path: root/les/fetcher.go

// Copyright 2016 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

// Package les implements the Light Ethereum Subprotocol.
package les

import (
    "math/big"
    "sync"
    "time"

    "github.com/dexon-foundation/dexon/common"
    "github.com/dexon-foundation/dexon/common/mclock"
    "github.com/dexon-foundation/dexon/consensus"
    "github.com/dexon-foundation/dexon/core/rawdb"
    "github.com/dexon-foundation/dexon/core/types"
    "github.com/dexon-foundation/dexon/light"
    "github.com/dexon-foundation/dexon/log"
)

const (
    blockDelayTimeout    = time.Second * 10 // timeout for a peer to announce a head that has already been confirmed by others
    maxNodeCount         = 20               // maximum number of fetcherTreeNode entries remembered for each peer
    serverStateAvailable = 100              // number of recent blocks where state availability is assumed
)

// lightFetcher implements retrieval of newly announced headers. It also provides a peerHasBlock function for the
// ODR system to ensure that we only request data related to a certain block from peers who have already processed
// and announced that block.
type lightFetcher struct {
    pm    *ProtocolManager
    odr   *LesOdr
    chain *light.LightChain

    lock            sync.Mutex // lock protects access to the fetcher's internal state variables except sent requests
    maxConfirmedTd  *big.Int
    peers           map[*peer]*fetcherPeerInfo
    lastUpdateStats *updateStatsEntry
    syncing         bool
    syncDone        chan *peer

    reqMu      sync.RWMutex // reqMu protects access to sent header fetch requests
    requested  map[uint64]fetchRequest
    deliverChn chan fetchResponse
    timeoutChn chan uint64
    requestChn chan bool // true if initiated from outside
}

// fetcherPeerInfo holds fetcher-specific information about each active peer
type fetcherPeerInfo struct {
    root, lastAnnounced *fetcherTreeNode
    nodeCnt             int
    confirmedTd         *big.Int
    bestConfirmed       *fetcherTreeNode
    nodeByHash          map[common.Hash]*fetcherTreeNode
    firstUpdateStats    *updateStatsEntry
}

// fetcherTreeNode is a node of a tree that holds information about blocks recently
// announced and confirmed by a certain peer. Each new announce message from a peer
// adds nodes to the tree, based on the previous announced head and the reorg depth.
// There are three possible states for a tree node:
// - announced: not downloaded (known) yet, but we know its head, number and td
// - intermediate: not known, hash and td are empty, they are filled out when it becomes known
// - known: both announced by this peer and downloaded (from any peer).
// This structure makes it possible to always know which peer has a certain block,
// which is necessary for selecting a suitable peer for ODR requests and also for
// canonizing new heads. It also helps to always download the minimum necessary
// amount of headers with a single request.
type fetcherTreeNode struct {
    hash             common.Hash
    number           uint64
    td               *big.Int
    known, requested bool
    parent           *fetcherTreeNode
    children         []*fetcherTreeNode
}

// fetchRequest represents a header download request
type fetchRequest struct {
    hash    common.Hash
    amount  uint64
    peer    *peer
    sent    mclock.AbsTime
    timeout bool
}

// fetchResponse represents a header download response
type fetchResponse struct {
    reqID   uint64
    headers []*types.Header
    peer    *peer
}

// newLightFetcher creates a new light fetcher
func newLightFetcher(pm *ProtocolManager) *lightFetcher {
    f := &lightFetcher{
        pm:             pm,
        chain:          pm.blockchain.(*light.LightChain),
        odr:            pm.odr,
        peers:          make(map[*peer]*fetcherPeerInfo),
        deliverChn:     make(chan fetchResponse, 100),
        requested:      make(map[uint64]fetchRequest),
        timeoutChn:     make(chan uint64),
        requestChn:     make(chan bool, 100),
        syncDone:       make(chan *peer),
        maxConfirmedTd: big.NewInt(0),
    }
    pm.peers.notify(f)

    f.pm.wg.Add(1)
    go f.syncLoop()
    return f
}

// syncLoop is the main event loop of the light fetcher
func (f *lightFetcher) syncLoop() {
    requesting := false
    defer f.pm.wg.Done()
    for {
        select {
        case <-f.pm.quitSync:
            return
        // when a new announce is received, request loop keeps running until
        // no further requests are necessary or possible
        case newAnnounce := <-f.requestChn:
            f.lock.Lock()
            s := requesting
            requesting = false
            var (
                rq      *distReq
                reqID   uint64
                syncing bool
            )
            if !f.syncing && !(newAnnounce && s) {
                rq, reqID, syncing = f.nextRequest()
            }
            f.lock.Unlock()

