]> git.bitcoin.ninja Git - rust-lightning/commitdiff
Implement non-strict forwarding
authorWillem Van Lint <noreply@wvanlint.dev>
Tue, 11 Jun 2024 23:05:57 +0000 (16:05 -0700)
committerWillem Van Lint <noreply@wvanlint.dev>
Thu, 20 Jun 2024 04:24:21 +0000 (21:24 -0700)
This change implements non-strict forwarding, allowing the node to
forward an HTLC along a channel other than the one specified
by short_channel_id in the onion message, so long as the receiver has
the same node public key intended by short_channel_id
([BOLT](https://github.com/lightning/bolts/blob/57ce4b1e05c996fa649f00dc13521f6d496a288f/04-onion-routing.md#non-strict-forwarding)).
This can improve payment reliability when there are multiple channels
with the same peer e.g. when outbound liquidity is replenished by
opening a new channel.

The implemented forwarding strategy now chooses the channel with the
lowest outbound liquidity that can forward an HTLC to maximize the
probability of being able to successfully forward a subsequent HTLC.

Fixes #1278.

fuzz/src/full_stack.rs
lightning/src/ln/channelmanager.rs
lightning/src/ln/payment_tests.rs

index 2807b6dd4195e6f14d9e434f8b8268199ae5c876..a2ae07a2149045e14742174f7df16f2888cfed61 100644 (file)
@@ -1366,8 +1366,8 @@ mod tests {
 
                // process the now-pending HTLC forward
                ext_from_hex("07", &mut test);
-               // Two feerate requests to check dust exposure
-               ext_from_hex("00fd00fd", &mut test);
+               // Three feerate requests to check dust exposure
+               ext_from_hex("00fd00fd00fd", &mut test);
                // client now sends id 1 update_add_htlc and commitment_signed (CHECK 7: UpdateHTLCs event for node 03020000 with 1 HTLCs for channel 3f000000)
 
                // we respond with commitment_signed then revoke_and_ack (a weird, but valid, order)
@@ -1478,8 +1478,8 @@ mod tests {
                // process the now-pending HTLC forward
                ext_from_hex("07", &mut test);
 
-               // Two feerate requests to check dust exposure
-               ext_from_hex("00fd00fd", &mut test);
+               // Three feerate requests to check dust exposure
+               ext_from_hex("00fd00fd00fd", &mut test);
 
                // client now sends id 1 update_add_htlc and commitment_signed (CHECK 7 duplicate)
                // we respond with revoke_and_ack, then commitment_signed, then update_fail_htlc
@@ -1602,8 +1602,8 @@ mod tests {
 
                // process the now-pending HTLC forward
                ext_from_hex("07", &mut test);
-               // Two feerate requests to check dust exposure
-               ext_from_hex("00fd00fd", &mut test);
+               // Three feerate requests to check dust exposure
+               ext_from_hex("00fd00fd00fd", &mut test);
                // client now sends id 1 update_add_htlc and commitment_signed (CHECK 7 duplicate)
 
