routing: abstract path finding interface

routing/missioncontrol.go

@@ -226,6 +226,7 @@ func (m *missionControl) NewPaymentSession(routeHints [][]zpay32.HopHint,
 		bandwidthHints:       bandwidthHints,
 		errFailedPolicyChans: make(map[EdgeLocator]struct{}),
 		mc:                   m,
+		pathFinder:           findPath,
 	}, nil
 }
 
@@ -240,6 +241,7 @@ func (m *missionControl) NewPaymentSessionFromRoutes(routes []*Route) *paymentSe
 		preBuiltRoutes:       routes,
 		errFailedPolicyChans: make(map[EdgeLocator]struct{}),
 		mc:                   m,
+		pathFinder:           findPath,
 	}
 }
 

routing/pathfind.go

@@ -39,6 +39,11 @@ const (
 	RiskFactorBillionths = 15
 )
 
+// pathFinder defines the interface of a path finding algorithm.
+type pathFinder = func(g *graphParams, r *RestrictParams,
+	source, target Vertex, amt lnwire.MilliSatoshi) (
+	[]*channeldb.ChannelEdgePolicy, error)
+
 // Hop represents an intermediate or final node of the route. This naming
 // is in line with the definition given in BOLT #4: Onion Routing Protocol.
 // The struct houses the channel along which this hop can be reached and

routing/payment_session.go

@@ -33,6 +33,8 @@ type paymentSession struct {
 
 	haveRoutes     bool
 	preBuiltRoutes []*Route
+
+	pathFinder pathFinder
 }
 
 // ReportVertexFailure adds a vertex to the graph prune view after a client
@@ -141,7 +143,7 @@ func (p *paymentSession) RequestRoute(payment *LightningPayment,
 	// Taking into account this prune view, we'll attempt to locate a path
 	// to our destination, respecting the recommendations from
 	// missionControl.
-	path, err := findPath(
+	path, err := p.pathFinder(
 		&graphParams{
 			graph:           p.mc.graph,
 			additionalEdges: p.additionalEdges,