go-ethereum/swarm/network/kademlia.go

// Copyright 2017 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 network

import (
	"bytes"
	"fmt"
	"math/rand"
	"strings"
	"sync"
	"time"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/swarm/log"
	"github.com/ethereum/go-ethereum/swarm/pot"
)

/*

Taking the proximity order relative to a fix point x classifies the points in
the space (n byte long byte sequences) into bins. Items in each are at
most half as distant from x as items in the previous bin. Given a sample of
uniformly distributed items (a hash function over arbitrary sequence) the
proximity scale maps onto series of subsets with cardinalities on a negative
exponential scale.

It also has the property that any two item belonging to the same bin are at
most half as distant from each other as they are from x.

If we think of random sample of items in the bins as connections in a network of
interconnected nodes then relative proximity can serve as the basis for local
decisions for graph traversal where the task is to find a route between two
points. Since in every hop, the finite distance halves, there is
a guaranteed constant maximum limit on the number of hops needed to reach one
node from the other.
*/

var Pof = pot.DefaultPof(256)

// KadParams holds the config params for Kademlia
type KadParams struct {
	// adjustable parameters
	MaxProxDisplay int   // number of rows the table shows
	MinProxBinSize int   // nearest neighbour core minimum cardinality
	MinBinSize     int   // minimum number of peers in a row
	MaxBinSize     int   // maximum number of peers in a row before pruning
	RetryInterval  int64 // initial interval before a peer is first redialed
	RetryExponent  int   // exponent to multiply retry intervals with
	MaxRetries     int   // maximum number of redial attempts
	// function to sanction or prevent suggesting a peer
	Reachable func(*BzzAddr) bool `json:"-"`
}

// NewKadParams returns a params struct with default values
func NewKadParams() *KadParams {
	return &KadParams{
		MaxProxDisplay: 16,
		MinProxBinSize: 2,
		MinBinSize:     2,
		MaxBinSize:     4,
		RetryInterval:  4200000000, // 4.2 sec
		MaxRetries:     42,
		RetryExponent:  2,
	}
}

// Kademlia is a table of live peers and a db of known peers (node records)
type Kademlia struct {
	lock       sync.RWMutex
	*KadParams          // Kademlia configuration parameters
	base       []byte   // immutable baseaddress of the table
	addrs      *pot.Pot // pots container for known peer addresses
	conns      *pot.Pot // pots container for live peer connections
	depth      uint8    // stores the last current depth of saturation
	nDepth     int      // stores the last neighbourhood depth
	nDepthC    chan int // returned by DepthC function to signal neighbourhood depth change
	addrCountC chan int // returned by AddrCountC function to signal peer count change
}

// NewKademlia creates a Kademlia table for base address addr
// with parameters as in params
// if params is nil, it uses default values
func NewKademlia(addr []byte, params *KadParams) *Kademlia {
	if params == nil {
		params = NewKadParams()
	}
	return &Kademlia{
		base:      addr,
		KadParams: params,
		addrs:     pot.NewPot(nil, 0),
		conns:     pot.NewPot(nil, 0),
	}
}

// entry represents a Kademlia table entry (an extension of BzzAddr)
type entry struct {
	*BzzAddr
	conn    *Peer
	seenAt  time.Time
	retries int
}

// newEntry creates a kademlia peer from a *Peer
func newEntry(p *BzzAddr) *entry {
	return &entry{
		BzzAddr: p,
		seenAt:  time.Now(),
	}
}

// Label is a short tag for the entry for debug
func Label(e *entry) string {
	return fmt.Sprintf("%s (%d)", e.Hex()[:4], e.retries)
}

// Hex is the hexadecimal serialisation of the entry address
func (e *entry) Hex() string {
	return fmt.Sprintf("%x", e.Address())
}