                // connect a block with one transaction of len 125
index 274edf6d7b4238495eb87390bb8453ad76ca3a8b..be2b599e6af31f8e45a7d7d17988ea2756d7defd 100644 (file)
@@ -4895,8 +4895,8 @@ where
                                if short_chan_id != 0 {
                                        let mut forwarding_counterparty = None;
                                        macro_rules! forwarding_channel_not_found {
-                                               () => {
-                                                       for forward_info in pending_forwards.drain(..) {
+                                               ($forward_infos: expr) => {
+                                                       for forward_info in $forward_infos {
                                                                match forward_info {
                                                                        HTLCForwardInfo::AddHTLC(PendingAddHTLCInfo {
                                                                                prev_short_channel_id, prev_htlc_id, prev_channel_id, prev_funding_outpoint,
@@ -5004,7 +5004,7 @@ where
                                        let (counterparty_node_id, forward_chan_id) = match chan_info_opt {
                                                Some((cp_id, chan_id)) => (cp_id, chan_id),
                                                None => {
-                                                       forwarding_channel_not_found!();
+                                                       forwarding_channel_not_found!(pending_forwards.drain(..));
                                                        continue;
                                                }
                                        };
@@ -5012,96 +5012,148 @@ where
                                        let per_peer_state = self.per_peer_state.read().unwrap();
                                        let peer_state_mutex_opt = per_peer_state.get(&counterparty_node_id);
                                        if peer_state_mutex_opt.is_none() {
-                                               forwarding_channel_not_found!();
+                                               forwarding_channel_not_found!(pending_forwards.drain(..));
                                                continue;
                                        }
                                        let mut peer_state_lock = peer_state_mutex_opt.unwrap().lock().unwrap();
                                        let peer_state = &mut *peer_state_lock;
-                                       if let Some(ChannelPhase::Funded(ref mut chan)) = peer_state.channel_by_id.get_mut(&forward_chan_id) {
-                                               let logger = WithChannelContext::from(&self.logger, &chan.context, None);
-                                               for forward_info in pending_forwards.drain(..) {
-                                                       let queue_fail_htlc_res = match forward_info {
-                                                               HTLCForwardInfo::AddHTLC(PendingAddHTLCInfo {
-                                                                       prev_short_channel_id, prev_htlc_id, prev_channel_id, prev_funding_outpoint,
-                                                                       prev_user_channel_id, forward_info: PendingHTLCInfo {
-                                                                               incoming_shared_secret, payment_hash, outgoing_amt_msat, outgoing_cltv_value,
-                                                                               routing: PendingHTLCRouting::Forward {
-                                                                                       onion_packet, blinded, ..
-                                                                               }, skimmed_fee_msat, ..
+                                       let mut draining_pending_forwards = pending_forwards.drain(..);
+                                       while let Some(forward_info) = draining_pending_forwards.next() {
+                                               let queue_fail_htlc_res = match forward_info {
+                                                       HTLCForwardInfo::AddHTLC(PendingAddHTLCInfo {
+                                                               prev_short_channel_id, prev_htlc_id, prev_channel_id, prev_funding_outpoint,
+                                                               prev_user_channel_id, forward_info: PendingHTLCInfo {
+                                                                       incoming_shared_secret, payment_hash, outgoing_amt_msat, outgoing_cltv_value,
+                                                                       routing: PendingHTLCRouting::Forward {
+                                                                               ref onion_packet, blinded, ..
+                                                                       }, skimmed_fee_msat, ..
+                                                               },
+                                                       }) => {
+                                                               let htlc_source = HTLCSource::PreviousHopData(HTLCPreviousHopData {
+                                                                       short_channel_id: prev_short_channel_id,
+                                                                       user_channel_id: Some(prev_user_channel_id),
+                                                                       channel_id: prev_channel_id,
+                                                                       outpoint: prev_funding_outpoint,
+                                                                       htlc_id: prev_htlc_id,
+                                                                       incoming_packet_shared_secret: incoming_shared_secret,
+                                                                       // Phantom payments are only PendingHTLCRouting::Receive.
+                                                                       phantom_shared_secret: None,
+                                                                       blinded_failure: blinded.map(|b| b.failure),
+                                                               });
+                                                               let next_blinding_point = blinded.and_then(|b| {
+                                                                       let encrypted_tlvs_ss = self.node_signer.ecdh(
+                                                                               Recipient::Node, &b.inbound_blinding_point, None
