Add trampoline_hops field to BlindedTail

Given that we do not intend to allow sending to unblinded recipients
via Trampoline hops, we're adding a vector of unblinded `TrampolineHop`s
directly to the `BlindedTail` struct.

Author: arik-so

lightning/src/ln/blinded_payment_tests.rs

@@ -1505,6 +1505,7 @@ fn route_blinding_spec_test_vector() {
 			maybe_announced_channel: false,
 		}],
 		blinded_tail: Some(BlindedTail {
+			trampoline_hops: vec![],
 			hops: blinded_hops,
 			blinding_point: bob_blinding_point,
 			excess_final_cltv_expiry_delta: 0,

lightning/src/routing/router.rs

@@ -399,13 +399,13 @@ pub struct RouteHop {
 	/// The fee taken on this hop (for paying for the use of the *next* channel in the path).
 	/// If this is the last hop in [`Path::hops`]:
 	/// * if we're sending to a [`BlindedPaymentPath`], this is the fee paid for use of the entire
-	///   blinded path
+	///   blinded path (including any Trampoline hops)
 	/// * otherwise, this is the full value of this [`Path`]'s part of the payment
 	pub fee_msat: u64,
 	/// The CLTV delta added for this hop.
 	/// If this is the last hop in [`Path::hops`]:
 	/// * if we're sending to a [`BlindedPaymentPath`], this is the CLTV delta for the entire blinded
-	///   path
+	///   path (including any Trampoline hops)
 	/// * otherwise, this is the CLTV delta expected at the destination
 	pub cltv_expiry_delta: u32,
 	/// Indicates whether this hop is possibly announced in the public network graph.
@@ -460,6 +460,13 @@ impl_writeable_tlv_based!(TrampolineHop, {
 /// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub struct BlindedTail {
+	/// The list of unblinded Trampoline hops. When using Trampoline, must contain at least one hop.
+	///
+	/// Note that the first [`TrampolineHop`] node must also be present as the last [`RouteHop`] node,
+	/// where the RouteHop's fee_msat is the total amount received by the first [`TrampolineHop`],
+	/// including any blinded path, whereas the [`TrampolineHop`]'s fee_msat is the fee that node
+	/// may use to route to the next Trampoline hop and pay itself for the trouble.
+	pub trampoline_hops: Vec<TrampolineHop>,
 	/// The hops of the [`BlindedPaymentPath`] provided by the recipient.
 	pub hops: Vec<BlindedHop>,
 	/// The blinding point of the [`BlindedPaymentPath`] provided by the recipient.
@@ -476,6 +483,7 @@ impl_writeable_tlv_based!(BlindedTail, {
 	(2, blinding_point, required),
 	(4, excess_final_cltv_expiry_delta, required),
 	(6, final_value_msat, required),
+	(8, trampoline_hops, optional_vec),
 });
 
 /// A path in a [`Route`] to the payment recipient. Must always be at least length one.
@@ -3404,6 +3412,8 @@ where L::Target: Logger {
 			if let Some(blinded_path) = h.candidate.blinded_path() {
 				final_cltv_delta = h.candidate.cltv_expiry_delta();
 				Some(BlindedTail {
+					// TODO: fill correctly
+					trampoline_hops: vec![],
 					hops: blinded_path.blinded_hops().to_vec(),
 					blinding_point: blinded_path.blinding_point(),
 					excess_final_cltv_expiry_delta: 0,
@@ -7750,6 +7760,7 @@ mod tests {
 				maybe_announced_channel: true,
 			}],
 			blinded_tail: Some(BlindedTail {
+				trampoline_hops: vec![],
 				hops: vec![
 					BlindedHop { blinded_node_id: ln_test_utils::pubkey(44), encrypted_payload: Vec::new() },
 					BlindedHop { blinded_node_id: ln_test_utils::pubkey(45), encrypted_payload: Vec::new() }
@@ -7776,6 +7787,7 @@ mod tests {
 
 		// (De)serialize a Route with two paths, each containing a blinded tail.
 		route.paths[1].blinded_tail = Some(BlindedTail {
+			trampoline_hops: vec![],
 			hops: vec![
 				BlindedHop { blinded_node_id: ln_test_utils::pubkey(48), encrypted_payload: Vec::new() },
 				BlindedHop { blinded_node_id: ln_test_utils::pubkey(49), encrypted_payload: Vec::new() }
@@ -7815,6 +7827,7 @@ mod tests {
 				maybe_announced_channel: false,
 			}],
 			blinded_tail: Some(BlindedTail {
+				trampoline_hops: vec![],
 				hops: vec![BlindedHop { blinded_node_id: ln_test_utils::pubkey(49), encrypted_payload: Vec::new() }],
 				blinding_point: ln_test_utils::pubkey(48),
 				excess_final_cltv_expiry_delta: 0,
@@ -7850,6 +7863,7 @@ mod tests {
 			}
 			],
 			blinded_tail: Some(BlindedTail {
+				trampoline_hops: vec![],
 				hops: vec![
 					BlindedHop { blinded_node_id: ln_test_utils::pubkey(45), encrypted_payload: Vec::new() },
 					BlindedHop { blinded_node_id: ln_test_utils::pubkey(46), encrypted_payload: Vec::new() }

lightning/src/routing/scoring.rs

@@ -3571,6 +3571,7 @@ mod tests {
 		let mut path = payment_path_for_amount(768);
 		let recipient_hop = path.hops.pop().unwrap();
 		path.blinded_tail = Some(BlindedTail {
+			trampoline_hops: vec![],
 			hops: vec![BlindedHop { blinded_node_id: test_utils::pubkey(44), encrypted_payload: Vec::new() }],
 			blinding_point: test_utils::pubkey(42),
 			excess_final_cltv_expiry_delta: recipient_hop.cltv_expiry_delta,

lightning/src/util/ser.rs

@@ -1044,6 +1044,7 @@ impl_for_vec!(crate::ln::msgs::SocketAddress);
 impl_for_vec!((A, B), A, B);
 impl_writeable_for_vec!(&crate::routing::router::BlindedTail);
 impl_readable_for_vec!(crate::routing::router::BlindedTail);
+impl_for_vec!(crate::routing::router::TrampolineHop);
 impl_for_vec_with_element_length_prefix!(crate::ln::msgs::UpdateAddHTLC);
 impl_writeable_for_vec_with_element_length_prefix!(&crate::ln::msgs::UpdateAddHTLC);