path: root/p2p/discover/table.go

// Copyright 2015 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 discover implements the Node Discovery Protocol.
//
// The Node Discovery protocol provides a way to find RLPx nodes that
// can be connected to. It uses a Kademlia-like protocol to maintain a
// distributed database of the IDs and endpoints of all listening
// nodes.
package discover

import (
    crand "crypto/rand"
    "encoding/binary"
    "errors"
    "fmt"
    mrand "math/rand"
    "net"
    "sort"
    "sync"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/crypto"
    "github.com/ethereum/go-ethereum/log"
    "github.com/ethereum/go-ethereum/p2p/netutil"
)

const (
    alpha           = 3  // Kademlia concurrency factor
    bucketSize      = 16 // Kademlia bucket size
    maxReplacements = 10 // Size of per-bucket replacement list

    // We keep buckets for the upper 1/15 of distances because
    // it's very unlikely we'll ever encounter a node that's closer.
    hashBits          = len(common.Hash{}) * 8
    nBuckets          = hashBits / 15       // Number of buckets
    bucketMinDistance = hashBits - nBuckets // Log distance of closest bucket

    // IP address limits.
    bucketIPLimit, bucketSubnet = 2, 24 // at most 2 addresses from the same /24
    tableIPLimit, tableSubnet   = 10, 24

    maxBondingPingPongs = 16 // Limit on the number of concurrent ping/pong interactions
    maxFindnodeFailures = 5  // Nodes exceeding this limit are dropped

    refreshInterval    = 30 * time.Minute
    revalidateInterval = 10 * time.Second
    copyNodesInterval  = 30 * time.Second
    seedMinTableTime   = 5 * time.Minute
    seedCount          = 30
    seedMaxAge         = 5 * 24 * time.Hour
)

type Table struct {
    mutex   sync.Mutex        // protects buckets, bucket content, nursery, rand
    buckets [nBuckets]*bucket // index of known nodes by distance
    nursery []*Node           // bootstrap nodes
    rand    *mrand.Rand       // source of randomness, periodically reseeded
    ips     netutil.DistinctNetSet

    db         *nodeDB // database of known nodes
    refreshReq chan chan struct{}
    initDone   chan struct{}
    closeReq   chan struct{}
    closed     chan struct{}

    bondmu    sync.Mutex
    bonding   map[NodeID]*bondproc
    bondslots chan struct{} // limits total number of active bonding processes

    nodeAddedHook func(*Node) // for testing

    net  transport
    self *Node // metadata of the local node
}

type bondproc struct {
    err  error
    n    *Node
    done chan struct{}
}

// transport is implemented by the UDP transport.
// it is an interface so we can test without opening lots of UDP
// sockets and without generating a private key.
type transport interface {
    ping(NodeID, *net.UDPAddr) error
    waitping(NodeID) error
    findnode(toid NodeID, addr *net.UDPAddr, target NodeID) ([]*Node, error)
    close()
}

// bucket contains nodes, ordered by their last activity. the entry
// that was most recently active is the first element in entries.
type bucket struct {
    entries      []*Node // live entries, sorted by time of last contact
    replacements []*Node // recently seen nodes to be used if revalidation fails
    ips          netutil.DistinctNetSet
}

