routing: add new NewPaymentSessionFromRoutes function instead of interface

In this commit, we modify the recent refactoring of the mission control
sub-system to overload the existing payment session, rather than create
a brand new one. This allows us to re-use more of the existing logic, and
also feedback into mission control the failures incurred by any user
selected routes.

routing/missioncontrol.go

@@ -1,6 +1,7 @@
 package routing
 
 import (
+	"fmt"
 	"sync"
 	"time"
 
@@ -71,8 +72,7 @@ type missionControl struct {
 // newMissionControl returns a new instance of missionControl.
 //
 // TODO(roasbeef): persist memory
-func newMissionControl(
+func newMissionControl(g *channeldb.ChannelGraph, selfNode *channeldb.LightningNode,
-	g *channeldb.ChannelGraph, selfNode *channeldb.LightningNode,
 	qb func(*channeldb.ChannelEdgeInfo) lnwire.MilliSatoshi) *missionControl {
 
 	return &missionControl{
@@ -165,6 +165,9 @@ type paymentSession struct {
 	bandwidthHints map[uint64]lnwire.MilliSatoshi
 
 	mc *missionControl
+
+	haveRoutes     bool
+	preBuiltRoutes []*Route
 }
 
 // NewPaymentSession creates a new payment session backed by the latest prune
@@ -240,6 +243,18 @@ func (m *missionControl) NewPaymentSession(routeHints [][]HopHint,
 	}, nil
 }
 
+// NewPaymentSessionFromRoutes creates a new paymentSession instance that will
+// skip all path finding, and will instead utilize a set of pre-built routes.
+// This constructor allows callers to specify their own routes which can be
+// used for things like channel rebalancing, and swaps.
+func (m *missionControl) NewPaymentSessionFromRoutes(routes []*Route) *paymentSession {
+	return &paymentSession{
+		haveRoutes:     true,
+		preBuiltRoutes: routes,
+		mc:             m,
+	}
+}
+
 // generateBandwidthHints is a helper function that's utilized the main
 // findPath function in order to obtain hints from the lower layer w.r.t to the
 // available bandwidth of edges on the network. Currently, we'll only obtain
@@ -326,9 +341,25 @@ func (p *paymentSession) ReportChannelFailure(e uint64) {
 func (p *paymentSession) RequestRoute(payment *LightningPayment,
 	height uint32, finalCltvDelta uint16) (*Route, error) {
 
-	// First, we'll obtain our current prune view snapshot. This view will
+	switch {
-	// only ever grow during the duration of this payment session, never
+	// If we have a set of pre-built routes, then we'll just pop off the
-	// shrinking.
+	// next route from the queue, and use it directly.
+	case p.haveRoutes && len(p.preBuiltRoutes) > 0:
+		nextRoute := p.preBuiltRoutes[0]
+		p.preBuiltRoutes[0] = nil // Set to nil to avoid GC leak.
+		p.preBuiltRoutes = p.preBuiltRoutes[1:]
+
+		return nextRoute, nil
+
+	// If we were instantiated with a set of pre-built routes, and we've
+	// run out, then we'll return a terminal error.
+	case p.haveRoutes && len(p.preBuiltRoutes) == 0:
+		return nil, fmt.Errorf("pre-built routes exhausted")
+	}
+
+	// Otherwise we actually need to perform path finding, so we'll obtain
+	// our current prune view snapshot. This view will only ever grow
+	// during the duration of this payment session, never shrinking.
 	pruneView := p.pruneViewSnapshot
 
 	log.Debugf("Mission Control session using prune view of %v "+
@@ -352,8 +383,9 @@ func (p *paymentSession) RequestRoute(payment *LightningPayment,
 	// With the next candidate path found, we'll attempt to turn this into
 	// a route by applying the time-lock and fee requirements.
 	sourceVertex := Vertex(p.mc.selfNode.PubKeyBytes)
-	route, err := newRoute(payment.Amount, sourceVertex, path, height,
+	route, err := newRoute(
-		finalCltvDelta)
+		payment.Amount, sourceVertex, path, height, finalCltvDelta,
+	)
 	if err != nil {
 		// TODO(roasbeef): return which edge/vertex didn't work
 		// out

routing/router.go

@@ -1532,37 +1532,6 @@ type LightningPayment struct {
 	// TODO(roasbeef): add e2e message?
 }
 
