兄弟連區塊鏈入門教程eth源碼分析p2p-udp.go源碼分析（二）

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ping方法與pending的處理，之前談到了pending是等待一個reply。 這裡通過代碼來分析是如何?等待reply的。
pending方法把pending結構體發送給addpending. 然後等待訊息的處理和接收。

// ping sends a ping message to the given node and waits for a reply.func (t *udp) ping(toid NodeID, toaddr *net.UDPAddr) error {    // TODO: maybe check for ReplyTo field in callback to measure RTT    errc := t.pending(toid, pongPacket, func(interface{}) bool { return true })    t.send(toaddr, pingPacket, &ping{        Version: Version,        From: t.ourEndpoint,        To: makeEndpoint(toaddr, 0), // TODO: maybe use known TCP port from DB        Expiration: uint64(time.Now().Add(expiration).Unix()),    })    return <-errc}// pending adds a reply callback to the pending reply queue.// see the documentation of type pending for a detailed explanation.func (t *udp) pending(id NodeID, ptype byte, callback func(interface{}) bool) <-chan error {    ch := make(chan error, 1)    p := &pending{from: id, ptype: ptype, callback: callback, errc: ch}    select {    case t.addpending <- p:        // loop will handle it    case <-t.closing:        ch <- errClosed    }    return ch}

addpending訊息的處理。 之前建立udp的時候調用了newUDP方法。裡面啟動了兩個goroutine。 其中的loop()就是用來處理pending訊息的。

// loop runs in its own goroutine. it keeps track of// the refresh timer and the pending reply queue.func (t *udp) loop() {    var (        plist = list.New()        timeout = time.NewTimer(0)        nextTimeout *pending // head of plist when timeout was last reset        contTimeouts = 0 // number of continuous timeouts to do NTP checks        ntpWarnTime = time.Unix(0, 0)    )    <-timeout.C // ignore first timeout    defer timeout.Stop()    resetTimeout := func() {        //這個方法的主要功能是查看隊列裡面是否有需要逾時的pending訊息。 如果有。那麼        //根據最先逾時的時間設定逾時醒來。        if plist.Front() == nil || nextTimeout == plist.Front().Value {            return        }        // Start the timer so it fires when the next pending reply has expired.        now := time.Now()        for el := plist.Front(); el != nil; el = el.Next() {            nextTimeout = el.Value.(*pending)            if dist := nextTimeout.deadline.Sub(now); dist < 2*respTimeout {                timeout.Reset(dist)                return            }            // Remove pending replies whose deadline is too far in the            // future. These can occur if the system clock jumped            // backwards after the deadline was assigned.            //如果有訊息的deadline在很遠的未來，那麼直接設定逾時，然後移除。            //這種情況在修改系統時間的時候有可能發生，如果不處理可能導致堵塞太長時間。            nextTimeout.errc <- errClockWarp            plist.Remove(el)        }        nextTimeout = nil        timeout.Stop()    }    for {        resetTimeout() //首先處理逾時。        select {        case <-t.closing: //收到關閉資訊。 逾時所有的堵塞的隊列            for el := plist.Front(); el != nil; el = el.Next() {                el.Value.(*pending).errc <- errClosed            }            return        case p := <-t.addpending: //增加一個pending 設定deadline            p.deadline = time.Now().Add(respTimeout)            plist.PushBack(p)        case r := <-t.gotreply: //收到一個reply 尋找匹配的pending            var matched bool            for el := plist.Front(); el != nil; el = el.Next() {                p := el.Value.(*pending)                if p.from == r.from && p.ptype == r.ptype { //如果來自同一個人。 而且類型相同                    matched = true                    // Remove the matcher if its callback indicates                    // that all replies have been received. This is                    // required for packet types that expect multiple                    // reply packets.                    if p.callback(r.data) { //如果callback傳回值是true 。說明pending已經完成。 給p.errc寫入nil。 pending完成。                        p.errc <- nil                        plist.Remove(el)                    }                    // Reset the continuous timeout counter (time drift detection)                    contTimeouts = 0                }            }            r.matched <- matched //寫入reply的matched        case now := <-timeout.C: //處理逾時資訊            nextTimeout = nil            // Notify and remove callbacks whose deadline is in the past.            for el := plist.Front(); el != nil; el = el.Next() {                p := el.Value.(*pending)                if now.After(p.deadline) || now.Equal(p.deadline) { //如果逾時寫入逾時資訊並移除                    p.errc <- errTimeout                    plist.Remove(el)                    contTimeouts++                }            }            // If we‘ve accumulated too many timeouts, do an NTP time sync check            if contTimeouts > ntpFailureThreshold {                //如果連續逾時很多次。 那麼查看是否是時間不同步。 和NTP伺服器進行同步。                if time.Since(ntpWarnTime) >= ntpWarningCooldown {                    ntpWarnTime = time.Now()                    go checkClockDrift()                }                contTimeouts = 0            }        }    }}

