Introduce HopConnector enum

In order to move the Trampoline-specific data from the onion decryption
layer to the PendingHTLCRouting enum, we need to make the
NextPacketDetails more descriptive, which we do here by virtue of
converting the outbound_scid field into an enum than is more expressive
in its Trampoline variant.

Author: arik-so

lightning/src/ln/channelmanager.rs

@@ -57,7 +57,7 @@ use crate::types::features::{Bolt12InvoiceFeatures, ChannelFeatures, ChannelType
 use crate::types::features::Bolt11InvoiceFeatures;
 use crate::routing::gossip::NodeId;
 use crate::routing::router::{BlindedTail, FixedRouter, InFlightHtlcs, Path, Payee, PaymentParameters, Route, RouteParameters, Router};
-use crate::ln::onion_payment::{check_incoming_htlc_cltv, create_recv_pending_htlc_info, create_fwd_pending_htlc_info, decode_incoming_update_add_htlc_onion, InboundHTLCErr, NextPacketDetails};
+use crate::ln::onion_payment::{check_incoming_htlc_cltv, create_recv_pending_htlc_info, create_fwd_pending_htlc_info, decode_incoming_update_add_htlc_onion, InboundHTLCErr, NextPacketDetails, HopConnector};
 use crate::ln::msgs;
 use crate::ln::onion_utils;
 use crate::ln::onion_utils::{HTLCFailReason, INVALID_ONION_BLINDING};
@@ -4261,11 +4261,15 @@ where
 			// we don't allow forwards outbound over them.
 			return Err(("Refusing to forward to a private channel based on our config.", 0x4000 | 10));
 		}
-		if chan.context.get_channel_type().supports_scid_privacy() && next_packet.outgoing_scid != chan.context.outbound_scid_alias() {
-			// `option_scid_alias` (referred to in LDK as `scid_privacy`) means
-			// "refuse to forward unless the SCID alias was used", so we pretend
-			// we don't have the channel here.
-			return Err(("Refusing to forward over real channel SCID as our counterparty requested.", 0x4000 | 10));
+		if let HopConnector::ShortChannelId(outgoing_scid) = next_packet.outgoing_connector {
+			if chan.context.get_channel_type().supports_scid_privacy() && outgoing_scid != chan.context.outbound_scid_alias() {
+				// `option_scid_alias` (referred to in LDK as `scid_privacy`) means
+				// "refuse to forward unless the SCID alias was used", so we pretend
+				// we don't have the channel here.
+				return Err(("Refusing to forward over real channel SCID as our counterparty requested.", 0x4000 | 10));
+			}
+		} else {
+			return Err(("Cannot forward by Node ID without SCID.", 0x4000 | 10));
 		}
 
 		// Note that we could technically not return an error yet here and just hope
@@ -4317,7 +4321,13 @@ where
 	fn can_forward_htlc(
 		&self, msg: &msgs::UpdateAddHTLC, next_packet_details: &NextPacketDetails
 	) -> Result<(), (&'static str, u16)> {
-		match self.do_funded_channel_callback(next_packet_details.outgoing_scid, |chan: &mut FundedChannel<SP>| {
+		let outgoing_scid = match next_packet_details.outgoing_connector {
+			HopConnector::ShortChannelId(scid) => scid,
+			HopConnector::Trampoline { .. } => {
+				return Err(("Cannot forward by Node ID without SCID.", 0x4000 | 10));
+			}
+		};
+		match self.do_funded_channel_callback(outgoing_scid, |chan: &mut FundedChannel<SP>| {
 			self.can_forward_htlc_to_outgoing_channel(chan, msg, next_packet_details)
 		}) {
 			Some(Ok(())) => {},
@@ -4326,8 +4336,8 @@ where
 				// If we couldn't find the channel info for the scid, it may be a phantom or
 				// intercept forward.
 				if (self.default_configuration.accept_intercept_htlcs &&
-					fake_scid::is_valid_intercept(&self.fake_scid_rand_bytes, next_packet_details.outgoing_scid, &self.chain_hash)) ||
-					fake_scid::is_valid_phantom(&self.fake_scid_rand_bytes, next_packet_details.outgoing_scid, &self.chain_hash)
+					fake_scid::is_valid_intercept(&self.fake_scid_rand_bytes, outgoing_scid, &self.chain_hash)) ||
+					fake_scid::is_valid_phantom(&self.fake_scid_rand_bytes, outgoing_scid, &self.chain_hash)
 				{} else {
 					return Err(("Don't have available channel for forwarding as requested.", 0x4000 | 10));
 				}
@@ -5679,7 +5689,12 @@ where
 				};
 
