吐量大到吞吐量小。
        for j := i + 1; j < len(idle); j++ {
            if throughput(idle[i]) < throughput(idle[j]) {
                idle[i], idle[j] = idle[j], idle[i]
            }
        }
    }
    return idle, total
}

medianRTT,求得peerset的RTT的中位數，

// medianRTT returns the median RTT of te peerset, considering only the tuning
// peers if there are more peers available.
func (ps *peerSet) medianRTT() time.Duration {
    // Gather all the currnetly measured round trip times
    ps.lock.RLock()
    defer ps.lock.RUnlock()

    rtts := make([]float64, 0, len(ps.peers))
    for _, p := range ps.peers {
        p.lock.RLock()
        rtts = append(rtts, float64(p.rtt))
        p.lock.RUnlock()
    }
    sort.Float64s(rtts)

    median := rttMaxEstimate
    if qosTuningPeers <= len(rtts) {
        median = time.Duration(rtts[qosTuningPeers/2]) // Median of our tuning peers
    } else if len(rtts) > 0 {
        median = time.Duration(rtts[len(rtts)/2]) // Median of our connected peers (maintain even like this some baseline qos)
    }
    // Restrict the RTT into some QoS defaults, irrelevant of true RTT
    if median < rttMinEstimate {
        median = rttMinEstimate
    }
    if median > rttMaxEstimate {
        median = rttMaxEstimate
    }
    return median
}