func newTable(t transport, ourID NodeID, ourAddr *net.UDPAddr, nodeDBPath string, bootnodes []*Node) (*Table, error) {
    // If no node database was given, use an in-memory one
    db, err := newNodeDB(nodeDBPath, Version, ourID)
    if err != nil {
        return nil, err
    }
    tab := &Table{
        net:        t,
        db:         db,
        self:       NewNode(ourID, ourAddr.IP, uint16(ourAddr.Port), uint16(ourAddr.Port)),
        bonding:    make(map[NodeID]*bondproc),
        bondslots:  make(chan struct{}, maxBondingPingPongs),
        refreshReq: make(chan chan struct{}),
        initDone:   make(chan struct{}),
        closeReq:   make(chan struct{}),
        closed:     make(chan struct{}),
        rand:       mrand.New(mrand.NewSource(0)),
        ips:        netutil.DistinctNetSet{Subnet: tableSubnet, Limit: tableIPLimit},
    }
    if err := tab.setFallbackNodes(bootnodes); err != nil {
        return nil, err
    }
    for i := 0; i < cap(tab.bondslots); i++ {
        tab.bondslots <- struct{}{}
    }
    for i := range tab.buckets {
        tab.buckets[i] = &bucket{
            ips: netutil.DistinctNetSet{Subnet: bucketSubnet, Limit: bucketIPLimit},
        }
    }
    tab.seedRand()
    tab.loadSeedNodes(false)
    // Start the background expiration goroutine after loading seeds so that the search for
    // seed nodes also considers older nodes that would otherwise be removed by the
    // expiration.
    tab.db.ensureExpirer()
    go tab.loop()
    return tab, nil
}

func (tab *Table) seedRand() {
    var b [8]byte
    crand.Read(b[:])

    tab.mutex.Lock()
    tab.rand.Seed(int64(binary.BigEndian.Uint64(b[:])))
    tab.mutex.Unlock()
}

// Self returns the local node.
// The returned node should not be modified by the caller.
func (tab *Table) Self() *Node {
    return tab.self
}

// ReadRandomNodes fills the given slice with random nodes from the
// table. It will not write the same node more than once. The nodes in
// the slice are copies and can be modified by the caller.
func (tab *Table) ReadRandomNodes(buf []*Node) (n int) {
    if !tab.isInitDone() {
        return 0
    }
    tab.mutex.Lock()
    defer tab.mutex.Unlock()

    // Find all non-empty buckets and get a fresh slice of their entries.
    var buckets [][]*Node
    for _, b := range tab.buckets {
        if len(b.entries) > 0 {
            buckets = append(buckets, b.entries[:])
        }
    }
    if len(buckets) == 0 {
        return 0
    }
    // Shuffle the buckets.
    for i := len(buckets) - 1; i > 0; i-- {
        j := tab.rand.Intn(len(buckets))
        buckets[i], buckets[j] = buckets[j], buckets[i]
    }
    // Move head of each bucket into buf, removing buckets that become empty.
    var i, j int
    for ; i < len(buf); i, j = i+1, (j+1)%len(buckets) {
        b := buckets[j]
        buf[i] = &(*b[0])
        buckets[j] = b[1:]
        if len(b) == 1 {
            buckets = append(buckets[:j], buckets[j+1:]...)
        }
        if len(buckets) == 0 {
            break
        }
    }
    return i + 1
}

// Close terminates the network listener and flushes the node database.
func (tab *Table) Close() {
    select {
    case <-tab.closed:
        // already closed.
    case tab.closeReq <- struct{}{}:
        <-tab.closed // wait for refreshLoop to end.
    }
}

// setFallbackNodes sets the initial points of contact. These nodes
// are used to connect to the network if the table is empty and there
// are no known nodes in the database.
func (tab *Table) setFallbackNodes(nodes []*Node) error {
    for _, n := range nodes {
        if err := n.validateComplete(); err != nil {
            return fmt.Errorf("bad bootstrap/fallback node %q (%v)", n, err)
        }
    }
    tab.nursery = make([]*Node, 0, len(nodes))
    for _, n := range nodes {
        cpy := *n
        // Recompute cpy.sha because the node might not have been
        // created by NewNode or ParseNode.
        cpy.sha = crypto.Keccak256Hash(n.ID[:])
        tab.nursery = append(tab.nursery, &cpy)
    }
    return nil
}

// isInitDone returns whether the table's initial seeding procedure has completed.
func (tab *Table) isInitDone() bool {
    select {
    case <-tab.initDone:
        return true
    default:
        return false
    }
}

