routing: add a caching layer in front of the KSP algorithm

This commit adds caching to our route finding. Caching is done on a
tuple-basis mapping a (dest, amt) pair to a previously calculated set
of shortest paths. The cache invalidated on two occasions: when a block
closes a set of transactions, or we received a new channel update or
channel announcement message.

With this change, payments are now snappier from the PoV of an
application developer as we no longer need to do a series of disk-seeks
before we dispatch each payment.

routing/router.go

@@ -86,6 +86,25 @@ type Config struct {
 		htlcAdd *lnwire.UpdateAddHTLC) ([32]byte, error)
 }
 
+// routeTuple is an entry within the ChannelRouter's route cache. We cache
+// prospective routes based on first the destination, and then the target
+// amount. We required the target amount as that will influence the available
+// set of paths for a payment.
+type routeTuple struct {
+	amt  btcutil.Amount
+	dest [33]byte
+}
+
+// newRouteTuple creates a new route tuple from the target and amount.
+func newRouteTuple(amt btcutil.Amount, dest *btcec.PublicKey) routeTuple {
+	r := routeTuple{
+		amt: amt,
+	}
+	copy(r.dest[:], dest.SerializeCompressed())
+
+	return r
+}
+
 // ChannelRouter is the layer 3 router within the Lightning stack. Below the
 // ChannelRouter is the HtlcSwitch, and below that is the Bitcoin blockchain
 // itself. The primary role of the ChannelRouter is to respond to queries for
@@ -110,10 +129,16 @@ type ChannelRouter struct {
 	// when doing any path finding.
 	selfNode *channeldb.LightningNode
 
-	// TODO(roasbeef): make LRU, invalidate upon new block connect
-	shortestPathCache map[[33]byte][]*Route
-	nodeCache         map[[33]byte]*channeldb.LightningNode
-	edgeCache         map[wire.OutPoint]*channeldb.ChannelEdgePolicy
+	// routeCache is a map that caches the k-shortest paths from ourselves
+	// to a given target destination for a particular payment amount. This
+	// map is used as an optimization to speed up subsequent payments to a
+	// particular destination. This map will be cleared each time a new
+	// channel announcement is accepted, or a new block arrives that
+	// results in channels being closed.
+	//
+	// TODO(roasbeef): make LRU
+	routeCacheMtx sync.RWMutex
+	routeCache    map[routeTuple][]*Route
 
 	// newBlocks is a channel in which new blocks connected to the end of
 	// the main chain are sent over.
@@ -194,6 +219,7 @@ func New(cfg Config) (*ChannelRouter, error) {
 		prematureAnnouncements: make(map[uint32][]lnwire.Message),
 		topologyClients:        make(map[uint64]topologyClient),
 		ntfnClientUpdates:      make(chan *topologyClientUpdate),
+		routeCache:             make(map[routeTuple][]*Route),
 		quit:                   make(chan struct{}),
 	}, nil
 }
@@ -486,6 +512,13 @@ func (r *ChannelRouter) networkHandler() {
 				continue
 			}
 
+			// Invalidate the route cache as channels within the
+			// graph have closed, which may affect our choice of
+			// the KSP's for a particular routeTuple.
+			r.routeCacheMtx.Lock()
+			r.routeCache = make(map[routeTuple][]*Route)
+			r.routeCacheMtx.Unlock()
+
 			// Notify all currently registered clients of the newly
 			// closed channels.
 			closeSummaries := createCloseSummaries(blockHeight, chansClosed...)
@@ -628,6 +661,8 @@ func (r *ChannelRouter) processNetworkAnnouncement(msg lnwire.Message) bool {
 		return chanID.BlockHeight > r.bestHeight
 	}
 
+	var invalidateCache bool
+
 	switch msg := msg.(type) {
 
 	// A new node announcement has arrived which either presents a new
@@ -749,6 +784,7 @@ func (r *ChannelRouter) processNetworkAnnouncement(msg lnwire.Message) bool {
 			return false
 		}
 
+		invalidateCache = true
 		log.Infof("New channel discovered! Link "+
 			"connects %x and %x with ChannelPoint(%v), chan_id=%v",
 			msg.FirstNodeID.SerializeCompressed(),
@@ -845,10 +881,20 @@ func (r *ChannelRouter) processNetworkAnnouncement(msg lnwire.Message) bool {
 			return false
 		}
 
+		invalidateCache = true
 		log.Infof("New channel update applied: %v",
 			spew.Sdump(chanUpdate))
 	}
 
+	// If we've received a channel update, then invalidate the route cache
+	// as channels within the graph have closed, which may affect our
+	// choice of the KSP's for a particular routeTuple.
+	if invalidateCache {
+		r.routeCacheMtx.Lock()
+		r.routeCache = make(map[routeTuple][]*Route)
+		r.routeCacheMtx.Unlock()
+	}
+
 	return true
 }
 
@@ -1222,23 +1268,44 @@ type LightningPayment struct {
 func (r *ChannelRouter) SendPayment(payment *LightningPayment) ([32]byte, *Route, error) {
 	log.Tracef("Dispatching route for lightning payment: %v",
 		newLogClosure(func() string {
+			payment.Target.Curve = nil
 			return spew.Sdump(payment)
 		}),
 	)
 
-	// TODO(roasbeef): consult KSP cache before dispatching
-
 	var (
 		sendError error
 		preImage  [32]byte
 	)
 
-	// Query the graph for a set of potential routes to the destination
-	// node that can support our payment amount. If no such routes can be
-	// found then an error will be returned.
-	routes, err := r.FindRoutes(payment.Target, payment.Amount)
-	if err != nil {
-		return preImage, nil, err
+	// TODO(roasbeef): consult KSP cache before dispatching
+
+	// Before attempting to perform a series of graph traversals to find
+	// the k-shortest paths to the destination, we'll first consult our
+	// path cache
+	rt := newRouteTuple(payment.Amount, payment.Target)
+
+	r.routeCacheMtx.RLock()
+	routes, ok := r.routeCache[rt]
+	r.routeCacheMtx.RUnlock()
+
+	// If we don't have a set of routes cached, we'll query the graph for a
+	// set of potential routes to the destination node that can support our
+	// payment amount. If no such routes can be found then an error will be
+	// returned.
+	if !ok {
+		freshRoutes, err := r.FindRoutes(payment.Target, payment.Amount)
+		if err != nil {
+			return preImage, nil, err
+		}
+
+		// Populate the cache with this set of fresh routes so we can
+		// reuse them in the future.
+		r.routeCacheMtx.Lock()
+		r.routeCache[rt] = freshRoutes
+		r.routeCacheMtx.Unlock()
+
+		routes = freshRoutes
 	}
 
 	// For each eligible path, we'll attempt to successfully send our
@@ -1246,7 +1313,7 @@ func (r *ChannelRouter) SendPayment(payment *LightningPayment) ([32]byte, *Route
 	// serially until either once succeeds, or we've exhausted our set of
 	// available paths.
 	for _, route := range routes {
-		log.Tracef("Attempting to send payment %x, using route: %#v",
+		log.Tracef("Attempting to send payment %x, using route: %v",
 			payment.PaymentHash, newLogClosure(func() string {
 				return spew.Sdump(route)
 			}),