discovery: reprocess premature ChannelUpdates

This commit makes the gossiper store received ChannelUpdates
that is not for any known channel in a map, such that they
can be reprocessed when the ChannelAnnouncement arrives.

This is done to handle the case where we receive a ChannelUpdate
from our channel counterparty before we have been able to process
our own local ChannelAnnouncement.

discovery/gossiper.go

@@ -139,6 +139,13 @@ type AuthenticatedGossiper struct {
 	// TODO(roasbeef): limit premature networkMsgs to N
 	prematureAnnouncements map[uint32][]*networkMsg
 
+	// prematureChannelUpdates is a map of ChannelUpdates we have
+	// received that wasn't associated with any channel we know about.
+	// We store them temporarily, such that we can reprocess them when
+	// a ChannelAnnouncement for the channel is received.
+	prematureChannelUpdates map[uint64][]*networkMsg
+	pChanUpdMtx             sync.Mutex
+
 	// waitingProofs is a persistent storage of partial channel proof
 	// announcement messages. We use it to buffer half of the material
 	// needed to reconstruct a full authenticated channel announcement. Once
@@ -174,13 +181,14 @@ func New(cfg Config, selfKey *btcec.PublicKey) (*AuthenticatedGossiper, error) {
 	}
 
 	return &AuthenticatedGossiper{
-		selfKey:                selfKey,
-		cfg:                    &cfg,
-		networkMsgs:            make(chan *networkMsg),
-		quit:                   make(chan struct{}),
-		feeUpdates:             make(chan *feeUpdateRequest),
-		prematureAnnouncements: make(map[uint32][]*networkMsg),
-		waitingProofs:          storage,
+		selfKey:                 selfKey,
+		cfg:                     &cfg,
+		networkMsgs:             make(chan *networkMsg),
+		quit:                    make(chan struct{}),
+		feeUpdates:              make(chan *feeUpdateRequest),
+		prematureAnnouncements:  make(map[uint32][]*networkMsg),
+		prematureChannelUpdates: make(map[uint64][]*networkMsg),
+		waitingProofs:           storage,
 	}, nil
 }
 
@@ -1013,6 +1021,60 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(nMsg *networkMsg) []l
 			return nil
 		}
 
+		// If we earlier received any ChannelUpdates for this channel,
+		// we can now process them, as the channel is added to the
+		// graph.
+		shortChanID := msg.ShortChannelID.ToUint64()
+		var channelUpdates []*networkMsg
+
+		d.pChanUpdMtx.Lock()
+		for _, cu := range d.prematureChannelUpdates[shortChanID] {
+			channelUpdates = append(channelUpdates, cu)
+		}
+
+		// Now delete the premature ChannelUpdates, since we added them
+		// all to the queue of network messages.
+
+		delete(d.prematureChannelUpdates, shortChanID)
+		d.pChanUpdMtx.Unlock()
+
+		// Launch a new goroutine to handle each ChannelUpdate,
+		// this to ensure we don't block here, as we can handle
+		// only one announcement at a time.
+		for _, cu := range channelUpdates {
+			go func(nMsg *networkMsg) {
+				switch msg := nMsg.msg.(type) {
+
+				case *lnwire.ChannelUpdate:
+					// We can safely wait for the error to
+					// be returned, as in case of shutdown,
+					// the gossiper will return an error.
+					var err error
+					if nMsg.isRemote {
+						err = <-d.ProcessRemoteAnnouncement(
+							msg, nMsg.peer)
+					} else {
+						err = <-d.ProcessLocalAnnouncement(
+							msg, nMsg.peer)
+					}
+					if err != nil {
+						log.Errorf("Failed reprocessing"+
+							" ChannelUpdate for "+
+							"shortChanID=%v: %v",
+							msg.ShortChannelID.ToUint64(),
+							err)
+						return
+					}
+
+				// We don't expect any other message type
+				// than ChannelUpdate to be in this map.
+				default:
+					log.Errorf("Unsupported message type "+
+						"found among ChannelUpdates: %T", msg)
+				}
+			}(cu)
+		}
+
 		// Channel announcement was successfully proceeded and know it
 		// might be broadcast to other connected nodes if it was
 		// announcement with proof (remote).
@@ -1063,12 +1125,44 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(nMsg *networkMsg) []l
 		// verify message signature.
 		chanInfo, _, _, err := d.cfg.Router.GetChannelByID(msg.ShortChannelID)
 		if err != nil {
-			err := errors.Errorf("unable to validate "+
-				"channel update short_chan_id=%v: %v",
-				shortChanID, err)
-			log.Error(err)
-			nMsg.err <- err
-			return nil
+			switch err {
+			case channeldb.ErrGraphNotFound:
+				fallthrough
+			case channeldb.ErrGraphNoEdgesFound:
+				fallthrough
+			case channeldb.ErrEdgeNotFound:
+				// If the edge corresponding to this
+				// ChannelUpdate was not found in the graph,
+				// this might be a channel in the process of
+				// being opened, and we haven't processed our
+				// own ChannelAnnouncement yet, hence it is not
+				// found in the graph. This usually gets
+				// resolved after the channel proofs are
+				// exchanged and the channel is broadcasted to
+				// the rest of the network, but in case this
+				// is a private channel this won't ever happen.
+				// Because of this, we temporarily add it to a
+				// map, and reprocess it after our own
+				// ChannelAnnouncement has been processed.
+				d.pChanUpdMtx.Lock()
+				d.prematureChannelUpdates[shortChanID] = append(
+					d.prematureChannelUpdates[shortChanID],
+					nMsg)
+				d.pChanUpdMtx.Unlock()
+				log.Infof("Got ChannelUpdate for edge not "+
+					"found in graph(shortChanID=%v), "+
+					"saving for reprocessing later",
+					shortChanID)
+				nMsg.err <- nil
+				return nil
+			default:
+				err := errors.Errorf("unable to validate "+
+					"channel update short_chan_id=%v: %v",
+					shortChanID, err)
+				log.Error(err)
+				nMsg.err <- err
+				return nil
+			}
 		}
 
 		// The least-significant bit in the flag on the channel update
@@ -1117,11 +1211,11 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(nMsg *networkMsg) []l
 			return nil
 		}
 
-		// If this is a local ChannelUpdate without an AuthProof, it means
-		// it is an update to a channel that is not (yet) supposed to be
-		// announced to the greater network. However, our channel counter
-		// party will need to be given the update, so we'll try sending
-		// the update directly to the remote peer.
+		// If this is a local ChannelUpdate without an AuthProof, it
+		// means it is an update to a channel that is not (yet)
+		// supposed to be announced to the greater network. However,
+		// our channel counter party will need to be given the update,
+		// 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 remotePeer *btcec.PublicKey