routerrpc: allow BuildRoute RPC to use arbitrary source

The routerrpc.Server and ChannelRouter assume that the source
public key for a route is always the node’s public key when calling
BuildRoute.

We update to allow overriding of this field to any arbitrary public
key. The list of hop public keys must be connected to the source
via channels in our graph view.

Author: calvinrzachman

lnrpc/routerrpc/router.pb.go

@@ -753,6 +753,12 @@ message BuildRouteRequest {
     base64.
     */
     map<uint64, bytes> first_hop_custom_records = 6;
+
+    /*
+    An optional source node public key from whose perspective the route is to be
+    built. If empty, the router's identity key is assumed.
+    */
+    bytes source_pubkey = 7;
 }
 
 message BuildRouteResponse {

lnrpc/routerrpc/router.proto

@@ -1313,6 +1313,11 @@
             "format": "byte"
           },
           "description": "An optional field that can be used to pass an arbitrary set of TLV records\nto the first hop peer of this payment. This can be used to pass application\nspecific data during the payment attempt. Record types are required to be in\nthe custom range \u003e= 65536. When using REST, the values must be encoded as\nbase64."
+        },
+        "source_pubkey": {
+          "type": "string",
+          "format": "byte",
+          "description": "An optional source node public key from whose perspective the route is to be\nbuilt. If empty, the router's identity key is assumed."
         }
       }
     },

lnrpc/routerrpc/router.swagger.json

@@ -1488,10 +1488,23 @@ func (s *Server) BuildRoute(_ context.Context,
 		firstHopBlob = fn.Some(firstHopData)
 	}
 
+	// Set the source node based on the request, using the router's
+	// self-node as the default if no source_pubkey is provided.
+	var sourceNode route.Vertex
+	var err error
+	if len(req.SourcePubkey) != 0 {
+		sourceNode, err = route.NewVertexFromBytes(req.SourcePubkey)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		sourceNode = s.cfg.Router.GetSelfNode()
+	}
+
 	// Build the route and return it to the caller.
 	route, err := s.cfg.Router.BuildRoute(
-		amt, hops, outgoingChan, req.FinalCltvDelta, payAddr,
-		firstHopBlob,
+		sourceNode, amt, hops, outgoingChan, req.FinalCltvDelta,
+		payAddr, firstHopBlob,
 	)
 	if err != nil {
 		return nil, err

lnrpc/routerrpc/router_server.go

@@ -1376,15 +1376,18 @@ func (e ErrNoChannel) Error() string {
 		"node index %v", e.position)
 }
 
-// BuildRoute returns a fully specified route based on a list of pubkeys. If
-// amount is nil, the minimum routable amount is used. To force a specific
-// outgoing channel, use the outgoingChan parameter.
-func (r *ChannelRouter) BuildRoute(amt fn.Option[lnwire.MilliSatoshi],
-	hops []route.Vertex, outgoingChan *uint64, finalCltvDelta int32,
+// BuildRoute builds a fully specified route based on a list of pubkeys from
+// the perspective of the provided source node. If amount is nil, the minimum
+// routable amount is used. To force a specific outgoing channel, use the
+// outgoingChan parameter.
+func (r *ChannelRouter) BuildRoute(sourceNode route.Vertex,
+	amt fn.Option[lnwire.MilliSatoshi], hops []route.Vertex,
+	outgoingChan *uint64, finalCltvDelta int32,
 	payAddr fn.Option[[32]byte], firstHopBlob fn.Option[[]byte]) (
 	*route.Route, error) {
 
-	log.Tracef("BuildRoute called: hopsCount=%v, amt=%v", len(hops), amt)
+	log.Tracef("BuildRoute called: sourceNode=%v, hopsCount=%v, amt=%v",
+		sourceNode, len(hops), amt)
 
 	var outgoingChans map[uint64]struct{}
 	if outgoingChan != nil {
@@ -1403,12 +1406,10 @@ func (r *ChannelRouter) BuildRoute(amt fn.Option[lnwire.MilliSatoshi],
 		return nil, err
 	}
 
-	sourceNode := r.cfg.SelfNode
-
 	// We check that each node in the route has a connection to others that
 	// can forward in principle.
 	unifiers, err := getEdgeUnifiers(
-		r.cfg.SelfNode, hops, outgoingChans, r.cfg.RoutingGraph,
+		sourceNode, hops, outgoingChans, r.cfg.RoutingGraph,
 	)
 	if err != nil {
 		return nil, err

routing/router.go

@@ -1661,17 +1661,21 @@ func TestBuildRoute(t *testing.T) {
 
 	noAmt := fn.None[lnwire.MilliSatoshi]()
 
