Filter route hints for phantom invoices

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

* Only one channel for every counterparty node per phantom
payment-receiving node in the invoice
* Always select the channel with the highest inbound capacity
* Filter out channels with a lower inbound capacity than the invoice amt
* If any private channels exists for a payment-receiving node, push a
single RouteHintHop with the phantom route and let the sender find the
path to the payment-receiving node through the public channels.

Author: ViktorTigerstrom

lightning-invoice/src/utils.rs

@@ -64,42 +64,32 @@ pub fn create_phantom_invoice<Signer: Sign, K: Deref>(
 	}
 
 	for hint in phantom_route_hints {
-		for channel in &hint.channels {
-			let short_channel_id = match channel.short_channel_id {
-				Some(id) => id,
-				None => continue,
-			};
-			let forwarding_info = match &channel.counterparty.forwarding_info {
-				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,
-					}])
-			);
+		let (mut route_hints, has_private_channels) = filter_channels(
+			hint.channels.clone(),
+			amt_msat
+		);
+
+		// If we have any private channels, the route hints from `filter_channels` will be empty.
+		// In that case we create a RouteHint on which we will push a single hop with the phantom
+		// route into the invoice, and let the sender find the path to the `hint.real_node_pubkey`
+		// node by looking at our public channels.
+		if has_private_channels && route_hints.is_empty(){
+			route_hints.push(RouteHint(vec![]))
+		}
+		for mut route_hint in route_hints {
+			route_hint.0.push(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,});
+			invoice = invoice.private_route(route_hint.clone());
 		}
 	}
-
 	let raw_invoice = match invoice.build_raw() {
 		Ok(inv) => inv,
 		Err(e) => return Err(SignOrCreationError::CreationError(e))