htlcswitch: exit early if no adds are in the fwd pkg #9911

ziggie1984 · 2025-06-06T11:28:51Z

This lead to the case that we would always record a HTLC
two times in the decayed log (batch-replay bucket) protection which is not necessary
in the first place.

We already do it for the settle/fail package:

lnd/htlcswitch/link.go

Lines 3626 to 3628 in 4a7cd00

    
           if len(fwdPkg.SettleFails) == 0 { 
        
           	return 
        
           }

coderabbitai · 2025-06-06T11:28:58Z

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yyforyongyu

would always record a HTLC two times in the decayed log (batch-replay bucket) protection

When there are no Adds it's a noop?

ziggie1984 · 2025-06-06T12:24:19Z

I actually verified on my local node and HTLC will be persisted 2 times in the batch-reply log, here is the flow:

lnd/htlcswitch/link.go

Line 3761 in 4a7cd00

decodeResps, sphinxErr := l.cfg.DecodeHopIterators(

(is called)

We start the batch_replay tx here:

lnd/htlcswitch/hop/iterator.go

Line 756 in 4a7cd00

tx := p.router.BeginTxn(id, batchSize)

it persist the empty batch in the batch-reply here:

lnd/htlcswitch/hop/iterator.go

Line 831 in 4a7cd00

packets, replays, err := tx.Commit()

which happens here:

https://github.com/lightningnetwork/lightning-onion/blob/8b40adf04ab9f081e198b05320843ebde96277c6/sphinx.go#L828

and writes it eventually here:

lnd/htlcswitch/decayedlog.go

Line 407 in 4a7cd00

return batchReplayBkt.Put(b.ID, replayBuf.Bytes())

yyforyongyu

I actually verified on my local node and HTLC will be persisted 2 times in the batch-reply log, here is the flow:

Nice analysis! I'm wondering why would call processRemoteAdds with empty ADDs?

yyforyongyu · 2025-06-08T23:44:49Z

htlcswitch/link.go

 func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg) {
+	// Exit early if there are no adds to process.
+	if len(fwdPkg.Adds) == 0 {
+		return


Let's add a log here since it's unexpected to have empty Adds here

It is not unexpeted tho, every HTLC will eventually be resolved and then the fwd has only Fail/Settles no ADDs, I can add a trace if you want ? In case no other HTLC is added while the first one is Removed.

FWD PKG 1 => adds the invoming HTLC onto the Channel

FWD PKG 2 => resolves the HTLC via an FAIL/Settle and has 0 ADDs in the single HTLC flow.

yeah right - we sort just move the empty check into the methods, so they are always called.

This lead to the case that we would always record a HTLC two times in the decayed log protection which is not necessary in the first place.

yyforyongyu

LGTM🙏

yyforyongyu · 2025-06-10T19:07:29Z

htlcswitch/link.go

 func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg) {
+	// Exit early if there are no adds to process.
+	if len(fwdPkg.Adds) == 0 {
+		return


yeah right - we sort just move the empty check into the methods, so they are always called.

Roasbeef

LGTM ☃️

ziggie1984 requested review from bhandras and yyforyongyu June 6, 2025 11:29

ziggie1984 self-assigned this Jun 6, 2025

ziggie1984 added the no-changelog label Jun 6, 2025

ziggie1984 added this to the v0.19.2 milestone Jun 6, 2025

ziggie1984 added the htlcswitch label Jun 6, 2025

yyforyongyu reviewed Jun 6, 2025

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ziggie1984 mentioned this pull request Jun 8, 2025

decayed log migration #9906

Closed

yyforyongyu reviewed Jun 8, 2025

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yyforyongyu added the bug fix label Jun 9, 2025

ziggie1984 force-pushed the exit-early-for-fwd-adds branch from a7219c0 to d82b1b8 Compare June 9, 2025 06:57

htlcswitch: exit early if no adds are in the fwd pkg

6feed32

This lead to the case that we would always record a HTLC two times in the decayed log protection which is not necessary in the first place.

ziggie1984 force-pushed the exit-early-for-fwd-adds branch from d82b1b8 to 6feed32 Compare June 9, 2025 07:17

yyforyongyu approved these changes Jun 10, 2025

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ziggie1984 requested review from Roasbeef and removed request for bhandras June 10, 2025 21:18

Roasbeef approved these changes Jun 10, 2025

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Roasbeef merged commit 92a5d35 into lightningnetwork:master Jun 10, 2025
33 of 38 checks passed

Roasbeef mentioned this pull request Jun 20, 2025

release: create v0.19.2-rc1 branch #9976

Closed

guggero mentioned this pull request Jun 24, 2025

release: create v0.19.2-rc1 branch #9986

Merged

ziggie1984 mentioned this pull request Jul 5, 2025

channeldb: gc fwd packgages which have zero ADDs #10041

Merged

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