上面看到了pending的處理。 不過loop()方法種還有一個gotreply的處理。 這個實在readLoop()這個goroutine中產生的。

// readLoop runs in its own goroutine. it handles incoming UDP packets.func (t *udp) readLoop() {    defer t.conn.Close()    // Discovery packets are defined to be no larger than 1280 bytes.    // Packets larger than this size will be cut at the end and treated    // as invalid because their hash won‘t match.    buf := make([]byte, 1280)    for {        nbytes, from, err := t.conn.ReadFromUDP(buf)        if netutil.IsTemporaryError(err) {            // Ignore temporary read errors.            log.Debug("Temporary UDP read error", "err", err)            continue        } else if err != nil {            // Shut down the loop for permament errors.            log.Debug("UDP read error", "err", err)            return        }        t.handlePacket(from, buf[:nbytes])    }}func (t *udp) handlePacket(from *net.UDPAddr, buf []byte) error {    packet, fromID, hash, err := decodePacket(buf)    if err != nil {        log.Debug("Bad discv4 packet", "addr", from, "err", err)        return err    }    err = packet.handle(t, from, fromID, hash)    log.Trace("<< "+packet.name(), "addr", from, "err", err)    return err}func (req *ping) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {    if expired(req.Expiration) {        return errExpired    }    t.send(from, pongPacket, &pong{        To: makeEndpoint(from, req.From.TCP),        ReplyTok: mac,        Expiration: uint64(time.Now().Add(expiration).Unix()),    })    if !t.handleReply(fromID, pingPacket, req) {        // Note: we‘re ignoring the provided IP address right now        go t.bond(true, fromID, from, req.From.TCP)    }    return nil}func (t *udp) handleReply(from NodeID, ptype byte, req packet) bool {    matched := make(chan bool, 1)    select {    case t.gotreply <- reply{from, ptype, req, matched}:        // loop will handle it        return <-matched    case <-t.closing:        return false    }}

上面介紹了udp的大致處理的流程。 下面介紹下udp的主要處理的業務。 udp主要發送兩種請求，對應的也會接收別人發送的這兩種請求， 對應這兩種請求又會產生兩種回應。

ping請求，可以看到ping請求希望得到一個pong回答。 然後返回。

// ping sends a ping message to the given node and waits for a reply.func (t *udp) ping(toid NodeID, toaddr *net.UDPAddr) error {    // TODO: maybe check for ReplyTo field in callback to measure RTT    errc := t.pending(toid, pongPacket, func(interface{}) bool { return true })    t.send(toaddr, pingPacket, &ping{        Version: Version,        From: t.ourEndpoint,        To: makeEndpoint(toaddr, 0), // TODO: maybe use known TCP port from DB        Expiration: uint64(time.Now().Add(expiration).Unix()),    })    return <-errc}

pong回答,如果pong回答沒有匹配到一個對應的ping請求。那麼返回errUnsolicitedReply異常。

func (req *pong) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {    if expired(req.Expiration) {        return errExpired    }    if !t.handleReply(fromID, pongPacket, req) {        return errUnsolicitedReply    }    return nil}

findnode請求, 發送findnode請求，然後等待node回應 k個鄰居。

// findnode sends a findnode request to the given node and waits until// the node has sent up to k neighbors.func (t *udp) findnode(toid NodeID, toaddr *net.UDPAddr, target NodeID) ([]*Node, error) {    nodes := make([]*Node, 0, bucketSize)    nreceived := 0    errc := t.pending(toid, neighborsPacket, func(r interface{}) bool {        reply := r.(*neighbors)        for _, rn := range reply.Nodes {            nreceived++            n, err := t.nodeFromRPC(toaddr, rn)            if err != nil {                log.Trace("Invalid neighbor node received", "ip", rn.IP, "addr", toaddr, "err", err)                continue            }            nodes = append(nodes, n)        }        return nreceived >= bucketSize    })    t.send(toaddr, findnodePacket, &findnode{        Target: target,        Expiration: uint64(time.Now().Add(expiration).Unix()),    })    err := <-errc    return nodes, err}

neighbors回應, 很簡單。 把回應發送給gotreply隊列。 如果沒有找到匹配的findnode請求。返回errUnsolicitedReply錯誤

func (req *neighbors) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {    if expired(req.Expiration) {        return errExpired    }    if !t.handleReply(fromID, neighborsPacket, req) {        return errUnsolicitedReply    }    return nil}

