weave/mesh/routes.go

package mesh

import (
	"math"
	"sync"
)

type unicastRoutes map[PeerName]PeerName
type broadcastRoutes map[PeerName][]PeerName

type Routes struct {
	sync.RWMutex
	ourself      *LocalPeer
	peers        *Peers
	onChange     []func()
	unicast      unicastRoutes
	unicastAll   unicastRoutes // [1]
	broadcast    broadcastRoutes
	broadcastAll broadcastRoutes // [1]
	recalculate  chan<- *struct{}
	wait         chan<- chan struct{}
	action       chan<- func()
	// [1] based on *all* connections, not just established &
	// symmetric ones
}

func NewRoutes(ourself *LocalPeer, peers *Peers) *Routes {
	recalculate := make(chan *struct{}, 1)
	wait := make(chan chan struct{})
	action := make(chan func())
	routes := &Routes{
		ourself:      ourself,
		peers:        peers,
		unicast:      make(unicastRoutes),
		unicastAll:   make(unicastRoutes),
		broadcast:    make(broadcastRoutes),
		broadcastAll: make(broadcastRoutes),
		recalculate:  recalculate,
		wait:         wait,
		action:       action}
	routes.unicast[ourself.Name] = UnknownPeerName
	routes.unicastAll[ourself.Name] = UnknownPeerName
	routes.broadcast[ourself.Name] = []PeerName{}
	routes.broadcastAll[ourself.Name] = []PeerName{}
	go routes.run(recalculate, wait, action)
	return routes
}

func (routes *Routes) OnChange(callback func()) {
	routes.Lock()
	defer routes.Unlock()
	routes.onChange = append(routes.onChange, callback)
}

func (routes *Routes) PeerNames() PeerNameSet {
	return routes.peers.Names()
}

func (routes *Routes) Unicast(name PeerName) (PeerName, bool) {
	routes.RLock()
	defer routes.RUnlock()
	hop, found := routes.unicast[name]
	return hop, found
}

func (routes *Routes) UnicastAll(name PeerName) (PeerName, bool) {
	routes.RLock()
	defer routes.RUnlock()
	hop, found := routes.unicastAll[name]
	return hop, found
}

func (routes *Routes) Broadcast(name PeerName) []PeerName {
	routes.RLock()
	hops, found := routes.broadcast[name]
	routes.RUnlock()
	if found {
		return hops
	}
	res := make(chan []PeerName)
	routes.action <- func() {
		routes.RLock()
		hops, found := routes.broadcast[name]
		routes.RUnlock()
		if found {
			res <- hops
		}
		routes.peers.RLock()
		routes.ourself.RLock()
		hops = routes.calculateBroadcast(name, true)
		routes.ourself.RUnlock()
		routes.peers.RUnlock()
		res <- hops
		routes.Lock()
		routes.broadcast[name] = hops
		routes.Unlock()
	}
	return <-res
}

func (routes *Routes) BroadcastAll(name PeerName) []PeerName {
	routes.RLock()
	hops, found := routes.broadcastAll[name]
	routes.RUnlock()
	if found {
		return hops
	}
	res := make(chan []PeerName)
	routes.action <- func() {
		routes.RLock()
		hops, found := routes.broadcastAll[name]
		routes.RUnlock()
		if found {
			res <- hops
		}
		routes.peers.RLock()
		routes.ourself.RLock()
		hops = routes.calculateBroadcast(name, false)
		routes.ourself.RUnlock()
		routes.peers.RUnlock()
		res <- hops
		routes.Lock()
		routes.broadcastAll[name] = hops
		routes.Unlock()
	}
	return <-res
}

// Choose min(log2(n_peers), n_neighbouring_peers) neighbours, with a
// random distribution that is topology-sensitive, favouring
// neighbours at the end of "bottleneck links". We determine the
// latter based on the unicast routing table. If a neighbour appears
// as the value more frequently than others - meaning that we reach a
// higher proportion of peers via that neighbour than other neighbours
// - then it is chosen with a higher probability.
//
// Note that we choose log2(n_peers) *neighbours*, not
// peers. Consequently, on sparsely connected peers this function
// returns a higher proportion of neighbours than elsewhere. In
// extremis, on peers with fewer than log2(n_peers) neighbours, all
// neighbours are returned.
func (routes *Routes) RandomNeighbours(except PeerName) []PeerName {
	destinations := make(PeerNameSet)
	routes.RLock()
	defer routes.RUnlock()
	count := int(math.Log2(float64(len(routes.unicastAll))))
	// depends on go's random map iteration
	for _, dst := range routes.unicastAll {
		if dst != UnknownPeerName && dst != except {
			destinations[dst] = void
			if len(destinations) >= count {
				break
			}
		}
	}
	res := make([]PeerName, 0, len(destinations))
	for dst := range destinations {
		res = append(res, dst)
	}
	return res
}

