Splicing Gossip

[ddustin]

@litch reports: I'm testing this in a mixed-nodes environment and it looks like a post-spliced channel is un-detectable by LND. I'm not 100% sure why at this point, and have some logs I can provide on request.

Workflow:

• Lnd1 is connected to cln1
• Open a channel between cln1 and cln2
• Verify that Lnd1 can see the channel between cln1 and cln2 in its describegraph
• Splice the channel
• the channel between cln1 and cln2 is now not listed in the describegraph
• Open a new channel between cln1 and cln2
• Verify that the new channel between cln1 and cln2 appears in the describegraph, whereas the spliced one does not

Happy to help debug!

EDIT TO ADD:

It looks the newly-created channel eventually does get broadcast. The ChannelAnnounce message wound up getting broadcast an hour after the splice happened. Even with dev-fast-gossip enabled and I restarted the CLN node within that hour window (at about 40 minutes). Unsure if that is related.

In any case: tACK (at least in my cluster of regtest nodes)

Originally posted by @litch in #6253 (comment)

[rustyrussell]

Yes, this is expected!

1. You can only announce a "new" channel after it's 6 blocks deep (to avoid reorg hell).
2. As of last year, you are supposed to wait for 12 blocks before seeing a channel "close" before discarding it.

Obviously LND didn't implement number 2, yet (and, without splices, I guess there was no reason to) so it sees a channel "vanish".

[litch]

Just to clarify, hundreds of blocks may pass without the spliced channel being broadcast (I test this in regtest) regularly.

I've tested this a number of times, and I've gotten a number of unique failure modes.

I wish it were a bit cleaner and I could give you a better precise set of observations, but it's different each time I run the experiment.

Observations:

• the channel doesn't show up in the LND node even after an hour (and many hundreds of blocks), and upon restarting the CLN node the channel gets force closed because the CLN node doesn't know it spliced it
• the channel gets broadcast after ~1 hour (and hundreds of blocks)
• the channel doesn't get broadcast no matter how many times I restart all the involved nodes, but I can close it and open a new one

[litch]

Oh nice find, 846cec4 - I'll keep experimenting with this