            if rq != nil {
                requesting = true
                if _, ok := <-f.pm.reqDist.queue(rq); ok {
                    if syncing {
                        f.lock.Lock()
                        f.syncing = true
                        f.lock.Unlock()
                    } else {
                        go func() {
                            time.Sleep(softRequestTimeout)
                            f.reqMu.Lock()
                            req, ok := f.requested[reqID]
                            if ok {
                                req.timeout = true
                                f.requested[reqID] = req
                            }
                            f.reqMu.Unlock()
                            // keep starting new requests while possible
                            f.requestChn <- false
                        }()
                    }
                } else {
                    f.requestChn <- false
                }
            }
        case reqID := <-f.timeoutChn:
            f.reqMu.Lock()
            req, ok := f.requested[reqID]
            if ok {
                delete(f.requested, reqID)
            }
            f.reqMu.Unlock()
            if ok {
                f.pm.serverPool.adjustResponseTime(req.peer.poolEntry, time.Duration(mclock.Now()-req.sent), true)
                req.peer.Log().Debug("Fetching data timed out hard")
                go f.pm.removePeer(req.peer.id)
            }
        case resp := <-f.deliverChn:
            f.reqMu.Lock()
            req, ok := f.requested[resp.reqID]
            if ok && req.peer != resp.peer {
                ok = false
            }
            if ok {
                delete(f.requested, resp.reqID)
            }
            f.reqMu.Unlock()
            if ok {
                f.pm.serverPool.adjustResponseTime(req.peer.poolEntry, time.Duration(mclock.Now()-req.sent), req.timeout)
            }
            f.lock.Lock()
            if !ok || !(f.syncing || f.processResponse(req, resp)) {
                resp.peer.Log().Debug("Failed processing response")
                go f.pm.removePeer(resp.peer.id)
            }
            f.lock.Unlock()
        case p := <-f.syncDone:
            f.lock.Lock()
            p.Log().Debug("Done synchronising with peer")
            f.checkSyncedHeaders(p)
            f.syncing = false
            f.lock.Unlock()
            f.requestChn <- false
        }
    }
}

// registerPeer adds a new peer to the fetcher's peer set
func (f *lightFetcher) registerPeer(p *peer) {
    p.lock.Lock()
    p.hasBlock = func(hash common.Hash, number uint64, hasState bool) bool {
        return f.peerHasBlock(p, hash, number, hasState)
    }
    p.lock.Unlock()

    f.lock.Lock()
    defer f.lock.Unlock()

    f.peers[p] = &fetcherPeerInfo{nodeByHash: make(map[common.Hash]*fetcherTreeNode)}
}

// unregisterPeer removes a new peer from the fetcher's peer set
func (f *lightFetcher) unregisterPeer(p *peer) {
    p.lock.Lock()
    p.hasBlock = nil
    p.lock.Unlock()

    f.lock.Lock()
    defer f.lock.Unlock()

    // check for potential timed out block delay statistics
    f.checkUpdateStats(p, nil)
    delete(f.peers, p)
}

// announce processes a new announcement message received from a peer, adding new
// nodes to the peer's block tree and removing old nodes if necessary
func (f *lightFetcher) announce(p *peer, head *announceData) {
    f.lock.Lock()
    defer f.lock.Unlock()
    p.Log().Debug("Received new announcement", "number", head.Number, "hash", head.Hash, "reorg", head.ReorgDepth)

    fp := f.peers[p]
    if fp == nil {
        p.Log().Debug("Announcement from unknown peer")
        return
    }

    if fp.lastAnnounced != nil && head.Td.Cmp(fp.lastAnnounced.td) <= 0 {
        // announced tds should be strictly monotonic
        p.Log().Debug("Received non-monotonic td", "current", head.Td, "previous", fp.lastAnnounced.td)
        go f.pm.removePeer(p.id)
        return
    }

