routing: fix fee limit condition

When iterating edges, pathfinding checks early whether using an edge
would violate the requested total fee limit for a route. This check is
done on the net amount (an amount the inbound fee is calculated with).
However, a possible next hop's fee discount leads to a reduction in fees
and as such using the net amount leads to assuming a higher cumulative
fee than the route really has, excluding the path erroneously. We
perform the fee limit check on the amount to send, which includes both
inbound and outbound fees. This should be possible as the first hop's
outbound fee is zero and therefore doesn't have to be checked in the
end.

Author: bitromortac

docs/release-notes/release-notes-0.18.3.md

@@ -36,6 +36,10 @@
 * [Fixed a bug](https://github.com/lightningnetwork/lnd/pull/8896) that caused
   LND to use a default fee rate for the batch channel opening flow.
 
+* The fee limit for payments [was made
+  compatible](https://github.com/lightningnetwork/lnd/pull/8941) with inbound
+  fees.
+
 # New Features
 ## Functional Enhancements
 ## RPC Additions
@@ -150,6 +154,7 @@
 # Contributors (Alphabetical Order)
 
 * Andras Banki-Horvath
+* bitromortac
 * Bufo
 * Elle Mouton
 * Matheus Degiovani

itest/lnd_routing_test.go

@@ -1397,12 +1397,8 @@ func testFeeLimitAfterQueryRoutes(ht *lntest.HarnessTest) {
 		FeeLimitMsat:   0,
 	}
 
-	// We assert that the payment fails because the fee limit doesn't work
-	// correctly. This is fixed in the next commit.
-	ht.SendPaymentAssertFail(
-		alice, sendReq,
-		lnrpc.PaymentFailureReason_FAILURE_REASON_NO_ROUTE,
-	)
+	// We assert that a route compatible with the fee limit is available.
+	ht.SendPaymentAssertSettled(alice, sendReq)
 
 	// Once we're done, close the channels.
 	ht.CloseChannel(alice, chanPointAliceBob)

routing/pathfind.go

@@ -723,6 +723,24 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		amountToSend := toNodeDist.netAmountReceived +
 			lnwire.MilliSatoshi(inboundFee)
 
+		// Check if accumulated fees would exceed fee limit when this
+		// node would be added to the path.
+		totalFee := int64(amountToSend) - int64(amt)
+
+		log.Trace(lnutils.NewLogClosure(func() string {
+			return fmt.Sprintf(
+				"Checking fromVertex (%v) with "+
+					"minInboundFee=%v, inboundFee=%v, "+
+					"amountToSend=%v, amt=%v, totalFee=%v",
+				fromVertex, minInboundFee, inboundFee,
+				amountToSend, amt, totalFee,
+			)
+		}))
+
+		if totalFee > 0 && lnwire.MilliSatoshi(totalFee) > r.FeeLimit {
+			return
+		}
+
 		// Request the success probability for this edge.
 		edgeProbability := r.ProbabilitySource(
 			fromVertex, toNodeDist.node, amountToSend,
@@ -779,24 +797,6 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 		netAmountToReceive := amountToSend +
 			lnwire.MilliSatoshi(outboundFee)
 
-		// Check if accumulated fees would exceed fee limit when this
-		// node would be added to the path.
-		totalFee := int64(netAmountToReceive) - int64(amt)
-
-		log.Trace(lnutils.NewLogClosure(func() string {
-			return fmt.Sprintf(
-				"Checking fromVertex (%v) with "+
-					"minInboundFee=%v, inboundFee=%v, "+
-					"amountToSend=%v, amt=%v, totalFee=%v",
-				fromVertex, minInboundFee, inboundFee,
-				amountToSend, amt, totalFee,
-			)
-		}))
-
-		if totalFee > 0 && lnwire.MilliSatoshi(totalFee) > r.FeeLimit {
-			return
-		}
-
 		// Calculate total probability of successfully reaching target
 		// by multiplying the probabilities. Both this edge and the rest
 		// of the route must succeed.