// Resolve searches for a specific node with the given ID.
// It returns nil if the node could not be found.
func (tab *Table) Resolve(targetID NodeID) *Node {
    // If the node is present in the local table, no
    // network interaction is required.
    hash := crypto.Keccak256Hash(targetID[:])
    tab.mutex.Lock()
    cl := tab.closest(hash, 1)
    tab.mutex.Unlock()
    if len(cl.entries) > 0 && cl.entries[0].ID == targetID {
        return cl.entries[0]
    }
    // Otherwise, do a network lookup.
    result := tab.Lookup(targetID)
    for _, n := range result {
        if n.ID == targetID {
            return n
        }
    }
    return nil
}

// Lookup performs a network search for nodes close
// to the given target. It approaches the target by querying
// nodes that are closer to it on each iteration.
// The given target does not need to be an actual node
// identifier.
func (tab *Table) Lookup(targetID NodeID) []*Node {
    return tab.lookup(targetID, true)
}

func (tab *Table) lookup(targetID NodeID, refreshIfEmpty bool) []*Node {
    var (
        target         = crypto.Keccak256Hash(targetID[:])
        asked          = make(map[NodeID]bool)
        seen           = make(map[NodeID]bool)
        reply          = make(chan []*Node, alpha)
        pendingQueries = 0
        result         *nodesByDistance
    )
    // don't query further if we hit ourself.
    // unlikely to happen often in practice.
    asked[tab.self.ID] = true

    for {
        tab.mutex.Lock()
        // generate initial result set
        result = tab.closest(target, bucketSize)
        tab.mutex.Unlock()
        if len(result.entries) > 0 || !refreshIfEmpty {
            break
        }
        // The result set is empty, all nodes were dropped, refresh.
        // We actually wait for the refresh to complete here. The very
        // first query will hit this case and run the bootstrapping
        // logic.
        <-tab.refresh()
        refreshIfEmpty = false
    }

    for {
        // ask the alpha closest nodes that we haven't asked yet
        for i := 0; i < len(result.entries) && pendingQueries < alpha; i++ {
            n := result.entries[i]
            if !asked[n.ID] {
                asked[n.ID] = true
                pendingQueries++
                go func() {
                    // Find potential neighbors to bond with
                    r, err := tab.net.findnode(n.ID, n.addr(), targetID)
                    if err != nil {
                        // Bump the failure counter to detect and evacuate non-bonded entries
                        fails := tab.db.findFails(n.ID) + 1
                        tab.db.updateFindFails(n.ID, fails)
                        log.Trace("Bumping findnode failure counter", "id", n.ID, "failcount", fails)

                        if fails >= maxFindnodeFailures {
                            log.Trace("Too many findnode failures, dropping", "id", n.ID, "failcount", fails)
                            tab.delete(n)
                        }
                    }
                    reply <- tab.bondall(r)
                }()
            }
        }
        if pendingQueries == 0 {
            // we have asked all closest nodes, stop the search
            break
        }
        // wait for the next reply
        for _, n := range <-reply {
            if n != nil && !seen[n.ID] {
                seen[n.ID] = true
                result.push(n, bucketSize)
            }
        }
        pendingQueries--
    }
    return result.entries
}

func (tab *Table) refresh() <-chan struct{} {
    done := make(chan struct{})
    select {
    case tab.refreshReq <- done:
    case <-tab.closed:
        close(done)
    }
    return done
}

// loop schedules refresh, revalidate runs and coordinates shutdown.
func (tab *Table) loop() {
    var (
        revalidate     = time.NewTimer(tab.nextRevalidateTime())
        refresh        = time.NewTicker(refreshInterval)
        copyNodes      = time.NewTicker(copyNodesInterval)
        revalidateDone = make(chan struct{})
        refreshDone    = make(chan struct{})           // where doRefresh reports completion
        waiting        = []chan struct{}{tab.initDone} // holds waiting callers while doRefresh runs
    )
    defer refresh.Stop()
    defer revalidate.Stop()
    defer copyNodes.Stop()