-// PaymentSession imitates paymentSession created from mission control
-// and enables SendPayment and SendToRoute to call sendPayment.
-type PaymentSession interface {
-	RequestRoute(payment *LightningPayment, currentHeight uint32,
-		finalCLTVDelta uint16) (*Route, error)
-
-	ReportVertexFailure(Vertex)
-	ReportChannelFailure(chanID uint64)
-}
-
-// sendToRoutePaymentSession implements PaymentSession.
-type sendToRoutePaymentSession struct {
-	currentRouteIndex int
-	routes            []*Route
-}
-
-func (s *sendToRoutePaymentSession) RequestRoute(payment *LightningPayment,
-	currentHeight uint32, finalCLTVDelta uint16) (*Route, error) {
-
-	defer func() { s.currentRouteIndex++ }()
-
-	if s.currentRouteIndex < len(s.routes) {
-		return s.routes[s.currentRouteIndex], nil
-	}
-	return nil, errors.New("unable to send payment along any of " +
-		"the given routes")
-}
-
-func (s *sendToRoutePaymentSession) ReportVertexFailure(v Vertex)       {}
-func (s *sendToRoutePaymentSession) ReportChannelFailure(chanID uint64) {}
-
 // SendPayment attempts to send a payment as described within the passed
 // LightningPayment. This function is blocking and will return either: when the
 // payment is successful, or all candidates routes have been attempted and
@@ -1594,9 +1563,9 @@ func (r *ChannelRouter) SendPayment(payment *LightningPayment) ([32]byte, *Route
 func (r *ChannelRouter) SendToRoute(routes []*Route,
 	payment *LightningPayment) ([32]byte, *Route, error) {
 
-	paySession := &sendToRoutePaymentSession{
+	paySession := r.missionControl.NewPaymentSessionFromRoutes(
-		routes: routes,
+		routes,
-	}
+	)
 
 	return r.sendPayment(payment, paySession)
 }
@@ -1609,7 +1578,7 @@ func (r *ChannelRouter) SendToRoute(routes []*Route,
 // within the network to reach the destination. Additionally, the payment
 // preimage will also be returned.
 func (r *ChannelRouter) sendPayment(payment *LightningPayment,
-	paySession PaymentSession) ([32]byte, *Route, error) {
+	paySession *paymentSession) ([32]byte, *Route, error) {
 
 	log.Tracef("Dispatching route for lightning payment: %v",
 		newLogClosure(func() string {
@@ -1960,7 +1929,7 @@ func (r *ChannelRouter) sendPayment(payment *LightningPayment,
 // pruneVertexFailure will attempt to prune a vertex from the current available
 // vertexes of the target payment session in response to an encountered routing
 // error.
-func pruneVertexFailure(paySession PaymentSession, route *Route,
+func pruneVertexFailure(paySession *paymentSession, route *Route,
 	errSource *btcec.PublicKey, nextNode bool) {
 
 	// By default, we'll try to prune the node that actually sent us the
@@ -1986,7 +1955,7 @@ func pruneVertexFailure(paySession PaymentSession, route *Route,
 // pruneEdgeFailure will attempts to prune an edge from the current available
 // edges of the target payment session in response to an encountered routing
 // error.
-func pruneEdgeFailure(paySession PaymentSession, route *Route,
+func pruneEdgeFailure(paySession *paymentSession, route *Route,
 	errSource *btcec.PublicKey) {
 
 	// As this error indicates that the target channel was unable to carry