 				let is_intro_node_blinded_forward = next_hop.is_intro_node_blinded_forward();
-				let outgoing_scid_opt = next_packet_details_opt.as_ref().map(|d| d.outgoing_scid);
+				let outgoing_scid_opt = next_packet_details_opt.as_ref().and_then(|d| {
+					match d.outgoing_connector {
+						HopConnector::ShortChannelId(scid) => { Some(scid) }
+						HopConnector::Trampoline { .. } => { None }
+					}
+				});
 
 				// Process the HTLC on the incoming channel.
 				match self.do_funded_channel_callback(incoming_scid, |chan: &mut FundedChannel<SP>| {

lightning/src/ln/onion_payment.rs

@@ -17,6 +17,7 @@ use crate::types::features::BlindedHopFeatures;
 use crate::ln::msgs;
 use crate::ln::onion_utils;
 use crate::ln::onion_utils::{HTLCFailReason, INVALID_ONION_BLINDING};
+use crate::routing::gossip::NodeId;
 use crate::sign::{NodeSigner, Recipient};
 use crate::util::logger::Logger;
 
@@ -298,7 +299,7 @@ where
 	Ok(match hop {
 		onion_utils::Hop::Forward { next_hop_data, next_hop_hmac, new_packet_bytes } => {
 			let NextPacketDetails {
-				next_packet_pubkey, outgoing_amt_msat: _, outgoing_scid: _, outgoing_cltv_value
+				next_packet_pubkey, outgoing_amt_msat: _, outgoing_connector: _, outgoing_cltv_value
 			} = match next_packet_details_opt {
 				Some(next_packet_details) => next_packet_details,
 				// Forward should always include the next hop details
@@ -336,9 +337,22 @@ where
 	})
 }
 
+pub(super) enum HopConnector {
+	// scid-based routing
+	ShortChannelId(u64),
+	// Trampoline-based routing
+	#[allow(unused)]
+	Trampoline {
+		// shared secret to derive keys for error decoding
+		shared_secret: [u8; 32],
+		// node ID to get to the next Trampoline hop
+		node_id: NodeId,
+	},
+}
+
 pub(super) struct NextPacketDetails {
 	pub(super) next_packet_pubkey: Result<PublicKey, secp256k1::Error>,
-	pub(super) outgoing_scid: u64,
+	pub(super) outgoing_connector: HopConnector,
 	pub(super) outgoing_amt_msat: u64,
 	pub(super) outgoing_cltv_value: u32,
 }
@@ -432,7 +446,7 @@ where
 			let next_packet_pubkey = onion_utils::next_hop_pubkey(secp_ctx,
 				msg.onion_routing_packet.public_key.unwrap(), &shared_secret);
 			NextPacketDetails {
-				next_packet_pubkey, outgoing_scid: short_channel_id,
+				next_packet_pubkey, outgoing_connector: HopConnector::ShortChannelId(short_channel_id),
 				outgoing_amt_msat: amt_to_forward, outgoing_cltv_value
 			}
 		},
@@ -453,7 +467,7 @@ where
 			let next_packet_pubkey = onion_utils::next_hop_pubkey(&secp_ctx,
 				msg.onion_routing_packet.public_key.unwrap(), &shared_secret);
 			NextPacketDetails {
-				next_packet_pubkey, outgoing_scid: short_channel_id, outgoing_amt_msat: amt_to_forward,
+				next_packet_pubkey, outgoing_connector: HopConnector::ShortChannelId(short_channel_id), outgoing_amt_msat: amt_to_forward,
 				outgoing_cltv_value
 			}
 		},