From 44c87ef1d74870323570517fd71193da0956f928 Mon Sep 17 00:00:00 2001
From: Keagan McClelland <keagan.mcclelland@gmail.com>
Date: Tue, 12 Dec 2023 13:11:05 -0800
Subject: [PATCH] htlcswitch: bounce packets when quiescent

---
 htlcswitch/link.go | 49 +++++++++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 11 deletions(-)

diff --git a/htlcswitch/link.go b/htlcswitch/link.go
index 46f06c9ab..b56b0897f 100644
--- a/htlcswitch/link.go
+++ b/htlcswitch/link.go
@@ -643,20 +643,37 @@ func (l *channelLink) WaitForShutdown() {
 // actively accept requests to forward HTLC's. We're able to forward HTLC's if
 // we are eligible to update AND the channel isn't currently flushing the
 // outgoing half of the channel.
+//
+// NOTE: MUST NOT be called from the main event loop.
 func (l *channelLink) EligibleToForward() bool {
-	return l.EligibleToUpdate() &&
-		!l.IsFlushing(Outgoing)
+	l.RLock()
+	defer l.RUnlock()
+
+	return l.eligibleToForward()
 }
 
-// EligibleToUpdate returns a bool indicating if the channel is able to update
+// eligibleToForward returns a bool indicating if the channel is able to
+// actively accept requests to forward HTLC's. We're able to forward HTLC's if
+// we are eligible to update AND the channel isn't currently flushing the
+// outgoing half of the channel.
+//
+// NOTE: MUST be called from the main event loop.
+func (l *channelLink) eligibleToForward() bool {
+	return l.eligibleToUpdate() && !l.IsFlushing(Outgoing)
+}
+
+// eligibleToUpdate returns a bool indicating if the channel is able to update
 // channel state. We're able to update channel state if we know the remote
 // party's next revocation point. Otherwise, we can't initiate new channel
 // state. We also require that the short channel ID not be the all-zero source
 // ID, meaning that the channel has had its ID finalized.
-func (l *channelLink) EligibleToUpdate() bool {
+//
+// NOTE: MUST be called from the main event loop.
+func (l *channelLink) eligibleToUpdate() bool {
 	return l.channel.RemoteNextRevocation() != nil &&
-		l.ShortChanID() != hop.Source &&
-		l.isReestablished()
+		l.channel.ShortChanID() != hop.Source &&
+		l.isReestablished() &&
+		l.quiescer.CanSendUpdates()
 }
 
 // EnableAdds sets the ChannelUpdateHandler state to allow UpdateAddHtlc's in
@@ -1599,13 +1616,14 @@ func (l *channelLink) handleDownstreamUpdateAdd(pkt *htlcPacket) error {
 		return errors.New("not an UpdateAddHTLC packet")
 	}
 
-	// If we are flushing the link in the outgoing direction we can't add
-	// new htlcs to the link and we need to bounce it
-	if l.IsFlushing(Outgoing) {
+	// If we are flushing the link in the outgoing direction or we have
+	// already sent Stfu, then we can't add new htlcs to the link and we
+	// need to bounce it.
+	if l.IsFlushing(Outgoing) || !l.quiescer.CanSendUpdates() {
 		l.mailBox.FailAdd(pkt)
 
 		return NewDetailedLinkError(
-			&lnwire.FailPermanentChannelFailure{},
+			&lnwire.FailTemporaryChannelFailure{},
 			OutgoingFailureLinkNotEligible,
 		)
 	}
@@ -1708,6 +1726,15 @@ func (l *channelLink) handleDownstreamUpdateAdd(pkt *htlcPacket) error {
 //
 // TODO(roasbeef): add sync ntfn to ensure switch always has consistent view?
 func (l *channelLink) handleDownstreamPkt(pkt *htlcPacket) {
+	if pkt.htlc.MsgType().IsChannelUpdate() &&
+		!l.quiescer.CanSendUpdates() {
+
+		l.log.Warnf("unable to process channel update. "+
+			"ChannelID=%v is quiescent.", l.ChanID)
+
+		return
+	}
+
 	switch htlc := pkt.htlc.(type) {
 	case *lnwire.UpdateAddHTLC:
 		// Handle add message. The returned error can be ignored,
@@ -3364,7 +3391,7 @@ func (l *channelLink) updateChannelFee(feePerKw chainfee.SatPerKWeight) error {
 
 	// We skip sending the UpdateFee message if the channel is not
 	// currently eligible to forward messages.
-	if !l.EligibleToUpdate() {
+	if !l.eligibleToUpdate() {
 		l.log.Debugf("skipping fee update for inactive channel")
 		return nil
 	}