ddustin

   Alice                           Bob
     |                              |
     | splice_locked       -------> |
     | update_add_htlc     -------> |
     | commit_sig          -------> | batch_size=2, funding_tx=funding_tx_1
     | commit_sig          -------> | batch_size=2, funding_tx=funding_tx_2
     |                splice_locked | (recieved)
     | <-------       splice_locked | (sent)
 | | NOTE: Bob now expects a single commit_sig

ddustin

2025-02-01T22:42:36.4316216Z Current runner version: '2.322.0'
2025-02-01T22:42:36.4345187Z ##[group]Operating System
2025-02-01T22:42:36.4345932Z Ubuntu
2025-02-01T22:42:36.4346362Z 22.04.5
2025-02-01T22:42:36.4346982Z LTS
2025-02-01T22:42:36.4347454Z ##[endgroup]
2025-02-01T22:42:36.4347914Z ##[group]Runner Image
2025-02-01T22:42:36.4348582Z Image: ubuntu-22.04
2025-02-01T22:42:36.4349089Z Version: 20250126.1.0
2025-02-01T22:42:36.4350035Z Included Software: https://github.com/actions/runner-images/blob/ubuntu22/20250126.1/images/ubuntu/Ubuntu2204-Readme.md

ddustin

1) How did you spend your time?

I accomplished two primary things with my time. The first is a large progression towards so called "inter-op" for Splicing with Eclair. Inter-op is when two separate lightning implementations support splicing and are able to perform a splice between the two implementations. Essentially this means having a channel between the Eclair and CLN implementations, then doing a splice over that channel. Once a single splice is completed, numerous edge and corner cases are tested and must be handled robustly by both implementations.

This step is a big deal because once completed splicing officially becomes part of the Lightning specification for BOLTs 2, 7, and 9.

In a test environment I setup Eclair and CLN nodes and formed a channel between them. Then I attempted splicing. Numerous issues and incompatibilities emerged and had to be addressed.

These tests are important to lightning for two reasons:

ddustin

Question of reestablish and `tx_abort`.

We two nodes, l1 and l2 who are doing an interactive splice. Both sides have sent `tx_complete` and the splice has reached the signing phase.

By signature ordering rules, l2 signs first.

```
l1                           l2
       <- commit_sig
         *l1 restarts*

ddustin

Imagine a three node network with two channels

A <-> B <-> C

Let's say B would like to perform two splices:

splice-out 10M sats from A <-> B
splice-out 10M sats from B <-> C

ddustin

IMMEDIATELY AFTER RECEIVING CHANNEL_REESTABLISH

• Do we have any active inflight splices?
  • Yes:
    • does latest splice inflight have commit and remote sig?
      • Yes: continue
      • No: send next_funding txid of latest inflight candidate, then, continue
    • peer sends next_funding value of:
      • None: Have I sent splice tx signatures?
        • Yes: Have I received splice tx signatures?
• Yes: send nothing

ddustin

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

/*
 * Often while rebasing, the git merge algorithm will leave these

ddustin

splice_locked     ->
                   <- splice_locked
(Whichever side has lower funding pubkey starts STFU)
STFU              ->
                   <- STFU (ack)
splice_locked_ack ->
                   <- splice_locked_ack
Receiving splice_locked_ack ends STFU mode and
means we can atomically move to the new channel
confirmed state.

ddustin

Legend:
Item -> means sent
Item <- means received
Chan X (implies a channel at block height X)
(Since these happen at different times)

Splice locked race condition example


Node A.               Node B.

ddustin

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <math.h>
#include "BTCUtil.h"
#include "../libraries/mnemonic/mnemonic.h"
#include "../libraries/mnemonic/wordlist.h"

static const char *answer = "bc1qh07vgylvs66k850vr5e7efxxemhdgczj8w72xq";
static int found = 0;