    n := fp.lastAnnounced
    for i := uint64(0); i < head.ReorgDepth; i++ {
        if n == nil {
            break
        }
        n = n.parent
    }
    // n is now the reorg common ancestor, add a new branch of nodes
    if n != nil && (head.Number >= n.number+maxNodeCount || head.Number <= n.number) {
        // if announced head block height is lower or same as n or too far from it to add
        // intermediate nodes then discard previous announcement info and trigger a resync
        n = nil
        fp.nodeCnt = 0
        fp.nodeByHash = make(map[common.Hash]*fetcherTreeNode)
    }
    if n != nil {
        // check if the node count is too high to add new nodes, discard oldest ones if necessary
        locked := false
        for uint64(fp.nodeCnt)+head.Number-n.number > maxNodeCount && fp.root != nil {
            if !locked {
                f.chain.LockChain()
                defer f.chain.UnlockChain()
                locked = true
            }
            // if one of root's children is canonical, keep it, delete other branches and root itself
            var newRoot *fetcherTreeNode
            for i, nn := range fp.root.children {
                if rawdb.ReadCanonicalHash(f.pm.chainDb, nn.number) == nn.hash {
                    fp.root.children = append(fp.root.children[:i], fp.root.children[i+1:]...)
                    nn.parent = nil
                    newRoot = nn
                    break
                }
            }
            fp.deleteNode(fp.root)
            if n == fp.root {
                n = newRoot
            }
            fp.root = newRoot
            if newRoot == nil || !f.checkKnownNode(p, newRoot) {
                fp.bestConfirmed = nil
                fp.confirmedTd = nil
            }

            if n == nil {
                break
            }
        }
        if n != nil {
            for n.number < head.Number {
                nn := &fetcherTreeNode{number: n.number + 1, parent: n}
                n.children = append(n.children, nn)
                n = nn
                fp.nodeCnt++
            }
            n.hash = head.Hash
            n.td = head.Td
            fp.nodeByHash[n.hash] = n
        }
    }
    if n == nil {
        // could not find reorg common ancestor or had to delete entire tree, a new root and a resync is needed
        if fp.root != nil {
            fp.deleteNode(fp.root)
        }
        n = &fetcherTreeNode{hash: head.Hash, number: head.Number, td: head.Td}
        fp.root = n
        fp.nodeCnt++
        fp.nodeByHash[n.hash] = n
        fp.bestConfirmed = nil
        fp.confirmedTd = nil
    }

    f.checkKnownNode(p, n)
    p.lock.Lock()
    p.headInfo = head
    fp.lastAnnounced = n
    p.lock.Unlock()
    f.checkUpdateStats(p, nil)
    f.requestChn <- true
}

// peerHasBlock returns true if we can assume the peer knows the given block
// based on its announcements
func (f *lightFetcher) peerHasBlock(p *peer, hash common.Hash, number uint64, hasState bool) bool {
    f.lock.Lock()
    defer f.lock.Unlock()

    fp := f.peers[p]
    if fp == nil || fp.root == nil {
        return false
    }

    if hasState {
        if fp.lastAnnounced == nil || fp.lastAnnounced.number > number+serverStateAvailable {
            return false
        }
    }

    if f.syncing {
        // always return true when syncing
        // false positives are acceptable, a more sophisticated condition can be implemented later
        return true
    }

    if number >= fp.root.number {
        // it is recent enough that if it is known, is should be in the peer's block tree
        return fp.nodeByHash[hash] != nil
    }
    f.chain.LockChain()
    defer f.chain.UnlockChain()
    // if it's older than the peer's block tree root but it's in the same canonical chain
    // as the root, we can still be sure the peer knows it
    //
    // when syncing, just check if it is part of the known chain, there is nothing better we
    // can do since we do not know the most recent block hash yet
    return rawdb.ReadCanonicalHash(f.pm.chainDb, fp.root.number) == fp.root.hash && rawdb.ReadCanonicalHash(f.pm.chainDb, number) == hash
}

// requestAmount calculates the amount of headers to be downloaded starting
// from a certain head backwards
func (f *lightFetcher) requestAmount(p *peer, n *fetcherTreeNode) uint64 {
    amount := uint64(0)
    nn := n
    for nn != nil && !f.checkKnownNode(p, nn) {
        nn = nn.parent
        amount++
    }
    if nn == nil {
        amount = n.number
    }
    return amount
}

// requestedID tells if a certain reqID has been requested by the fetcher
func (f *lightFetcher) requestedID(reqID uint64) bool {
    f.reqMu.RLock()
    _, ok := f.requested[reqID]
    f.reqMu.RUnlock()
    return ok
}