+                                                                       ).unwrap().secret_bytes();
+                                                                       onion_utils::next_hop_pubkey(
+                                                                               &self.secp_ctx, b.inbound_blinding_point, &encrypted_tlvs_ss
+                                                                       ).ok()
+                                                               });
+
+                                                               // Forward the HTLC over the most appropriate channel with the corresponding peer,
+                                                               // applying non-strict forwarding.
+                                                               // The channel with the least amount of outbound liquidity will be used to maximize the
+                                                               // probability of being able to successfully forward a subsequent HTLC.
+                                                               let maybe_optimal_channel = peer_state.channel_by_id.values_mut().filter_map(|phase| match phase {
+                                                                       ChannelPhase::Funded(chan) => {
+                                                                               let balances = chan.context.get_available_balances(&self.fee_estimator);
+                                                                               if outgoing_amt_msat <= balances.next_outbound_htlc_limit_msat &&
+                                                                                       outgoing_amt_msat >= balances.next_outbound_htlc_minimum_msat &&
+                                                                                       chan.context.is_usable() {
+                                                                                       Some((chan, balances))
+                                                                               } else {
+                                                                                       None
+                                                                               }
                                                                        },
-                                                               }) => {
-                                                                       let logger = WithChannelContext::from(&self.logger, &chan.context, Some(payment_hash));
-                                                                       log_trace!(logger, "Adding HTLC from short id {} with payment_hash {} to channel with short id {} after delay", prev_short_channel_id, &payment_hash, short_chan_id);
-                                                                       let htlc_source = HTLCSource::PreviousHopData(HTLCPreviousHopData {
-                                                                               short_channel_id: prev_short_channel_id,
-                                                                               user_channel_id: Some(prev_user_channel_id),
-                                                                               channel_id: prev_channel_id,
-                                                                               outpoint: prev_funding_outpoint,
-                                                                               htlc_id: prev_htlc_id,
-                                                                               incoming_packet_shared_secret: incoming_shared_secret,
-                                                                               // Phantom payments are only PendingHTLCRouting::Receive.
-                                                                               phantom_shared_secret: None,
-                                                                               blinded_failure: blinded.map(|b| b.failure),
-                                                                       });
-                                                                       let next_blinding_point = blinded.and_then(|b| {
-                                                                               let encrypted_tlvs_ss = self.node_signer.ecdh(
-                                                                                       Recipient::Node, &b.inbound_blinding_point, None
-                                                                               ).unwrap().secret_bytes();
-                                                                               onion_utils::next_hop_pubkey(
-                                                                                       &self.secp_ctx, b.inbound_blinding_point, &encrypted_tlvs_ss
-                                                                               ).ok()
-                                                                       });
-                                                                       if let Err(e) = chan.queue_add_htlc(outgoing_amt_msat,
-                                                                               payment_hash, outgoing_cltv_value, htlc_source.clone(),
-                                                                               onion_packet, skimmed_fee_msat, next_blinding_point, &self.fee_estimator,
-                                                                               &&logger)
-                                                                       {
-                                                                               if let ChannelError::Ignore(msg) = e {
-                                                                                       log_trace!(logger, "Failed to forward HTLC with payment_hash {}: {}", &payment_hash, msg);
+                                                                       _ => None,
+                                                               }).min_by_key(|(_, balances)| balances.next_outbound_htlc_limit_msat).map(|(c, _)| c);
+                                                               let optimal_channel = match maybe_optimal_channel {
+                                                                       Some(chan) => chan,
+                                                                       None => {
+                                                                               // Fall back to the specified channel to return an appropriate error.
