Fix sender double-including shadow offset in outgoing CLTV expiry.

The excess delta is included in the final RouteHop::cltv_expiry_delta, so by
adding it explicitly to cur_cltv we were erroneously including it twice in the
total cltv expiry.

This could've add up to an extra MAX_SHADOW_CLTV_DELTA_OFFSET (432) blocks to
the total cltv expiry.

lightning/src/ln/blinded_payment_tests.rs

@@ -572,6 +572,7 @@ fn do_multi_hop_receiver_fail(check: ReceiveCheckFail) {
 	let route = if check == ReceiveCheckFail::ProcessPendingHTLCsCheck {
 		let mut route = get_route(&nodes[0], &route_params).unwrap();
 		// Set the final CLTV expiry too low to trigger the failure in process_pending_htlc_forwards.
+		route.paths[0].hops.last_mut().map(|h| h.cltv_expiry_delta += excess_final_cltv_delta_opt.unwrap() as u32);
 		route.paths[0].blinded_tail.as_mut().map(|bt| bt.excess_final_cltv_expiry_delta = excess_final_cltv_delta_opt.unwrap() as u32);
 		route
 	} else if check == ReceiveCheckFail::PaymentConstraints {
@@ -680,6 +681,7 @@ fn do_multi_hop_receiver_fail(check: ReceiveCheckFail) {
 			commitment_signed_dance!(nodes[2], nodes[1], (), false, true, false, false);
 		},
 		ReceiveCheckFail::ProcessPendingHTLCsCheck => {
+			assert_eq!(payment_event_1_2.msgs[0].cltv_expiry, nodes[0].best_block_info().1 + 1 + excess_final_cltv_delta_opt.unwrap() as u32);
 			nodes[2].node.handle_update_add_htlc(&nodes[1].node.get_our_node_id(), &payment_event_1_2.msgs[0]);
 			check_added_monitors!(nodes[2], 0);
 			do_commitment_signed_dance(&nodes[2], &nodes[1], &payment_event_1_2.commitment_msg, true, true);

lightning/src/ln/onion_utils.rs

@@ -188,11 +188,10 @@ pub(super) fn build_onion_payloads(path: &Path, total_msat: u64, mut recipient_o
 				for (i, blinded_hop) in hops.iter().enumerate() {
 					if i == hops.len() - 1 {
 						cur_value_msat += final_value_msat;
-						cur_cltv += excess_final_cltv_expiry_delta;
 						res.push(msgs::OutboundOnionPayload::BlindedReceive {
 							amt_msat: *final_value_msat,
 							total_msat,
-							outgoing_cltv_value: cur_cltv,
+							outgoing_cltv_value: cur_cltv + excess_final_cltv_expiry_delta,
 							encrypted_tlvs: blinded_hop.encrypted_payload.clone(),
 							intro_node_blinding_point: blinding_point.take(),
 						});