Filter route hints for phantom invoices

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

* Only one channel per real participating node in the phantom invoice
* Always select the channel with the highest inbound capacity
* Filter out channels with a lower inbound capacity than the invoice amt

Author: ViktorT-11

lightning-invoice/src/utils.rs

@@ -62,9 +62,17 @@ pub fn create_phantom_invoice<Signer: Sign, K: Deref>(
 	if let Some(amt) = amt_msat {
 		invoice = invoice.amount_milli_satoshis(amt);
 	}
+	let mut route_hints: HashMap<PublicKey, (u64, RouteHint)> = HashMap::new();
+	let min_capacity_required = match amt_msat {
+		Some(amt) => amt,
+		None => 0,
+	};
 
 	for hint in phantom_route_hints {
 		for channel in &hint.channels {
+			// Filter the channels to ensure that the `route_hints` only include the highest
+			// inbound capacity channel per real node in the phantom invoice, and that channels
+			// with a lower inbound capacity than the invoice amount aren't included in the hints.
 			let short_channel_id = match channel.short_channel_id {
 				Some(id) => id,
 				None => continue,
@@ -73,33 +81,75 @@ pub fn create_phantom_invoice<Signer: Sign, K: Deref>(
 				Some(info) => info.clone(),
 				None => continue,
 			};
-			invoice = invoice.private_route(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,
-					},
-					RouteHintHop {
-						src_node_id: hint.real_node_pubkey,
-						short_channel_id: hint.phantom_scid,
-						fees: RoutingFees {
-							base_msat: 0,
-							proportional_millionths: 0,
-						},
-						cltv_expiry_delta: MIN_CLTV_EXPIRY_DELTA,
-						htlc_minimum_msat: None,
-						htlc_maximum_msat: None,
-					}])
-			);
+			if channel.inbound_capacity_msat < min_capacity_required {
+				continue;
+			};
+			match route_hints.entry(hint.real_node_pubkey) {
+				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,
+							},
+							RouteHintHop {
+								src_node_id: hint.real_node_pubkey,
+								short_channel_id: hint.phantom_scid,
+								fees: RoutingFees {
+									base_msat: 0,
+									proportional_millionths: 0,
+								},
+								cltv_expiry_delta: MIN_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,
+							},
+							RouteHintHop {
+								src_node_id: hint.real_node_pubkey,
+								short_channel_id: hint.phantom_scid,
+								fees: RoutingFees {
+									base_msat: 0,
+									proportional_millionths: 0,
+								},
+								cltv_expiry_delta: MIN_CLTV_EXPIRY_DELTA,
+								htlc_minimum_msat: None,
+								htlc_maximum_msat: None,
+					}])));
+				}
+			}
 		}
 	}
 
+	for (_, (_, hint)) in &route_hints {
+		invoice = invoice.private_route(hint.clone());
+	}
 	let raw_invoice = match invoice.build_raw() {
 		Ok(inv) => inv,
 		Err(e) => return Err(SignOrCreationError::CreationError(e))