Filter route hints for create invoice

Filter the route hints in `create_invoice_from_channelmanager` based on
the following criteria:

* Only one channel per counterparty node
* Always select the channel with the highest inbound capacity
* Filter out channels with a lower inbound capacity than the invoice amt
* If any private channel exists, the invoice rout_hints should be empty

Author: ViktorT-11

lightning-invoice/src/utils.rs

@@ -160,8 +160,21 @@ where
 {
 	// Marshall route hints.
 	let our_channels = channelmanager.list_usable_channels();
-	let mut route_hints = vec![];
+	let mut route_hints: HashMap<PublicKey, (u64, RouteHint)> = HashMap::new();
+	let min_capacity_required = match amt_msat {
+		Some(amt) => amt,
+		None => 0,
+	};
 	for channel in our_channels {
+		// Filter the channels to ensure that the `route_hints` only include the highest inbound
+		// capacity channel per counterparty node, and that channels with a lower inbound capacity
+		// than the invoice amount aren't included in the hints.
+		if !channel.is_public {
+			// If any of our channels are private, the invoice shouldn't include any `route_hints`
+			// and the sender will need to look at our public channels instead.
+			route_hints.clear();
+			break;
+		}
 		let short_channel_id = match channel.short_channel_id {
 			Some(id) => id,
 			None => continue,
@@ -170,17 +183,45 @@ where
 			Some(info) => info,
 			None => continue,
 		};
-		route_hints.push(RouteHint(vec![RouteHintHop {
-			src_node_id: channel.counterparty.node_id,
-			short_channel_id,
-			fees: RoutingFees {
-				base_msat: forwarding_info.fee_base_msat,
-				proportional_millionths: forwarding_info.fee_proportional_millionths,
-			},
-			cltv_expiry_delta: forwarding_info.cltv_expiry_delta,
-			htlc_minimum_msat: None,
-			htlc_maximum_msat: None,
-		}]));
+		if channel.inbound_capacity_msat < min_capacity_required {
+			continue;
+		};
+		match route_hints.entry(channel.counterparty.node_id) {
+			hash_map::Entry::Occupied(mut entry) => {
+				let current_max_capacity = entry.get().0;
+				if channel.inbound_capacity_msat < current_max_capacity {
+					continue;
+				}
+				entry.insert((
+					channel.inbound_capacity_msat,
+					RouteHint(vec![RouteHintHop {
+						src_node_id: channel.counterparty.node_id,
+						short_channel_id,
+						fees: RoutingFees {
+							base_msat: forwarding_info.fee_base_msat,
+							proportional_millionths: forwarding_info.fee_proportional_millionths,
+						},
+						cltv_expiry_delta: forwarding_info.cltv_expiry_delta,
+						htlc_minimum_msat: None,
+						htlc_maximum_msat: None,
+				}])));
+			}
+			hash_map::Entry::Vacant(entry) => {
+				entry.insert((
+					channel.inbound_capacity_msat,
+					RouteHint(vec![RouteHintHop {
+						src_node_id: channel.counterparty.node_id,
+						short_channel_id,
+						fees: RoutingFees {
+							base_msat: forwarding_info.fee_base_msat,
+							proportional_millionths: forwarding_info.fee_proportional_millionths,
+						},
+						cltv_expiry_delta: forwarding_info.cltv_expiry_delta,
+						htlc_minimum_msat: None,
+						htlc_maximum_msat: None,
+				}])));
+			}
+		}
 	}
 
 	// `create_inbound_payment` only returns an error if the amount is greater than the total bitcoin
@@ -200,8 +241,8 @@ where
 	if let Some(amt) = amt_msat {
 		invoice = invoice.amount_milli_satoshis(amt);
 	}
-	for hint in route_hints {
-		invoice = invoice.private_route(hint);
+	for (_, (_, hint)) in &route_hints {
+		invoice = invoice.private_route(hint.clone());
 	}
 
 	let raw_invoice = match invoice.build_raw() {