lnrpc: add HtlcIndex to ForwardingEvents #9813

Abdulkbk · 2025-05-15T22:00:34Z

Change Description

This PR adds a new HtlcIndex field to the ForwardingEvent to improve HTLC tracking and reconciliation.

The HtlcIndex field is populated using the incoming HtlcId from the circuit when creating forwarding events. This allows for matching HTLCs between channel state and forwarding log.

To ensure everything works well together, we handle EOF errors by setting the HtlcIndex field to a default value of "0" for older ForwardingEvents that do not have this field.

Steps to Test

Start LND node
Route payments through the node
Verify that the HtlcIndex of the payments' htlc are present in the response from lncli fwdinghistory

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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.

yyforyongyu · 2025-05-20T11:53:32Z

lnrpc/lightning.proto

@@ -4710,6 +4710,10 @@ message ForwardingEvent {
    // The peer alias of the outgoing channel.
    string peer_alias_out = 13;

+    // The index of the HTLC in the channel. This allows for better tracking


nit: typo in the commit fwdinghiatory -> fwdinghistory

yyforyongyu · 2025-05-20T12:01:06Z

htlcswitch/switch.go

@@ -3074,6 +3074,7 @@ func (s *Switch) handlePacketSettle(packet *htlcPacket) error {
 				OutgoingChanID: circuit.Outgoing.ChanID,
 				AmtIn:          circuit.IncomingAmount,
 				AmtOut:         circuit.OutgoingAmount,
+				HtlcIndex:      circuit.Incoming.HtlcID,


For forwarding events I think we need to have both indices? like what we have in the htlcPacket, which has both incomingHTLCID and outgoingHTLCID. Or like the RPC routerrpc.HtlcEvent

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 👍

yyforyongyu · 2025-05-20T12:19:17Z

channeldb/forwarding_log_test.go

+
+// TestForwardingLogDecodeForwardingEvent tests that we're able to decode
+// forwarding events that don't have the new HtlcIndex field.
+func TestForwardingLogDecodeForwardingEvent(t *testing.T) {


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.

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?

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

View reviewed changes

saubyk assigned Abdulkbk May 20, 2025

htlcswitch+channeldb: add htlcidx to fwding log

3f14155

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

Abdulkbk force-pushed the htlcidx-to-fwdingevent branch from a775db2 to a6343fe Compare May 21, 2025 11:48

Abdulkbk added 2 commits May 21, 2025 14:08

lnd+lnrpc: update fwdinghistory message

673aee6

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

docs: add release note

5b070cb

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

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

lnrpc: add HtlcIndex to ForwardingEvents #9813

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