Add test for only creating tlv format onions

Author: ViktorT-11

lightning/src/ln/onion_route_tests.rs

@@ -790,6 +790,45 @@ fn test_onion_failure_stale_channel_update() {
 	do_test_onion_failure_stale_channel_update(true);
 }
 
+#[test]
+fn test_always_create_onion_payload_tlv_format() {
+	let chanmon_cfgs = create_chanmon_cfgs(3);
+	let mut node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
+
+	// Set `node[1]` config to `InitFeatures::empty()` which return `false` for
+	// `supports_variable_length_onion()`
+	let mut node_1_cfg = &mut node_cfgs[1];
+	node_1_cfg.features = InitFeatures::empty();
+
+	let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[None, None, None]);
+	let mut nodes = create_network(3, &node_cfgs, &node_chanmgrs);
+
+	create_announced_chan_between_nodes(&nodes, 0, 1, InitFeatures::empty(), InitFeatures::empty());
+	create_announced_chan_between_nodes(&nodes, 1, 2, InitFeatures::empty(), InitFeatures::empty());
+
+	let (route, _payment_hash, _payment_preimage, _payment_secret) = get_route_and_payment_hash!(nodes[0], nodes[2], 40000);
+
+	let hops = &route.paths[0];
+	// Asserts that the first hop to `node[1]` returns `false` for
+	// `node_features.supports_variable_length_onion()`
+	assert!(!hops[0].node_features.supports_variable_length_onion());
+	// Asserts that the second hop to `node[2]` returns `true` for
+	// `node_features.supports_variable_length_onion()`
+	assert!(hops[1].node_features.supports_variable_length_onion());
+
+	let cur_height = nodes[0].best_block_info().1 + 1;
+	let (onion_payloads, _htlc_msat, _htlc_cltv) = onion_utils::build_onion_payloads(&route.paths[0], 40000, &None, cur_height, &None).unwrap();
+
+	for onion_payload in onion_payloads.iter() {
+		match onion_payload.format {
+			msgs::OnionHopDataFormat::Legacy {..} => {
+				panic!("`onion_utils::build_onion_payloads` generated a `msgs::OnionHopDataFormat::Legacy` payload");
+			}
+			_ => {}
+		}
+	}
+}
+
 macro_rules! get_phantom_route {
 	($nodes: expr, $amt: expr, $channel: expr) => {{
 		let secp_ctx = Secp256k1::new();