routing: add the option to set the income channel on blinded path

It was made general enough to specify chained channels hops.

routing/pathfind.go

@@ -1165,6 +1165,10 @@ type blindedPathRestrictions struct {
 	// nodeOmissionSet holds a set of node IDs of nodes that we should
 	// ignore during blinded path selection.
 	nodeOmissionSet fn.Set[route.Vertex]
+
+	// channelIncomeSet holds a chained channels that we
+	// should use as income hops during blinded path selection.
+	channelIncomeSet []uint64
 }
 
 // blindedHop holds the information about a hop we have selected for a blinded
@@ -1300,6 +1304,16 @@ func processNodeForBlindedPath(g Graph, node route.Vertex,
 	// node that can be used for blinded paths
 	err = g.ForEachNodeChannel(node,
 		func(channel *channeldb.DirectedChannel) error {
+			// If this channel was included  into the chained
+			// channel route.
+			chanInSet := restrictions.channelIncomeSet
+			chanIx := len(alreadyVisited)
+			if chanIx < len(chanInSet) {
+				if chanInSet[chanIx] != channel.ChannelID {
+					return nil
+				}
+			}
+
 			// Keep track of how many incoming channels this node
 			// has. We only use a node as an introduction node if it
 			// has channels other than the one that lead us to it.

routing/router.go

@@ -612,6 +612,10 @@ type BlindedPathRestrictions struct {
 	// NodeOmissionSet is a set of nodes that should not be used within any
 	// of the blinded paths that we generate.
 	NodeOmissionSet fn.Set[route.Vertex]
+
+	// channelIncomeSet holds a chained channels route that we
+	// should use as income hops during blinded path selection.
+	ChannelIncomeSet []uint64
 }
 
 // FindBlindedPaths finds a selection of paths to the destination node that can
@@ -624,9 +628,10 @@ func (r *ChannelRouter) FindBlindedPaths(destination route.Vertex,
 	// path length restrictions.
 	paths, err := findBlindedPaths(
 		r.cfg.RoutingGraph, destination, &blindedPathRestrictions{
-			minNumHops:      restrictions.MinDistanceFromIntroNode,
-			maxNumHops:      restrictions.NumHops,
-			nodeOmissionSet: restrictions.NodeOmissionSet,
+			minNumHops:       restrictions.MinDistanceFromIntroNode,
+			maxNumHops:       restrictions.NumHops,
+			nodeOmissionSet:  restrictions.NodeOmissionSet,
+			channelIncomeSet: restrictions.ChannelIncomeSet,
 		},
 	)
 	if err != nil {