Ability to add custom TLV to update_add_htlc message about lightning HOT 1 OPEN

I tried adding the extra_fee record to the returned payload in the htlc_accepted hook, but I get an error: Failing HTLC because of an invalid payload. I assume this is because the TLV is not recognized.

That is expected, since the payload is the TLV stream for the local node. As such lightningd will fail the HTLC if the payload contains any field that is not understood by lightningd. The onion would contain the payload for the next hop (but is unmodifiable due to the encrypted and HMACd nature of the onion). There is currently no way return value that allows adding fields to the TLVs in the update_add_htlc because it was not used by anyone so far.

I will add this new return field to the htlc_accepted hook call, and wire it through to the appropriate place.

from lightning.