Unintuitive treatment of min-capacity-sat config

[whitslack]

Issue and Steps to Reproduce

This is not exactly a bug since min-capacity-sat does appear to be functioning as described in the documentation. Rather, this is more of a complaint about a quality-of-life issue. Feel free to close this report unceremoniously if you disagree.

I was trying to participate in a triangle liquidity swap on LightningNetwork+. The swap contract was for 1 Msat of liquidity. My node's min-capacity-sat is set to 1000000, so I thought I would be fine. However, my peer was unable to open a channel to me because min-capacity-sat is actually defined as the minimal effective channel capacity to accept.

Options for resolving this issue:

• Add a complementary config option that bounds the nominal channel capacity. Default: unbounded. An incoming proposed channel would need to satisfy both bounds in order to be accepted.
• Change the interpretation of min-capacity-sat so that it bounds the nominal channel capacity rather than the effective channel capacity. This would be a breaking change and would eliminate a potentially useful safeguard, so I am not advocating for it, but I am mentioning it for the sake of completeness.
• Do nothing, and dismiss my complaint as ravings of an idiot user.

I am willing to author a PR to implement the first option if that's agreeable.

getinfo output

This is with CLN 24.08.2.

[rustyrussell]

First: I want to thank you for this issue! Not enough people file "I tripped on this" bug reports, but they're incredibly valuable for QoL issues! ❤️❤️❤️

The existence of this option is a compromise, itself. You can totally have a plug-in make calls on what channels to allow, based on arbitrary criteria. I would love to see a plug-in which allowed these kind of fine-grained controls, in the plugins repo.

Also, this config option should be dynamic.

[whitslack]

Also, this config option should be dynamic.

I believe it is dynamic, as my trying to change it via setconfig crashed CLN due to #7964.