// nextRequest selects the peer and announced head to be requested next, amount
// to be downloaded starting from the head backwards is also returned
func (f *lightFetcher) nextRequest() (*distReq, uint64, bool) {
    var (
        bestHash   common.Hash
        bestAmount uint64
    )
    bestTd := f.maxConfirmedTd
    bestSyncing := false

    for p, fp := range f.peers {
        for hash, n := range fp.nodeByHash {
            if !f.checkKnownNode(p, n) && !n.requested && (bestTd == nil || n.td.Cmp(bestTd) >= 0) {
                amount := f.requestAmount(p, n)
                if bestTd == nil || n.td.Cmp(bestTd) > 0 || amount < bestAmount {
                    bestHash = hash
                    bestAmount = amount
                    bestTd = n.td
                    bestSyncing = fp.bestConfirmed == nil || fp.root == nil || !f.checkKnownNode(p, fp.root)
                }
            }
        }
    }
    if bestTd == f.maxConfirmedTd {
        return nil, 0, false
    }

    var rq *distReq
    reqID := genReqID()
    if bestSyncing {
        rq = &distReq{
            getCost: func(dp distPeer) uint64 {
                return 0
            },
            canSend: func(dp distPeer) bool {
                p := dp.(*peer)
                f.lock.Lock()
                defer f.lock.Unlock()

                fp := f.peers[p]
                return fp != nil && fp.nodeByHash[bestHash] != nil
            },
            request: func(dp distPeer) func() {
                go func() {
                    p := dp.(*peer)
                    p.Log().Debug("Synchronisation started")
                    f.pm.synchronise(p)
                    f.syncDone <- p
                }()
                return nil
            },
        }
    } else {
        rq = &distReq{
            getCost: func(dp distPeer) uint64 {
                p := dp.(*peer)
                return p.GetRequestCost(GetBlockHeadersMsg, int(bestAmount))
            },
            canSend: func(dp distPeer) bool {
                p := dp.(*peer)
                f.lock.Lock()
                defer f.lock.Unlock()

                fp := f.peers[p]
                if fp == nil {
                    return false
                }
                n := fp.nodeByHash[bestHash]
                return n != nil && !n.requested
            },
            request: func(dp distPeer) func() {
                p := dp.(*peer)
                f.lock.Lock()
                fp := f.peers[p]
                if fp != nil {
                    n := fp.nodeByHash[bestHash]
                    if n != nil {
                        n.requested = true
                    }
                }
                f.lock.Unlock()

                cost := p.GetRequestCost(GetBlockHeadersMsg, int(bestAmount))
                p.fcServer.QueueRequest(reqID, cost)
                f.reqMu.Lock()
                f.requested[reqID] = fetchRequest{hash: bestHash, amount: bestAmount, peer: p, sent: mclock.Now()}
                f.reqMu.Unlock()
                go func() {
                    time.Sleep(hardRequestTimeout)
                    f.timeoutChn <- reqID
                }()
                return func() { p.RequestHeadersByHash(reqID, cost, bestHash, int(bestAmount), 0, true) }
            },
        }
    }
    return rq, reqID, bestSyncing
}

// deliverHeaders delivers header download request responses for processing
func (f *lightFetcher) deliverHeaders(peer *peer, reqID uint64, headers []*types.Header) {
    f.deliverChn <- fetchResponse{reqID: reqID, headers: headers, peer: peer}
}

// processResponse processes header download request responses, returns true if successful
func (f *lightFetcher) processResponse(req fetchRequest, resp fetchResponse) bool {
    if uint64(len(resp.headers)) != req.amount || resp.headers[0].Hash() != req.hash {
        req.peer.Log().Debug("Response content mismatch", "requested", len(resp.headers), "reqfrom", resp.headers[0], "delivered", req.amount, "delfrom", req.hash)
        return false
    }
    headers := make([]*types.Header, req.amount)
    for i, header := range resp.headers {
        headers[int(req.amount)-1-i] = header
    }
    if _, err := f.chain.InsertHeaderChain(headers, 1); err != nil {
        if err == consensus.ErrFutureBlock {
            return true
        }
        log.Debug("Failed to insert header chain", "err", err)
        return false
    }
    tds := make([]*big.Int, len(headers))
    for i, header := range headers {
        td := f.chain.GetTd(header.Hash(), header.Number.Uint64())
        if td == nil {
            log.Debug("Total difficulty not found for header", "index", i+1, "number", header.Number, "hash", header.Hash())
            return false
        }
        tds[i] = td
    }
    f.newHeaders(headers, tds)
    return true
}

