How to find compromise in restarted fee negotiation (coop close)?

[C-Otto]

Background

If both nodes run lnd 0.15+, it is possible to retry a failed fee negotiation (as part of a cooperative channel close). If one node is running an older version of lnd, the channel has to be force-closed.

However, in my observation this doesn't help as long as both peers stick to the same values: my node still suggests numbers far below my peer's suggestions, and after a few attempts the negotiation fails.

How can I tell my lnd (without restart!) to accept/suggest higher fee rates for a specific peer/channel?

See lightninglabs/loop#516 for an example of this.

Your environment

• version of lnd: lnd v0.15.0-beta with a few patches from master (and some of my own)
• which operating system (uname -a on *Nix): Linux server 5.10.0-13-amd64 #1 SMP Debian 5.10.106-1 (2022-03-17) x86_64 GNU/Linux
• version of btcd, bitcoind, or other backend: bitcoind v23

Steps to reproduce

Configure very low fee rates (bitcoind.estimatemode=ECONOMICAL, coop-close-target-confs=1000). Open channel to LOOP. Let LOOP coop-close channel.

Expected behaviour

Nodes agree on fee rate, especially after manual intervention on my side.

Actual behaviour

Fee rate negotiation fails repeatedly.

[Roasbeef]

This'll be fixed by #6770

Long story short: we went to catch up w/ co-op close semantics in the spec, rolled that out, realized that caused issues with co-op close in the wild (we went to no longer accept fees above the commitment fee, but w/ anchors that can be very low), realized that requirement was removed from the spec, now have a fix to roll back our prior change and make things more flexible.

[Roasbeef]

Fee negotiation isn't super well defined in the spec (basically just says agree on a fee). By 0.16 we'll have the fee range feature implemented, and for taproot we might actually just try to re-do things all together (round based, only send a sig at the very end, etc).