routing: fix mc blinded path behaviour.

docs/release-notes/release-notes-0.19.0.md

@@ -60,6 +60,9 @@
 * [Make the contract resolutions for the channel arbitrator optional](
   https://github.com/lightningnetwork/lnd/pull/9253)
 
+* [Make sure blinded payment failures are handled correctly in the mission
+controller](https://github.com/lightningnetwork/lnd/pull/9316).
+
 # New Features
 ## Functional Enhancements
 ## RPC Additions

routing/result_interpretation.go

@@ -517,11 +517,25 @@ func (i *interpretedResult) processPaymentOutcomeIntermediate(route *mcRoute,
 		if introIdx == len(route.hops.Val)-1 {
 			i.finalFailureReason = &reasonError
 		} else {
-			// If there are other hops between the recipient and
-			// introduction node, then we just penalize the last
-			// hop in the blinded route to minimize the storage of
-			// results for ephemeral keys.
-			i.failPairBalance(route, len(route.hops.Val)-1)
+			// Penalize the first hop after the introduction node.
+			// This makes sure we do not retry this blinded path
+			// in our path finding logic with the same amount.
+			// In the pathfinding logic for blinded routes we swap
+			// the target pubkey for a general one so that
+			// multi-path payment can be attempted, so the last
+			// hop is not the one that we are considering during
+			// pathfinding. The case where the next hop after the
+			// introduction node is the final recipient is handled
+			// above, where we punish nothing but abort the payment
+			// by reporting a final failure reason.
+			//
+			// TODO(ziggie): Make sure we only keep mc data for
+			// blinded paths, in both the success and failure case,
+			// in memory during the time of the payment and remove
+			// it afterwards. Blinded paths and their blinded hop
+			// keys are always changing per blinded route so there
+			// is no point in persisting this data.
+			i.failPairBalance(route, introIdx)
 		}
 
 	// In all other cases, we penalize the reporting node. These are all

routing/result_interpretation_test.go

@@ -552,7 +552,7 @@ var resultTestCases = []resultTestCase{
 			pairResults: map[DirectedNodePair]pairResult{
 				getTestPair(0, 1): successPairResult(100),
 				getTestPair(1, 2): successPairResult(99),
-				getTestPair(3, 4): failPairResult(88),
+				getTestPair(2, 3): failPairResult(95),
 			},
 		},
 	},
@@ -567,7 +567,7 @@ var resultTestCases = []resultTestCase{
 		expectedResult: &interpretedResult{
 			pairResults: map[DirectedNodePair]pairResult{
 				getTestPair(0, 1): successPairResult(100),
-				getTestPair(2, 3): failPairResult(75),
+				getTestPair(1, 2): failPairResult(90),
 			},
 		},
 	},