// newHeaders updates the block trees of all active peers according to a newly
// downloaded and validated batch or headers
func (f *lightFetcher) newHeaders(headers []*types.Header, tds []*big.Int) {
    var maxTd *big.Int
    for p, fp := range f.peers {
        if !f.checkAnnouncedHeaders(fp, headers, tds) {
            p.Log().Debug("Inconsistent announcement")
            go f.pm.removePeer(p.id)
        }
        if fp.confirmedTd != nil && (maxTd == nil || maxTd.Cmp(fp.confirmedTd) > 0) {
            maxTd = fp.confirmedTd
        }
    }
    if maxTd != nil {
        f.updateMaxConfirmedTd(maxTd)
    }
}

// checkAnnouncedHeaders updates peer's block tree if necessary after validating
// a batch of headers. It searches for the latest header in the batch that has a
// matching tree node (if any), and if it has not been marked as known already,
// sets it and its parents to known (even those which are older than the currently
// validated ones). Return value shows if all hashes, numbers and Tds matched
// correctly to the announced values (otherwise the peer should be dropped).
func (f *lightFetcher) checkAnnouncedHeaders(fp *fetcherPeerInfo, headers []*types.Header, tds []*big.Int) bool {
    var (
        n      *fetcherTreeNode
        header *types.Header
        td     *big.Int
    )

    for i := len(headers) - 1; ; i-- {
        if i < 0 {
            if n == nil {
                // no more headers and nothing to match
                return true
            }
            // we ran out of recently delivered headers but have not reached a node known by this peer yet, continue matching
            hash, number := header.ParentHash, header.Number.Uint64()-1
            td = f.chain.GetTd(hash, number)
            header = f.chain.GetHeader(hash, number)
            if header == nil || td == nil {
                log.Error("Missing parent of validated header", "hash", hash, "number", number)
                return false
            }
        } else {
            header = headers[i]
            td = tds[i]
        }
        hash := header.Hash()
        number := header.Number.Uint64()
        if n == nil {
            n = fp.nodeByHash[hash]
        }
        if n != nil {
            if n.td == nil {
                // node was unannounced
                if nn := fp.nodeByHash[hash]; nn != nil {
                    // if there was already a node with the same hash, continue there and drop this one
                    nn.children = append(nn.children, n.children...)
                    n.children = nil
                    fp.deleteNode(n)
                    n = nn
                } else {
                    n.hash = hash
                    n.td = td
                    fp.nodeByHash[hash] = n
                }
            }
            // check if it matches the header
            if n.hash != hash || n.number != number || n.td.Cmp(td) != 0 {
                // peer has previously made an invalid announcement
                return false
            }
            if n.known {
                // we reached a known node that matched our expectations, return with success
                return true
            }
            n.known = true
            if fp.confirmedTd == nil || td.Cmp(fp.confirmedTd) > 0 {
                fp.confirmedTd = td
                fp.bestConfirmed = n
            }
            n = n.parent
            if n == nil {
                return true
            }
        }
    }
}

// checkSyncedHeaders updates peer's block tree after synchronisation by marking
// downloaded headers as known. If none of the announced headers are found after
// syncing, the peer is dropped.
func (f *lightFetcher) checkSyncedHeaders(p *peer) {
    fp := f.peers[p]
    if fp == nil {
        p.Log().Debug("Unknown peer to check sync headers")
        return
    }
    n := fp.lastAnnounced
    var td *big.Int
    for n != nil {
        if td = f.chain.GetTd(n.hash, n.number); td != nil {
            break
        }
        n = n.parent
    }
    // now n is the latest downloaded header after syncing
    if n == nil {
        p.Log().Debug("Synchronisation failed")
        go f.pm.removePeer(p.id)
    } else {
        header := f.chain.GetHeader(n.hash, n.number)
        f.newHeaders([]*types.Header{header}, []*big.Int{td})
    }
}

// checkKnownNode checks if a block tree node is known (downloaded and validated)
// If it was not known previously but found in the database, sets its known flag
func (f *lightFetcher) checkKnownNode(p *peer, n *fetcherTreeNode) bool {
    if n.known {
        return true
    }
    td := f.chain.GetTd(n.hash, n.number)
    if td == nil {
        return false
    }
    header := f.chain.GetHeader(n.hash, n.number)
    // check the availability of both header and td because reads are not protected by chain db mutex
    // Note: returning false is always safe here
    if header == nil {
        return false
    }

    fp := f.peers[p]
    if fp == nil {
        p.Log().Debug("Unknown peer to check known nodes")
        return false
    }
    if !f.checkAnnouncedHeaders(fp, []*types.Header{header}, []*big.Int{td}) {
        p.Log().Debug("Inconsistent announcement")
        go f.pm.removePeer(p.id)
    }
    if fp.confirmedTd != nil {
        f.updateMaxConfirmedTd(fp.confirmedTd)
    }
    return n.known
}