// Register enters each address as kademlia peer record into the
// database of known peer addresses
func (k *Kademlia) Register(peers ...*BzzAddr) error {
	k.lock.Lock()
	defer k.lock.Unlock()
	var known, size int
	for _, p := range peers {
		// error if self received, peer should know better
		// and should be punished for this
		if bytes.Equal(p.Address(), k.base) {
			return fmt.Errorf("add peers: %x is self", k.base)
		}
		var found bool
		k.addrs, _, found, _ = pot.Swap(k.addrs, p, Pof, func(v pot.Val) pot.Val {
			// if not found
			if v == nil {
				// insert new offline peer into conns
				return newEntry(p)
			}
			// found among known peers, do nothing
			return v
		})
		if found {
			known++
		}
		size++
	}
	// send new address count value only if there are new addresses
	if k.addrCountC != nil && size-known > 0 {
		k.addrCountC <- k.addrs.Size()
	}

	k.sendNeighbourhoodDepthChange()
	return nil
}

// SuggestPeer returns a known peer for the lowest proximity bin for the
// lowest bincount below depth
// naturally if there is an empty row it returns a peer for that
func (k *Kademlia) SuggestPeer() (a *BzzAddr, o int, want bool) {
	k.lock.Lock()
	defer k.lock.Unlock()
	minsize := k.MinBinSize
	depth := depthForPot(k.conns, k.MinProxBinSize, k.base)
	// if there is a callable neighbour within the current proxBin, connect
	// this makes sure nearest neighbour set is fully connected
	var ppo int
	k.addrs.EachNeighbour(k.base, Pof, func(val pot.Val, po int) bool {
		if po < depth {
			return false
		}
		e := val.(*entry)
		c := k.callable(e)
		if c {
			a = e.BzzAddr
		}
		ppo = po
		return !c
	})
	if a != nil {
		log.Trace(fmt.Sprintf("%08x candidate nearest neighbour found: %v (%v)", k.BaseAddr()[:4], a, ppo))
		return a, 0, false
	}

	var bpo []int
	prev := -1
	k.conns.EachBin(k.base, Pof, 0, func(po, size int, f func(func(val pot.Val, i int) bool) bool) bool {
		prev++
		for ; prev < po; prev++ {
			bpo = append(bpo, prev)
			minsize = 0
		}
		if size < minsize {
			bpo = append(bpo, po)
			minsize = size
		}
		return size > 0 && po < depth
	})
	// all buckets are full, ie., minsize == k.MinBinSize
	if len(bpo) == 0 {
		return nil, 0, false
	}
	// as long as we got candidate peers to connect to
	// dont ask for new peers (want = false)
	// try to select a candidate peer
	// find the first callable peer
	nxt := bpo[0]
	k.addrs.EachBin(k.base, Pof, nxt, func(po, _ int, f func(func(pot.Val, int) bool) bool) bool {
		// for each bin (up until depth) we find callable candidate peers
		if po >= depth {
			return false
		}
		return f(func(val pot.Val, _ int) bool {
			e := val.(*entry)
			c := k.callable(e)
			if c {
				a = e.BzzAddr
			}
			return !c
		})
	})
	// found a candidate
	if a != nil {
		return a, 0, false
	}
	// no candidate peer found, request for the short bin
	var changed bool
	if uint8(nxt) < k.depth {
		k.depth = uint8(nxt)
		changed = true
	}
	return a, nxt, changed
}

// On inserts the peer as a kademlia peer into the live peers
func (k *Kademlia) On(p *Peer) (uint8, bool) {
	k.lock.Lock()
	defer k.lock.Unlock()
	var ins bool
	k.conns, _, _, _ = pot.Swap(k.conns, p, Pof, func(v pot.Val) pot.Val {
		// if not found live
		if v == nil {
			ins = true
			// insert new online peer into conns
			return p
		}
		// found among live peers, do nothing
		return v
	})
	if ins && !p.BzzPeer.LightNode {
		a := newEntry(p.BzzAddr)
		a.conn = p
		// insert new online peer into addrs
		k.addrs, _, _, _ = pot.Swap(k.addrs, p, Pof, func(v pot.Val) pot.Val {
			return a
		})
		// send new address count value only if the peer is inserted
		if k.addrCountC != nil {
			k.addrCountC <- k.addrs.Size()
		}
	}
	log.Trace(k.string())
	// calculate if depth of saturation changed
	depth := uint8(k.saturation())
	var changed bool
	if depth != k.depth {
		changed = true
		k.depth = depth
	}
	k.sendNeighbourhoodDepthChange()
	return k.depth, changed
}

// NeighbourhoodDepthC returns the channel that sends a new kademlia
// neighbourhood depth on each change.
// Not receiving from the returned channel will block On function
// when the neighbourhood depth is changed.
// TODO: Why is this exported, and if it should be; why can't we have more subscribers than one?
func (k *Kademlia) NeighbourhoodDepthC() <-chan int {
	k.lock.Lock()
	defer k.lock.Unlock()
	if k.nDepthC == nil {
		k.nDepthC = make(chan int)
	}
	return k.nDepthC
}

