Don't modify path when advance_path_by_one errors

When using advance_path_by_one when we are the introduction node, any
error will result having the first hop of the input blinded path
removed. Instead, only remove the first hop on success. Otherwise, the
path will be invalid.

Author: jkczyz

lightning/src/blinded_path/message.rs

@@ -106,6 +106,8 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 
 // 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>(
 	path: &mut BlindedPath, node_signer: &NS, node_id_lookup: &NL, secp_ctx: &Secp256k1<T>
 ) -> Result<(), ()>
@@ -116,8 +118,8 @@ where
 {
 	let control_tlvs_ss = node_signer.ecdh(Recipient::Node, &path.blinding_point, None)?;
 	let rho = onion_utils::gen_rho_from_shared_secret(&control_tlvs_ss.secret_bytes());
-	let encrypted_control_tlvs = path.blinded_hops.remove(0).encrypted_payload;
-	let mut s = Cursor::new(&encrypted_control_tlvs);
+	let encrypted_control_tlvs = &path.blinded_hops.get(0).ok_or(())?.encrypted_payload;
+	let mut s = Cursor::new(encrypted_control_tlvs);
 	let mut reader = FixedLengthReader::new(&mut s, encrypted_control_tlvs.len() as u64);
 	match ChaChaPolyReadAdapter::read(&mut reader, rho) {
 		Ok(ChaChaPolyReadAdapter {
@@ -139,6 +141,7 @@ where
 			};
 			mem::swap(&mut path.blinding_point, &mut new_blinding_point);
 			path.introduction_node = IntroductionNode::NodeId(next_node_id);
+			path.blinded_hops.remove(0);
 			Ok(())
 		},
 		_ => Err(())