Add test for partial-send MPP due to monitor update failure

Relatively simple test that, after a monitor update fails, we get
the right return value and continue with the bits of the MPP that
did not send after the monitor updating is restored.

Author: TheBlueMatt

lightning/src/ln/chanmon_update_fail_tests.rs

@@ -4,7 +4,7 @@
 //! here. See also the chanmon_fail_consistency fuzz test.
 
 use chain::transaction::OutPoint;
-use ln::channelmanager::{RAACommitmentOrder, PaymentPreimage, PaymentHash, PaymentSendFailure};
+use ln::channelmanager::{RAACommitmentOrder, PaymentPreimage, PaymentHash, PaymentSecret, PaymentSendFailure};
 use ln::channelmonitor::ChannelMonitorUpdateErr;
 use ln::features::InitFeatures;
 use ln::msgs;
@@ -1762,3 +1762,53 @@ fn during_funding_monitor_fail() {
 	do_during_funding_monitor_fail(true, true, true, true, false);
 	do_during_funding_monitor_fail(true, true, false, false, false);
 }
+
+#[test]
+fn test_path_paused_mpp() {
+	// Simple test of sending a multi-path payment where one path is currently blocked awaiting
+	// monitor update
+	let chanmon_cfgs = create_chanmon_cfgs(4);
+	let node_cfgs = create_node_cfgs(4, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(4, &node_cfgs, &[None, None, None, None]);
+	let mut nodes = create_network(4, &node_cfgs, &node_chanmgrs);
+
+	let chan_1_id = create_announced_chan_between_nodes(&nodes, 0, 1, InitFeatures::supported(), InitFeatures::supported()).0.contents.short_channel_id;
+	let (chan_2_ann, _, chan_2_id, _) = create_announced_chan_between_nodes(&nodes, 0, 2, InitFeatures::supported(), InitFeatures::supported());
+	let chan_3_id = create_announced_chan_between_nodes(&nodes, 1, 3, InitFeatures::supported(), InitFeatures::supported()).0.contents.short_channel_id;
+	let chan_4_id = create_announced_chan_between_nodes(&nodes, 2, 3, InitFeatures::supported(), InitFeatures::supported()).0.contents.short_channel_id;
+
+	let (payment_preimage, payment_hash) = get_payment_preimage_hash!(&nodes[0]);
+	let payment_secret = PaymentSecret([0xdb; 32]);
+	let mut route = nodes[0].router.get_route(&nodes[3].node.get_our_node_id(), None, &[], 100000, TEST_FINAL_CLTV).unwrap();
+	let path = route.paths[0].clone();
+	route.paths.push(path);
+	route.paths[0][0].pubkey = nodes[1].node.get_our_node_id();
+	route.paths[0][0].short_channel_id = chan_1_id;
+	route.paths[0][1].short_channel_id = chan_3_id;
+	route.paths[1][0].pubkey = nodes[2].node.get_our_node_id();
+	route.paths[1][0].short_channel_id = chan_2_ann.contents.short_channel_id;
+	route.paths[1][1].short_channel_id = chan_4_id;
+
+	*nodes[0].chan_monitor.update_ret.lock().unwrap() = Ok(());
+	*nodes[0].chan_monitor.next_update_ret.lock().unwrap() = Some(Err(ChannelMonitorUpdateErr::TemporaryFailure));
+
+	if let Err(PaymentSendFailure::PartialFailure(fails)) = nodes[0].node.send_payment(&route, payment_hash, &Some(payment_secret)) {
+		assert_eq!(fails.len(), 2);
+		if let Ok(()) = fails[0] {} else { panic!(); }
+		if let Err(APIError::MonitorUpdateFailed) = fails[1] {} else { panic!(); }
+	} else { panic!(); }
+	check_added_monitors!(nodes[0], 2);
+	*nodes[0].chan_monitor.update_ret.lock().unwrap() = Ok(());
+
+	let mut events = nodes[0].node.get_and_clear_pending_msg_events();
+	assert_eq!(events.len(), 1);
+	pass_along_path(&nodes[0], &[&nodes[1], &nodes[3]], 0, payment_hash.clone(), Some(payment_secret), events.pop().unwrap(), false);
+
+	let (outpoint, latest_update) = nodes[0].chan_monitor.latest_monitor_update_id.lock().unwrap().get(&chan_2_id).unwrap().clone();
+	nodes[0].node.channel_monitor_updated(&outpoint, latest_update);
+	let mut events = nodes[0].node.get_and_clear_pending_msg_events();
+	assert_eq!(events.len(), 1);
+	pass_along_path(&nodes[0], &[&nodes[2], &nodes[3]], 200_000, payment_hash.clone(), Some(payment_secret), events.pop().unwrap(), true);
+
+	claim_payment_along_route_with_secret(&nodes[0], &[&[&nodes[1], &nodes[3]], &[&nodes[2], &nodes[3]]], false, payment_preimage, Some(payment_secret), 200_000);
+}

