routing: limit routing info size during pathfinding

Also the max hop count check can be removed, because the real bound is
the payload size. By moving the check inside the search loop, we now
also backtrack when we hit the limit.

routing/heap.go

@@ -40,6 +40,10 @@ type nodeWithDist struct {
 
 	// nextHop is the edge this route comes from.
 	nextHop *channeldb.ChannelEdgePolicy
+
+	// routingInfoSize is the total size requirement for the payloads field
+	// in the onion packet from this hop towards the final destination.
+	routingInfoSize uint64
 }
 
 // distanceHeap is a min-distance heap that's used within our path finding

routing/pathfind.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/coreos/bbolt"
+	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/feature"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -16,12 +17,6 @@ import (
 )
 
 const (
-	// HopLimit is the maximum number hops that is permissible as a route.
-	// Any potential paths found that lie above this limit will be rejected
-	// with an error. This value is computed using the current fixed-size
-	// packet length of the Sphinx construction.
-	HopLimit = 20
-
 	// infinity is used as a starting distance in our shortest path search.
 	infinity = math.MaxInt64
 
@@ -79,10 +74,6 @@ var (
 	// not exist in the graph.
 	errNoPathFound = errors.New("unable to find a path to destination")
 
-	// errMaxHopsExceeded is returned when a candidate path is found, but
-	// the length of that path exceeds HopLimit.
-	errMaxHopsExceeded = errors.New("potential path has too many hops")
-
 	// errInsufficientLocalBalance is returned when none of the local
 	// channels have enough balance for the payment.
 	errInsufficientBalance = errors.New("insufficient local balance")
@@ -529,6 +520,23 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		}
 	}
 
+	// Build a preliminary destination hop structure to obtain the payload
+	// size.
+	var mpp *record.MPP
+	if r.PaymentAddr != nil {
+		mpp = record.NewMPP(amt, *r.PaymentAddr)
+	}
+
+	finalHop := route.Hop{
+		AmtToForward:     amt,
+		OutgoingTimeLock: uint32(finalHtlcExpiry),
+		CustomRecords:    r.DestCustomRecords,
+		LegacyPayload: !features.HasFeature(
+			lnwire.TLVOnionPayloadOptional,
+		),
+		MPP: mpp,
+	}
+
 	// We can't always assume that the end destination is publicly
 	// advertised to the network so we'll manually include the target node.
 	// The target node charges no fee. Distance is set to 0, because this is
@@ -545,6 +553,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		amountToReceive: amt,
 		incomingCltv:    finalHtlcExpiry,
 		probability:     1,
+		routingInfoSize: finalHop.PayloadSize(0),
 	}
 
 	// Calculate the absolute cltv limit. Use uint64 to prevent an overflow
@@ -554,6 +563,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 	// processEdge is a helper closure that will be used to make sure edges
 	// satisfy our specific requirements.
 	processEdge := func(fromVertex route.Vertex,
+		fromFeatures *lnwire.FeatureVector,
 		edge *channeldb.ChannelEdgePolicy, toNodeDist *nodeWithDist) {
 
 		edgesExpanded++
@@ -674,6 +684,34 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 				edge.ChannelID)
 		}
 
+		// Calculate the total routing info size if this hop were to be
+		// included. If we are coming from the source hop, the payload
+		// size is zero, because the original htlc isn't in the onion
+		// blob.
+		var payloadSize uint64
+		if fromVertex != source {
+			supportsTlv := fromFeatures.HasFeature(
+				lnwire.TLVOnionPayloadOptional,
+			)
+
+			hop := route.Hop{
+				AmtToForward: amountToSend,
+				OutgoingTimeLock: uint32(
+					toNodeDist.incomingCltv,
+				),
+				LegacyPayload: !supportsTlv,
+			}
+
+			payloadSize = hop.PayloadSize(edge.ChannelID)
+		}
+
+		routingInfoSize := toNodeDist.routingInfoSize + payloadSize
+
+		// Skip paths that would exceed the maximum routing info size.
+		if routingInfoSize > sphinx.MaxPayloadSize {
+			return
+		}
+
 		// All conditions are met and this new tentative distance is
 		// better than the current best known distance to this node.
 		// The new better distance is recorded, and also our "next hop"
@@ -686,6 +724,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 			incomingCltv:    incomingCltv,
 			probability:     probability,
 			nextHop:         edge,
+			routingInfoSize: routingInfoSize,
 		}
 		distance[fromVertex] = withDist
 
@@ -796,7 +835,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 
 			// Check if this candidate node is better than what we
 			// already have.
-			processEdge(fromNode, policy, partialPath)
+			processEdge(fromNode, fromFeatures, policy, partialPath)
 		}
 
 		if nodeHeap.Len() == 0 {
@@ -854,17 +893,8 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 	// findPath, and avoid using ChannelEdgePolicy altogether.
 	pathEdges[len(pathEdges)-1].Node.Features = features
 
-	// The route is invalid if it spans more than 20 hops. The current
-	// Sphinx (onion routing) implementation can only encode up to 20 hops
-	// as the entire packet is fixed size. If this route is more than 20
-	// hops, then it's invalid.
-	numEdges := len(pathEdges)
-	if numEdges > HopLimit {
-		return nil, errMaxHopsExceeded
-	}
-
 	log.Debugf("Found route: probability=%v, hops=%v, fee=%v\n",
-		distance[source].probability, numEdges,
+		distance[source].probability, len(pathEdges),
 		distance[source].amountToReceive-amt)
 
 	return pathEdges, nil

routing/pathfind_test.go

@@ -1393,8 +1393,19 @@ func TestNewRoutePathTooLong(t *testing.T) {
 	// Assert that finding a 21 hop route fails.
 	node21 := ctx.keyFromAlias("node-21")
 	_, err = ctx.findPath(node21, payAmt)
-	if err != errMaxHopsExceeded {
-		t.Fatalf("expected route too long, but got %v", err)
+	if err != errNoPathFound {
+		t.Fatalf("not route error expected, but got %v", err)
+	}
+
+	// Assert that we can't find a 20 hop route if custom records make it
+	// exceed the maximum payload size.
+	ctx.restrictParams.DestFeatures = tlvFeatures
+	ctx.restrictParams.DestCustomRecords = map[uint64][]byte{
+		100000: bytes.Repeat([]byte{1}, 100),
+	}
+	_, err = ctx.findPath(node20, payAmt)
+	if err != errNoPathFound {
+		t.Fatalf("not route error expected, but got %v", err)
 	}
 }
 

routing/payment_lifecycle.go

@@ -196,7 +196,6 @@ func errorToPaymentFailure(err error) channeldb.FailureReason {
 		errNoTlvPayload,
 		errNoPaymentAddr,
 		errNoPathFound,
-		errMaxHopsExceeded,
 		errPrebuiltRouteTried:
 
 		return channeldb.FailureReasonNoRoute