Fix a few new (and one old) issues in the new channel_update

We need to always set lowest_inbound_channel_fees to None if there
are no channels (and we should ignore the channels which are
disabled for the purpose of lowest-fee calculations). Further, we
cannot unwrap the channel lookups as they may be for channels which
are not related to the channel we are processing an update for.
Finally, we can satisfy borrowck much easier since things are on
self instead of on a MutexGuard.

Author: TheBlueMatt

lightning/src/routing/network_graph.rs

@@ -378,8 +378,8 @@ impl Readable for NodeAnnouncementInfo {
 pub struct NodeInfo {
 	/// All valid channels a node has announced
 	pub channels: Vec<u64>,
-	/// Lowest fees enabling routing via any of the known channels to a node.
- 	/// The two fields (flat and proportional fee) are independent,
+	/// Lowest fees enabling routing via any of the enabled, known channels to a node.
+	/// The two fields (flat and proportional fee) are independent,
 	/// meaning they don't have to refer to the same channel.
 	pub lowest_inbound_channel_fees: Option<RoutingFees>,
 	/// More information about a node from node_announcement.
@@ -712,34 +712,28 @@ impl NetworkGraph {
 				proportional_millionths
 			});
 		} else if chan_was_enabled {
-			let mut lowest_inbound_channel_fee_base_msat = u32::max_value();
-			let mut lowest_inbound_channel_fee_proportional_millionths = u32::max_value();
-
-			{
-				let node = self.nodes.get(&dest_node_id).unwrap();
-
-				for chan_id in node.channels.iter() {
-					let chan = self.channels.get(chan_id).unwrap();
-					// Since direction was enabled, the channel indeed had directional info
-					let chan_info;
-					if chan.node_one == dest_node_id {
-						chan_info = chan.two_to_one.as_ref().unwrap();
-					} else {
-						chan_info = chan.one_to_two.as_ref().unwrap();
+			let mut node = self.nodes.get_mut(&dest_node_id).unwrap();
+			let mut lowest_inbound_channel_fees = None;
+
+			for chan_id in node.channels.iter() {
+				let chan = self.channels.get(chan_id).unwrap();
+				let chan_info_opt;
+				if chan.node_one == dest_node_id {
+					chan_info_opt = chan.two_to_one.as_ref();
+				} else {
+					chan_info_opt = chan.one_to_two.as_ref();
+				}
+				if let Some(chan_info) = chan_info_opt {
+					if chan_info.enabled {
+						let fees = lowest_inbound_channel_fees.get_or_insert(RoutingFees {
+							base_msat: u32::max_value(), proportional_millionths: u32::max_value() });
+						fees.base_msat = cmp::min(fees.base_msat, chan_info.fees.base_msat);
+						fees.proportional_millionths = cmp::min(fees.proportional_millionths, chan_info.fees.proportional_millionths);
 					}
-					lowest_inbound_channel_fee_base_msat = cmp::min(lowest_inbound_channel_fee_base_msat, chan_info.fees.base_msat);
-					lowest_inbound_channel_fee_proportional_millionths = cmp::min(lowest_inbound_channel_fee_proportional_millionths, chan_info.fees.proportional_millionths);
 				}
 			}
 
-			//TODO: satisfy the borrow-checker without a double-map-lookup :(
-			let mut_node = self.nodes.get_mut(&dest_node_id).unwrap();
-			if mut_node.channels.len() > 0 {
-				mut_node.lowest_inbound_channel_fees = Some(RoutingFees {
-					base_msat: lowest_inbound_channel_fee_base_msat,
-					proportional_millionths: lowest_inbound_channel_fee_proportional_millionths
-				});
-			}
+			node.lowest_inbound_channel_fees = lowest_inbound_channel_fees;
 		}
 
 		Ok(msg.contents.excess_data.is_empty())