Rename and expose message-specific NextHop

`onion::message::messenger::PeeledOnion` is a public enum which
included the private enum `NextHop`, which is not acceptable. Thus,
we here expose `NextHop` but rename it `NextMessageHop` to make
clear that it is specific to messages.

lightning/src/blinded_path/message.rs

@@ -3,7 +3,7 @@ use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey};
 #[allow(unused_imports)]
 use crate::prelude::*;
 
-use crate::blinded_path::{BlindedHop, BlindedPath, IntroductionNode, NodeIdLookUp};
+use crate::blinded_path::{BlindedHop, BlindedPath, IntroductionNode, NextMessageHop, NodeIdLookUp};
 use crate::blinded_path::utils;
 use crate::io;
 use crate::io::Cursor;
@@ -20,7 +20,7 @@ use core::ops::Deref;
 /// route, they are encoded into [`BlindedHop::encrypted_payload`].
 pub(crate) struct ForwardTlvs {
 	/// The next hop in the onion message's path.
-	pub(crate) next_hop: NextHop,
+	pub(crate) next_hop: NextMessageHop,
 	/// Senders to a blinded path use this value to concatenate the route they find to the
 	/// introduction node with the blinded path.
 	pub(crate) next_blinding_override: Option<PublicKey>,
@@ -34,20 +34,11 @@ pub(crate) struct ReceiveTlvs {
 	pub(crate) path_id: Option<[u8; 32]>,
 }
 
-/// The next hop to forward the onion message along its path.
-#[derive(Debug)]
-pub enum NextHop {
-	/// The node id of the next hop.
-	NodeId(PublicKey),
-	/// The short channel id leading to the next hop.
-	ShortChannelId(u64),
-}
-
 impl Writeable for ForwardTlvs {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
 		let (next_node_id, short_channel_id) = match self.next_hop {
-			NextHop::NodeId(pubkey) => (Some(pubkey), None),
-			NextHop::ShortChannelId(scid) => (None, Some(scid)),
+			NextMessageHop::NodeId(pubkey) => (Some(pubkey), None),
+			NextMessageHop::ShortChannelId(scid) => (None, Some(scid)),
 		};
 		// TODO: write padding
 		encode_tlv_stream!(writer, {
@@ -75,7 +66,7 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 ) -> Result<Vec<BlindedHop>, secp256k1::Error> {
 	let blinded_tlvs = unblinded_path.iter()
 		.skip(1) // The first node's TLVs contains the next node's pubkey
-		.map(|pk| ForwardTlvs { next_hop: NextHop::NodeId(*pk), next_blinding_override: None })
+		.map(|pk| ForwardTlvs { next_hop: NextMessageHop::NodeId(*pk), next_blinding_override: None })
 		.map(|tlvs| ControlTlvs::Forward(tlvs))
 		.chain(core::iter::once(ControlTlvs::Receive(ReceiveTlvs { path_id: None })));
 
@@ -102,8 +93,8 @@ where
 			readable: ControlTlvs::Forward(ForwardTlvs { next_hop, next_blinding_override })
 		}) => {
 			let next_node_id = match next_hop {
-				NextHop::NodeId(pubkey) => pubkey,
-				NextHop::ShortChannelId(scid) => match node_id_lookup.next_node_id(scid) {
+				NextMessageHop::NodeId(pubkey) => pubkey,
+				NextMessageHop::ShortChannelId(scid) => match node_id_lookup.next_node_id(scid) {
 					Some(pubkey) => pubkey,
 					None => return Err(()),
 				},

lightning/src/blinded_path/mod.rs

@@ -24,6 +24,17 @@ use crate::util::ser::{Readable, Writeable, Writer};
 use crate::io;
 use crate::prelude::*;
 
+/// The next hop to forward an onion message along its path.
+///
+/// Note that payment blinded paths always specify their next hop using an explicit node id.
+#[derive(Debug)]
+pub enum NextMessageHop {
+	/// The node id of the next hop.
+	NodeId(PublicKey),
+	/// The short channel id leading to the next hop.
+	ShortChannelId(u64),
+}
+
 /// Onion messages and payments can be sent and received to blinded paths, which serve to hide the
 /// identity of the recipient.
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]

lightning/src/onion_message/messenger.rs

@@ -15,8 +15,8 @@ use bitcoin::hashes::hmac::{Hmac, HmacEngine};
 use bitcoin::hashes::sha256::Hash as Sha256;
 use bitcoin::secp256k1::{self, PublicKey, Scalar, Secp256k1, SecretKey};
 
