rpc+htlcswitch: add htlc transformation capabilities to the interceptor

[ffranr]

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Change Description

Closes https://github.com/lightningnetwork/lnd/issues/8619

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[ffranr]

There are two things left to do here:

1. Add support for modifying (via HTLC interception) p2p message lnwire.UpdateFulfillHTLC.
2. Write received UpdateAddHTLC to log so that the itest can ensure that Carol's node receives the correctly modified message.

I would be keen on feedback on what I have so far to check that I'm still on the right track.

Note that I couldn't include the record.CustomSet type as a field in UpdateAddHTLC because of an import cycle. Therefore, the code is currently serializing record.CustomSet into a blob before attaching it to UpdateAddHTLC.

It seems to me that much of package record should be merged into lnwire, is that a good idea?

[ffranr]

There are two things left to do here:

1. Add support for modifying (via HTLC interception) p2p message `lnwire.UpdateFulfillHTLC`.

2. Write received `UpdateAddHTLC` to log so that the itest can ensure that Carol's node receives the correctly modified message.

I would be keen on feedback on what I have so far to check that I'm still on the right track.

Note that I couldn't include the record.CustomSet type as a field in UpdateAddHTLC because of an import cycle. Therefore, the code is currently serializing record.CustomSet into a blob before attaching it to UpdateAddHTLC.

It seems to me that much of package record should be merged into lnwire, is that a good idea?

1. I'm not sure if we need to modify lnwire.UpdateFulfillHTLC also. We can leave that for a separate PR.

2. This is complete. We're checking Carol's log in the itest.

[ffranr]

New message encode/decode changes are in place.

Still need to modify lnwire.UpdateFulfillHTLC, but a review at this point would be useful to ensure correct encode/decode.

[ffranr]

Latest on this PR:

1. I've modified the custom records field encode/decode procedure. It's now serialized "flat" in the ExtraData field.

Things to consider:

Similar modifications are (probably) necessary for lnwire.UpdateFulfillHTLC. Will do that now that the overall pattern has been approved.

[ffranr]

The latest on this PR:

1. I've added a custom record field to UpdateFulfillHtlc. The HTLC interceptor can now modify the custom records field on update_fulfill_htlc and update_add_htlc messages.

2. I've removed a side effect from the UpdateAddHtlc.Encode method. Previously, when using this method, the blind point and custom records fields were added into the ExtraData field in the UpdateAddHtlc instance. Now they are encoded/decode as part of the message extra data field only. Effectively, UpdateAddHtlc.ExtraData now only contains TLV types which do not have corresponding fields in UpdateAddHtlc and are not within the custom records range.

3. The ExtraOpaqueData.PackRecords method has been made more robust with respect to canonical TLV record ordering.

TODO:

1. Add encoding/decoding unit tests for UpdateFulfillHtlc. (Corresponding unit tests already exist for UpdateAddHtlc.)

[ffranr]

I've just add encoding/decoding unit tests for UpdateFulfillHtlc.

PR should be ready for merge, IMO.

[ffranr]

I've included changes which were found necessary from the proof-of-concept investigation.

[guggero]

I made some additional changes to the custom records here: https://github.com/lightningnetwork/lnd/commit/8a4531fc9680f85040fdebce93e1fef3541482c2#diff-dcf3ed2a4ce04d1e140efd7a8d84967d7b7d04e69be69fdeb3a95c0350ac3b58 Would be nice to get those in as well.

[ffranr]

I made some additional changes to the custom records here: 8a4531f#diff-dcf3ed2a4ce04d1e140efd7a8d84967d7b7d04e69be69fdeb3a95c0350ac3b58 Would be nice to get those in as well.

Latest commits should include the custom record changes found above.