Expose blinded payment 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/payment.rs

@@ -281,11 +281,12 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 	utils::construct_blinded_hops(secp_ctx, pks, tlvs, session_priv)
 }
 
-// Advance the blinded onion payment 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 payment path by one hop, so make the second hop into the new
+/// introduction node. Useful if we are paying to a [`BlindedPath`] where we are the introduction
+/// node, since the payment should then be forwarded to the node after us in the path.
+///
+/// Will only modify `path` when returning `Ok`.
+pub fn advance_payment_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/ln/outbound_payment.rs

@@ -14,7 +14,7 @@ use bitcoin::hashes::sha256::Hash as Sha256;
 use bitcoin::secp256k1::{self, Secp256k1, SecretKey};
 
 use crate::blinded_path::{IntroductionNode, NodeIdLookUp};
-use crate::blinded_path::payment::advance_path_by_one;
+use crate::blinded_path::payment::advance_payment_path_by_one;
 use crate::events::{self, PaymentFailureReason};
 use crate::ln::types::{PaymentHash, PaymentPreimage, PaymentSecret};
 use crate::ln::channel_state::ChannelDetails;
@@ -834,7 +834,7 @@ impl OutboundPayments {
 					},
 				};
 				if introduction_node_id == our_node_id {
-					let _ = advance_path_by_one(path, node_signer, node_id_lookup, secp_ctx);
+					let _ = advance_payment_path_by_one(path, node_signer, node_id_lookup, secp_ctx);
 				}
 			}
 		}