json_sendpay() can pick the wrong local channel

[wtogami]

Version: pre-0.12 da0b651

Unexpected behavior: json_sendpay() can pick the wrong channel despite you specifying a particular local channel for firsthop. You expect json_sendpay() to be the low-level function that does exactly what you ask of it.

Currently send_payment_core() is used by both json_sendpay() RPC and also onion forwards where it uses the function find_channel_for_htlc_add() for non-strict forwarding as required by BOLT 4.

Christian Decker approved a plan where we pass a new boolean flag named strict_first to differentiate firsthop from onion forwarding.

nepet@e49f647
Draft patch v0 Feedback welcome!

• Currently missing: tests
• Maybe the implementation could be cleaner

Please confirm this logic:

• We want json_sendpay() to fail if the firsthop channel you specify is incapable. In this case we do not want it to pick a different channel. Right?

[wtogami]

#5505 fixed this in a different way.