Add CounterpartyForwardingInfo field to channel.

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

Also update the cltv_expiry_deta comment in msgs::ChannelUpdate

Co-authored-by: Valentine Wallace <[email protected]>
Co-authored-by: Antoine Riard <[email protected]>

Author: valentinewallace

lightning/src/ln/channel.rs

@@ -281,6 +281,18 @@ impl HTLCCandidate {
 	}
 }
 
+/// Information needed for constructing an invoice route hint for this channel.
+pub struct CounterpartyForwardingInfo {
+	/// Base routing fee in millisatoshis.
+	pub fee_base_msat: u32,
+	/// Amount in millionths of a satoshi the channel will charge per transferred satoshi.
+	pub fee_proportional_millionths: u32,
+	/// The minimum difference in cltv_expiry between an ingoing HTLC and its outgoing counterpart,
+	/// such that the outgoing HTLC is forwardable to this counterparty. See `msgs::ChannelUpdate`'s
+	/// `cltv_expiry_delta` for more details.
+	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 +403,8 @@ 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 +591,8 @@ 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 +829,8 @@ 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 +4455,16 @@ 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 +4625,16 @@ 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 +4705,8 @@ 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,
 

lightning/src/ln/msgs.rs

@@ -543,7 +543,14 @@ pub struct UnsignedChannelUpdate {
 	pub timestamp: u32,
 	/// Channel flags
 	pub flags: u8,
-	/// The number of blocks to subtract from incoming HTLC cltv_expiry values
+	/// The number of blocks such that if:
+	/// `incoming_htlc.cltv_expiry < outgoing_htlc.cltv_expiry + cltv_expiry_delta`
+	/// then we need to fail the HTLC backwards. When forwarding an HTLC, cltv_expiry_delta determines
+	/// the outgoing HTLC's minimum cltv_expiry value -- so, if an incoming HTLC comes in with a
+	/// cltv_expiry of 100000, and the node we're forwarding to has a cltv_expiry_delta value of 10,
+	/// then we'll check that the outgoing HTLC's cltv_expiry value is at least 100010 before
+	/// forwarding. Note that the HTLC sender is the one who originally sets this value when
+	/// constructing the route.
 	pub cltv_expiry_delta: u16,
 	/// The minimum HTLC size incoming to sender, in milli-satoshi
 	pub htlc_minimum_msat: u64,