lnrpc: add HtlcIndex to ForwardingEvents #9813

Abdulkbk · 2025-05-15T22:00:34Z

Change Description

This PR adds incoming and outgoing HTLC indices fields to the ForwardingEvent to improve HTLC tracking and reconciliation.

When creating forwarding events, the HTLC indices fields are populated using the incoming and outgoing HTLCId from the payment circuit.

Steps to Test

Start LND node
Route payments through the node
Verify that the incoming and outgoing htlc indices of the payments' HTLC are present in the response from lncli fwdinghistory

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Any new lncli commands have appropriate tags in the comments for the rpc in the proto file.
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guggero · 2025-05-16T06:37:37Z

Thanks for the PR.
Just a heads up, this won't work like that. For everything that touches the database, you need to assume there's already data in there with the old format. So we either have to write a migration script or deal with the fact that you might get an EOF error when reading a new field.

Abdulkbk · 2025-05-16T08:20:23Z

Thanks for the PR. Just a heads up, this won't work like that. For everything that touches the database, you need to assume there's already data in there with the old format. So we either have to write a migration script or deal with the fact that you might get an EOF error when reading a new field.

Thanks for the heads up. I will check how this was handled before in previous PRs. For now, I will change this to a draft.

Abdulkbk · 2025-05-19T13:34:20Z

Thanks for the PR. Just a heads up, this won't work like that. For everything that touches the database, you need to assume there's already data in there with the old format. So we either have to write a migration script or deal with the fact that you might get an EOF error when reading a new field.

Thanks again for the early feedback, @guggero. I thought about writing a migration script or handling the EOF error, but I decided to handle the EOF error when decoding the events (Let me know if a migration is preferred instead).

Also, does it make sense to set a default value of "0" when returning older event logs?. This means if you execute fwdinghistory and you have events before this change, you'd a get a bunch returned with HtlcIndex set to 0. But I'm thinking maybe updating the command's docs to explicitly state this would help?

guggero · 2025-05-20T06:54:11Z

Definitely looks better now, thanks for the update. I think because HtlcIndex: 0 is a valid value, we shouldn't use that. We should probably use a fn.Option to signify whether it's set or not (and then on the RPC level we can use the optional keyword, which makes the value nilable).

yyforyongyu

Thanks for the PR!

For forwarding events we need to have both incoming and outgoing htlc index.

A migration would be nice, tho I don't think it's possible since we cannot get the htlc index info. Alternative to fn.Option, we can use a tlv record instead, sth like tlv.BigSizeT to save the space since for a busy node the logs can be huge.

We should also update the RPC docs, and release docs to mention this behavior.

lnrpc/lightning.proto

htlcswitch/switch.go

yyforyongyu · 2025-05-20T12:11:46Z

rpcserver.go

@@ -8060,6 +8060,7 @@ func (r *rpcServer) ForwardingHistory(ctx context.Context,
 			FeeMsat:     uint64(feeMsat),
 			AmtInMsat:   uint64(amtInMsat),
 			AmtOutMsat:  uint64(amtOutMsat),
+			HtlcIndex:   event.HtlcIndex,


let's also update the test here to check the change,

lnd/itest/lnd_multi-hop-payments_test.go

Lines 183 to 184 in 3707b1f

fwdingHistory := dave.RPC.ForwardingHistory(nil)

require.Len(ht, fwdingHistory.ForwardingEvents, numPayments)

Updated to check for the change 👍

channeldb/forwarding_log_test.go

Abdulkbk · 2025-05-21T13:12:02Z

Thanks for the PR. Just a heads up, this won't work like that. For everything that touches the database, you need to assume there's already data in there with the old format. So we either have to write a migration script or deal with the fact that you might get an EOF error when reading a new field.

Now using fn.Option 👍

Abdulkbk · 2025-05-21T13:21:42Z

Thanks for the PR!

For forwarding events we need to have both incoming and outgoing htlc index.

A migration would be nice, tho I don't think it's possible since we cannot get the htlc index info. Alternative to fn.Option, we can use a tlv record instead, sth like tlv.BigSizeT to save the space since for a busy node the logs can be huge.

We should also update the RPC docs, and release docs to mention this behavior.

Thanks for the review @yyforyongyu.

The tlv is quite new to me, but I've done some research on it, and I want to clarify something. By using tlv.BigSizeT, do you mean doing something like below here:

....
       // IncomingHtlcID is the ID of the incoming HTLC in the payment circuit.
	// If the HTLC ID is not set, the value will be nil.
	IncomingHtlcID tlv.OptionalRecordT[tlv.TlvType0, tlv.BigSizeT[uint64]]

	// OutgoingHtlcID is the ID of the outgoing HTLC in the payment circuit.
	// If the HTLC ID is not set, the value will be nil.
	OutgoingHtlcID tlv.OptionalRecordT[tlv.TlvType0, tlv.BigSizeT[uint64]]
...

and then checking if the indices are set in the encode and decode functions?

yyforyongyu

Changes looking good! My main comment is to provide more detailed docs, and minor suggestion on the tests.

