server+discovery: send channel updates to remote peers reliably

In this commit, we also allow channel updates for our channels to be
sent reliably to our channel counterparty. This is especially crucial
for private channels, since they're not announced, in order to ensure
each party can receive funds from the other side.

discovery/gossiper.go

@@ -95,11 +95,6 @@ type Config struct {
 	Broadcast func(skips map[routing.Vertex]struct{},
 		msg ...lnwire.Message) error
 
-	// FindPeer returns the actively registered peer for a given remote
-	// public key. An error is returned if the peer was not found or a
-	// shutdown has been requested.
-	FindPeer func(identityKey *btcec.PublicKey) (lnpeer.Peer, error)
-
 	// NotifyWhenOnline is a function that allows the gossiper to be
 	// notified when a certain peer comes online, allowing it to
 	// retry sending a peer message.
@@ -1927,30 +1922,26 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(
 		// so we'll try sending the update directly to the remote peer.
 		if !nMsg.isRemote && chanInfo.AuthProof == nil {
 			// Get our peer's public key.
-			var remotePub *btcec.PublicKey
+			var remotePubKey [33]byte
 			switch {
 			case msg.ChannelFlags&lnwire.ChanUpdateDirection == 0:
-				remotePub, _ = chanInfo.NodeKey2()
+				remotePubKey = chanInfo.NodeKey2Bytes
 			case msg.ChannelFlags&lnwire.ChanUpdateDirection == 1:
-				remotePub, _ = chanInfo.NodeKey1()
+				remotePubKey = chanInfo.NodeKey1Bytes
 			}
 
-			sPeer, err := d.cfg.FindPeer(remotePub)
+			// Now, we'll attempt to send the channel update message
+			// reliably to the remote peer in the background, so
+			// that we don't block if the peer happens to be offline
+			// at the moment.
+			err := d.reliableSender.sendMessage(msg, remotePubKey)
 			if err != nil {
-				log.Errorf("unable to send channel update -- "+
-					"could not find peer %x: %v",
-					remotePub.SerializeCompressed(),
-					err)
-			} else {
-				// Send ChannelUpdate directly to remotePeer.
-				// TODO(halseth): make reliable send?
-				err = sPeer.SendMessage(false, msg)
-				if err != nil {
-					log.Errorf("unable to send channel "+
-						"update message to peer %x: %v",
-						remotePub.SerializeCompressed(),
-						err)
-				}
+				err := fmt.Errorf("unable to reliably send %v "+
+					"for channel=%v to peer=%x: %v",
+					msg.MsgType(), msg.ShortChannelID,
+					remotePubKey, err)
+				nMsg.err <- err
+				return nil
 			}
 		}