// sendNeighbourhoodDepthChange sends new neighbourhood depth to k.nDepth channel
// if it is initialized.
func (k *Kademlia) sendNeighbourhoodDepthChange() {
	// nDepthC is initialized when NeighbourhoodDepthC is called and returned by it.
	// It provides signaling of neighbourhood depth change.
	// This part of the code is sending new neighbourhood depth to nDepthC if that condition is met.
	if k.nDepthC != nil {
		nDepth := depthForPot(k.conns, k.MinProxBinSize, k.base)
		if nDepth != k.nDepth {
			k.nDepth = nDepth
			k.nDepthC <- nDepth
		}
	}
}

// AddrCountC returns the channel that sends a new
// address count value on each change.
// Not receiving from the returned channel will block Register function
// when address count value changes.
func (k *Kademlia) AddrCountC() <-chan int {
	if k.addrCountC == nil {
		k.addrCountC = make(chan int)
	}
	return k.addrCountC
}

// Off removes a peer from among live peers
func (k *Kademlia) Off(p *Peer) {
	k.lock.Lock()
	defer k.lock.Unlock()
	var del bool
	if !p.BzzPeer.LightNode {
		k.addrs, _, _, _ = pot.Swap(k.addrs, p, Pof, func(v pot.Val) pot.Val {
			// v cannot be nil, must check otherwise we overwrite entry
			if v == nil {
				panic(fmt.Sprintf("connected peer not found %v", p))
			}
			del = true
			return newEntry(p.BzzAddr)
		})
	} else {
		del = true
	}

	if del {
		k.conns, _, _, _ = pot.Swap(k.conns, p, Pof, func(_ pot.Val) pot.Val {
			// v cannot be nil, but no need to check
			return nil
		})
		// send new address count value only if the peer is deleted
		if k.addrCountC != nil {
			k.addrCountC <- k.addrs.Size()
		}
		k.sendNeighbourhoodDepthChange()
	}
}

// EachBin is a two level nested iterator
// The outer iterator returns all bins that have known peers, in order from shallowest to deepest
// The inner iterator returns all peers per bin returned by the outer iterator, in no defined order
// TODO the po returned by the inner iterator is not reliable. However, it is not being used in this method
func (k *Kademlia) EachBin(base []byte, pof pot.Pof, o int, eachBinFunc func(conn *Peer, po int) bool) {
	k.lock.RLock()
	defer k.lock.RUnlock()

	var startPo int
	var endPo int
	kadDepth := depthForPot(k.conns, k.MinProxBinSize, k.base)

	k.conns.EachBin(base, Pof, o, func(po, size int, f func(func(val pot.Val, i int) bool) bool) bool {
		if startPo > 0 && endPo != k.MaxProxDisplay {
			startPo = endPo + 1
		}
		if po < kadDepth {
			endPo = po
		} else {
			endPo = k.MaxProxDisplay
		}

		for bin := startPo; bin <= endPo; bin++ {
			f(func(val pot.Val, _ int) bool {
				return eachBinFunc(val.(*Peer), bin)
			})
		}
		return true
	})
}

// EachConn is an iterator with args (base, po, f) applies f to each live peer
// that has proximity order po or less as measured from the base
// if base is nil, kademlia base address is used
func (k *Kademlia) EachConn(base []byte, o int, f func(*Peer, int) bool) {
	k.lock.RLock()
	defer k.lock.RUnlock()
	k.eachConn(base, o, f)
}

func (k *Kademlia) eachConn(base []byte, o int, f func(*Peer, int) bool) {
	if len(base) == 0 {
		base = k.base
	}
	k.conns.EachNeighbour(base, Pof, func(val pot.Val, po int) bool {
		if po > o {
			return true
		}
		return f(val.(*Peer), po)
	})
}