    // Start initial refresh.
    go tab.doRefresh(refreshDone)

loop:
    for {
        select {
        case <-refresh.C:
            tab.seedRand()
            if refreshDone == nil {
                refreshDone = make(chan struct{})
                go tab.doRefresh(refreshDone)
            }
        case req := <-tab.refreshReq:
            waiting = append(waiting, req)
            if refreshDone == nil {
                refreshDone = make(chan struct{})
                go tab.doRefresh(refreshDone)
            }
        case <-refreshDone:
            for _, ch := range waiting {
                close(ch)
            }
            waiting, refreshDone = nil, nil
        case <-revalidate.C:
            go tab.doRevalidate(revalidateDone)
        case <-revalidateDone:
            revalidate.Reset(tab.nextRevalidateTime())
        case <-copyNodes.C:
            go tab.copyBondedNodes()
        case <-tab.closeReq:
            break loop
        }
    }

    if tab.net != nil {
        tab.net.close()
    }
    if refreshDone != nil {
        <-refreshDone
    }
    for _, ch := range waiting {
        close(ch)
    }
    tab.db.close()
    close(tab.closed)
}

// doRefresh performs a lookup for a random target to keep buckets
// full. seed nodes are inserted if the table is empty (initial
// bootstrap or discarded faulty peers).
func (tab *Table) doRefresh(done chan struct{}) {
    defer close(done)

    // Load nodes from the database and insert
    // them. This should yield a few previously seen nodes that are
    // (hopefully) still alive.
    tab.loadSeedNodes(true)

    // Run self lookup to discover new neighbor nodes.
    tab.lookup(tab.self.ID, false)

    // The Kademlia paper specifies that the bucket refresh should
    // perform a lookup in the least recently used bucket. We cannot
    // adhere to this because the findnode target is a 512bit value
    // (not hash-sized) and it is not easily possible to generate a
    // sha3 preimage that falls into a chosen bucket.
    // We perform a few lookups with a random target instead.
    for i := 0; i < 3; i++ {
        var target NodeID
        crand.Read(target[:])
        tab.lookup(target, false)
    }
}

func (tab *Table) loadSeedNodes(bond bool) {
    seeds := tab.db.querySeeds(seedCount, seedMaxAge)
    seeds = append(seeds, tab.nursery...)
    if bond {
        seeds = tab.bondall(seeds)
    }
    for i := range seeds {
        seed := seeds[i]
        age := log.Lazy{Fn: func() interface{} { return time.Since(tab.db.bondTime(seed.ID)) }}
        log.Debug("Found seed node in database", "id", seed.ID, "addr", seed.addr(), "age", age)
        tab.add(seed)
    }
}

// doRevalidate checks that the last node in a random bucket is still live
// and replaces or deletes the node if it isn't.
func (tab *Table) doRevalidate(done chan<- struct{}) {
    defer func() { done <- struct{}{} }()

    last, bi := tab.nodeToRevalidate()
    if last == nil {
        // No non-empty bucket found.
        return
    }

    // Ping the selected node and wait for a pong.
    err := tab.ping(last.ID, last.addr())

    tab.mutex.Lock()
    defer tab.mutex.Unlock()
    b := tab.buckets[bi]
    if err == nil {
        // The node responded, move it to the front.
        log.Trace("Revalidated node", "b", bi, "id", last.ID)
        b.bump(last)
        return
    }
    // No reply received, pick a replacement or delete the node if there aren't
    // any replacements.
    if r := tab.replace(b, last); r != nil {
        log.Trace("Replaced dead node", "b", bi, "id", last.ID, "ip", last.IP, "r", r.ID, "rip", r.IP)
    } else {
        log.Trace("Removed dead node", "b", bi, "id", last.ID, "ip", last.IP)
    }
}

