Persist update_add sender skimmed fee in Channel

Author: valentinewallace

lightning/src/ln/channel.rs

@@ -222,6 +222,7 @@ struct OutboundHTLCOutput {
 	payment_hash: PaymentHash,
 	state: OutboundHTLCState,
 	source: HTLCSource,
+	skimmed_fee_msat: Option<u64>,
 }
 
 /// See AwaitingRemoteRevoke ChannelState for more info
@@ -233,6 +234,8 @@ enum HTLCUpdateAwaitingACK {
 		payment_hash: PaymentHash,
 		source: HTLCSource,
 		onion_routing_packet: msgs::OnionPacket,
+		// The extra fee skimmed by the sender of this update_add.
+		skimmed_fee_msat: Option<u64>,
 	},
 	ClaimHTLC {
 		payment_preimage: PaymentPreimage,
@@ -5928,6 +5931,7 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 				cltv_expiry,
 				source,
 				onion_routing_packet,
+				skimmed_fee_msat: None,
 			});
 			return Ok(None);
 		}
@@ -5939,6 +5943,7 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 			cltv_expiry,
 			state: OutboundHTLCState::LocalAnnounced(Box::new(onion_routing_packet.clone())),
 			source,
+			skimmed_fee_msat: None,
 		});
 
 		let res = msgs::UpdateAddHTLC {
@@ -6458,18 +6463,23 @@ impl<Signer: WriteableEcdsaChannelSigner> Writeable for Channel<Signer> {
 					reason.write(writer)?;
 				}
 			}
+			htlc.skimmed_fee_msat.write(writer)?;
 		}
 
 		(self.holding_cell_htlc_updates.len() as u64).write(writer)?;
 		for update in self.holding_cell_htlc_updates.iter() {
 			match update {
-				&HTLCUpdateAwaitingACK::AddHTLC { ref amount_msat, ref cltv_expiry, ref payment_hash, ref source, ref onion_routing_packet } => {
+				&HTLCUpdateAwaitingACK::AddHTLC {
+					ref amount_msat, ref cltv_expiry, ref payment_hash, ref source, ref onion_routing_packet,
+					ref skimmed_fee_msat,
+				} => {
 					0u8.write(writer)?;
 					amount_msat.write(writer)?;
 					cltv_expiry.write(writer)?;
 					payment_hash.write(writer)?;
 					source.write(writer)?;
 					onion_routing_packet.write(writer)?;
+					skimmed_fee_msat.write(writer)?;
 				},
 				&HTLCUpdateAwaitingACK::ClaimHTLC { ref payment_preimage, ref htlc_id } => {
 					1u8.write(writer)?;
@@ -6746,6 +6756,7 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 					},
 					_ => return Err(DecodeError::InvalidValue),
 				},
+				skimmed_fee_msat: Readable::read(reader)?,
 			});
 		}
 
@@ -6759,6 +6770,7 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 					payment_hash: Readable::read(reader)?,
 					source: Readable::read(reader)?,
 					onion_routing_packet: Readable::read(reader)?,
+					skimmed_fee_msat: Readable::read(reader)?,
 				},
 				1 => HTLCUpdateAwaitingACK::ClaimHTLC {
 					payment_preimage: Readable::read(reader)?,
@@ -7317,7 +7329,8 @@ mod tests {
 				session_priv: SecretKey::from_slice(&hex::decode("0fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff").unwrap()[..]).unwrap(),
 				first_hop_htlc_msat: 548,
 				payment_id: PaymentId([42; 32]),
-			}
+			},
+			skimmed_fee_msat: None,
 		});
 
 		// Make sure when Node A calculates their local commitment transaction, none of the HTLCs pass

pending_changelog/accept-htlc-underpay.txt

@@ -0,0 +1,4 @@
+## Backwards Compat
+
+* 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