[c-lightning] Towards atomic multipath payments

[Rusty Russell]

ZmnSCPxj <ZmnSCPxj at protonmail.com> writes:
> Good morning c-lightningers,
>
> I would like to propose a JSON-RPC interface for low-level atomic multipath payments.

Sorry I've been MIA on this thread, but I was in the middle of moving
`pay` out to a plugin.

A result of that is a new `paystatus` command which details all payment
attempts in full detail (not yet persistent across restarts).

I agree that we need to change the APIs, but I think we should do it
differently: have the pay plugin provide `listpays` which merges
`listpayments` back into whole payments.

To do AMP, I think we should make sendpay and listpayments AMP-aware.

1. Add a 'total_amount' and 'attempt_id' optional fields to
   sendpay. Defaults are final amount and 0.
2. Change "if we have a concurrent payment, refuse another" to "if total
   of payments in flight >= total_amount, refuse".
3. payments table is now indexed by payment_hash, attempt_id.

It's up to the pay plugin to merge these back together for the higher
level users.

Cheers,
Rusty.