// deleteNode deletes a node and its child subtrees from a peer's block tree
func (fp *fetcherPeerInfo) deleteNode(n *fetcherTreeNode) {
    if n.parent != nil {
        for i, nn := range n.parent.children {
            if nn == n {
                n.parent.children = append(n.parent.children[:i], n.parent.children[i+1:]...)
                break
            }
        }
    }
    for {
        if n.td != nil {
            delete(fp.nodeByHash, n.hash)
        }
        fp.nodeCnt--
        if len(n.children) == 0 {
            return
        }
        for i, nn := range n.children {
            if i == 0 {
                n = nn
            } else {
                fp.deleteNode(nn)
            }
        }
    }
}

// updateStatsEntry items form a linked list that is expanded with a new item every time a new head with a higher Td
// than the previous one has been downloaded and validated. The list contains a series of maximum confirmed Td values
// and the time these values have been confirmed, both increasing monotonically. A maximum confirmed Td is calculated
// both globally for all peers and also for each individual peer (meaning that the given peer has announced the head
// and it has also been downloaded from any peer, either before or after the given announcement).
// The linked list has a global tail where new confirmed Td entries are added and a separate head for each peer,
// pointing to the next Td entry that is higher than the peer's max confirmed Td (nil if it has already confirmed
// the current global head).
type updateStatsEntry struct {
    time mclock.AbsTime
    td   *big.Int
    next *updateStatsEntry
}

// updateMaxConfirmedTd updates the block delay statistics of active peers. Whenever a new highest Td is confirmed,
// adds it to the end of a linked list together with the time it has been confirmed. Then checks which peers have
// already confirmed a head with the same or higher Td (which counts as zero block delay) and updates their statistics.
// Those who have not confirmed such a head by now will be updated by a subsequent checkUpdateStats call with a
// positive block delay value.
func (f *lightFetcher) updateMaxConfirmedTd(td *big.Int) {
    if f.maxConfirmedTd == nil || td.Cmp(f.maxConfirmedTd) > 0 {
        f.maxConfirmedTd = td
        newEntry := &updateStatsEntry{
            time: mclock.Now(),
            td:   td,
        }
        if f.lastUpdateStats != nil {
            f.lastUpdateStats.next = newEntry
        }
        f.lastUpdateStats = newEntry
        for p := range f.peers {
            f.checkUpdateStats(p, newEntry)
        }
    }
}

// checkUpdateStats checks those peers who have not confirmed a certain highest Td (or a larger one) by the time it
// has been confirmed by another peer. If they have confirmed such a head by now, their stats are updated with the
// block delay which is (this peer's confirmation time)-(first confirmation time). After blockDelayTimeout has passed,
// the stats are updated with blockDelayTimeout value. In either case, the confirmed or timed out updateStatsEntry
// items are removed from the head of the linked list.
// If a new entry has been added to the global tail, it is passed as a parameter here even though this function
// assumes that it has already been added, so that if the peer's list is empty (all heads confirmed, head is nil),
// it can set the new head to newEntry.
func (f *lightFetcher) checkUpdateStats(p *peer, newEntry *updateStatsEntry) {
    now := mclock.Now()
    fp := f.peers[p]
    if fp == nil {
        p.Log().Debug("Unknown peer to check update stats")
        return
    }
    if newEntry != nil && fp.firstUpdateStats == nil {
        fp.firstUpdateStats = newEntry
    }
    for fp.firstUpdateStats != nil && fp.firstUpdateStats.time <= now-mclock.AbsTime(blockDelayTimeout) {
        f.pm.serverPool.adjustBlockDelay(p.poolEntry, blockDelayTimeout)
        fp.firstUpdateStats = fp.firstUpdateStats.next
    }
    if fp.confirmedTd != nil {
        for fp.firstUpdateStats != nil && fp.firstUpdateStats.td.Cmp(fp.confirmedTd) <= 0 {
            f.pm.serverPool.adjustBlockDelay(p.poolEntry, time.Duration(now-fp.firstUpdateStats.time))
            fp.firstUpdateStats = fp.firstUpdateStats.next
        }
    }
}