收到別的節點發送的ping請求，發送pong回答。 如果沒有匹配上一個pending(說明不是自己方請求的結果)。 就調用bond方法把這個節點加入自己的bucket緩衝。(這部分原理在table.go裡面會詳細介紹)

func (req *ping) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {    if expired(req.Expiration) {        return errExpired    }    t.send(from, pongPacket, &pong{        To: makeEndpoint(from, req.From.TCP),        ReplyTok: mac,        Expiration: uint64(time.Now().Add(expiration).Unix()),    })    if !t.handleReply(fromID, pingPacket, req) {        // Note: we‘re ignoring the provided IP address right now        go t.bond(true, fromID, from, req.From.TCP)    }    return nil}

收到別人發送的findnode請求。這個請求希望把和target距離相近的k個節點發送回去。 演算法的詳細請參考references目錄下面的pdf文檔。

func (req *findnode) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {    if expired(req.Expiration) {        return errExpired    }    if t.db.node(fromID) == nil {        // No bond exists, we don‘t process the packet. This prevents        // an attack vector where the discovery protocol could be used        // to amplify traffic in a DDOS attack. A malicious actor        // would send a findnode request with the IP address and UDP        // port of the target as the source address. The recipient of        // the findnode packet would then send a neighbors packet        // (which is a much bigger packet than findnode) to the victim.        return errUnknownNode    }    target := crypto.Keccak256Hash(req.Target[:])    t.mutex.Lock()    //擷取bucketSize個和target距離相近的節點。 這個方法在table.go內部實現。後續會詳細介紹    closest := t.closest(target, bucketSize).entries    t.mutex.Unlock()    p := neighbors{Expiration: uint64(time.Now().Add(expiration).Unix())}    // Send neighbors in chunks with at most maxNeighbors per packet    // to stay below the 1280 byte limit.    for i, n := range closest {        if netutil.CheckRelayIP(from.IP, n.IP) != nil {            continue        }        p.Nodes = append(p.Nodes, nodeToRPC(n))        if len(p.Nodes) == maxNeighbors || i == len(closest)-1 {            t.send(from, neighborsPacket, &p)            p.Nodes = p.Nodes[:0]        }    }    return nil}

udp資訊加密和安全問題

discover協議因為沒有承載什麼敏感性資料，所以資料是以明文傳輸，但是為了確保資料的完整性和不被篡改，所以在資料包的包頭加上了數位簽章。

func encodePacket(priv *ecdsa.PrivateKey, ptype byte, req interface{}) ([]byte, error) {    b := new(bytes.Buffer)    b.Write(headSpace)    b.WriteByte(ptype)    if err := rlp.Encode(b, req); err != nil {        log.Error("Can‘t encode discv4 packet", "err", err)        return nil, err    }    packet := b.Bytes()    sig, err := crypto.Sign(crypto.Keccak256(packet[headSize:]), priv)    if err != nil {        log.Error("Can‘t sign discv4 packet", "err", err)        return nil, err    }    copy(packet[macSize:], sig)    // add the hash to the front. Note: this doesn‘t protect the    // packet in any way. Our public key will be part of this hash in    // The future.    copy(packet, crypto.Keccak256(packet[macSize:]))    return packet, nil}func decodePacket(buf []byte) (packet, NodeID, []byte, error) {    if len(buf) < headSize+1 {        return nil, NodeID{}, nil, errPacketTooSmall    }    hash, sig, sigdata := buf[:macSize], buf[macSize:headSize], buf[headSize:]    shouldhash := crypto.Keccak256(buf[macSize:])    if !bytes.Equal(hash, shouldhash) {        return nil, NodeID{}, nil, errBadHash    }    fromID, err := recoverNodeID(crypto.Keccak256(buf[headSize:]), sig)    if err != nil {        return nil, NodeID{}, hash, err    }    var req packet    switch ptype := sigdata[0]; ptype {    case pingPacket:        req = new(ping)    case pongPacket:        req = new(pong)    case findnodePacket:        req = new(findnode)    case neighborsPacket:        req = new(neighbors)    default:        return nil, fromID, hash, fmt.Errorf("unknown type: %d", ptype)    }    s := rlp.NewStream(bytes.NewReader(sigdata[1:]), 0)    err = s.Decode(req)    return req, fromID, hash, err}

兄弟連區塊鏈入門教程eth源碼分析p2p-udp.go源碼分析（二）