peer: avoid chansync resend loop

If a peer receives a channel reestablish message shortly after the
channel has been closed, it will resend its own channel reestablish
message. In the meantime the other peer could also have seen the channel
being closed and will also resend its own message. This leads to a
resend loop that never terminates.

To avoid two peers getting into this situation, we now allow only one
such resent message per conection.

peer.go

@@ -210,6 +210,12 @@ type peer struct {
 	// the connection handshake.
 	remoteFeatures *lnwire.FeatureVector
 
+	// resentChanSyncMsg is a set that keeps track of which channels we
+	// have re-sent channel reestablishment messages for. This is done to
+	// avoid getting into loop where both peers will respond to the other
+	// peer's chansync message with its own over and over again.
+	resentChanSyncMsg map[lnwire.ChannelID]struct{}
+
 	// writePool is the task pool to that manages reuse of write buffers.
 	// Write tasks are submitted to the pool in order to conserve the total
 	// number of write buffers allocated at any one time, and decouple write
@@ -266,6 +272,7 @@ func newPeer(conn net.Conn, connReq *connmgr.ConnReq, server *server,
 		localCloseChanReqs: make(chan *htlcswitch.ChanClose),
 		linkFailures:       make(chan linkFailureReport),
 		chanCloseMsgs:      make(chan *closeMsg),
+		resentChanSyncMsg:  make(map[lnwire.ChannelID]struct{}),
 
 		chanActiveTimeout: chanActiveTimeout,
 
@@ -2506,6 +2513,12 @@ func (p *peer) sendInitMsg() error {
 // resendChanSyncMsg will attempt to find a channel sync message for the closed
 // channel and resend it to our peer.
 func (p *peer) resendChanSyncMsg(cid lnwire.ChannelID) error {
+	// If we already re-sent the mssage for this channel, we won't do it
+	// again.
+	if _, ok := p.resentChanSyncMsg[cid]; ok {
+		return nil
+	}
+
 	// Check if we have any channel sync messages stored for this channel.
 	c, err := p.server.chanDB.FetchClosedChannelForID(cid)
 	if err != nil {
@@ -2534,6 +2547,10 @@ func (p *peer) resendChanSyncMsg(cid lnwire.ChannelID) error {
 	peerLog.Debugf("Re-sent channel sync message for channel %v to peer "+
 		"%v", cid, p)
 
+	// Note down that we sent the message, so we won't resend it again for
+	// this connection.
+	p.resentChanSyncMsg[cid] = struct{}{}
+
 	return nil
 }