routing: dont include final hop cltv in blinded path

Only include the final hop's cltv delta in the total timelock
calculation if the route does not include a blinded path. This is
because in a blinded path, the final hops final cltv delta will be
included in the blinded path's accumlated cltv delta value.

With this commit, we remove the responsibility of remembering not to set
the `finalHop.cltvDelta` from the caller of `newRoute`. The relevant
test is updated accordingly.

cmd/lncli/cmd_payments.go

@@ -1099,8 +1099,13 @@ var queryRoutesCommand = cli.Command{
 		},
 		cli.Int64Flag{
 			Name: "final_cltv_delta",
-			Usage: "(optional) number of blocks the last hop has to reveal " +
-				"the preimage",
+			Usage: "(optional) number of blocks the last hop has " +
+				"to reveal the preimage. Note that this " +
+				"should not be set in the case where the " +
+				"path includes a blinded path since in " +
+				"that case, the receiver will already have " +
+				"accounted for this value in the " +
+				"blinded_cltv value",
 		},
 		cli.BoolFlag{
 			Name:  "use_mc",
@@ -1238,6 +1243,13 @@ func parseBlindedPaymentParameters(ctx *cli.Context) (
 		return nil, nil
 	}
 
+	// If a blinded path has been provided, then the final_cltv_delta flag
+	// should not be provided since this value will be ignored.
+	if ctx.IsSet("final_cltv_delta") {
+		return nil, fmt.Errorf("`final_cltv_delta` should not be " +
+			"provided if a blinded path is provided")
+	}
+
 	// If any one of our blinding related flags is set, we expect the
 	// full set to be set and we'll error out accordingly.
 	introNode, err := route.NewVertexFromStr(

routing/pathfind.go

@@ -98,7 +98,15 @@ type edgePolicyWithSource struct {
 type finalHopParams struct {
 	amt      lnwire.MilliSatoshi
 	totalAmt lnwire.MilliSatoshi
+
+	// cltvDelta is the final hop's minimum CLTV expiry delta.
+	//
+	// NOTE that in the case of paying to a blinded path, this value will
+	// be set to a duplicate of the blinded path's accumulated CLTV value.
+	// We would then only need to use this value in the case where the
+	// introduction node of the path is also the destination node.
 	cltvDelta uint16
+
 	records     record.CustomSet
 	paymentAddr *[32]byte
 
@@ -190,10 +198,21 @@ func newRoute(sourceVertex route.Vertex,
 			// reporting through RPC. Set to zero for the final hop.
 			fee = 0
 
-			// As this is the last hop, we'll use the specified
-			// final CLTV delta value instead of the value from the
-			// last link in the route.
+			// Only include the final hop CLTV delta in the total
+			// time lock value if this is not a route to a blinded
+			// path. For blinded paths, the total time-lock from the
+			// whole path will be deduced from the introduction
+			// node's CLTV delta. The exception is for the case
+			// where the final hop is the blinded path introduction
+			// node.
+			if blindedPath == nil ||
+				len(blindedPath.BlindedHops) == 1 {
+
+				// As this is the last hop, we'll use the
+				// specified final CLTV delta value instead of
+				// the value from the last link in the route.
 				totalTimeLock += uint32(finalHop.cltvDelta)
+			}
 			outgoingTimeLock = totalTimeLock
 
 			// Attach any custom records to the final hop.

routing/pathfind_test.go

@@ -3322,16 +3322,19 @@ func TestBlindedRouteConstruction(t *testing.T) {
 			ToNodeFeatures: tlvFeatures,
 		}
 
-		// Create final hop parameters for payment amount = 110. Note
-		// that final cltv delta is not set because blinded paths
-		// include this final delta in their aggregate delta. A
-		// sender-set delta may be added to account for block arrival
-		// during payment, but we do not set it in this test.
+		// Create final hop parameters for payment amount = 110.
 		totalAmt       lnwire.MilliSatoshi = 110
 		finalHopParams                     = finalHopParams{
 			amt:      totalAmt,
 			totalAmt: totalAmt,
 			metadata: metadata,
+
+			// We set a CLTV delta here just to test that this will
+			// be ignored by newRoute since this is a blinded path
+			// where the accumulated CLTV delta for the route
+			// communicated in the blinded path should be assumed to
+			// include the CLTV delta of the final hop.
+			cltvDelta: MaxCLTVDelta,
 		}
 	)
 

zpay32/invoice.go

@@ -156,6 +156,10 @@ type Invoice struct {
 	// This field is un-exported and can only be read by the
 	// MinFinalCLTVExpiry() method. By forcing callers to read via this
 	// method, we can easily enforce the default if not specified.
+	//
+	// NOTE: this field is ignored in the case that the invoice contains
+	// blinded paths since then the final minimum cltv expiry delta is
+	// expected to be included in the route's accumulated CLTV delta value.
 	minFinalCLTVExpiry *uint64
 
 	// Description is a short description of the purpose of this invoice.