Persist update_add sender skimmed fee in Channel

This will always be None as of this commit, but once we start setting the field
it will break backwards compat with LDK versions prior to 0.0.116.

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 we're skimming off the top of this HTLC.
+		skimmed_fee_msat: Option<u64>,
 	},
 	ClaimHTLC {
 		payment_preimage: PaymentPreimage,
@@ -6003,6 +6006,7 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 				cltv_expiry,
 				source,
 				onion_routing_packet,
+				skimmed_fee_msat: None,
 			});
 			return Ok(None);
 		}
@@ -6014,6 +6018,7 @@ 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 {
@@ -6533,18 +6538,23 @@ 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)?;
@@ -6821,6 +6831,7 @@ 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)?,
 			});
 		}
 
@@ -6834,6 +6845,7 @@ 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)?,
@@ -7393,7 +7405,8 @@ 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
@@ -7927,6 +7940,7 @@ mod tests {
 				payment_hash: PaymentHash([0; 32]),
 				state: OutboundHTLCState::Committed,
 				source: HTLCSource::dummy(),
+				skimmed_fee_msat: None,
 			};
 			out.payment_hash.0 = Sha256::hash(&hex::decode("0202020202020202020202020202020202020202020202020202020202020202").unwrap()).into_inner();
 			out
@@ -7939,6 +7953,7 @@ mod tests {
 				payment_hash: PaymentHash([0; 32]),
 				state: OutboundHTLCState::Committed,
 				source: HTLCSource::dummy(),
+				skimmed_fee_msat: None,
 			};
 			out.payment_hash.0 = Sha256::hash(&hex::decode("0303030303030303030303030303030303030303030303030303030303030303").unwrap()).into_inner();
 			out
@@ -8340,6 +8355,7 @@ mod tests {
 				payment_hash: PaymentHash([0; 32]),
 				state: OutboundHTLCState::Committed,
 				source: HTLCSource::dummy(),
+				skimmed_fee_msat: None,
 			};
 			out.payment_hash.0 = Sha256::hash(&hex::decode("0505050505050505050505050505050505050505050505050505050505050505").unwrap()).into_inner();
 			out
@@ -8352,6 +8368,7 @@ mod tests {
 				payment_hash: PaymentHash([0; 32]),
 				state: OutboundHTLCState::Committed,
 				source: HTLCSource::dummy(),
+				skimmed_fee_msat: None,
 			};
 			out.payment_hash.0 = Sha256::hash(&hex::decode("0505050505050505050505050505050505050505050505050505050505050505").unwrap()).into_inner();
 			out