lightning/src/util/test_utils.rs

@@ -51,6 +51,9 @@ pub struct TestChannelMonitor<'a> {
 	pub latest_monitor_update_id: Mutex<HashMap<[u8; 32], (OutPoint, u64)>>,
 	pub simple_monitor: channelmonitor::SimpleManyChannelMonitor<OutPoint, EnforcingChannelKeys, &'a chaininterface::BroadcasterInterface, &'a TestFeeEstimator>,
 	pub update_ret: Mutex<Result<(), channelmonitor::ChannelMonitorUpdateErr>>,
+	// If this is set to Some(), after the next return, we'll always return this until update_ret
+	// is changed:
+	pub next_update_ret: Mutex<Option<Result<(), channelmonitor::ChannelMonitorUpdateErr>>>,
 }
 impl<'a> TestChannelMonitor<'a> {
 	pub fn new(chain_monitor: Arc<chaininterface::ChainWatchInterface>, broadcaster: &'a chaininterface::BroadcasterInterface, logger: Arc<Logger>, fee_estimator: &'a TestFeeEstimator) -> Self {
@@ -59,6 +62,7 @@ impl<'a> TestChannelMonitor<'a> {
 			latest_monitor_update_id: Mutex::new(HashMap::new()),
 			simple_monitor: channelmonitor::SimpleManyChannelMonitor::new(chain_monitor, broadcaster, logger, fee_estimator),
 			update_ret: Mutex::new(Ok(())),
+			next_update_ret: Mutex::new(None),
 		}
 	}
 }
@@ -76,7 +80,12 @@ impl<'a> channelmonitor::ManyChannelMonitor<EnforcingChannelKeys> for TestChanne
 		self.latest_monitor_update_id.lock().unwrap().insert(funding_txo.to_channel_id(), (funding_txo, monitor.get_latest_update_id()));
 		self.added_monitors.lock().unwrap().push((funding_txo, monitor));
 		assert!(self.simple_monitor.add_monitor(funding_txo, new_monitor).is_ok());
-		self.update_ret.lock().unwrap().clone()
+
+		let ret = self.update_ret.lock().unwrap().clone();
+		if let Some(next_ret) = self.next_update_ret.lock().unwrap().take() {
+			*self.update_ret.lock().unwrap() = next_ret;
+		}
+		ret
 	}
 
 	fn update_monitor(&self, funding_txo: OutPoint, update: channelmonitor::ChannelMonitorUpdate) -> Result<(), channelmonitor::ChannelMonitorUpdateErr> {
@@ -100,7 +109,12 @@ impl<'a> channelmonitor::ManyChannelMonitor<EnforcingChannelKeys> for TestChanne
 		w.0.clear();
 		monitor.write_for_watchtower(&mut w).unwrap(); // This at least shouldn't crash...
 		self.added_monitors.lock().unwrap().push((funding_txo, new_monitor));
-		self.update_ret.lock().unwrap().clone()
+
+		let ret = self.update_ret.lock().unwrap().clone();
+		if let Some(next_ret) = self.next_update_ret.lock().unwrap().take() {
+			*self.update_ret.lock().unwrap() = next_ret;
+		}
+		ret
 	}
 
 	fn get_and_clear_pending_htlcs_updated(&self) -> Vec<HTLCUpdate> {