The tlv is quite new to me, but I've done some research on it, and I want to clarify something.

Yeah sth like that, I've played a bit locally without the optional record, here's the diff. Previously I was concerned about the size of the records, but since each TLV record requires extra 2 bytes to store the type and length info, plus one byte to store the total length of the TLV records, in this specific case, it would require extra 5 bytes. If the value stored is less than 0xffffffff (roughly 4.3 billion), then each htlc index takes 5 bytes, so a total of 5*2 + 5 = 15 bytes, which takes one byte less than two uint64.

However I don't think it's necessary to use TLV here now, as I'm looking forward to sunset the forwarding log bucket and relying on revocation log only one day - the latter plus the channel commitments gives us enough info to construct the forwarding events on the fly, maybe a simple query would be sufficient once we've done SQLizing the channeldb.

Also not requesting for changes, but just give you my thoughts on the most proper way of handling adding new fields, for this specific case,

add new fields, without the fn.Option.
define a flag, sth like NoHtlcIndexValue = math.MaxUint64.
create a migration to migrate all forwarding events using this flag.
at the RPC level, check the index value, and only set the optional field there if the value is not the above flag value.

This way we can avoid all the if-else branches and catching EOF in our serializer/deserializer, tho there's a bit more work to create the migration.

channeldb/forwarding_log.go

yyforyongyu · 2025-05-27T08:07:37Z

channeldb/forwarding_log.go

+		return err
+	}
+
+	// Then we write the optional HTLC IDs if they are set.


To be more defensive - I would return an error here if f.IncomingHtlcID.IsNone() to make it clear that they are mandatory.

Makes sense since new events now should always have them. Now returning an error if the fields are not set.

docs/release-notes/release-notes-0.20.0.md

lnrpc/lightning.proto

yyforyongyu · 2025-05-27T08:16:51Z

itest/lnd_multi-hop-payments_test.go

@@ -213,6 +214,17 @@ func testMultiHopPayments(ht *lntest.HarnessTest) {
 		require.Equal(ht, aliceAlias, event.PeerAliasOut)
 	}

+	// Verify HTLC IDs are not nil and unique across all forwarding events.


Nice addition! I think we can make this test more rigorous by asserting the htlc ids are exactly [0, 1, 2, 3, 4].

I tried using require.Equal to compare the htlc indices strictly, the itest passed locally but all failed in the CI, seems it's a bit hard to track the exact indices.

yeah the events order is not guaranteed, so we'll need to collect the indices in a map and compare them after the iteration.

Now comparing the indices

Abdulkbk · 2025-05-28T13:49:30Z

Thanks again for the review @yyforyongyu 👍.

Changes looking good! My main comment is to provide more detailed docs, and minor suggestion on the tests.

Added more details to the doc, giving more context to this change.

Also not requesting for changes, but just give you my thoughts on the most proper way of handling adding new fields, for this specific case,

add new fields, without the fn.Option.
define a flag, sth like NoHtlcIndexValue = math.MaxUint64.
create a migration to migrate all forwarding events using this flag.
at the RPC level, check the index value, and only set the optional field there if the value is not the above flag value.
This way we can avoid all the if-else branches and catching EOF in our serializer/deserializer, tho there's a bit more work to create the migration.

Thanks for the information. I will certainly keep this in mind!

yyforyongyu

LGTM🌹 (Need to squash that fixup commit before merging ofc)

channeldb/forwarding_log.go

yyforyongyu · 2025-06-03T10:22:45Z

itest/lnd_multi-hop-payments_test.go

@@ -213,6 +214,17 @@ func testMultiHopPayments(ht *lntest.HarnessTest) {
 		require.Equal(ht, aliceAlias, event.PeerAliasOut)
 	}

+	// Verify HTLC IDs are not nil and unique across all forwarding events.


yeah the events order is not guaranteed, so we'll need to collect the indices in a map and compare them after the iteration.