// Request recalculation of the routing table. This is async but can
// effectively be made synchronous with a subsequent call to
// EnsureRecalculated.
func (routes *Routes) Recalculate() {
	// The use of a 1-capacity channel in combination with the
	// non-blocking send is an optimisation that results in multiple
	// requests being coalesced.
	select {
	case routes.recalculate <- nil:
	default:
		log.Println("route recalculation coalesced")
	}
}

// Wait for any preceding Recalculate requests to be processed.
func (routes *Routes) EnsureRecalculated() {
	done := make(chan struct{})
	routes.wait <- done
	<-done
}

func (routes *Routes) run(recalculate <-chan *struct{}, wait <-chan chan struct{}, action <-chan func()) {
	for {
		select {
		case <-recalculate:
			routes.calculate()
		case done := <-wait:
			select {
			case <-recalculate:
				routes.calculate()
			default:
			}
			close(done)
		case f := <-action:
			f()
		}
	}
}

func (routes *Routes) calculate() {
	routes.peers.RLock()
	routes.ourself.RLock()
	var (
		unicast      = routes.calculateUnicast(true)
		unicastAll   = routes.calculateUnicast(false)
		broadcast    = make(broadcastRoutes)
		broadcastAll = make(broadcastRoutes)
	)
	broadcast[routes.ourself.Name] = routes.calculateBroadcast(routes.ourself.Name, true)
	broadcastAll[routes.ourself.Name] = routes.calculateBroadcast(routes.ourself.Name, false)
	routes.ourself.RUnlock()
	routes.peers.RUnlock()

	routes.Lock()
	routes.unicast = unicast
	routes.unicastAll = unicastAll
	routes.broadcast = broadcast
	routes.broadcastAll = broadcastAll
	onChange := routes.onChange
	routes.Unlock()

	for _, callback := range onChange {
		callback()
	}
}

// Calculate all the routes for the question: if *we* want to send a
// packet to Peer X, what is the next hop?
//
// When we sniff a packet, we determine the destination peer
// ourself. Consequently, we can relay the packet via any
// arbitrary peers - the intermediate peers do not have to have
// any knowledge of the MAC address at all. Thus there's no need
// to exchange knowledge of MAC addresses, nor any constraints on
// the routes that we construct.
func (routes *Routes) calculateUnicast(establishedAndSymmetric bool) unicastRoutes {
	_, unicast := routes.ourself.Routes(nil, establishedAndSymmetric)
	return unicast
}

// Calculate the route to answer the question: if we receive a
// broadcast originally from Peer X, which peers should we pass the
// frames on to?
//
// When the topology is stable, and thus all peers perform route
// calculations based on the same data, the algorithm ensures that
// broadcasts reach every peer exactly once.
//
// This is largely due to properties of the Peer.Routes algorithm. In
// particular:
//
// ForAll X,Y,Z in Peers.
//     X.Routes(Y) <= X.Routes(Z) \/
//     X.Routes(Z) <= X.Routes(Y)
// ForAll X,Y,Z in Peers.
//     Y =/= Z /\ X.Routes(Y) <= X.Routes(Z) =>
//     X.Routes(Y) u [P | Y.HasSymmetricConnectionTo(P)] <= X.Routes(Z)
// where <= is the subset relationship on keys of the returned map.
func (routes *Routes) calculateBroadcast(name PeerName, establishedAndSymmetric bool) []PeerName {
	hops := []PeerName{}
	peer, found := routes.peers.byName[name]
	if !found {
		return hops
	}
	if found, reached := peer.Routes(routes.ourself.Peer, establishedAndSymmetric); found {
		routes.ourself.ForEachConnectedPeer(establishedAndSymmetric, reached,
			func(remotePeer *Peer) { hops = append(hops, remotePeer.Name) })
	}
	return hops
}