From fb95458a1bd53b931ff9b9f2231bffe6976b8351 Mon Sep 17 00:00:00 2001
From: yyforyongyu <yong2452@gmail.com>
Date: Wed, 11 Jun 2025 12:16:09 +0800
Subject: [PATCH] htlcswitch: skip checking replays for reforwarded packets

We now rely on the forwarding package's state to decide whether a given
packet is a reforwarding or not. If we know it's a reforwarding packet,
there's no need to check for replays in the `sharedHashes` bucket, which
behaves the same as if we are querying the `batchReplayBkt`.
---
 htlcswitch/hop/iterator.go | 14 ++++++++------
 htlcswitch/link.go         | 18 ++++++++++--------
 htlcswitch/mock.go         |  2 +-
 3 files changed, 19 insertions(+), 15 deletions(-)

diff --git a/htlcswitch/hop/iterator.go b/htlcswitch/hop/iterator.go
index 2cf2adb54..314f05209 100644
--- a/htlcswitch/hop/iterator.go
+++ b/htlcswitch/hop/iterator.go
@@ -745,7 +745,8 @@ func (r *DecodeHopIteratorResponse) Result() (Iterator, lnwire.FailCode) {
 // the presented readers and rhashes *NEVER* deviate across invocations for the
 // same id.
 func (p *OnionProcessor) DecodeHopIterators(id []byte,
-	reqs []DecodeHopIteratorRequest) ([]DecodeHopIteratorResponse, error) {
+	reqs []DecodeHopIteratorRequest,
+	reforward bool) ([]DecodeHopIteratorResponse, error) {
 
 	var (
 		batchSize = len(reqs)
@@ -864,11 +865,12 @@ func (p *OnionProcessor) DecodeHopIterators(id []byte,
 			continue
 		}
 
-		// If this index is contained in the replay set, mark it with a
-		// temporary channel failure error code. We infer that the
-		// offending error was due to a replayed packet because this
-		// index was found in the replay set.
-		if replays.Contains(uint16(i)) {
+		// If this index is contained in the replay set, and it is not a
+		// reforwarding on startup, mark it with a permanent channel
+		// failure error code. We infer that the offending error was due
+		// to a replayed packet because this index was found in the
+		// replay set.
+		if !reforward && replays.Contains(uint16(i)) {
 			log.Errorf("unable to process onion packet: %v",
 				sphinx.ErrReplayedPacket)
 
diff --git a/htlcswitch/link.go b/htlcswitch/link.go
index 72b506986..ec7cb1539 100644
--- a/htlcswitch/link.go
+++ b/htlcswitch/link.go
@@ -108,8 +108,10 @@ type ChannelLinkConfig struct {
 	// blobs, which are then used to inform how to forward an HTLC.
 	//
 	// NOTE: This function assumes the same set of readers and preimages
-	// are always presented for the same identifier.
-	DecodeHopIterators func([]byte, []hop.DecodeHopIteratorRequest) (
+	// are always presented for the same identifier. The last boolean is
+	// used to decide whether this is a reforwarding or not - when it's
+	// reforwarding, we skip the replay check enforced in our decay log.
+	DecodeHopIterators func([]byte, []hop.DecodeHopIteratorRequest, bool) (
 		[]hop.DecodeHopIteratorResponse, error)
 
 	// ExtractErrorEncrypter function is responsible for decoding HTLC
@@ -3764,12 +3766,14 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg) {
 		}
 	}
 
+	reforward := fwdPkg.State != channeldb.FwdStateLockedIn
+
 	// Atomically decode the incoming htlcs, simultaneously checking for
 	// replay attempts. A particular index in the returned, spare list of
 	// channel iterators should only be used if the failure code at the
 	// same index is lnwire.FailCodeNone.
 	decodeResps, sphinxErr := l.cfg.DecodeHopIterators(
-		fwdPkg.ID(), decodeReqs,
+		fwdPkg.ID(), decodeReqs, reforward,
 	)
 	if sphinxErr != nil {
 		l.failf(LinkFailureError{code: ErrInternalError},
@@ -4120,17 +4124,15 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg) {
 		return
 	}
 
-	replay := fwdPkg.State != channeldb.FwdStateLockedIn
-
-	l.log.Debugf("forwarding %d packets to switch: replay=%v",
-		len(switchPackets), replay)
+	l.log.Debugf("forwarding %d packets to switch: reforward=%v",
+		len(switchPackets), reforward)
 
 	// NOTE: This call is made synchronous so that we ensure all circuits
 	// are committed in the exact order that they are processed in the link.
 	// Failing to do this could cause reorderings/gaps in the range of
 	// opened circuits, which violates assumptions made by the circuit
 	// trimming.
-	l.forwardBatch(replay, switchPackets...)
+	l.forwardBatch(reforward, switchPackets...)
 }
 
 // experimentalEndorsement returns the value to set for our outgoing
diff --git a/htlcswitch/mock.go b/htlcswitch/mock.go
index cb814e95c..136d5c39b 100644
--- a/htlcswitch/mock.go
+++ b/htlcswitch/mock.go
@@ -522,7 +522,7 @@ func (p *mockIteratorDecoder) DecodeHopIterator(r io.Reader, rHash []byte,
 }
 
 func (p *mockIteratorDecoder) DecodeHopIterators(id []byte,
-	reqs []hop.DecodeHopIteratorRequest) (
+	reqs []hop.DecodeHopIteratorRequest, _ bool) (
 	[]hop.DecodeHopIteratorResponse, error) {
 
 	idHash := sha256.Sum256(id)