Expose `skimmed_fee_msat` in `PaymentForwarded`
authorElias Rohrer <dev@tnull.de>
Mon, 29 Jan 2024 15:16:08 +0000 (16:16 +0100)
committerElias Rohrer <dev@tnull.de>
Thu, 1 Feb 2024 08:21:15 +0000 (09:21 +0100)
We generally allow routing nodes to forward less than the expected HTLC
amount, if the receiver knowingly accepts this and claims the
underpaying HTLC (see `ChannelConfig::accept_underpaying_htlcs`). This
use case is in particular useful for the LSPS2/JIT channel setting where
the intial underpaying HTLC pays for the channel open.

While we previously exposed the withheld amount as
`PaymentClaimable::counterparty_skimmed_fee_msat` on the receiver side,
we did not individually provide it on the forwarding node's side.
Here, we therefore expose this additionally withheld amount via
`PaymentForwarded::skimmed_fee_msat`.

lightning/src/events/mod.rs
lightning/src/ln/chanmon_update_fail_tests.rs
lightning/src/ln/channel.rs
lightning/src/ln/channelmanager.rs
lightning/src/ln/functional_test_utils.rs
lightning/src/ln/functional_tests.rs

index 201431bbed70fde3df320ba09c9418e742dbf77a..801e8695c8194f9671c9094dd9efc3730f21963c 100644 (file)
@@ -797,6 +797,20 @@ pub enum Event {
                /// `PaymentForwarded` events are generated for the same payment iff `total_fee_earned_msat` is
                /// `None`.
                total_fee_earned_msat: Option<u64>,
+               /// The share of the total fee, in milli-satoshis, which was withheld in addition to the
+               /// forwarding fee.
+               ///
+               /// This will only be `Some` if we forwarded an intercepted HTLC with less than the
+               /// expected amount. This means our counterparty accepted to receive less than the invoice
+               /// amount, e.g., by claiming the payment featuring a corresponding
+               /// [`PaymentClaimable::counterparty_skimmed_fee_msat`].
+               ///
+               /// Will also always be `None` for events serialized with LDK prior to version 0.0.122.
+               ///
+               /// The caveat described above the `total_fee_earned_msat` field applies here as well.
+               ///
+               /// [`PaymentClaimable::counterparty_skimmed_fee_msat`]: Self::PaymentClaimable::counterparty_skimmed_fee_msat
+               skimmed_fee_msat: Option<u64>,
                /// If this is `true`, the forwarded HTLC was claimed by our counterparty via an on-chain
                /// transaction.
                claim_from_onchain_tx: bool,
@@ -1084,7 +1098,7 @@ impl Writeable for Event {
                        }
                        &Event::PaymentForwarded {
                                total_fee_earned_msat, prev_channel_id, claim_from_onchain_tx,
-                               next_channel_id, outbound_amount_forwarded_msat
+                               next_channel_id, outbound_amount_forwarded_msat, skimmed_fee_msat,
                        } => {
                                7u8.write(writer)?;
                                write_tlv_fields!(writer, {
@@ -1093,6 +1107,7 @@ impl Writeable for Event {
                                        (2, claim_from_onchain_tx, required),
                                        (3, next_channel_id, option),
                                        (5, outbound_amount_forwarded_msat, option),
+                                       (7, skimmed_fee_msat, option),
                                });
                        },
                        &Event::ChannelClosed { ref channel_id, ref user_channel_id, ref reason,
@@ -1389,16 +1404,18 @@ impl MaybeReadable for Event {
                                        let mut claim_from_onchain_tx = false;
                                        let mut next_channel_id = None;
                                        let mut outbound_amount_forwarded_msat = None;
+                                       let mut skimmed_fee_msat = None;
                                        read_tlv_fields!(reader, {
                                                (0, total_fee_earned_msat, option),
                                                (1, prev_channel_id, option),
                                                (2, claim_from_onchain_tx, required),
                                                (3, next_channel_id, option),
                                                (5, outbound_amount_forwarded_msat, option),
+                                               (7, skimmed_fee_msat, option),
                                        });
                                        Ok(Some(Event::PaymentForwarded {
                                                total_fee_earned_msat, prev_channel_id, claim_from_onchain_tx, next_channel_id,
-                                               outbound_amount_forwarded_msat
+                                               outbound_amount_forwarded_msat, skimmed_fee_msat,
                                        }))
                                };
                                f()
index 02253a003fb0ea5b24dbcbd397d427fdff6e9c1c..5f7c72be1dc7a09c9ce21f16a445cddd426adbc0 100644 (file)
@@ -3404,7 +3404,8 @@ fn do_test_reload_mon_update_completion_actions(close_during_reload: bool) {
        let bc_update_id = nodes[1].chain_monitor.latest_monitor_update_id.lock().unwrap().get(&chan_id_bc).unwrap().2;
        let mut events = nodes[1].node.get_and_clear_pending_events();
        assert_eq!(events.len(), if close_during_reload { 2 } else { 1 });
-       expect_payment_forwarded(events.pop().unwrap(), &nodes[1], &nodes[0], &nodes[2], Some(1000), close_during_reload, false);
+       expect_payment_forwarded(events.pop().unwrap(), &nodes[1], &nodes[0], &nodes[2], Some(1000),
+               None, close_during_reload, false);
        if close_during_reload {
                match events[0] {
                        Event::ChannelClosed { .. } => {},
index 2007a9e772c780cafe02a308bbabf5ccd5142fca..e56fa978953b2d9d90769ff172691d341017741f 100644 (file)
@@ -3327,7 +3327,7 @@ impl<SP: Deref> Channel<SP> where
                Err(ChannelError::Close("Remote tried to fulfill/fail an HTLC we couldn't find".to_owned()))
        }
 