+                                                                               if let Some(ChannelPhase::Funded(ref mut chan)) = peer_state.channel_by_id.get_mut(&forward_chan_id) {
+                                                                                       chan
                                                                                } else {
-                                                                                       panic!("Stated return value requirements in send_htlc() were not met");
+                                                                                       forwarding_channel_not_found!(core::iter::once(forward_info).chain(draining_pending_forwards));
+                                                                                       break;
                                                                                }
+                                                                       }
+                                                               };
+
+                                                               let logger = WithChannelContext::from(&self.logger, &optimal_channel.context, Some(payment_hash));
+                                                               let channel_description = if optimal_channel.context.get_short_channel_id() == Some(short_chan_id) {
+                                                                       "specified"
+                                                               } else {
+                                                                       "alternate"
+                                                               };
+                                                               log_trace!(logger, "Forwarding HTLC from SCID {} with payment_hash {} and next hop SCID {} over {} channel {} with corresponding peer {}",
+                                                                       prev_short_channel_id, &payment_hash, short_chan_id, channel_description, optimal_channel.context.channel_id(), &counterparty_node_id);
+                                                               if let Err(e) = optimal_channel.queue_add_htlc(outgoing_amt_msat,
+                                                                               payment_hash, outgoing_cltv_value, htlc_source.clone(),
+                                                                               onion_packet.clone(), skimmed_fee_msat, next_blinding_point, &self.fee_estimator,
+                                                                               &&logger)
+                                                               {
+                                                                       if let ChannelError::Ignore(msg) = e {
+                                                                               log_trace!(logger, "Failed to forward HTLC with payment_hash {} to peer {}: {}", &payment_hash, &counterparty_node_id, msg);
+                                                                       } else {
+                                                                               panic!("Stated return value requirements in send_htlc() were not met");
+                                                                       }
+
+                                                                       if let Some(ChannelPhase::Funded(ref mut chan)) = peer_state.channel_by_id.get_mut(&forward_chan_id) {
                                                                                let (failure_code, data) = self.get_htlc_temp_fail_err_and_data(0x1000|7, short_chan_id, chan);
                                                                                failed_forwards.push((htlc_source, payment_hash,
                                                                                        HTLCFailReason::reason(failure_code, data),
                                                                                        HTLCDestination::NextHopChannel { node_id: Some(chan.context.get_counterparty_node_id()), channel_id: forward_chan_id }
                                                                                ));
-                                                                               continue;
+                                                                       } else {
+                                                                               forwarding_channel_not_found!(core::iter::once(forward_info).chain(draining_pending_forwards));
+                                                                               break;
                                                                        }
-                                                                       None
-                                                               },
-                                                               HTLCForwardInfo::AddHTLC { .. } => {
-                                                                       panic!("short_channel_id != 0 should imply any pending_forward entries are of type Forward");
-                                                               },
-                                                               HTLCForwardInfo::FailHTLC { htlc_id, err_packet } => {
+                                                               }
+                                                               None
+                                                       },
+                                                       HTLCForwardInfo::AddHTLC { .. } => {
+                                                               panic!("short_channel_id != 0 should imply any pending_forward entries are of type Forward");
+                                                       },
+                                                       HTLCForwardInfo::FailHTLC { htlc_id, ref err_packet } => {
+                                                               if let Some(ChannelPhase::Funded(ref mut chan)) = peer_state.channel_by_id.get_mut(&forward_chan_id) {
+                                                                       let logger = WithChannelContext::from(&self.logger, &chan.context, None);
                                                                        log_trace!(logger, "Failing HTLC back to channel with short id {} (backward HTLC ID {}) after delay", short_chan_id, htlc_id);
-                                                                       Some((chan.queue_fail_htlc(htlc_id, err_packet, &&logger), htlc_id))
-                                                               },
-                                                               HTLCForwardInfo::FailMalformedHTLC { htlc_id, failure_code, sha256_of_onion } => {
