discovery/gossiper: send edge NodeAnnouncements upon announcing channel

In this commit, we also include the node announcements for both parties
of the channel if we happen to know about them. We do this to ensure
that other nodes in the network have properly processed the node
announcement of each party after successfully announcing the channel.

discovery/gossiper.go

@@ -2351,13 +2351,51 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(
 				source: nMsg.source,
 				msg:    e1Ann,
 			})
+
+			// In addition to sending this edge's policy, we'll also
+			// attempt to send the backing node's announcement if we
+			// happen to know about it.
+			pubKey, _ := chanInfo.NodeKey1()
+			node, err := d.cfg.Router.FetchLightningNode(pubKey)
+			if err == nil {
+				nodeAnn, err := CreateNodeAnnouncement(node)
+				if err == nil {
+					networkMsg := networkMsg{
+						peer:   nMsg.peer,
+						source: nMsg.source,
+						msg:    nodeAnn,
+					}
+					announcements = append(
+						announcements, networkMsg,
+					)
+				}
+			}
 		}
 		if e2Ann != nil {
 			announcements = append(announcements, networkMsg{
 				peer:   nMsg.peer,
 				source: nMsg.source,
 				msg:    e2Ann,
 			})
+
+			// In addition to sending this edge's policy, we'll also
+			// attempt to send the backing node's announcement if we
+			// happen to know about it.
+			pubKey, _ := chanInfo.NodeKey2()
+			node, err := d.cfg.Router.FetchLightningNode(pubKey)
+			if err == nil {
+				nodeAnn, err := CreateNodeAnnouncement(node)
+				if err == nil {
+					networkMsg := networkMsg{
+						peer:   nMsg.peer,
+						source: nMsg.source,
+						msg:    nodeAnn,
+					}
+					announcements = append(
+						announcements, networkMsg,
+					)
+				}
+			}
 		}
 
 		nMsg.err <- nil