peer: define resendChanSyncMsg

This method is used to fetch channel sync messages for closed channels
from the db, and respond to the peer.

peer.go

@@ -874,7 +874,33 @@ func newChanMsgStream(p *peer, cid lnwire.ChannelID) *msgStream {
 			// active goroutine dedicated to this channel.
 			if chanLink == nil {
 				link, err := p.server.htlcSwitch.GetLink(cid)
-				if err != nil {
+				switch {
+
+				// If we failed to find the link in question,
+				// and the message received was a channel sync
+				// message, then this might be a peer trying to
+				// resync closed channel. In this case we'll
+				// try to resend our last channel sync message,
+				// such that the peer can recover funds from
+				// the closed channel.
+				case err != nil && isChanSyncMsg:
+					peerLog.Debugf("Unable to find "+
+						"link(%v) to handle channel "+
+						"sync, attempting to resend "+
+						"last ChanSync message", cid)
+
+					err := p.resendChanSyncMsg(cid)
+					if err != nil {
+						// TODO(halseth): send error to
+						// peer?
+						peerLog.Errorf(
+							"resend failed: %v",
+							err,
+						)
+					}
+					return
+
+				case err != nil:
 					peerLog.Errorf("recv'd update for "+
 						"unknown channel %v from %v: "+
 						"%v", cid, p, err)
@@ -2144,6 +2170,35 @@ func (p *peer) sendInitMsg() error {
 	return p.writeMessage(msg)
 }
 
+// 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 {
+	// Check if we have any channel sync messages stored for this channel.
+	c, err := p.server.chanDB.FetchClosedChannelForID(cid)
+	if err != nil {
+		return fmt.Errorf("unable to fetch channel sync messages for "+
+			"peer %v: %v", p, err)
+	}
+
+	if c.LastChanSyncMsg == nil {
+		return fmt.Errorf("no chan sync message stored for channel %v",
+			cid)
+	}
+
+	peerLog.Debugf("Re-sending channel sync message for channel %v to "+
+		"peer %v", cid, p)
+
+	if err := p.SendMessage(true, c.LastChanSyncMsg); err != nil {
+		return fmt.Errorf("Failed resending channel sync "+
+			"message to peer %v: %v", p, err)
+	}
+
+	peerLog.Debugf("Re-sent channel sync message for channel %v to peer "+
+		"%v", cid, p)
+
+	return nil
+}
+
 // SendMessage sends a variadic number of message to remote peer. The first
 // argument denotes if the method should block until the message has been sent
 // to the remote peer.