// nodeToRevalidate returns the last node in a random, non-empty bucket.
func (tab *Table) nodeToRevalidate() (n *Node, bi int) {
    tab.mutex.Lock()
    defer tab.mutex.Unlock()

    for _, bi = range tab.rand.Perm(len(tab.buckets)) {
        b := tab.buckets[bi]
        if len(b.entries) > 0 {
            last := b.entries[len(b.entries)-1]
            return last, bi
        }
    }
    return nil, 0
}

func (tab *Table) nextRevalidateTime() time.Duration {
    tab.mutex.Lock()
    defer tab.mutex.Unlock()

    return time.Duration(tab.rand.Int63n(int64(revalidateInterval)))
}

// copyBondedNodes adds nodes from the table to the database if they have been in the table
// longer then minTableTime.
func (tab *Table) copyBondedNodes() {
    tab.mutex.Lock()
    defer tab.mutex.Unlock()

    now := time.Now()
    for _, b := range tab.buckets {
        for _, n := range b.entries {
            if now.Sub(n.addedAt) >= seedMinTableTime {
                tab.db.updateNode(n)
            }
        }
    }
}

// closest returns the n nodes in the table that are closest to the
// given id. The caller must hold tab.mutex.
func (tab *Table) closest(target common.Hash, nresults int) *nodesByDistance {
    // This is a very wasteful way to find the closest nodes but
    // obviously correct. I believe that tree-based buckets would make
    // this easier to implement efficiently.
    close := &nodesByDistance{target: target}
    for _, b := range tab.buckets {
        for _, n := range b.entries {
            close.push(n, nresults)
        }
    }
    return close
}

func (tab *Table) len() (n int) {
    for _, b := range tab.buckets {
        n += len(b.entries)
    }
    return n
}

// bondall bonds with all given nodes concurrently and returns
// those nodes for which bonding has probably succeeded.
func (tab *Table) bondall(nodes []*Node) (result []*Node) {
    rc := make(chan *Node, len(nodes))
    for i := range nodes {
        go func(n *Node) {
            nn, _ := tab.bond(false, n.ID, n.addr(), n.TCP)
            rc <- nn
        }(nodes[i])
    }
    for range nodes {
        if n := <-rc; n != nil {
            result = append(result, n)
        }
    }
    return result
}

// bond ensures the local node has a bond with the given remote node.
// It also attempts to insert the node into the table if bonding succeeds.
// The caller must not hold tab.mutex.
//
// A bond is must be established before sending findnode requests.
// Both sides must have completed a ping/pong exchange for a bond to
// exist. The total number of active bonding processes is limited in
// order to restrain network use.
//
// bond is meant to operate idempotently in that bonding with a remote
// node which still remembers a previously established bond will work.
// The remote node will simply not send a ping back, causing waitping
// to time out.
//
// If pinged is true, the remote node has just pinged us and one half
// of the process can be skipped.
func (tab *Table) bond(pinged bool, id NodeID, addr *net.UDPAddr, tcpPort uint16) (*Node, error) {
    if id == tab.self.ID {
        return nil, errors.New("is self")
    }
    if pinged && !tab.isInitDone() {
        return nil, errors.New("still initializing")
    }
    // Start bonding if we haven't seen this node for a while or if it failed findnode too often.
    node, fails := tab.db.node(id), tab.db.findFails(id)
    age := time.Since(tab.db.bondTime(id))
    var result error
    if fails > 0 || age > nodeDBNodeExpiration {
        log.Trace("Starting bonding ping/pong", "id", id, "known", node != nil, "failcount", fails, "age", age)