+                                                                       Some((chan.queue_fail_htlc(htlc_id, err_packet.clone(), &&logger), htlc_id))
+                                                               } else {
+                                                                       forwarding_channel_not_found!(core::iter::once(forward_info).chain(draining_pending_forwards));
+                                                                       break;
+                                                               }
+                                                       },
+                                                       HTLCForwardInfo::FailMalformedHTLC { htlc_id, failure_code, sha256_of_onion } => {
+                                                               if let Some(ChannelPhase::Funded(ref mut chan)) = peer_state.channel_by_id.get_mut(&forward_chan_id) {
+                                                                       let logger = WithChannelContext::from(&self.logger, &chan.context, None);
                                                                        log_trace!(logger, "Failing malformed HTLC back to channel with short id {} (backward HTLC ID {}) after delay", short_chan_id, htlc_id);
                                                                        let res = chan.queue_fail_malformed_htlc(
                                                                                htlc_id, failure_code, sha256_of_onion, &&logger
                                                                        );
                                                                        Some((res, htlc_id))
-                                                               },
-                                                       };
-                                                       if let Some((queue_fail_htlc_res, htlc_id)) = queue_fail_htlc_res {
-                                                               if let Err(e) = queue_fail_htlc_res {
-                                                                       if let ChannelError::Ignore(msg) = e {
+                                                               } else {
+                                                                       forwarding_channel_not_found!(core::iter::once(forward_info).chain(draining_pending_forwards));
+                                                                       break;
+                                                               }
+                                                       },
+                                               };
+                                               if let Some((queue_fail_htlc_res, htlc_id)) = queue_fail_htlc_res {
+                                                       if let Err(e) = queue_fail_htlc_res {
+                                                               if let ChannelError::Ignore(msg) = e {
+                                                                       if let Some(ChannelPhase::Funded(ref mut chan)) = peer_state.channel_by_id.get_mut(&forward_chan_id) {
+                                                                               let logger = WithChannelContext::from(&self.logger, &chan.context, None);
                                                                                log_trace!(logger, "Failed to fail HTLC with ID {} backwards to short_id {}: {}", htlc_id, short_chan_id, msg);
-                                                                       } else {
-                                                                               panic!("Stated return value requirements in queue_fail_{{malformed_}}htlc() were not met");
                                                                        }
-                                                                       // fail-backs are best-effort, we probably already have one
-                                                                       // pending, and if not that's OK, if not, the channel is on
-                                                                       // the chain and sending the HTLC-Timeout is their problem.
-                                                                       continue;
+                                                               } else {
+                                                                       panic!("Stated return value requirements in queue_fail_{{malformed_}}htlc() were not met");
                                                                }
+                                                               // fail-backs are best-effort, we probably already have one
+                                                               // pending, and if not that's OK, if not, the channel is on
+                                                               // the chain and sending the HTLC-Timeout is their problem.
+                                                               continue;
                                                        }
                                                }
-                                       } else {
-                                               forwarding_channel_not_found!();
-                                               continue;
                                        }
                                } else {
                                        'next_forwardable_htlc: for forward_info in pending_forwards.drain(..) {
index dcfb6ed8de77768abb8d49d6de292d36344185f4..fcfdd5cb5a02f64e688ddd7cdb9b375029907226 100644 (file)
@@ -2053,22 +2053,28 @@ fn accept_underpaying_htlcs_config() {
 fn do_accept_underpaying_htlcs_config(num_mpp_parts: usize) {
        let chanmon_cfgs = create_chanmon_cfgs(3);
        let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
+       let max_in_flight_percent = 10;
        let mut intercept_forwards_config = test_default_channel_config();
        intercept_forwards_config.accept_intercept_htlcs = true;
+       intercept_forwards_config.channel_handshake_config.max_inbound_htlc_value_in_flight_percent_of_channel = max_in_flight_percent;
        let mut underpay_config = test_default_channel_config();
        underpay_config.channel_config.accept_underpaying_htlcs = true;
+       underpay_config.channel_handshake_config.max_inbound_htlc_value_in_flight_percent_of_channel = max_in_flight_percent;
        let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[None, Some(intercept_forwards_config), Some(underpay_config)]);
        let nodes = create_network(3, &node_cfgs, &node_chanmgrs);
 
