Introduce HopConnector enum

When parsing inbound HTLCs, we need to provide supplemental information
about potential outgoing hops, which hitherto always required the next
SCID. However, Trampoline uproots the approach because the sender did
not specify the SCID; rather, they rely on us to do the intermediate
routing. Thus we now need to distinguish between SCID-based and node-
ID-based outgoing routing.

Author: arik-so

lightning/src/ln/channelmanager.rs

@@ -58,7 +58,7 @@ use crate::types::features::Bolt11InvoiceFeatures;
 #[cfg(trampoline)]
 use crate::routing::gossip::NodeId;
 use crate::routing::router::{BlindedTail, FixedRouter, InFlightHtlcs, Path, Payee, PaymentParameters, Route, RouteParameters, RouteParametersConfig, 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, HopConnector, InboundHTLCErr, NextPacketDetails};
 use crate::ln::msgs;
 use crate::ln::onion_utils;
 use crate::ln::onion_utils::{HTLCFailReason, INVALID_ONION_BLINDING};
@@ -4290,11 +4290,15 @@ 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
@@ -4346,7 +4350,13 @@ 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(())) => {},
@@ -4355,8 +4365,8 @@ 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));
 				}
@@ -5708,7 +5718,12 @@ 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 }
+					}
+				});
 				let shared_secret = next_hop.shared_secret().secret_bytes();
 
 				// Process the HTLC on the incoming channel.

lightning/src/ln/onion_payment.rs

@@ -16,6 +16,7 @@ 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
 		onion_utils::Hop::Forward { shared_secret, next_hop_hmac, new_packet_bytes, .. } |
 		onion_utils::Hop::BlindedForward { shared_secret, 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 +337,17 @@ where
 	})
 }
 
+pub(super) enum HopConnector {
+	// scid-based routing
+	ShortChannelId(u64),
+	// Trampoline-based routing
+	#[allow(unused)]
+	Trampoline(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,
 }
@@ -415,7 +424,7 @@ where
 			let next_packet_pubkey = onion_utils::next_hop_pubkey(secp_ctx,
 				msg.onion_routing_packet.public_key.unwrap(), &shared_secret.secret_bytes());
 			Some(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
 			})
 		}
@@ -432,7 +441,7 @@ where
 			let next_packet_pubkey = onion_utils::next_hop_pubkey(&secp_ctx,
 				msg.onion_routing_packet.public_key.unwrap(), &shared_secret.secret_bytes());
 			Some(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
 			})
 		}