Add CounterpartyForwardingInfo field to channel.

This will be filled in in upcoming commits, then exposed in ChannelDetails
to allow constructing route hints for invoices.

Author: valentinewallace

lightning/src/ln/channel.rs

@@ -281,6 +281,12 @@ impl HTLCCandidate {
 	}
 }
 
+pub struct CounterpartyForwardingInfo {
+	pub fee_base_msat: u32,
+	pub fee_proportional_millionths: u32,
+	pub cltv_expiry_delta: u16,
+}
+
 // TODO: We should refactor this to be an Inbound/OutboundChannel until initial setup handshaking
 // has been completed, and then turn into a Channel to get compiler-time enforcement of things like
 // calling channel_id() before we're set up or things like get_outbound_funding_signed on an
@@ -391,6 +397,8 @@ pub(super) struct Channel<Signer: Sign> {
 	//implied by OUR_MAX_HTLCS: max_accepted_htlcs: u16,
 	minimum_depth: u32,
 
+	counterparty_forwarding_info: Option<CounterpartyForwardingInfo>,
+
 	pub(crate) channel_transaction_parameters: ChannelTransactionParameters,
 
 	counterparty_cur_commitment_point: Option<PublicKey>,
@@ -577,6 +585,8 @@ impl<Signer: Sign> Channel<Signer> {
 			counterparty_max_accepted_htlcs: 0,
 			minimum_depth: 0, // Filled in in accept_channel
 
+			counterparty_forwarding_info: None,
+
 			channel_transaction_parameters: ChannelTransactionParameters {
 				holder_pubkeys: pubkeys,
 				holder_selected_contest_delay: config.own_channel_config.our_to_self_delay,
@@ -813,6 +823,8 @@ impl<Signer: Sign> Channel<Signer> {
 			counterparty_max_accepted_htlcs: msg.max_accepted_htlcs,
 			minimum_depth: config.own_channel_config.minimum_depth,
 
+			counterparty_forwarding_info: None,
+
 			channel_transaction_parameters: ChannelTransactionParameters {
 				holder_pubkeys: pubkeys,
 				holder_selected_contest_delay: config.own_channel_config.our_to_self_delay,
@@ -4437,6 +4449,16 @@ impl<Signer: Sign> Writeable for Channel<Signer> {
 		self.counterparty_max_accepted_htlcs.write(writer)?;
 		self.minimum_depth.write(writer)?;
 
+		match &self.counterparty_forwarding_info {
+			Some(info) => {
+				1u8.write(writer)?;
+				info.fee_base_msat.write(writer)?;
+				info.fee_proportional_millionths.write(writer)?;
+				info.cltv_expiry_delta.write(writer)?;
+			},
+			None => 0u8.write(writer)?
+		}
+
 		self.channel_transaction_parameters.write(writer)?;
 		self.counterparty_cur_commitment_point.write(writer)?;
 
@@ -4597,6 +4619,16 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<&'a K> for Channel<Signer>
 		let counterparty_max_accepted_htlcs = Readable::read(reader)?;
 		let minimum_depth = Readable::read(reader)?;
 
+		let counterparty_forwarding_info = match <u8 as Readable>::read(reader)? {
+			0 => None,
+			1 => Some(CounterpartyForwardingInfo {
+				fee_base_msat: Readable::read(reader)?,
+				fee_proportional_millionths: Readable::read(reader)?,
+				cltv_expiry_delta: Readable::read(reader)?,
+			}),
+			_ => return Err(DecodeError::InvalidValue),
+		};
+
 		let channel_parameters = Readable::read(reader)?;
 		let counterparty_cur_commitment_point = Readable::read(reader)?;
 
@@ -4667,6 +4699,8 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<&'a K> for Channel<Signer>
 			counterparty_max_accepted_htlcs,
 			minimum_depth,
 
+			counterparty_forwarding_info,
+
 			channel_transaction_parameters: channel_parameters,
 			counterparty_cur_commitment_point,