OMs: fix panic sending to a two-hop blinded route where we are the intro node

Author: valentinewallace

lightning/src/onion_message/functional_tests.rs

@@ -174,14 +174,20 @@ fn too_big_packet_error() {
 fn we_are_intro_node() {
 	// If we are sending straight to a blinded route and we are the introduction node, we need to
 	// advance the blinded route by 1 hop so the second hop is the new introduction node.
-	let nodes = create_nodes(3);
+	let mut nodes = create_nodes(3);
 	let test_msg = TestCustomMessage {};
 
 	let secp_ctx = Secp256k1::new();
 	let blinded_route = BlindedRoute::new(&[nodes[0].get_node_pk(), nodes[1].get_node_pk(), nodes[2].get_node_pk()], &*nodes[2].keys_manager, &secp_ctx).unwrap();
 
 	nodes[0].messenger.send_onion_message(&[], Destination::BlindedRoute(blinded_route), OnionMessageContents::Custom(test_msg.clone()), None).unwrap();
 	pass_along_path(&nodes, None);
+
+	// Try with a two-hop blinded route where we are the introduction node.
+	let blinded_route = BlindedRoute::new(&[nodes[0].get_node_pk(), nodes[1].get_node_pk()], &*nodes[1].keys_manager, &secp_ctx).unwrap();
+	nodes[0].messenger.send_onion_message(&[], Destination::BlindedRoute(blinded_route), OnionMessageContents::Custom(test_msg), None).unwrap();
+	nodes.remove(2);
+	pass_along_path(&nodes, None);
 }
 
 #[test]

lightning/src/onion_message/messenger.rs

@@ -513,12 +513,8 @@ fn packet_payloads_and_keys<T: CustomOnionMessageContents, S: secp256k1::Signing
 					next_blinding_override: Some(blinding_pt),
 				})), control_tlvs_ss));
 			}
-			if let Some(encrypted_payload) = enc_payload_opt {
-				payloads.push((Payload::Forward(ForwardControlTlvs::Blinded(encrypted_payload)),
-					control_tlvs_ss));
-			} else { debug_assert!(false); }
-			blinded_path_idx += 1;
-		} else if blinded_path_idx < num_blinded_hops - 1 && enc_payload_opt.is_some() {
+		}
+		if blinded_path_idx < num_blinded_hops.saturating_sub(1) && enc_payload_opt.is_some() {
 			payloads.push((Payload::Forward(ForwardControlTlvs::Blinded(enc_payload_opt.unwrap())),
 				control_tlvs_ss));
 			blinded_path_idx += 1;