discovery: properly store and retrieve edge policies within mockGraphSource

In this commit, we address an issue with our router mock in which it was
not properly storing and retrieving edge policies. Previously, they were
being appended to a slice of policies, but this doesn't always work like
when you attempt to update the same edge twice. Instead, the slice can
only contain up to two entries, each one being the latest version of
each direction.

discovery/gossiper_test.go

@@ -155,7 +155,16 @@ func (r *mockGraphSource) UpdateEdge(edge *channeldb.ChannelEdgePolicy) error {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 
-	r.edges[edge.ChannelID] = append(r.edges[edge.ChannelID], *edge)
+	if len(r.edges[edge.ChannelID]) == 0 {
+		r.edges[edge.ChannelID] = make([]channeldb.ChannelEdgePolicy, 2)
+	}
+
+	if edge.ChannelFlags&lnwire.ChanUpdateDirection == 0 {
+		r.edges[edge.ChannelID][0] = *edge
+	} else {
+		r.edges[edge.ChannelID][1] = *edge
+	}
+
 	return nil
 }
 
@@ -226,13 +235,17 @@ func (r *mockGraphSource) GetChannelByID(chanID lnwire.ShortChannelID) (
 		return &chanInfo, nil, nil, nil
 	}
 
-	if len(edges) == 1 {
+	var edge1 *channeldb.ChannelEdgePolicy
-		edge1 := edges[0]
+	if !reflect.DeepEqual(edges[0], channeldb.ChannelEdgePolicy{}) {
-		return &chanInfo, &edge1, nil, nil
+		edge1 = &edges[0]
 	}
 
-	edge1, edge2 := edges[0], edges[1]
+	var edge2 *channeldb.ChannelEdgePolicy
-	return &chanInfo, &edge1, &edge2, nil
+	if !reflect.DeepEqual(edges[1], channeldb.ChannelEdgePolicy{}) {
+		edge2 = &edges[1]
+	}
+
+	return &chanInfo, edge1, edge2, nil
 }
 
 func (r *mockGraphSource) FetchLightningNode(
@@ -327,11 +340,15 @@ func (r *mockGraphSource) IsStaleEdgePolicy(chanID lnwire.ShortChannelID,
 	}
 
 	switch {
-	case len(edges) >= 1 && edges[0].ChannelFlags == flags:
+	case flags&lnwire.ChanUpdateDirection == 0 &&
-		return !edges[0].LastUpdate.Before(timestamp)
+		!reflect.DeepEqual(edges[0], channeldb.ChannelEdgePolicy{}):
 
-	case len(edges) >= 2 && edges[1].ChannelFlags == flags:
+		return !timestamp.After(edges[0].LastUpdate)
-		return !edges[1].LastUpdate.Before(timestamp)
+
+	case flags&lnwire.ChanUpdateDirection == 1 &&
+		!reflect.DeepEqual(edges[1], channeldb.ChannelEdgePolicy{}):
+
+		return !timestamp.After(edges[1].LastUpdate)
 
 	default:
 		return false