        tab.bondmu.Lock()
        w := tab.bonding[id]
        if w != nil {
            // Wait for an existing bonding process to complete.
            tab.bondmu.Unlock()
            <-w.done
        } else {
            // Register a new bonding process.
            w = &bondproc{done: make(chan struct{})}
            tab.bonding[id] = w
            tab.bondmu.Unlock()
            // Do the ping/pong. The result goes into w.
            tab.pingpong(w, pinged, id, addr, tcpPort)
            // Unregister the process after it's done.
            tab.bondmu.Lock()
            delete(tab.bonding, id)
            tab.bondmu.Unlock()
        }
        // Retrieve the bonding results
        result = w.err
        if result == nil {
            node = w.n
        }
    }
    // Add the node to the table even if the bonding ping/pong
    // fails. It will be relaced quickly if it continues to be
    // unresponsive.
    if node != nil {
        tab.add(node)
        tab.db.updateFindFails(id, 0)
    }
    return node, result
}

func (tab *Table) pingpong(w *bondproc, pinged bool, id NodeID, addr *net.UDPAddr, tcpPort uint16) {
    // Request a bonding slot to limit network usage
    <-tab.bondslots
    defer func() { tab.bondslots <- struct{}{} }()

    // Ping the remote side and wait for a pong.
    if w.err = tab.ping(id, addr); w.err != nil {
        close(w.done)
        return
    }
    if !pinged {
        // Give the remote node a chance to ping us before we start
        // sending findnode requests. If they still remember us,
        // waitping will simply time out.
        tab.net.waitping(id)
    }
    // Bonding succeeded, update the node database.
    w.n = NewNode(id, addr.IP, uint16(addr.Port), tcpPort)
    close(w.done)
}

// ping a remote endpoint and wait for a reply, also updating the node
// database accordingly.
func (tab *Table) ping(id NodeID, addr *net.UDPAddr) error {
    tab.db.updateLastPing(id, time.Now())
    if err := tab.net.ping(id, addr); err != nil {
        return err
    }
    tab.db.updateBondTime(id, time.Now())
    return nil
}

// bucket returns the bucket for the given node ID hash.
func (tab *Table) bucket(sha common.Hash) *bucket {
    d := logdist(tab.self.sha, sha)
    if d <= bucketMinDistance {
        return tab.buckets[0]
    }
    return tab.buckets[d-bucketMinDistance-1]
}

// add attempts to add the given node its corresponding bucket. If the
// bucket has space available, adding the node succeeds immediately.
// Otherwise, the node is added if the least recently active node in
// the bucket does not respond to a ping packet.
//
// The caller must not hold tab.mutex.
func (tab *Table) add(new *Node) {
    tab.mutex.Lock()
    defer tab.mutex.Unlock()

    b := tab.bucket(new.sha)
    if !tab.bumpOrAdd(b, new) {
        // Node is not in table. Add it to the replacement list.
        tab.addReplacement(b, new)
    }
}

// stuff adds nodes the table to the end of their corresponding bucket
// if the bucket is not full. The caller must not hold tab.mutex.
func (tab *Table) stuff(nodes []*Node) {
    tab.mutex.Lock()
    defer tab.mutex.Unlock()

    for _, n := range nodes {
        if n.ID == tab.self.ID {
            continue // don't add self
        }
        b := tab.bucket(n.sha)
        if len(b.entries) < bucketSize {
            tab.bumpOrAdd(b, n)
        }
    }
}

// delete removes an entry from the node table (used to evacuate
// failed/non-bonded discovery peers).
func (tab *Table) delete(node *Node) {
    tab.mutex.Lock()
    defer tab.mutex.Unlock()

    tab.deleteInBucket(tab.bucket(node.sha), node)
}

func (tab *Table) addIP(b *bucket, ip net.IP) bool {
    if netutil.IsLAN(ip) {
        return true
    }
    if !tab.ips.Add(ip) {
        log.Debug("IP exceeds table limit", "ip", ip)
        return false
    }
    if !b.ips.Add(ip) {
        log.Debug("IP exceeds bucket limit", "ip", ip)
        tab.ips.Remove(ip)
        return false
    }
    return true
}

