Include all channel route hints if no connected channels exist

Mobile clients often take a second or two before they reconnect to
their peers as its not always clear immediately that connections
have been killed (especially on iOS). This can cause us to
spuriously fail to include route hints in our invoices if we're on
mobile.

The fix is simple, if we're selecting channels to include in route
hints and we're not not connected to the peer for any of our
channels, we should simply include the hints for all channels, even
though we're disconencted.

Fixes #1768.

Author: TheBlueMatt

lightning-invoice/src/utils.rs

@@ -326,7 +326,7 @@ where
 	F::Target: FeeEstimator,
 	L::Target: Logger,
 {
-	let route_hints = filter_channels(channelmanager.list_usable_channels(), amt_msat);
+	let route_hints = filter_channels(channelmanager.list_channels(), amt_msat);
 
 	// `create_inbound_payment` only returns an error if the amount is greater than the total bitcoin
 	// supply.
@@ -377,16 +377,18 @@ where
 /// The filtering is based on the following criteria:
 /// * Only one channel per counterparty node
 /// * Always select the channel with the highest inbound capacity per counterparty node
-/// * Filter out channels with a lower inbound capacity than `min_inbound_capacity_msat`, if any
-/// channel with a higher or equal inbound capacity than `min_inbound_capacity_msat` exists
+/// * Prefer channels with capacity at least `min_inbound_capacity_msat` and where the channel
+///   `is_usable` (i.e. the peer is connected).
 /// * If any public channel exists, the returned `RouteHint`s will be empty, and the sender will
-/// need to find the path by looking at the public channels instead
+///   need to find the path by looking at the public channels instead
 fn filter_channels(channels: Vec<ChannelDetails>, min_inbound_capacity_msat: Option<u64>) -> Vec<RouteHint>{
-	let mut filtered_channels: HashMap<PublicKey, &ChannelDetails> = HashMap::new();
+	let mut filtered_channels: HashMap<PublicKey, ChannelDetails> = HashMap::new();
 	let min_inbound_capacity = min_inbound_capacity_msat.unwrap_or(0);
 	let mut min_capacity_channel_exists = false;
+	let mut online_channel_exists = false;
+	let mut online_min_capacity_channel_exists = false;
 
-	for channel in channels.iter() {
+	for channel in channels.into_iter().filter(|chan| chan.is_channel_ready) {
 		if channel.get_inbound_payment_scid().is_none() || channel.counterparty.forwarding_info.is_none() {
 			continue;
 		}
@@ -399,7 +401,13 @@ fn filter_channels(channels: Vec<ChannelDetails>, min_inbound_capacity_msat: Opt
 
 		if channel.inbound_capacity_msat >= min_inbound_capacity {
 			min_capacity_channel_exists = true;
-		};
+			if channel.is_usable {
+				online_min_capacity_channel_exists = true;
+			}
+		}
+		if channel.is_usable {
+			online_channel_exists = true;
+		}
 		match filtered_channels.entry(channel.counterparty.node_id) {
 			hash_map::Entry::Occupied(mut entry) => {
 				let current_max_capacity = entry.get().inbound_capacity_msat;
@@ -414,7 +422,7 @@ fn filter_channels(channels: Vec<ChannelDetails>, min_inbound_capacity_msat: Opt
 		}
 	}
 
-	let route_hint_from_channel = |channel: &ChannelDetails| {
+	let route_hint_from_channel = |channel: ChannelDetails| {
 		let forwarding_info = channel.counterparty.forwarding_info.as_ref().unwrap();
 		RouteHint(vec![RouteHintHop {
 			src_node_id: channel.counterparty.node_id,
@@ -427,15 +435,26 @@ fn filter_channels(channels: Vec<ChannelDetails>, min_inbound_capacity_msat: Opt
 			htlc_minimum_msat: channel.inbound_htlc_minimum_msat,
 			htlc_maximum_msat: channel.inbound_htlc_maximum_msat,}])
 	};
-	// If all channels are private, return the route hint for the highest inbound capacity channel
-	// per counterparty node. If channels with an higher inbound capacity than the
-	// min_inbound_capacity exists, filter out the channels with a lower capacity than that.
+	// If all channels are private, prefer to return route hints which have a higher capacity than
+	// the payment value and where we're currently connected to the channel counterparty.
+	// Even if we cannot satisfy both goals, always ensure we include *some* hints, preferring
+	// those which meet at least one criteria.
 	filtered_channels.into_iter()
 		.filter(|(_counterparty_id, channel)| {
-			min_capacity_channel_exists && channel.inbound_capacity_msat >= min_inbound_capacity ||
-			!min_capacity_channel_exists
+			if online_min_capacity_channel_exists {
+				channel.inbound_capacity_msat >= min_inbound_capacity && channel.is_usable
+			} else if min_capacity_channel_exists && online_channel_exists {
+				// If there are some online channels and some min_capacity channels, but no
+				// online-and-min_capacity channels, just include the min capacity ones and ignore
+				// online-ness.
+				channel.inbound_capacity_msat >= min_inbound_capacity
+			} else if min_capacity_channel_exists {
+				channel.inbound_capacity_msat >= min_inbound_capacity
+			} else if online_channel_exists {
+				channel.is_usable
+			} else { true }
 		})
-		.map(|(_counterparty_id, channel)| route_hint_from_channel(&channel))
+		.map(|(_counterparty_id, channel)| route_hint_from_channel(channel))
 		.collect::<Vec<RouteHint>>()
 }
 
@@ -723,6 +742,35 @@ mod test {
 		match_invoice_routes(Some(5000), &nodes[0], scid_aliases);
 	}
 
+	#[test]
+	fn test_hints_has_only_online_channels() {
+		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 nodes = create_network(4, &node_cfgs, &node_chanmgrs);
+		let chan_a = create_unannounced_chan_between_nodes_with_value(&nodes, 1, 0, 10_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features());
+		let chan_b = create_unannounced_chan_between_nodes_with_value(&nodes, 2, 0, 10_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features());
+		let _chan_c = create_unannounced_chan_between_nodes_with_value(&nodes, 3, 0, 1_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features());
+
+		// With all peers connected we should get all hints that have sufficient value
+		let mut scid_aliases = HashSet::new();
+		scid_aliases.insert(chan_a.0.short_channel_id_alias.unwrap());
+		scid_aliases.insert(chan_b.0.short_channel_id_alias.unwrap());
+
+		match_invoice_routes(Some(1_000_000_000), &nodes[0], scid_aliases.clone());
+
+		// With only one sufficient-value peer connected we should only get its hint
+		scid_aliases.remove(&chan_b.0.short_channel_id_alias.unwrap());
+		nodes[0].node.peer_disconnected(&nodes[2].node.get_our_node_id(), false);
+		match_invoice_routes(Some(1_000_000_000), &nodes[0], scid_aliases.clone());
+
+		// If we don't have any sufficient-value peers connected we should get all hints with
+		// sufficient value, even though there is a connected insufficient-value peer.
+		scid_aliases.insert(chan_b.0.short_channel_id_alias.unwrap());
+		nodes[0].node.peer_disconnected(&nodes[1].node.get_our_node_id(), false);
+		match_invoice_routes(Some(1_000_000_000), &nodes[0], scid_aliases);
+	}
+
 	#[test]
 	fn test_forwarding_info_not_assigned_channel_excluded_from_hints() {
 		let chanmon_cfgs = create_chanmon_cfgs(3);