+	selfNode := ctx.router.GetSelfNode()
+
 	// Test that we can't build a route when no hops are given.
 	hops = []route.Vertex{}
 	_, err = ctx.router.BuildRoute(
-		noAmt, hops, nil, 40, fn.None[[32]byte](), fn.None[[]byte](),
+		selfNode, noAmt, hops, nil, 40,
+		fn.None[[32]byte](), fn.None[[]byte](),
 	)
 	require.Error(t, err)
 
 	// Create hop list for an unknown destination.
 	hops := []route.Vertex{ctx.aliases["b"], ctx.aliases["y"]}
 	_, err = ctx.router.BuildRoute(
-		noAmt, hops, nil, 40, fn.Some(payAddr), fn.None[[]byte](),
+		selfNode, noAmt, hops, nil, 40,
+		fn.Some(payAddr), fn.None[[]byte](),
 	)
 	noChanErr := ErrNoChannel{}
 	require.ErrorAs(t, err, &noChanErr)
@@ -1683,8 +1687,8 @@ func TestBuildRoute(t *testing.T) {
 
 	// Build the route for the given amount.
 	rt, err := ctx.router.BuildRoute(
-		fn.Some(amt), hops, nil, 40, fn.Some(payAddr),
-		fn.None[[]byte](),
+		selfNode, fn.Some(amt), hops, nil, 40,
+		fn.Some(payAddr), fn.None[[]byte](),
 	)
 	require.NoError(t, err)
 
@@ -1696,7 +1700,8 @@ func TestBuildRoute(t *testing.T) {
 
 	// Build the route for the minimum amount.
 	rt, err = ctx.router.BuildRoute(
-		noAmt, hops, nil, 40, fn.Some(payAddr), fn.None[[]byte](),
+		selfNode, noAmt, hops, nil, 40,
+		fn.Some(payAddr), fn.None[[]byte](),
 	)
 	require.NoError(t, err)
 
@@ -1714,7 +1719,8 @@ func TestBuildRoute(t *testing.T) {
 	// There is no amount that can pass through both channel 5 and 4.
 	hops = []route.Vertex{ctx.aliases["e"], ctx.aliases["c"]}
 	_, err = ctx.router.BuildRoute(
-		noAmt, hops, nil, 40, fn.None[[32]byte](), fn.None[[]byte](),
+		selfNode, noAmt, hops, nil, 40,
+		fn.None[[32]byte](), fn.None[[]byte](),
 	)
 	require.Error(t, err)
 	noChanErr = ErrNoChannel{}
@@ -1734,7 +1740,8 @@ func TestBuildRoute(t *testing.T) {
 	// policy of channel 3.
 	hops = []route.Vertex{ctx.aliases["b"], ctx.aliases["z"]}
 	rt, err = ctx.router.BuildRoute(
-		noAmt, hops, nil, 40, fn.Some(payAddr), fn.None[[]byte](),
+		selfNode, noAmt, hops, nil, 40,
+		fn.Some(payAddr), fn.None[[]byte](),
 	)
 	require.NoError(t, err)
 	checkHops(rt, []uint64{1, 8}, payAddr)
@@ -1748,8 +1755,8 @@ func TestBuildRoute(t *testing.T) {
 	hops = []route.Vertex{ctx.aliases["d"], ctx.aliases["f"]}
 	amt = lnwire.NewMSatFromSatoshis(100)
 	rt, err = ctx.router.BuildRoute(
-		fn.Some(amt), hops, nil, 40, fn.Some(payAddr),
-		fn.None[[]byte](),
+		selfNode, fn.Some(amt), hops, nil, 40,
+		fn.Some(payAddr), fn.None[[]byte](),
 	)
 	require.NoError(t, err)
 	checkHops(rt, []uint64{9, 10}, payAddr)
@@ -1765,11 +1772,22 @@ func TestBuildRoute(t *testing.T) {
 	// is a third pass through newRoute in which this gets corrected to end
 	hops = []route.Vertex{ctx.aliases["d"], ctx.aliases["f"]}
 	rt, err = ctx.router.BuildRoute(
-		noAmt, hops, nil, 40, fn.Some(payAddr), fn.None[[]byte](),
+		selfNode, noAmt, hops, nil, 40,
+		fn.Some(payAddr), fn.None[[]byte](),
 	)
 	require.NoError(t, err)
 	checkHops(rt, []uint64{9, 10}, payAddr)
 	require.EqualValues(t, 20180, rt.TotalAmount, "%v", rt.TotalAmount)
+
+	// Test a route built from an alternate source node (eg: b --> c).
+	hops = []route.Vertex{ctx.aliases["c"]}
+	rt, err = ctx.router.BuildRoute(
+		ctx.aliases["b"], fn.Some(amt), hops, nil, 40,
+		fn.Some(payAddr), fn.None[[]byte](),
+	)
+	require.NoError(t, err)
+	checkHops(rt, []uint64{7}, payAddr)
+
 }
 
 // TestReceiverAmtForwardPass tests that the forward pass returns the expected