func (tab *Table) removeIP(b *bucket, ip net.IP) {
    if netutil.IsLAN(ip) {
        return
    }
    tab.ips.Remove(ip)
    b.ips.Remove(ip)
}

func (tab *Table) addReplacement(b *bucket, n *Node) {
    for _, e := range b.replacements {
        if e.ID == n.ID {
            return // already in list
        }
    }
    if !tab.addIP(b, n.IP) {
        return
    }
    var removed *Node
    b.replacements, removed = pushNode(b.replacements, n, maxReplacements)
    if removed != nil {
        tab.removeIP(b, removed.IP)
    }
}

// replace removes n from the replacement list and replaces 'last' with it if it is the
// last entry in the bucket. If 'last' isn't the last entry, it has either been replaced
// with someone else or became active.
func (tab *Table) replace(b *bucket, last *Node) *Node {
    if len(b.entries) == 0 || b.entries[len(b.entries)-1].ID != last.ID {
        // Entry has moved, don't replace it.
        return nil
    }
    // Still the last entry.
    if len(b.replacements) == 0 {
        tab.deleteInBucket(b, last)
        return nil
    }
    r := b.replacements[tab.rand.Intn(len(b.replacements))]
    b.replacements = deleteNode(b.replacements, r)
    b.entries[len(b.entries)-1] = r
    tab.removeIP(b, last.IP)
    return r
}

// bump moves the given node to the front of the bucket entry list
// if it is contained in that list.
func (b *bucket) bump(n *Node) bool {
    for i := range b.entries {
        if b.entries[i].ID == n.ID {
            // move it to the front
            copy(b.entries[1:], b.entries[:i])
            b.entries[0] = n
            return true
        }
    }
    return false
}

// bumpOrAdd moves n to the front of the bucket entry list or adds it if the list isn't
// full. The return value is true if n is in the bucket.
func (tab *Table) bumpOrAdd(b *bucket, n *Node) bool {
    if b.bump(n) {
        return true
    }
    if len(b.entries) >= bucketSize || !tab.addIP(b, n.IP) {
        return false
    }
    b.entries, _ = pushNode(b.entries, n, bucketSize)
    b.replacements = deleteNode(b.replacements, n)
    n.addedAt = time.Now()
    if tab.nodeAddedHook != nil {
        tab.nodeAddedHook(n)
    }
    return true
}

func (tab *Table) deleteInBucket(b *bucket, n *Node) {
    b.entries = deleteNode(b.entries, n)
    tab.removeIP(b, n.IP)
}

// pushNode adds n to the front of list, keeping at most max items.
func pushNode(list []*Node, n *Node, max int) ([]*Node, *Node) {
    if len(list) < max {
        list = append(list, nil)
    }
    removed := list[len(list)-1]
    copy(list[1:], list)
    list[0] = n
    return list, removed
}

// deleteNode removes n from list.
func deleteNode(list []*Node, n *Node) []*Node {
    for i := range list {
        if list[i].ID == n.ID {
            return append(list[:i], list[i+1:]...)
        }
    }
    return list
}

// nodesByDistance is a list of nodes, ordered by
// distance to target.
type nodesByDistance struct {
    entries []*Node
    target  common.Hash
}

// push adds the given node to the list, keeping the total size below maxElems.
func (h *nodesByDistance) push(n *Node, maxElems int) {
    ix := sort.Search(len(h.entries), func(i int) bool {
        return distcmp(h.target, h.entries[i].sha, n.sha) > 0
    })
    if len(h.entries) < maxElems {
        h.entries = append(h.entries, n)
    }
    if ix == len(h.entries) {
        // farther away than all nodes we already have.
        // if there was room for it, the node is now the last element.
    } else {
        // slide existing entries down to make room
        // this will overwrite the entry we just appended.
        copy(h.entries[ix+1:], h.entries[ix:])
        h.entries[ix] = n
    }
}