Lnd: Support invoices without amount specified.

[asocjal]

According to this:

https://github.com/lightningnetwork/lightning-rfc/blob/master/11-payment-encoding.md

amout is optional for payment invoice. Unfortunatelly lncli doesn't give option to create invoice without amout:

lncli addinvoice
[lncli] value argument missing

In lncli there is sendpayment method that gives possibility for sender to specify amount, but I don't see command that creates payment_hash without invoice.

How can I query for payment without amout, and send payment with amout specified by sender?

Also when trying to create invoice by RPC I got error message:

Caused by: io.grpc.StatusRuntimeException: UNKNOWN: zero value invoices are disallowed

[halseth]

Zero value invoices are currently not supported by the LN spec. The invoice format makes in optional to allow for this in the future :)

[halseth]

Correction: it is supported by the LN spec, but lnd does not currently support it.

[halseth]

Sorry, I misunderstood your initial description. Per the discussion on the mailing list, I believe this is a feature request? :)

[halseth]

Summary:

It is possible by the LN specification to pay invoices that has no specified amount, allowing "donation type" payments. lnd currently requires an invoice to specify the amount.

Steps to completion:

• [ ] Allow the creation of invoices with no amount.
• [ ] Handle incoming HTLCs paying an invoice without any amount.
• [ ] Allow payment using invoices with no amount, where the amount is specified at time of payment.

[wpaulino]

Since I'm working on #636, which covers some of the same code, I'll pick this up as well :)