router: return probability from findPath

The returned probability can then be used in QueryRoutes to not having
to reconstruct the probability.

lnrpc/routerrpc/router_backend.go

@@ -57,7 +57,7 @@ type RouterBackend struct {
 		restrictions *routing.RestrictParams,
 		destCustomRecords record.CustomSet,
 		routeHints map[route.Vertex][]*channeldb.CachedEdgePolicy,
-		finalExpiry uint16) (*route.Route, error)
+		finalExpiry uint16) (*route.Route, float64, error)
 
 	MissionControl MissionControl
 
@@ -325,7 +325,7 @@ func (r *RouterBackend) QueryRoutes(ctx context.Context,
 	// Query the channel router for a possible path to the destination that
 	// can carry `in.Amt` satoshis _including_ the total fee required on
 	// the route.
-	route, err := r.FindRoute(
+	route, _, err := r.FindRoute(
 		sourcePubKey, targetPubKey, amt, in.TimePref, restrictions,
 		customRecords, routeHintEdges, finalCLTVDelta,
 	)

lnrpc/routerrpc/router_backend_test.go

@@ -126,7 +126,7 @@ func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool,
 		amt lnwire.MilliSatoshi, _ float64,
 		restrictions *routing.RestrictParams, _ record.CustomSet,
 		routeHints map[route.Vertex][]*channeldb.CachedEdgePolicy,
-		finalExpiry uint16) (*route.Route, error) {
+		finalExpiry uint16) (*route.Route, float64, error) {
 
 		if int64(amt) != amtSat*1000 {
 			t.Fatal("unexpected amount")
@@ -187,7 +187,9 @@ func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool,
 		}
 
 		hops := []*route.Hop{{}}
-		return route.NewRouteFromHops(amt, 144, source, hops)
+		route, err := route.NewRouteFromHops(amt, 144, source, hops)
+
+		return route, expectedProb, err
 	}
 
 	backend := &RouterBackend{

lnrpc/routerrpc/router_server.go

@@ -366,7 +366,7 @@ func (s *Server) EstimateRouteFee(ctx context.Context,
 	// restriction for the default CLTV limit, otherwise we can find a route
 	// that exceeds it and is useless to us.
 	mc := s.cfg.RouterBackend.MissionControl
-	route, err := s.cfg.Router.FindRoute(
+	route, _, err := s.cfg.Router.FindRoute(
 		s.cfg.RouterBackend.SelfNode, destNode, amtMsat, 0,
 		&routing.RestrictParams{
 			FeeLimit:          feeLimit,