-       pub fn update_fulfill_htlc(&mut self, msg: &msgs::UpdateFulfillHTLC) -> Result<(HTLCSource, u64), ChannelError> {
+       pub fn update_fulfill_htlc(&mut self, msg: &msgs::UpdateFulfillHTLC) -> Result<(HTLCSource, u64, Option<u64>), ChannelError> {
                if !matches!(self.context.channel_state, ChannelState::ChannelReady(_)) {
                        return Err(ChannelError::Close("Got fulfill HTLC message when channel was not in an operational state".to_owned()));
                }
@@ -3335,7 +3335,7 @@ impl<SP: Deref> Channel<SP> where
                        return Err(ChannelError::Close("Peer sent update_fulfill_htlc when we needed a channel_reestablish".to_owned()));
                }
 
-               self.mark_outbound_htlc_removed(msg.htlc_id, Some(msg.payment_preimage), None).map(|htlc| (htlc.source.clone(), htlc.amount_msat))
+               self.mark_outbound_htlc_removed(msg.htlc_id, Some(msg.payment_preimage), None).map(|htlc| (htlc.source.clone(), htlc.amount_msat, htlc.skimmed_fee_msat))
        }
 
        pub fn update_fail_htlc(&mut self, msg: &msgs::UpdateFailHTLC, fail_reason: HTLCFailReason) -> Result<(), ChannelError> {
index 43bd4efd5daef7693bafe3393246ffa832e4f9a3..d61e0005fea9ab3cdd0bf0189a898a4b24ca76b3 100644 (file)
@@ -5690,9 +5690,9 @@ where
        }
 
        fn claim_funds_internal(&self, source: HTLCSource, payment_preimage: PaymentPreimage,
-               forwarded_htlc_value_msat: Option<u64>, from_onchain: bool, startup_replay: bool,
-               next_channel_counterparty_node_id: Option<PublicKey>, next_channel_outpoint: OutPoint,
-               next_channel_id: ChannelId,
+               forwarded_htlc_value_msat: Option<u64>, skimmed_fee_msat: Option<u64>, from_onchain: bool,
+               startup_replay: bool, next_channel_counterparty_node_id: Option<PublicKey>,
+               next_channel_outpoint: OutPoint, next_channel_id: ChannelId,
        ) {
                match source {
                        HTLCSource::OutboundRoute { session_priv, payment_id, path, .. } => {
@@ -5793,6 +5793,8 @@ where
                                                                        Some(claimed_htlc_value - forwarded_htlc_value)
                                                                } else { None }
                                                        } else { None };
+                                                       debug_assert!(skimmed_fee_msat <= total_fee_earned_msat,
+                                                               "skimmed_fee_msat must always be included in total_fee_earned_msat");
                                                        Some(MonitorUpdateCompletionAction::EmitEventAndFreeOtherChannel {
                                                                event: events::Event::PaymentForwarded {
                                                                        total_fee_earned_msat,
@@ -5800,6 +5802,7 @@ where
                                                                        prev_channel_id: Some(prev_channel_id),
                                                                        next_channel_id: Some(next_channel_id),
                                                                        outbound_amount_forwarded_msat: forwarded_htlc_value_msat,
+                                                                       skimmed_fee_msat,
                                                                },
                                                                downstream_counterparty_and_funding_outpoint: chan_to_release,
                                                        })
@@ -6739,7 +6742,7 @@ where
 