// EachAddr called with (base, po, f) is an iterator applying f to each known peer
// that has proximity order o or less as measured from the base
// if base is nil, kademlia base address is used
func (k *Kademlia) EachAddr(base []byte, o int, f func(*BzzAddr, int) bool) {
	k.lock.RLock()
	defer k.lock.RUnlock()
	k.eachAddr(base, o, f)
}

func (k *Kademlia) eachAddr(base []byte, o int, f func(*BzzAddr, int) bool) {
	if len(base) == 0 {
		base = k.base
	}
	k.addrs.EachNeighbour(base, Pof, func(val pot.Val, po int) bool {
		if po > o {
			return true
		}
		return f(val.(*entry).BzzAddr, po)
	})
}

func (k *Kademlia) NeighbourhoodDepth() (depth int) {
	k.lock.RLock()
	defer k.lock.RUnlock()
	return depthForPot(k.conns, k.MinProxBinSize, k.base)
}

// depthForPot returns the proximity order that defines the distance of
// the nearest neighbour set with cardinality >= MinProxBinSize
// if there is altogether less than MinProxBinSize peers it returns 0
// caller must hold the lock
func depthForPot(p *pot.Pot, minProxBinSize int, pivotAddr []byte) (depth int) {
	if p.Size() <= minProxBinSize {
		return 0
	}

	// total number of peers in iteration
	var size int

	// determining the depth is a two-step process
	// first we find the proximity bin of the shallowest of the MinProxBinSize peers
	// the numeric value of depth cannot be higher than this
	var maxDepth int

	f := func(v pot.Val, i int) bool {
		// po == 256 means that addr is the pivot address(self)
		if i == 256 {
			return true
		}
		size++

		// this means we have all nn-peers.
		// depth is by default set to the bin of the farthest nn-peer
		if size == minProxBinSize {
			maxDepth = i
			return false
		}

		return true
	}
	p.EachNeighbour(pivotAddr, Pof, f)

	// the second step is to test for empty bins in order from shallowest to deepest
	// if an empty bin is found, this will be the actual depth
	// we stop iterating if we hit the maxDepth determined in the first step
	p.EachBin(pivotAddr, Pof, 0, func(po int, _ int, f func(func(pot.Val, int) bool) bool) bool {
		if po == depth {
			if maxDepth == depth {
				return false
			}
			depth++
			return true
		}
		return false
	})

	return depth
}

// callable decides if an address entry represents a callable peer
func (k *Kademlia) callable(e *entry) bool {
	// not callable if peer is live or exceeded maxRetries
	if e.conn != nil || e.retries > k.MaxRetries {
		return false
	}
	// calculate the allowed number of retries based on time lapsed since last seen
	timeAgo := int64(time.Since(e.seenAt))
	div := int64(k.RetryExponent)
	div += (150000 - rand.Int63n(300000)) * div / 1000000
	var retries int
	for delta := timeAgo; delta > k.RetryInterval; delta /= div {
		retries++
	}
	// this is never called concurrently, so safe to increment
	// peer can be retried again
	if retries < e.retries {
		log.Trace(fmt.Sprintf("%08x: %v long time since last try (at %v) needed before retry %v, wait only warrants %v", k.BaseAddr()[:4], e, timeAgo, e.retries, retries))
		return false
	}
	// function to sanction or prevent suggesting a peer
	if k.Reachable != nil && !k.Reachable(e.BzzAddr) {
		log.Trace(fmt.Sprintf("%08x: peer %v is temporarily not callable", k.BaseAddr()[:4], e))
		return false
	}
	e.retries++
	log.Trace(fmt.Sprintf("%08x: peer %v is callable", k.BaseAddr()[:4], e))

	return true
}

// BaseAddr return the kademlia base address
func (k *Kademlia) BaseAddr() []byte {
	return k.base
}

// String returns kademlia table + kaddb table displayed with ascii
func (k *Kademlia) String() string {
	k.lock.RLock()
	defer k.lock.RUnlock()
	return k.string()
}

// string returns kademlia table + kaddb table displayed with ascii
// caller must hold the lock
func (k *Kademlia) string() string {
	wsrow := "                          "
	var rows []string

	rows = append(rows, "=========================================================================")
	rows = append(rows, fmt.Sprintf("%v KΛÐΞMLIΛ hive: queen's address: %x", time.Now().UTC().Format(time.UnixDate), k.BaseAddr()[:3]))
	rows = append(rows, fmt.Sprintf("population: %d (%d), MinProxBinSize: %d, MinBinSize: %d, MaxBinSize: %d", k.conns.Size(), k.addrs.Size(), k.MinProxBinSize, k.MinBinSize, k.MaxBinSize))

	liverows := make([]string, k.MaxProxDisplay)
	peersrows := make([]string, k.MaxProxDisplay)

