lnwallet: respect local dust limit in cooperative close

[JssDWt]

Change Description

When the remote dustlimit is lower than the local one, a cooperative closure could create an output that doesn't respect the local dustlimit. By comparing the remote balance to the local dustlimit we're protected against creating invalid cooperative close transactions according to the local standards.

This requirement is part of the BOLTs: https://github.com/lightning/bolts/blob/5f8fea8dc3c8c612167dd9645c4a21fe9de2f147/03-transactions.md?plain=1#L373

Unfortunately this now leads to failure to close the channel. But that's better than crafting invalid transactions in my opinion.

Next steps will be

• allowing the remote peer to eliminate its own output (spec).
• potentially having a ruleset to in some circumstances eliminate the local output if it's small enough.

Steps to Test

• Create a channel with Alice and Bob
• Alice has dustlimit 10000
• Bob has dustlimit 2000
• Bob's balance is 2500
• Attempt to cooperatively close the channel
• Alice should not include Bob's output in the closing transaction

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[yyforyongyu]

It's probably better to have an agreed value on dust_limit_satoshis during the channel opening - the funder proposes dust_limit_satoshis via open_channel msg, and the fundee should set it to the same value in accept_channel msg. Otherwise we'd always end up using max(local_dust_limit, remote_dust_limit) since we'd also need to make sure ourBalance >= remoteDust?

[JssDWt]

Otherwise we'd always end up using max(local_dust_limit, remote_dust_limit) since we'd also need to make sure ourBalance >= remoteDust?

That's a decision you can make. To either fail the closing procedure, because you have in invalid signature according to the remote party, or to omit your own output. Probably a good reasoning to decide whether you should omit your own output is:

• is my output below the remote dust limit? If yes, would it cost me more if I would force close the channel?

For now, this PR only ensures you never create a transaction below your own dust limit in the first place. Deciding to omit your own output can be future improvements.

[Crypt-iQ]

The reason we haven't done this is because iirc each implementation does something slightly different. I'd defer work on this PR until we can all agree on the logic here. Additionally, changing this now will result in incompatibility with older LND nodes.

[JssDWt]

Additionally, changing this now will result in incompatibility with older LND nodes.

I agree there will be an incompatibility with older LND nodes. In fact, there will be an incompatibility between upgraded LND nodes too. If one party omits the peers output, but the peer doesn't, there's an incompatibility. But it also doesn't ever make sense to construct a closing transaction with an output below your own dustlimit. So the incompatibility already exists. This change just makes it more obvious.

[Crypt-iQ]

technically, the wording of the spec means this should be if theirBalance >= localDust. I think some clarification in the spec meeting would be helpful here. You're right an incompatibility exists even with this change which is why I think it's important to nail down the specifics and see if we can get rid of that incompatibility for coop close v1. I don't think a partial fix is worth doing. When coop close v2 is deployed (slated for 0.18.1 but may get pushed to 0.19), this logic is likely to get superseded but may be toggle-able at first.

[Roasbeef]

FWIW rbf-coop close will make this explicit, as the ender of a signature states which outputs is does/doesn't cover: #8453