        fn internal_update_fulfill_htlc(&self, counterparty_node_id: &PublicKey, msg: &msgs::UpdateFulfillHTLC) -> Result<(), MsgHandleErrInternal> {
                let funding_txo;
-               let (htlc_source, forwarded_htlc_value) = {
+               let (htlc_source, forwarded_htlc_value, skimmed_fee_msat) = {
                        let per_peer_state = self.per_peer_state.read().unwrap();
                        let peer_state_mutex = per_peer_state.get(counterparty_node_id)
                                .ok_or_else(|| {
@@ -6777,8 +6780,11 @@ where
                                hash_map::Entry::Vacant(_) => return Err(MsgHandleErrInternal::send_err_msg_no_close(format!("Got a message for a channel from the wrong node! No such channel for the passed counterparty_node_id {}", counterparty_node_id), msg.channel_id))
                        }
                };
-               self.claim_funds_internal(htlc_source, msg.payment_preimage.clone(), Some(forwarded_htlc_value),
-                       false, false, Some(*counterparty_node_id), funding_txo, msg.channel_id);
+               self.claim_funds_internal(htlc_source, msg.payment_preimage.clone(),
+                       Some(forwarded_htlc_value), skimmed_fee_msat, false, false, Some(*counterparty_node_id),
+                       funding_txo, msg.channel_id
+               );
+
                Ok(())
        }
 
@@ -7276,7 +7282,9 @@ where
                                                let logger = WithContext::from(&self.logger, counterparty_node_id, Some(channel_id));
                                                if let Some(preimage) = htlc_update.payment_preimage {
                                                        log_trace!(logger, "Claiming HTLC with preimage {} from our monitor", preimage);
-                                                       self.claim_funds_internal(htlc_update.source, preimage, htlc_update.htlc_value_satoshis.map(|v| v * 1000), true, false, counterparty_node_id, funding_outpoint, channel_id);
+                                                       self.claim_funds_internal(htlc_update.source, preimage,
+                                                               htlc_update.htlc_value_satoshis.map(|v| v * 1000), None, true,
+                                                               false, counterparty_node_id, funding_outpoint, channel_id);
                                                } else {
                                                        log_trace!(logger, "Failing HTLC with hash {} from our monitor", &htlc_update.payment_hash);
                                                        let receiver = HTLCDestination::NextHopChannel { node_id: counterparty_node_id, channel_id };
@@ -11168,7 +11176,7 @@ where
                        // We use `downstream_closed` in place of `from_onchain` here just as a guess - we
                        // don't remember in the `ChannelMonitor` where we got a preimage from, but if the
                        // channel is closed we just assume that it probably came from an on-chain claim.
-                       channel_manager.claim_funds_internal(source, preimage, Some(downstream_value),
+                       channel_manager.claim_funds_internal(source, preimage, Some(downstream_value), None,
                                downstream_closed, true, downstream_node_id, downstream_funding, downstream_channel_id);
                }
 
index 45e5de51a0ef770dfbcf03d54705233b67dbf904..e53fd6b0c1e61908ab7c8f758c34555852fe6ac6 100644 (file)
@@ -2203,14 +2203,19 @@ macro_rules! expect_payment_path_successful {
 