	depth := depthForPot(k.conns, k.MinProxBinSize, k.base)
	rest := k.conns.Size()
	k.conns.EachBin(k.base, Pof, 0, func(po, size int, f func(func(val pot.Val, i int) bool) bool) bool {
		var rowlen int
		if po >= k.MaxProxDisplay {
			po = k.MaxProxDisplay - 1
		}
		row := []string{fmt.Sprintf("%2d", size)}
		rest -= size
		f(func(val pot.Val, vpo int) bool {
			e := val.(*Peer)
			row = append(row, fmt.Sprintf("%x", e.Address()[:2]))
			rowlen++
			return rowlen < 4
		})
		r := strings.Join(row, " ")
		r = r + wsrow
		liverows[po] = r[:31]
		return true
	})

	k.addrs.EachBin(k.base, Pof, 0, func(po, size int, f func(func(val pot.Val, i int) bool) bool) bool {
		var rowlen int
		if po >= k.MaxProxDisplay {
			po = k.MaxProxDisplay - 1
		}
		if size < 0 {
			panic("wtf")
		}
		row := []string{fmt.Sprintf("%2d", size)}
		// we are displaying live peers too
		f(func(val pot.Val, vpo int) bool {
			e := val.(*entry)
			row = append(row, Label(e))
			rowlen++
			return rowlen < 4
		})
		peersrows[po] = strings.Join(row, " ")
		return true
	})

	for i := 0; i < k.MaxProxDisplay; i++ {
		if i == depth {
			rows = append(rows, fmt.Sprintf("============ DEPTH: %d ==========================================", i))
		}
		left := liverows[i]
		right := peersrows[i]
		if len(left) == 0 {
			left = " 0                             "
		}
		if len(right) == 0 {
			right = " 0"
		}
		rows = append(rows, fmt.Sprintf("%03d %v | %v", i, left, right))
	}
	rows = append(rows, "=========================================================================")
	return "\n" + strings.Join(rows, "\n")
}

// PeerPot keeps info about expected nearest neighbours
// used for testing only
// TODO move to separate testing tools file
type PeerPot struct {
	NNSet [][]byte
}

// NewPeerPotMap creates a map of pot record of *BzzAddr with keys
// as hexadecimal representations of the address.
// the MinProxBinSize of the passed kademlia is used
// used for testing only
// TODO move to separate testing tools file
func NewPeerPotMap(minProxBinSize int, addrs [][]byte) map[string]*PeerPot {

	// create a table of all nodes for health check
	np := pot.NewPot(nil, 0)
	for _, addr := range addrs {
		np, _, _ = pot.Add(np, addr, Pof)
	}
	ppmap := make(map[string]*PeerPot)

	// generate an allknowing source of truth for connections
	// for every kademlia passed
	for i, a := range addrs {

		// actual kademlia depth
		depth := depthForPot(np, minProxBinSize, a)

		// all nn-peers
		var nns [][]byte

		// iterate through the neighbours, going from the deepest to the shallowest
		np.EachNeighbour(a, Pof, func(val pot.Val, po int) bool {
			addr := val.([]byte)
			// po == 256 means that addr is the pivot address(self)
			// we do not include self in the map
			if po == 256 {
				return true
			}
			// append any neighbors found
			// a neighbor is any peer in or deeper than the depth
			if po >= depth {
				nns = append(nns, addr)
				return true
			}
			return false
		})

		log.Trace(fmt.Sprintf("%x PeerPotMap NNS: %s", addrs[i][:4], LogAddrs(nns)))
		ppmap[common.Bytes2Hex(a)] = &PeerPot{
			NNSet: nns,
		}
	}
	return ppmap
}

// saturation iterates through all peers and
// returns the smallest po value in which the node has less than n peers
// if the iterator reaches depth, then value for depth is returned
// TODO move to separate testing tools file
// TODO this function will stop at the first bin with less than MinBinSize peers, even if there are empty bins between that bin and the depth. This may not be correct behavior
func (k *Kademlia) saturation() int {
	prev := -1
	k.addrs.EachBin(k.base, Pof, 0, func(po, size int, f func(func(val pot.Val, i int) bool) bool) bool {
		prev++
		return prev == po && size >= k.MinBinSize
	})
	// TODO evaluate whether this check cannot just as well be done within the eachbin
	depth := depthForPot(k.conns, k.MinProxBinSize, k.base)
	if depth < prev {
		return depth
	}
	return prev
}

