f: fix edge case only when trampoline is also receiver

Author: a-mpch

lightning/src/ln/blinded_payment_tests.rs

@@ -2958,6 +2958,6 @@ fn test_trampoline_forward_rejection() {
 		// Expect UnknownNextPeer error while we are unable to route forwarding Trampoline payments.
 		let payment_failed_conditions = PaymentFailedConditions::new()
 			.expected_htlc_error_data(LocalHTLCFailureReason::UnknownNextPeer, &[0; 0]);
-		expect_payment_failed_conditions(&nodes[0], payment_hash, true, payment_failed_conditions);
+		expect_payment_failed_conditions(&nodes[0], payment_hash, false, payment_failed_conditions);
 	}
 }

lightning/src/ln/onion_utils.rs

@@ -1107,6 +1107,8 @@ where
 	// Keep track of the first hop for which the attribution data failed to check out.
 	let mut attribution_failed_channel = None;
 
+	let trampoline_hop_count = path.blinded_tail.as_ref().map_or(0, |bt| bt.trampoline_hops.len());
+
 	// Handle packed channel/node updates for passing back for the route handler
 	let mut iter = nontrampolines.chain(trampolines.into_iter().flatten()).enumerate().peekable();
 	while let Some((route_hop_idx, (route_hop_option, shared_secret))) = iter.next() {
@@ -1373,17 +1375,19 @@ where
 			short_channel_id = route_hop.short_channel_id()
 		}
 
-		// If next hop is from a trampoline we are already inside the trampoline
-		// route. If we found a permanent failure inside the trampoline route, we
-		// fail the payment permanently.
+		// If next hop is from a trampoline and there is only one trampoline hop
+		// in the trampoline route, means that the trampoline hop is also a
+		// final non-blinded node.
 		let is_next_hop_from_trampoline =
 			matches!(next_hop, Some((_, (Some(ErrorHop::TrampolineHop(..)), _))));
+		if is_next_hop_from_trampoline && trampoline_hop_count == 1 {
+			is_from_final_non_blinded_node = true;
+		}
 
 		res = Some(FailureLearnings {
 			network_update,
 			short_channel_id,
-			payment_failed_permanently: error_code.is_permanent()
-				&& (is_from_final_non_blinded_node || is_next_hop_from_trampoline),
+			payment_failed_permanently: error_code.is_permanent() && is_from_final_non_blinded_node,
 			failed_within_blinded_path: false,
 		});