+       let amt_msat = 900_000;
+
        let mut chan_ids = Vec::new();
        for _ in 0..num_mpp_parts {
-               let _ = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 10_000, 0);
-               let channel_id = create_unannounced_chan_between_nodes_with_value(&nodes, 1, 2, 2_000_000, 0).0.channel_id;
+               // We choose the channel size so that there can be at most one part pending on each channel.
+               let channel_size = amt_msat / 1000 / num_mpp_parts as u64 * 100 / max_in_flight_percent as u64 + 100;
+               let _ = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, channel_size, 0);
+               let channel_id = create_unannounced_chan_between_nodes_with_value(&nodes, 1, 2, channel_size, 0).0.channel_id;
                chan_ids.push(channel_id);
        }
 
        // Send the initial payment.
-       let amt_msat = 900_000;
        let skimmed_fee_msat = 20;
        let mut route_hints = Vec::new();
        for _ in 0..num_mpp_parts {
@@ -4098,7 +4104,7 @@ fn do_test_payment_metadata_consistency(do_reload: bool, do_modify: bool) {
 
        // Create a new channel between C and D as A will refuse to retry on the existing one because
        // it just failed.
-       let chan_id_cd_2 = create_announced_chan_between_nodes_with_value(&nodes, 2, 3, 1_000_000, 0).2;
+       create_announced_chan_between_nodes_with_value(&nodes, 2, 3, 1_000_000, 0);
 
