lnwire: hard reject positive inbound fees set over RPC

[Roasbeef]

In #6703, we'll add initial support for inbound fees. Due to deployment considerations, only inbound fees should be used, as they're backwards compatible. An unupgraded sender will end up missing out on the discount, but the payment will go through if negative fees are used. If positive fees are used, then the forwarding node may reject a payment from an unupgraded sender. Additionally, in order for the sender to be able to properly retry, they'll also need the inbound fee policy in addition to the outbound fee policy. lnd isn't yet able to send both policies until #6967 is merged.

To ensure the base feature minimized disruption in the network, we should also modify the RPC code to reject positive inbound fees. The CLI code will already do this.

[joostjager]

The CLI update should indeed prevent an ad-hoc unintentional positive inbound fee from being set.

But for applications that are using the API and to which inbound fee support is added specifically, this safe guard is perhaps a bit too much hand-holding? The down side of the API restriction is that even if someone would want to try experimenting with positive inbound fees (perhaps they are confident that enough senders have upgraded to 0.18), they cannot do it and need to wait for and upgrade to a future version of lnd. Enabling experimentation was the main goal of the inbound fee change, so I'd argue to keep the option open on the API level.

#6967 isn't just relevant for positive inbound fees, but also for negative inbound fees that are made less negative.

[feelancer21]

If external tools have implemented incoming fees, unintentional use can also occur via the API. Imho this is also more likely as many users do their policy settings with tools like lndg or charge-lnd.

I suggest that positive inbound charges are disabled by default, but the user has the option to enable the experimental use via the configuration.

[Roasbeef]

If external tools have implemented incoming fees, unintentional use can also occur via the API. Imho this is also more likely as many users do their policy settings with tools like lndg or charge-lnd.

Yeah this captures my fear: users see the value, set it to somethign positive, then wonder why all forwards are being/failed rejected.

[joostjager]

I just meant to say that someone developing a 3rd party tool and using the api is probably more aware of the consequences of using positive inbound fees and will make a sensible decision how to offer the functionality to the tool's users. Where as with a casual cli user, the risk of making an unintentional change might be higher. Although setting a positive fee and seeing traffic go away isn't the end of the world either.

How about adding a bool flag to the api call allow_positive_inbound_fee, surrounded with warningful docs?

[feelancer21]

@joostjager I have already started Pull Request #8627. I think it is better to have an activation flag for the positive fees in lnd.conf. Then it is disabled per default and the users who want to play around with this feature can enable it.

[joostjager]

Ah didn't notice that one. Why do you think it is better?