Add RevokeAndACK::release_htlc_message_paths

As part of supporting sending payments as an often-offline sender, the sender
needs to send held_htlc_available onion messages where the reply path
terminates at their always-online channel counterparty that is holding the HTLC
until the recipient comes online. That way when the recipient sends
release_held_htlc, the sender's counterparty will receive that message.

To accomplish this, the sender's always-online counterparty includes said reply
path in the revoke_and_ack message corresponding to the held HTLC. Here we add
support for this field, though we don't set it yet.

We also had to tweak the ser macros for this because impl_writeable_msg had
never had to write a Vec in a message TLV field before.

Author: valentinewallace

lightning/src/ln/channel.rs

@@ -8998,6 +8998,7 @@ where
 					next_per_commitment_point: self.holder_commitment_point.next_point(),
 					#[cfg(taproot)]
 					next_local_nonce: None,
+					release_htlc_message_paths: Vec::new(),
 				});
 			}
 		}

lightning/src/ln/functional_tests.rs

@@ -6548,6 +6548,7 @@ pub fn test_counterparty_raa_skip_no_crash() {
 		next_per_commitment_point,
 		#[cfg(taproot)]
 		next_local_nonce: None,
+		release_htlc_message_paths: Vec::new(),
 	};
 	nodes[1].node.handle_revoke_and_ack(node_a_id, &raa);
 	assert_eq!(

lightning/src/ln/htlc_reserve_unit_tests.rs

@@ -936,6 +936,7 @@ pub fn do_test_fee_spike_buffer(cfg: Option<UserConfig>, htlc_fails: bool) {
 		next_per_commitment_point: next_local_point,
 		#[cfg(taproot)]
 		next_local_nonce: None,
+		release_htlc_message_paths: Vec::new(),
 	};
 	nodes[1].node.handle_revoke_and_ack(node_a_id, &raa_msg);
 	expect_and_process_pending_htlcs(&nodes[1], false);
@@ -2381,6 +2382,7 @@ pub fn do_test_dust_limit_fee_accounting(can_afford: bool) {
 			next_per_commitment_point: next_local_point,
 			#[cfg(taproot)]
 			next_local_nonce: None,
+			release_htlc_message_paths: Vec::new(),
 		};
 		nodes[1].node.handle_revoke_and_ack(node_a_id, &raa_msg);
 

lightning/src/ln/msgs.rs

@@ -31,6 +31,7 @@ use bitcoin::secp256k1::ecdsa::Signature;
 use bitcoin::secp256k1::PublicKey;
 use bitcoin::{secp256k1, Transaction, Witness};
 
+use crate::blinded_path::message::BlindedMessagePath;
 use crate::blinded_path::payment::{
 	BlindedPaymentTlvs, ForwardTlvs, ReceiveTlvs, UnauthenticatedReceiveTlvs,
 };
@@ -888,6 +889,13 @@ pub struct RevokeAndACK {
 	#[cfg(taproot)]
 	/// Musig nonce the recipient should use in their next commitment signature message
 	pub next_local_nonce: Option<musig2::types::PublicNonce>,
+	/// A list of `(htlc_id, blinded_path)`. The receiver of this message will use the blinded paths
+	/// as reply paths to [`HeldHtlcAvailable`] onion messages that they send to the often-offline
+	/// receiver of this HTLC. The `htlc_id` is used by the receiver of this message to identify which
+	/// held HTLC a given blinded path corresponds to.
+	///
+	/// [`HeldHtlcAvailable`]: crate::onion_message::async_payments::HeldHtlcAvailable
+	pub release_htlc_message_paths: Vec<(u64, BlindedMessagePath)>,
 }
 
 /// An [`update_fee`] message to be sent to or received from a peer
@@ -3260,15 +3268,18 @@ impl_writeable_msg!(RevokeAndACK, {
 	channel_id,
 	per_commitment_secret,
 	next_per_commitment_point
-}, {});
+}, {
+	(75537, release_htlc_message_paths, optional_vec)
+});
 
 #[cfg(taproot)]
 impl_writeable_msg!(RevokeAndACK, {
 	channel_id,
 	per_commitment_secret,
 	next_per_commitment_point
 }, {
-	(4, next_local_nonce, option)
+	(4, next_local_nonce, option),
+	(75537, release_htlc_message_paths, optional_vec)
 });
 
 impl_writeable_msg!(Shutdown, {
@@ -5976,6 +5987,7 @@ mod tests {
 			next_per_commitment_point: pubkey_1,
 			#[cfg(taproot)]
 			next_local_nonce: None,
+			release_htlc_message_paths: Vec::new(),
 		};
 		let encoded_value = raa.encode();
 		let target_value = <Vec<u8>>::from_hex("02020202020202020202020202020202020202020202020202020202020202020101010101010101010101010101010101010101010101010101010101010101031b84c5567b126440995d3ed5aaba0565d71e1834604819ff9c17f5e9d5dd078f").unwrap();

lightning/src/util/ser_macros.rs

@@ -700,7 +700,7 @@ macro_rules! impl_writeable_msg {
 		impl $crate::util::ser::Writeable for $st {
 			fn write<W: $crate::util::ser::Writer>(&self, w: &mut W) -> Result<(), $crate::io::Error> {
 				$( self.$field.write(w)?; )*
-				$crate::encode_tlv_stream!(w, {$(($type, self.$tlvfield.as_ref(), $fieldty)),*});
+				$crate::encode_tlv_stream!(w, {$(($type, &self.$tlvfield, $fieldty)),*});
 				Ok(())
 			}
 		}
@@ -713,7 +713,7 @@ macro_rules! impl_writeable_msg {
 				$crate::decode_tlv_stream!(r, {$(($type, $tlvfield, $fieldty)),*});
 				Ok(Self {
 					$($field,)*
-					$($tlvfield),*
+					$($tlvfield: $crate::_init_tlv_based_struct_field!($tlvfield, $fieldty)),*
 				})
 			}
 		}