Abdulkbk · 2025-06-04T21:22:32Z

LGTM🌹 (Need to squash that fixup commit before merging ofc)

It will be squashed (used fixup because the contribution guide says it's preferable)

guggero · 2025-06-05T06:45:42Z

The doc mentions "auto squash the fix up commits on rebase". Which is something the PR author needs to do before merge, upon request by the reviewers, which is what @yyforyongyu meant.
We could use "rebase and merge", but that would mean all the commits have a different committer because they are changed by GitHub (which we don't prefer). So yeah, maybe we could make the docs a bit more clear.

guggero

Very nice! Looks good, just two ideas/requests.

channeldb/forwarding_log.go

In this commit we add htlcindex field to the forwardingevent struct, which is persisted alongside the other event fields.

In this commit we update the returned message for fwdinghistory to include the htlcindex for all forwarded htlcs.

ziggie1984 · 2025-06-06T13:16:52Z

channeldb/forwarding_log.go

@@ -78,14 +81,44 @@ type ForwardingEvent struct {
 	// AmtOut is the amount of the outgoing HTLC. Subtracting the incoming
 	// amount from this gives the total fees for this payment circuit.
 	AmtOut lnwire.MilliSatoshi
+
+	// IncomingHtlcID is the ID of the incoming HTLC in the payment circuit.
+	// If this is not set, the value will be nil. This field is added in


I do not understand this comment, this value cannot be nil, it is of type fn.Option.

probably you meant in lnrpc.ForwardingEvent

I do not understand this comment, this value cannot be nil, it is of type fn.Option.

Since fn.Option represents a value that may or may not be present, I thought I could refer to it as nil when the value isn't present?. But I think if you find it confusing then many would too, which means I should've framed this better.

ziggie1984 · 2025-06-06T13:17:09Z

channeldb/forwarding_log.go

+	IncomingHtlcID fn.Option[uint64]
+
+	// OutgoingHtlcID is the ID of the outgoing HTLC in the payment circuit.
+	// If this is not set, the value will be nil. This field is added in


Abdulkbk force-pushed the htlcidx-to-fwdingevent branch from f4c5fd9 to 009584f Compare May 15, 2025 22:29

Abdulkbk marked this pull request as draft May 16, 2025 08:20

Abdulkbk force-pushed the htlcidx-to-fwdingevent branch from 009584f to cb09e8c Compare May 19, 2025 12:48

Abdulkbk marked this pull request as ready for review May 19, 2025 12:54

Abdulkbk force-pushed the htlcidx-to-fwdingevent branch from cb09e8c to a775db2 Compare May 19, 2025 12:55

yyforyongyu requested changes May 20, 2025

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Abdulkbk force-pushed the htlcidx-to-fwdingevent branch 2 times, most recently from a6343fe to 5b070cb Compare May 21, 2025 13:08

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saubyk added this to the v0.20.0 milestone May 27, 2025

saubyk added this to lnd v0.20 May 27, 2025

saubyk moved this to In progress in lnd v0.20 May 27, 2025

Abdulkbk force-pushed the htlcidx-to-fwdingevent branch 5 times, most recently from 90b2f24 to f06a99c Compare May 28, 2025 13:05

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Abdulkbk added 3 commits June 6, 2025 12:21

htlcswitch+channeldb: add htlcidx to fwding log

f79e613

In this commit we add htlcindex field to the forwardingevent struct, which is persisted alongside the other event fields.

lnd+lnrpc: update fwdinghistory message

a0634e4

In this commit we update the returned message for fwdinghistory to include the htlcindex for all forwarded htlcs.

docs: add release note

ccf3a28

Abdulkbk force-pushed the htlcidx-to-fwdingevent branch from 1a7298d to ccf3a28 Compare June 6, 2025 11:26

guggero merged commit e1c9825 into lightningnetwork:master Jun 6, 2025
34 of 37 checks passed

github-project-automation bot moved this from In progress to Done in lnd v0.20 Jun 6, 2025

Abdulkbk mentioned this pull request Jun 6, 2025

[feature]: Bring FwdLog into SQL #9882

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ziggie1984 reviewed Jun 6, 2025

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guggero modified the milestones: v0.20.0, v0.19.2 Jun 6, 2025

feelancer21 mentioned this pull request Jun 6, 2025

[feature]: Add htlc_id to lnrpc.HTLCAttempt and lnrpc.ForwardingEvent #9656

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saubyk removed this from lnd v0.20 Jun 6, 2025

Roasbeef mentioned this pull request Jun 20, 2025

release: create v0.19.2-rc1 branch #9976

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guggero mentioned this pull request Jun 24, 2025

release: create v0.19.2-rc1 branch #9986

Merged

	fwdingHistory := dave.RPC.ForwardingHistory(nil)
	require.Len(ht, fwdingHistory.ForwardingEvents, numPayments)

lnrpc: add HtlcIndex to ForwardingEvents #9813

lnrpc: add HtlcIndex to ForwardingEvents #9813

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