// knowNeighbours tests if all neighbours in the peerpot
// are found among the peers known to the kademlia
// It is used in Healthy function for testing only
// TODO move to separate testing tools file
func (k *Kademlia) knowNeighbours(addrs [][]byte) (got bool, n int, missing [][]byte) {
	pm := make(map[string]bool)
	depth := depthForPot(k.conns, k.MinProxBinSize, k.base)
	// create a map with all peers at depth and deeper known in the kademlia
	k.eachAddr(nil, 255, func(p *BzzAddr, po int) bool {
		// in order deepest to shallowest compared to the kademlia base address
		// all bins (except self) are included (0 <= bin <= 255)
		if po < depth {
			return false
		}
		pk := common.Bytes2Hex(p.Address())
		pm[pk] = true
		return true
	})

	// iterate through nearest neighbors in the peerpot map
	// if we can't find the neighbor in the map we created above
	// then we don't know all our neighbors
	// (which sadly is all too common in modern society)
	var gots int
	var culprits [][]byte
	for _, p := range addrs {
		pk := common.Bytes2Hex(p)
		if pm[pk] {
			gots++
		} else {
			log.Trace(fmt.Sprintf("%08x: known nearest neighbour %s not found", k.base, pk))
			culprits = append(culprits, p)
		}
	}
	return gots == len(addrs), gots, culprits
}

// connectedNeighbours tests if all neighbours in the peerpot
// are currently connected in the kademlia
// It is used in Healthy function for testing only
func (k *Kademlia) connectedNeighbours(peers [][]byte) (got bool, n int, missing [][]byte) {
	pm := make(map[string]bool)
	depth := depthForPot(k.conns, k.MinProxBinSize, k.base)
	k.eachConn(nil, 255, func(p *Peer, po int) bool {
		if po < depth {
			return false
		}
		pk := common.Bytes2Hex(p.Address())
		pm[pk] = true
		return true
	})

	// iterate through nearest neighbors in the peerpot map
	// if we can't find the neighbor in the map we created above
	// then we don't know all our neighbors
	var gots int
	var culprits [][]byte
	for _, p := range peers {
		pk := common.Bytes2Hex(p)
		if pm[pk] {
			gots++
		} else {
			log.Trace(fmt.Sprintf("%08x: ExpNN: %s not found", k.base, pk))
			culprits = append(culprits, p)
		}
	}
	return gots == len(peers), gots, culprits
}

// Health state of the Kademlia
// used for testing only
type Health struct {
	KnowNN           bool     // whether node knows all its neighbours
	CountKnowNN      int      // amount of neighbors known
	MissingKnowNN    [][]byte // which neighbours we should have known but we don't
	ConnectNN        bool     // whether node is connected to all its neighbours
	CountConnectNN   int      // amount of neighbours connected to
	MissingConnectNN [][]byte // which neighbours we should have been connected to but we're not
	Saturated        bool     // whether we are connected to all the peers we would have liked to
	Hive             string
}

// Healthy reports the health state of the kademlia connectivity
//
// The PeerPot argument provides an all-knowing view of the network
// The resulting Health object is a result of comparisons between
// what is the actual composition of the kademlia in question (the receiver), and
// what SHOULD it have been when we take all we know about the network into consideration.
//
// used for testing only
func (k *Kademlia) Healthy(pp *PeerPot) *Health {
	k.lock.RLock()
	defer k.lock.RUnlock()
	gotnn, countgotnn, culpritsgotnn := k.connectedNeighbours(pp.NNSet)
	knownn, countknownn, culpritsknownn := k.knowNeighbours(pp.NNSet)
	depth := depthForPot(k.conns, k.MinProxBinSize, k.base)
	saturated := k.saturation() < depth
	log.Trace(fmt.Sprintf("%08x: healthy: knowNNs: %v, gotNNs: %v, saturated: %v\n", k.base, knownn, gotnn, saturated))
	return &Health{
		KnowNN:           knownn,
		CountKnowNN:      countknownn,
		MissingKnowNN:    culpritsknownn,
		ConnectNN:        gotnn,
		CountConnectNN:   countgotnn,
		MissingConnectNN: culpritsgotnn,
		Saturated:        saturated,
		Hive:             k.string(),
	}
}