 pub fn expect_payment_forwarded<CM: AChannelManager, H: NodeHolder<CM=CM>>(
        event: Event, node: &H, prev_node: &H, next_node: &H, expected_fee: Option<u64>,
-       upstream_force_closed: bool, downstream_force_closed: bool
+       expected_extra_fees_msat: Option<u64>, upstream_force_closed: bool,
+       downstream_force_closed: bool
 ) {
        match event {
                Event::PaymentForwarded {
                        total_fee_earned_msat, prev_channel_id, claim_from_onchain_tx, next_channel_id,
-                       outbound_amount_forwarded_msat: _
+                       outbound_amount_forwarded_msat: _, skimmed_fee_msat
                } => {
                        assert_eq!(total_fee_earned_msat, expected_fee);
+
+                       // Check that the (knowingly) withheld amount is always less or equal to the expected
+                       // overpaid amount.
+                       assert!(skimmed_fee_msat == expected_extra_fees_msat);
                        if !upstream_force_closed {
                                // Is the event prev_channel_id in one of the channels between the two nodes?
                                assert!(node.node().list_channels().iter().any(|x| x.counterparty.node_id == prev_node.node().get_our_node_id() && x.channel_id == prev_channel_id.unwrap()));
@@ -2226,13 +2231,15 @@ pub fn expect_payment_forwarded<CM: AChannelManager, H: NodeHolder<CM=CM>>(
        }
 }
 
+#[macro_export]
 macro_rules! expect_payment_forwarded {
        ($node: expr, $prev_node: expr, $next_node: expr, $expected_fee: expr, $upstream_force_closed: expr, $downstream_force_closed: expr) => {
                let mut events = $node.node.get_and_clear_pending_events();
                assert_eq!(events.len(), 1);
                $crate::ln::functional_test_utils::expect_payment_forwarded(
-                       events.pop().unwrap(), &$node, &$prev_node, &$next_node, $expected_fee,
-                       $upstream_force_closed, $downstream_force_closed);
+                       events.pop().unwrap(), &$node, &$prev_node, &$next_node, $expected_fee, None,
+                       $upstream_force_closed, $downstream_force_closed
+               );
        }
 }
 
@@ -2696,11 +2703,17 @@ pub fn pass_claimed_payment_along_route<'a, 'b, 'c, 'd>(args: ClaimAlongRouteArg
                                                        channel.context().config().forwarding_fee_base_msat
                                                }
                                        };
+
+                                       let mut expected_extra_fee = None;
                                        if $idx == 1 {
                                                fee += expected_extra_fees[i];
                                                fee += expected_min_htlc_overpay[i];
+                                               expected_extra_fee = if expected_extra_fees[i] > 0 { Some(expected_extra_fees[i] as u64) } else { None };
                                        }
-                                       expect_payment_forwarded!(*$node, $next_node, $prev_node, Some(fee as u64), false, false);
+                                       let mut events = $node.node.get_and_clear_pending_events();
+                                       assert_eq!(events.len(), 1);
+                                       expect_payment_forwarded(events.pop().unwrap(), *$node, $next_node, $prev_node,
+                                               Some(fee as u64), expected_extra_fee, false, false);
                                        expected_total_fee_msat += fee as u64;
                                        check_added_monitors!($node, 1);
                                        let new_next_msgs = if $new_msgs {
index c98b7231a218a7e4416938a2930465e22da832db..5f98910317070aa6fdef34fb7ac6fddbc3db79af 100644 (file)
@@ -2890,7 +2890,7 @@ fn test_htlc_on_chain_success() {
        let chan_id = Some(chan_1.2);
        match forwarded_events[1] {
                Event::PaymentForwarded { total_fee_earned_msat, prev_channel_id, claim_from_onchain_tx,
-                       next_channel_id, outbound_amount_forwarded_msat
+                       next_channel_id, outbound_amount_forwarded_msat, ..
                } => {
                        assert_eq!(total_fee_earned_msat, Some(1000));
                        assert_eq!(prev_channel_id, chan_id);
@@ -2902,7 +2902,7 @@ fn test_htlc_on_chain_success() {
        }
        match forwarded_events[2] {
                Event::PaymentForwarded { total_fee_earned_msat, prev_channel_id, claim_from_onchain_tx,
-                       next_channel_id, outbound_amount_forwarded_msat
+                       next_channel_id, outbound_amount_forwarded_msat, ..
                } => {
                        assert_eq!(total_fee_earned_msat, Some(1000));
                        assert_eq!(prev_channel_id, chan_id);
@@ -4917,7 +4917,7 @@ fn test_onchain_to_onchain_claim() {
        }
        match events[1] {
                Event::PaymentForwarded { total_fee_earned_msat, prev_channel_id, claim_from_onchain_tx,
-                       next_channel_id, outbound_amount_forwarded_msat
+                       next_channel_id, outbound_amount_forwarded_msat, ..
                } => {
                        assert_eq!(total_fee_earned_msat, Some(1000));
                        assert_eq!(prev_channel_id, Some(chan_1.2));