routing: add last hop restriction

routing/pathfind.go

@@ -263,6 +263,10 @@ type RestrictParams struct {
 	// hop. If nil, any channel may be used.
 	OutgoingChannelID *uint64
 
+	// LastHop is the pubkey of the last node before the final destination
+	// is reached. If nil, any node may be used.
+	LastHop *route.Vertex
+
 	// CltvLimit is the maximum time lock of the route excluding the final
 	// ctlv. After path finding is complete, the caller needs to increase
 	// all cltv expiry heights with the required final cltv delta.
@@ -562,6 +566,13 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		// Expand all connections using the optimal policy for each
 		// connection.
 		for fromNode, unifiedPolicy := range u.policies {
+			// Apply last hop restriction if set.
+			if r.LastHop != nil &&
+				pivot == target && fromNode != *r.LastHop {
+
+				continue
+			}
+
 			policy := unifiedPolicy.getPolicy(
 				amtToSend, g.bandwidthHints,
 			)

routing/pathfind_test.go

@@ -1874,6 +1874,51 @@ func TestRestrictOutgoingChannel(t *testing.T) {
 	}
 }
 
+// TestRestrictLastHop asserts that a last hop restriction is obeyed by the path
+// finding algorithm.
+func TestRestrictLastHop(t *testing.T) {
+	t.Parallel()
+
+	// Set up a test graph with three possible paths from roasbeef to
+	// target. The path via channel 1 and 2 is the lowest cost path.
+	testChannels := []*testChannel{
+		symmetricTestChannel("source", "a", 100000, &testChannelPolicy{
+			Expiry: 144,
+		}, 1),
+		symmetricTestChannel("a", "target", 100000, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 400,
+		}, 2),
+		symmetricTestChannel("source", "b", 100000, &testChannelPolicy{
+			Expiry: 144,
+		}, 3),
+		symmetricTestChannel("b", "target", 100000, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 800,
+		}, 4),
+	}
+
+	ctx := newPathFindingTestContext(t, testChannels, "source")
+	defer ctx.cleanup()
+
+	paymentAmt := lnwire.NewMSatFromSatoshis(100)
+	target := ctx.keyFromAlias("target")
+	lastHop := ctx.keyFromAlias("b")
+
+	// Find the best path given the restriction to use b as the last hop.
+	// This should force pathfinding to not take the lowest cost option.
+	ctx.restrictParams.LastHop = &lastHop
+	path, err := ctx.findPath(target, paymentAmt)
+	if err != nil {
+		t.Fatalf("unable to find path: %v", err)
+	}
+	if path[0].ChannelID != 3 {
+		t.Fatalf("expected route to pass through channel 3, "+
+			"but channel %v was selected instead",
+			path[0].ChannelID)
+	}
+}
+
 // TestCltvLimit asserts that a cltv limit is obeyed by the path finding
 // algorithm.
 func TestCltvLimit(t *testing.T) {