        // Now retry the failed HTLC.
        nodes[0].node.process_pending_htlc_forwards();
@@ -4110,6 +4116,7 @@ fn do_test_payment_metadata_consistency(do_reload: bool, do_modify: bool) {
        expect_pending_htlcs_forwardable!(nodes[2]);
        check_added_monitors(&nodes[2], 1);
        let cs_forward = SendEvent::from_node(&nodes[2]);
+       let cd_channel_used = cs_forward.msgs[0].channel_id;
        nodes[3].node.handle_update_add_htlc(&nodes[2].node.get_our_node_id(), &cs_forward.msgs[0]);
        commitment_signed_dance!(nodes[3], nodes[2], cs_forward.commitment_msg, false, true);
 
@@ -4129,7 +4136,7 @@ fn do_test_payment_metadata_consistency(do_reload: bool, do_modify: bool) {
                nodes[2].node.handle_update_fail_htlc(&nodes[3].node.get_our_node_id(), &ds_fail.update_fail_htlcs[0]);
                commitment_signed_dance!(nodes[2], nodes[3], ds_fail.commitment_signed, false, true);
                expect_pending_htlcs_forwardable_conditions(nodes[2].node.get_and_clear_pending_events(),
-                       &[HTLCDestination::NextHopChannel { node_id: Some(nodes[3].node.get_our_node_id()), channel_id: chan_id_cd_2 }]);
+                       &[HTLCDestination::NextHopChannel { node_id: Some(nodes[3].node.get_our_node_id()), channel_id: cd_channel_used }]);
        } else {
                expect_pending_htlcs_forwardable!(nodes[3]);
                expect_payment_claimable!(nodes[3], payment_hash, payment_secret, amt_msat);
@@ -4294,3 +4301,94 @@ fn peel_payment_onion_custom_tlvs() {
                _ => panic!()
        }
 }
+
+#[test]
+fn test_non_strict_forwarding() {
+       let chanmon_cfgs = create_chanmon_cfgs(3);
+       let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
+       let mut config = test_default_channel_config();
+       config.channel_handshake_config.max_inbound_htlc_value_in_flight_percent_of_channel = 100;
+       let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[Some(config), Some(config), Some(config)]);
+       let nodes = create_network(3, &node_cfgs, &node_chanmgrs);
+
+       // Create a routing node with two outbound channels, each of which can forward 2 payments of
+       // the given value.
+       let payment_value = 1_500_000;
+       create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100_000, 0);
+       let (chan_update_1, _, channel_id_1, _) = create_announced_chan_between_nodes_with_value(&nodes, 1, 2, 4_950, 0);
+       let (chan_update_2, _, channel_id_2, _) = create_announced_chan_between_nodes_with_value(&nodes, 1, 2, 5_000, 0);
+
+       // Create a route once.
+       let payment_params = PaymentParameters::from_node_id(nodes[2].node.get_our_node_id(), TEST_FINAL_CLTV)
+               .with_bolt11_features(nodes[2].node.bolt11_invoice_features()).unwrap();
+       let route_params = RouteParameters::from_payment_params_and_value(payment_params, payment_value);
+       let route = functional_test_utils::get_route(&nodes[0], &route_params).unwrap();
+
+       // Send 4 payments over the same route.
+       for i in 0..4 {
+               let (payment_preimage, payment_hash, payment_secret) = get_payment_preimage_hash(&nodes[2], Some(payment_value), None);
+               nodes[0].node.send_payment_with_route(&route, payment_hash,
+                       RecipientOnionFields::secret_only(payment_secret), PaymentId(payment_hash.0)).unwrap();
+               check_added_monitors!(nodes[0], 1);
+               let mut msg_events = nodes[0].node.get_and_clear_pending_msg_events();
+               assert_eq!(msg_events.len(), 1);
+               let mut send_event = SendEvent::from_event(msg_events.remove(0));
+               nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &send_event.msgs[0]);
+               commitment_signed_dance!(nodes[1], nodes[0], &send_event.commitment_msg, false);
+
+               expect_pending_htlcs_forwardable!(nodes[1]);
+               check_added_monitors!(nodes[1], 1);
+               msg_events = nodes[1].node.get_and_clear_pending_msg_events();
+               assert_eq!(msg_events.len(), 1);
+               send_event = SendEvent::from_event(msg_events.remove(0));
+               // The HTLC will be forwarded over the most appropriate channel with the corresponding peer,
+               // applying non-strict forwarding.
+               // The channel with the least amount of outbound liquidity will be used to maximize the
+               // probability of being able to successfully forward a subsequent HTLC.
+               assert_eq!(send_event.msgs[0].channel_id, if i < 2 {
+                       channel_id_1
+               } else {
+                       channel_id_2
+               });
+               nodes[2].node.handle_update_add_htlc(&nodes[1].node.get_our_node_id(), &send_event.msgs[0]);
+               commitment_signed_dance!(nodes[2], nodes[1], &send_event.commitment_msg, false);
+
+               expect_pending_htlcs_forwardable!(nodes[2]);
+               let events = nodes[2].node.get_and_clear_pending_events();
+               assert_eq!(events.len(), 1);
+               assert!(matches!(events[0], Event::PaymentClaimable { .. }));
+
+               claim_payment_along_route(
+                       ClaimAlongRouteArgs::new(&nodes[0], &[&[&nodes[1], &nodes[2]]], payment_preimage)
+               );
+       }
+
+       // Send a 5th payment which will fail.
+       let (_, payment_hash, payment_secret) = get_payment_preimage_hash(&nodes[2], Some(payment_value), None);
+       nodes[0].node.send_payment_with_route(&route, payment_hash,
+               RecipientOnionFields::secret_only(payment_secret), PaymentId(payment_hash.0)).unwrap();
+       check_added_monitors!(nodes[0], 1);
+       let mut msg_events = nodes[0].node.get_and_clear_pending_msg_events();
+       assert_eq!(msg_events.len(), 1);
+       let mut send_event = SendEvent::from_event(msg_events.remove(0));
+       nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &send_event.msgs[0]);
+       commitment_signed_dance!(nodes[1], nodes[0], &send_event.commitment_msg, false);
+
+       expect_pending_htlcs_forwardable!(nodes[1]);
+       check_added_monitors!(nodes[1], 1);
+       let routed_scid = route.paths[0].hops[1].short_channel_id;
+       let routed_channel_id = match routed_scid {
+               scid if scid == chan_update_1.contents.short_channel_id => channel_id_1,
+               scid if scid == chan_update_2.contents.short_channel_id => channel_id_2,
+               _ => panic!("Unexpected short channel id in route"),
+       };
+       // The failure to forward will refer to the channel given in the onion.
+       expect_pending_htlcs_forwardable_conditions(nodes[1].node.get_and_clear_pending_events(),
+               &[HTLCDestination::NextHopChannel { node_id: Some(nodes[2].node.get_our_node_id()), channel_id: routed_channel_id }]);
+
+       let updates = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
+       nodes[0].node.handle_update_fail_htlc(&nodes[1].node.get_our_node_id(), &updates.update_fail_htlcs[0]);
+       commitment_signed_dance!(nodes[0], nodes[1], updates.commitment_signed, false);
+       let events = nodes[0].node.get_and_clear_pending_events();
+       expect_payment_failed_conditions_event(events, payment_hash, false, PaymentFailedConditions::new().blamed_scid(routed_scid));
+}