discovery: check if stale within isMsgStale for ChannelUpdate messages

In this commit, we address an assumption of the gossiper's recently
introduce reliable sender. The reliable sender is currently only used
for messages of unannounced channels. This makes sense as peers should
be able to retrieve messages from the network if they've previously
announced. However, within isMsgStale, we assumed that the reliable
sender would be used for every ChannelUpdate being sent, even if the
channel is already announced. Due to this, checking if the policy is
stale was unnecessary. But since this isn't the case, we should actually
be checking whether it is stale to prevent sending it later on.

discovery/gossiper.go

@@ -221,7 +221,10 @@ type AuthenticatedGossiper struct {
 	peerSyncers map[routing.Vertex]*gossipSyncer
 
 	// reliableSender is a subsystem responsible for handling reliable
-	// message send requests to peers.
+	// message send requests to peers. This should only be used for channels
+	// that are unadvertised at the time of handling the message since if it
+	// is advertised, then peers should be able to get the message from the
+	// network.
 	reliableSender *reliableSender
 
 	sync.Mutex
@@ -2364,16 +2367,32 @@ func (d *AuthenticatedGossiper) isMsgStale(msg lnwire.Message) bool {
 		return chanInfo.AuthProof != nil
 
 	case *lnwire.ChannelUpdate:
-		// The MessageStore will always store the latest ChannelUpdate
-		// as it is not aware of its timestamp (by design), so it will
-		// never be stale. We should still however check if the channel
-		// is part of our graph. If it's not, we can mark it as stale.
-		_, _, _, err := d.cfg.Router.GetChannelByID(msg.ShortChannelID)
-		if err != nil && err != channeldb.ErrEdgeNotFound {
-			log.Debugf("Unable to retrieve channel=%v from graph: "+
-				"%v", err)
+		_, p1, p2, err := d.cfg.Router.GetChannelByID(msg.ShortChannelID)
+
+		// If the channel cannot be found, it is most likely a leftover
+		// message for a channel that was closed, so we can consider it
+		// stale.
+		if err == channeldb.ErrEdgeNotFound {
+			return true
 		}
-		return err == channeldb.ErrEdgeNotFound
+		if err != nil {
+			log.Debugf("Unable to retrieve channel=%v from graph: "+
+				"%v", msg.ShortChannelID, err)
+			return false
+		}
+
+		// Otherwise, we'll retrieve the correct policy that we
+		// currently have stored within our graph to check if this
+		// message is stale by comparing its timestamp.
+		var p *channeldb.ChannelEdgePolicy
+		if msg.ChannelFlags&lnwire.ChanUpdateDirection == 0 {
+			p = p1
+		} else {
+			p = p2
+		}
+
+		timestamp := time.Unix(int64(msg.Timestamp), 0)
+		return p.LastUpdate.After(timestamp)
 
 	default:
 		// We'll make sure to not mark any unsupported messages as stale