-use crate::blinded_path::{BlindedPath, IntroductionNode, NodeIdLookUp};
-use crate::blinded_path::message::{advance_path_by_one, ForwardTlvs, NextHop, ReceiveTlvs};
+use crate::blinded_path::{BlindedPath, IntroductionNode, NextMessageHop, NodeIdLookUp};
+use crate::blinded_path::message::{advance_path_by_one, ForwardTlvs, ReceiveTlvs};
 use crate::blinded_path::utils;
 use crate::events::{Event, EventHandler, EventsProvider};
 use crate::sign::{EntropySource, NodeSigner, Recipient};
@@ -572,7 +572,7 @@ pub trait CustomOnionMessageHandler {
 #[derive(Debug)]
 pub enum PeeledOnion<T: OnionMessageContents> {
 	/// Forwarded onion, with the next node id and a new onion
-	Forward(NextHop, OnionMessage),
+	Forward(NextMessageHop, OnionMessage),
 	/// Received onion message, with decrypted contents, path_id, and reply path
 	Receive(ParsedOnionMessageContents<T>, Option<[u8; 32]>, Option<BlindedPath>)
 }
@@ -1050,8 +1050,8 @@ where
 			},
 			Ok(PeeledOnion::Forward(next_hop, onion_message)) => {
 				let next_node_id = match next_hop {
-					NextHop::NodeId(pubkey) => pubkey,
-					NextHop::ShortChannelId(scid) => match self.node_id_lookup.next_node_id(scid) {
+					NextMessageHop::NodeId(pubkey) => pubkey,
+					NextMessageHop::ShortChannelId(scid) => match self.node_id_lookup.next_node_id(scid) {
 						Some(pubkey) => pubkey,
 						None => {
 							log_trace!(self.logger, "Dropping forwarded onion messager: unable to resolve next hop using SCID {}", scid);
@@ -1255,7 +1255,7 @@ fn packet_payloads_and_keys<T: OnionMessageContents, S: secp256k1::Signing + sec
 				if let Some(ss) = prev_control_tlvs_ss.take() {
 					payloads.push((Payload::Forward(ForwardControlTlvs::Unblinded(
 						ForwardTlvs {
-							next_hop: NextHop::NodeId(unblinded_pk_opt.unwrap()),
+							next_hop: NextMessageHop::NodeId(unblinded_pk_opt.unwrap()),
 							next_blinding_override: None,
 						}
 					)), ss));
@@ -1265,7 +1265,7 @@ fn packet_payloads_and_keys<T: OnionMessageContents, S: secp256k1::Signing + sec
 			} else if let Some((intro_node_id, blinding_pt)) = intro_node_id_blinding_pt.take() {
 				if let Some(control_tlvs_ss) = prev_control_tlvs_ss.take() {
 					payloads.push((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs {
-						next_hop: NextHop::NodeId(intro_node_id),
+						next_hop: NextMessageHop::NodeId(intro_node_id),
 						next_blinding_override: Some(blinding_pt),
 					})), control_tlvs_ss));
 				}

lightning/src/onion_message/packet.rs

@@ -12,8 +12,8 @@
 use bitcoin::secp256k1::PublicKey;
 use bitcoin::secp256k1::ecdh::SharedSecret;
 
-use crate::blinded_path::BlindedPath;
-use crate::blinded_path::message::{ForwardTlvs, NextHop, ReceiveTlvs};
+use crate::blinded_path::{BlindedPath, NextMessageHop};
+use crate::blinded_path::message::{ForwardTlvs, ReceiveTlvs};
 use crate::blinded_path::utils::Padding;
 use crate::ln::msgs::DecodeError;
 use crate::ln::onion_utils;
@@ -293,8 +293,8 @@ impl Readable for ControlTlvs {
 
 		let next_hop = match (short_channel_id, next_node_id) {
 			(Some(_), Some(_)) => return Err(DecodeError::InvalidValue),
-			(Some(scid), None) => Some(NextHop::ShortChannelId(scid)),
-			(None, Some(pubkey)) => Some(NextHop::NodeId(pubkey)),
+			(Some(scid), None) => Some(NextMessageHop::ShortChannelId(scid)),
+			(None, Some(pubkey)) => Some(NextMessageHop::NodeId(pubkey)),
 			(None, None) => None,
 		};