API feature request: return `feesPaid` in `payinvoice`

[callebtc]

Hi and thank you for building this human-friendly software.

We're developing against Eclair and I would appreciate if the payinvoice endpoint would return feesPaid.

Right now, we need to call getsentinfo right after payinvoice just to get this additional piece of information. Please let me know if my approach is wrong.

[t-bast]

It actually can return this information, depending on how you call it.

By default, payinvoice is fully asynchronous and returns as soon as HTLCs are sent out, before they are fulfilled, so it's impossible to return this information because we don't know it yet.

But there is a --blocking=true parameter which changes this behavior, and blocks until the payment is complete. If you use this parameter and the payment succeeds, the responses should contains feesPaid. But there is a trade-off: if the payment is stuck, you won't get a response and the CLI will be stuck waiting for 30 seconds (our default timeout). Also, if MPP is used, we may return a response as soon as we get the preimage from one of the HTLCs, which won't contain feesPaid, but that is something we can fix if you'd like.