Merge pull request #2715 from valentinewallace/2023-11-skimmed-fees

Complete underpaying HTLCs support

Author: TheBlueMatt

lightning/src/events/mod.rs

@@ -102,9 +102,15 @@ pub struct ClaimedHTLC {
 	pub cltv_expiry: u32,
 	/// The amount (in msats) of this part of an MPP.
 	pub value_msat: u64,
+	/// The extra fee our counterparty skimmed off the top of this HTLC, if any.
+	///
+	/// This value will always be 0 for [`ClaimedHTLC`]s serialized with LDK versions prior to
+	/// 0.0.119.
+	pub counterparty_skimmed_fee_msat: u64,
 }
 impl_writeable_tlv_based!(ClaimedHTLC, {
 	(0, channel_id, required),
+	(1, counterparty_skimmed_fee_msat, (default_value, 0u64)),
 	(2, user_channel_id, required),
 	(4, cltv_expiry, required),
 	(6, value_msat, required),

lightning/src/ln/channelmanager.rs

@@ -238,6 +238,7 @@ impl From<&ClaimableHTLC> for events::ClaimedHTLC {
 			user_channel_id: val.prev_hop.user_channel_id.unwrap_or(0),
 			cltv_expiry: val.cltv_expiry,
 			value_msat: val.value,
+			counterparty_skimmed_fee_msat: val.counterparty_skimmed_fee_msat.unwrap_or(0),
 		}
 	}
 }

lightning/src/util/config.rs

@@ -462,8 +462,9 @@ pub struct ChannelConfig {
 	/// - The counterparty will get an [`HTLCIntercepted`] event upon payment forward, and call
 	///   [`forward_intercepted_htlc`] with less than the amount provided in
 	///   [`HTLCIntercepted::expected_outbound_amount_msat`]. The difference between the expected and
-	///   actual forward amounts is their fee.
-	// TODO: link to LSP JIT channel invoice generation spec when it's merged
+	///   actual forward amounts is their fee. See
+	///   <https://github.com/BitcoinAndLightningLayerSpecs/lsp/tree/main/LSPS2#flow-lsp-trusts-client-model>
+	///   for how this feature may be used in the LSP use case.
 	///
 	/// # Note
 	/// It's important for payee wallet software to verify that [`PaymentClaimable::amount_msat`] is