Correctly handle any UPDATE errors to phandom invoices

If we try to send any onion error with the `UPDATE` flag in
response to a phantom receipt, we should always swap it for
something generic that doesn't require a `channel_update` in it.
Here we use `temporary_node_failure`.

Test provided by Valentine Wallace <[email protected]>

lightning/src/ln/channelmanager.rs

@@ -2222,7 +2222,7 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 			// with a short_channel_id of 0. This is important as various things later assume
 			// short_channel_id is non-0 in any ::Forward.
 			if let &PendingHTLCRouting::Forward { ref short_channel_id, .. } = routing {
-				if let Some((err, code, chan_update)) = loop {
+				if let Some((err, mut code, chan_update)) = loop {
 					let id_option = self.short_to_chan_info.read().unwrap().get(&short_channel_id).cloned();
 					let mut channel_state = self.channel_state.lock().unwrap();
 					let forwarding_id_opt = match id_option {
@@ -2331,6 +2331,11 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 						(chan_update.serialized_length() as u16 + 2).write(&mut res).expect("Writes cannot fail");
 						msgs::ChannelUpdate::TYPE.write(&mut res).expect("Writes cannot fail");
 						chan_update.write(&mut res).expect("Writes cannot fail");
+					} else if code & 0x1000 == 0x1000 {
+						// If we're trying to return an error that requires a `channel_update` but
+						// we're forwarding to a phantom "channel" (i.e. cannot generate an
+						// update), just use the generic "temporary_node_failure" instead.
+						code = 0x2000 | 2;
 					}
 					return_err!(err, code, &res.0[..]);
 				}

lightning/src/ln/onion_route_tests.rs

@@ -1150,6 +1150,45 @@ fn test_phantom_failure_modified_cltv() {
 	expect_payment_failed_conditions(&nodes[0], payment_hash, false, fail_conditions);
 }
 
+#[test]
+fn test_phantom_failure_expires_too_soon() {
+	// Test that we fail back phantoms if the HTLC got delayed and we got blocks in between with
+	// the correct error code.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	let channel = create_announced_chan_between_nodes(&nodes, 0, 1, channelmanager::provided_init_features(), channelmanager::provided_init_features());
+
+	// Get the route.
+	let recv_value_msat = 10_000;
+	let (_, payment_hash, payment_secret) = get_payment_preimage_hash!(nodes[1], Some(recv_value_msat));
+	let (mut route, phantom_scid) = get_phantom_route!(nodes, recv_value_msat, channel);
+
+	// Route the HTLC through to the destination.
+	nodes[0].node.send_payment(&route, payment_hash, &Some(payment_secret), PaymentId(payment_hash.0)).unwrap();
+	check_added_monitors!(nodes[0], 1);
+	let update_0 = get_htlc_update_msgs!(nodes[0], nodes[1].node.get_our_node_id());
+	let mut update_add = update_0.update_add_htlcs[0].clone();
+
+	connect_blocks(&nodes[1], CLTV_FAR_FAR_AWAY);
+	nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &update_add);
+	commitment_signed_dance!(nodes[1], nodes[0], &update_0.commitment_signed, false, true);
+
+	let update_1 = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
+	assert!(update_1.update_fail_htlcs.len() == 1);
+	let fail_msg = update_1.update_fail_htlcs[0].clone();
+	nodes[0].node.handle_update_fail_htlc(&nodes[1].node.get_our_node_id(), &fail_msg);
+	commitment_signed_dance!(nodes[0], nodes[1], update_1.commitment_signed, false);
+
+	// Ensure the payment fails with the expected error.
+	let mut fail_conditions = PaymentFailedConditions::new()
+		.blamed_scid(phantom_scid)
+		.expected_htlc_error_data(0x2000 | 2, &[]);
+		expect_payment_failed_conditions(&nodes[0], payment_hash, false, fail_conditions);
+}
+
 #[test]
 fn test_phantom_failure_too_low_recv_amt() {
 	let chanmon_cfgs = create_chanmon_cfgs(2);