Add PaymentClaimable::counterparty_skimmed_fee_msat

See its docs

Author: valentinewallace

lightning/src/events/mod.rs

@@ -387,8 +387,24 @@ pub enum Event {
 		///
 		/// Payments received on LDK versions prior to 0.0.115 will have this field unset.
 		onion_fields: Option<RecipientOnionFields>,
-		/// The value, in thousandths of a satoshi, that this payment is for.
+		/// The value, in thousandths of a satoshi, that this payment is claimable for.
+		///
+		/// May be less than the invoice amount if [`ChannelConfig::accept_underpaying_htlcs`] is set
+		/// and the previous hop took an extra fee.
+		///
+		/// # Note
+		/// If [`ChannelConfig::accept_underpaying_htlcs`] is set and you claim without verifying this
+		/// field, you may lose money!
+		///
+		/// [`ChannelConfig::accept_underpaying_htlcs`]: crate::util::config::ChannelConfig::accept_underpaying_htlcs
 		amount_msat: u64,
+		/// The value, in thousands of a satoshi, that was skimmed off of this payment as an extra fee
+		/// taken by our channel counterparty.
+		///
+		/// Will always be 0 unless [`ChannelConfig::accept_underpaying_htlcs`] is set.
+		///
+		/// [`ChannelConfig::accept_underpaying_htlcs`]: crate::util::config::ChannelConfig::accept_underpaying_htlcs
+		counterparty_skimmed_fee_msat: u64,
 		/// Information for claiming this received payment, based on whether the purpose of the
 		/// payment is to pay an invoice or to send a spontaneous payment.
 		purpose: PaymentPurpose,
@@ -832,8 +848,8 @@ impl Writeable for Event {
 				// We never write out FundingGenerationReady events as, upon disconnection, peers
 				// drop any channels which have not yet exchanged funding_signed.
 			},
-			&Event::PaymentClaimable { ref payment_hash, ref amount_msat, ref purpose,
-				ref receiver_node_id, ref via_channel_id, ref via_user_channel_id,
+			&Event::PaymentClaimable { ref payment_hash, ref amount_msat, counterparty_skimmed_fee_msat,
+				ref purpose, ref receiver_node_id, ref via_channel_id, ref via_user_channel_id,
 				ref claim_deadline, ref onion_fields
 			} => {
 				1u8.write(writer)?;
@@ -848,6 +864,8 @@ impl Writeable for Event {
 						payment_preimage = Some(*preimage);
 					}
 				}
+				let skimmed_fee_opt = if counterparty_skimmed_fee_msat == 0 { None }
+					else { Some(counterparty_skimmed_fee_msat) };
 				write_tlv_fields!(writer, {
 					(0, payment_hash, required),
 					(1, receiver_node_id, option),
@@ -859,6 +877,7 @@ impl Writeable for Event {
 					(7, claim_deadline, option),
 					(8, payment_preimage, option),
 					(9, onion_fields, option),
+					(10, skimmed_fee_opt, option),
 				});
 			},
 			&Event::PaymentSent { ref payment_id, ref payment_preimage, ref payment_hash, ref fee_paid_msat } => {
@@ -1058,6 +1077,7 @@ impl MaybeReadable for Event {
 					let mut payment_preimage = None;
 					let mut payment_secret = None;
 					let mut amount_msat = 0;
+					let mut counterparty_skimmed_fee_msat_opt = None;
 					let mut receiver_node_id = None;
 					let mut _user_payment_id = None::<u64>; // For compatibility with 0.0.103 and earlier
 					let mut via_channel_id = None;
@@ -1075,6 +1095,7 @@ impl MaybeReadable for Event {
 						(7, claim_deadline, option),
 						(8, payment_preimage, option),
 						(9, onion_fields, option),
+						(10, counterparty_skimmed_fee_msat_opt, option),
 					});
 					let purpose = match payment_secret {
 						Some(secret) => PaymentPurpose::InvoicePayment {
@@ -1088,6 +1109,7 @@ impl MaybeReadable for Event {
 						receiver_node_id,
 						payment_hash,
 						amount_msat,
+						counterparty_skimmed_fee_msat: counterparty_skimmed_fee_msat_opt.unwrap_or(0),
 						purpose,
 						via_channel_id,
 						via_user_channel_id,

lightning/src/ln/channelmanager.rs

@@ -3918,6 +3918,9 @@ where
 												payment_hash,
 												purpose: $purpose,
 												amount_msat,
+												counterparty_skimmed_fee_msat:
+													cmp::max(total_value.saturating_sub(amount_msat), htlcs.iter()
+														.map(|htlc| htlc.counterparty_skimmed_fee_msat.unwrap_or(0)).sum()),
 												via_channel_id: Some(prev_channel_id),
 												via_user_channel_id: Some(prev_user_channel_id),
 												claim_deadline: Some(earliest_expiry - HTLC_FAIL_BACK_BUFFER),

lightning/src/ln/functional_test_utils.rs

@@ -2124,7 +2124,7 @@ pub fn do_pass_along_path<'a, 'b, 'c>(origin_node: &Node<'a, 'b, 'c>, expected_p
 				match &events_2[0] {
 					Event::PaymentClaimable { ref payment_hash, ref purpose, amount_msat,
 						receiver_node_id, ref via_channel_id, ref via_user_channel_id,
-						claim_deadline, onion_fields,
+						claim_deadline, onion_fields, ..
 					} => {
 						assert_eq!(our_payment_hash, *payment_hash);
 						assert_eq!(node.node.get_our_node_id(), receiver_node_id.unwrap());

lightning/src/util/config.rs

@@ -412,6 +412,8 @@ pub struct ChannelConfig {
 	/// # Note
 	/// It's important for payee wallet software to verify that [`PaymentClaimable::amount_msat`] is
 	/// as-expected if this feature is activated, otherwise they may lose money!
+	/// [`PaymentClaimable::counterparty_skimmed_fee_msat`] provides the fee taken by the
+	/// counterparty.
 	///
 	/// # Note
 	/// Switching this config flag on may break compatibility with versions of LDK prior to 0.0.116.
@@ -423,6 +425,7 @@ pub struct ChannelConfig {
 	/// [`HTLCIntercepted`]: crate::events::Event::HTLCIntercepted
 	/// [`HTLCIntercepted::expected_outbound_amount_msat`]: crate::events::Event::HTLCIntercepted::expected_outbound_amount_msat
 	/// [`PaymentClaimable::amount_msat`]: crate::events::Event::PaymentClaimable::amount_msat
+	/// [`PaymentClaimable::counterparty_skimmed_fee_msat`]: crate::events::Event::PaymentClaimable::counterparty_skimmed_fee_msat
 	//  TODO: link to bLIP when it's merged
 	pub accept_underpaying_htlcs: bool,
 }

pending_changelog/forward-underpaying-htlc.txt

@@ -2,3 +2,5 @@
 
 * Forwarding less than the expected amount in `ChannelManager::forward_intercepted_htlc` may break
 	compatibility with versions of LDK prior to 0.0.116
+* Setting `ChannelConfig::accept_underpaying_htlcs` may break compatibility with versions of LDK
+	prior to 0.0.116, and unsetting the feature between restarts may lead to payment failures.