Expose blinded message paths' advance_path_by_one.

Useful for LDK users that are only using the onion messages module, like LNDK.

Author: valentinewallace

lightning/src/blinded_path/message.rs

@@ -199,11 +199,13 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 	utils::construct_blinded_hops(secp_ctx, pks, tlvs, session_priv)
 }
 
-// Advance the blinded onion message path by one hop, so make the second hop into the new
-// introduction node.
-//
-// Will only modify `path` when returning `Ok`.
-pub(crate) fn advance_path_by_one<NS: Deref, NL: Deref, T>(
+/// Advance the blinded onion message path by one hop, so make the second hop into the new
+/// introduction node. Useful if we are sending an onion message to a [`BlindedPath`] where we are
+/// the introduction node, since the message should then be forwarded to the node after us in the
+/// path.
+///
+/// Will only modify `path` when returning `Ok`.
+pub fn advance_message_path_by_one<NS: Deref, NL: Deref, T>(
 	path: &mut BlindedPath, node_signer: &NS, node_id_lookup: &NL, secp_ctx: &Secp256k1<T>
 ) -> Result<(), ()>
 where

lightning/src/onion_message/messenger.rs

@@ -16,7 +16,7 @@ use bitcoin::hashes::sha256::Hash as Sha256;
 use bitcoin::secp256k1::{self, PublicKey, Scalar, Secp256k1, SecretKey};
 
 use crate::blinded_path::{BlindedPath, IntroductionNode, NextMessageHop, NodeIdLookUp};
-use crate::blinded_path::message::{advance_path_by_one, ForwardNode, ForwardTlvs, MessageContext, ReceiveTlvs};
+use crate::blinded_path::message::{advance_message_path_by_one, ForwardNode, ForwardTlvs, MessageContext, ReceiveTlvs};
 use crate::blinded_path::utils;
 use crate::events::{Event, EventHandler, EventsProvider, ReplayEvent};
 use crate::sign::{EntropySource, NodeSigner, Recipient};
@@ -874,7 +874,7 @@ where
 				},
 			};
 			if introduction_node_id == our_node_id {
-				advance_path_by_one(blinded_path, node_signer, node_id_lookup, &secp_ctx)
+				advance_message_path_by_one(blinded_path, node_signer, node_id_lookup, &secp_ctx)
 					.map_err(|()| SendError::BlindedPathAdvanceFailed)?;
 			}
 		}