From 944f16255a60f52ecaa27c40417048853e39aaa3 Mon Sep 17 00:00:00 2001
From: yyforyongyu <yong2452@gmail.com>
Date: Sun, 10 Nov 2024 17:18:14 +0800
Subject: [PATCH] htlcswitch: handle nil circuit properly when settling

We have two sources which can call `handlePacketSettle`, either through
the link's `<-s.htlcPlex`, or the `<-s.resolutionMsgs`, which means the
`closeCircuit` could be call twice. Previously we already caught this
case inside `closeCircuit`, in that we would return a nil circuit upon
seeing `ErrUnknownCircuit`, indicating the circuit was removed. However,
we still need to account the case when the circuit is the process of
being closed, which is now fixed as we will ignore when seeing
`ErrCircuitClosing`.
---
 htlcswitch/packet.go | 11 +++++++++++
 htlcswitch/switch.go | 11 ++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/htlcswitch/packet.go b/htlcswitch/packet.go
index e99185851..ed5f82588 100644
--- a/htlcswitch/packet.go
+++ b/htlcswitch/packet.go
@@ -1,6 +1,8 @@
 package htlcswitch
 
 import (
+	"fmt"
+
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
@@ -136,3 +138,12 @@ func (p *htlcPacket) keystone() Keystone {
 		OutKey: p.outKey(),
 	}
 }
+
+// String returns a human-readable description of the packet.
+func (p *htlcPacket) String() string {
+	return fmt.Sprintf("keystone=%v, sourceRef=%v, destRef=%v, "+
+		"incomingAmount=%v, amount=%v, localFailure=%v, hasSource=%v "+
+		"isResolution=%v", p.keystone(), p.sourceRef, p.destRef,
+		p.incomingAmount, p.amount, p.localFailure, p.hasSource,
+		p.isResolution)
+}
diff --git a/htlcswitch/switch.go b/htlcswitch/switch.go
index 4c54fab0a..704887e8e 100644
--- a/htlcswitch/switch.go
+++ b/htlcswitch/switch.go
@@ -2994,6 +2994,15 @@ func (s *Switch) handlePacketSettle(packet *htlcPacket) error {
 	// If the source of this packet has not been set, use the circuit map
 	// to lookup the origin.
 	circuit, err := s.closeCircuit(packet)
+
+	// If the circuit is in the process of closing, we will return a nil as
+	// there's another packet handling undergoing.
+	if errors.Is(err, ErrCircuitClosing) {
+		log.Debugf("Circuit is closing for packet=%v", packet)
+		return nil
+	}
+
+	// Exit early if there's another error.
 	if err != nil {
 		return err
 	}
@@ -3005,7 +3014,7 @@ func (s *Switch) handlePacketSettle(packet *htlcPacket) error {
 	// and when `UpdateFulfillHTLC` is received. After which `RevokeAndAck`
 	// is received, which invokes `processRemoteSettleFails` in its link.
 	if circuit == nil {
-		log.Debugf("Found nil circuit: packet=%v", spew.Sdump(packet))
+		log.Debugf("Circuit already closed for packet=%v", packet)
 		return nil
 	}