the funding pubkey is allowed to vary in the splice, and we need to be involved in that. I'm wondering where that happens and how we will be informed of the remote one (and asked about the local one).

(note that the funding keys will be the same for all splice candidates)

we need to be involved in that

Are you saying that roundtrips/protocol is missing?

how we will be informed of the remote one

I think we are being informed with this call. Am I missing something?

funding pubkey is allowed to vary in the splice

https://github.com/lightning-signer/c-lightning/blob/2023-08-explore-splicing/channeld/channeld.c#L3536-L3539

That seems to imply it is not changing?

This is where we are telling the other side our key:

https://github.com/lightning-signer/c-lightning/blob/2023-08-explore-splicing/channeld/channeld.c#L3548-L3552

each of the prior is one side, presume there are similar in the other "direction"

the spec allows for it to change (for the whole splice, not per RBF candidate). we should ask the CLN team about this.

I just scoured the spec looking for where it is allowed to change ... can you point me to it?

https://github.com/lightning/bolts/pull/863/files#diff-ed04ca2c673fd6aabde69389511fa9ee60cb44d6b2ef6c88b549ffaa753d6afeR522

    type: 74 (splice)

    data:
        [chain_hash:chain_hash]
        [channel_id:channel_id]
        [u64:funding_satoshis]
        [u32:funding_feerate_perkw]
        [u32:locktime]
        [point:funding_pubkey]

    type: 76 (splice_ack)

    data:
        [chain_hash:chain_hash]
        [channel_id:channel_id]
        [u64:funding_satoshis]
        [point:funding_pubkey]

looks like CLN explicitly doesn't support changing them (they send an error to the peer if the key changes), but I assume that will be fixed before release. need to ask them.

the amounts are in peer->splicing->*_relative

So in this case the amount that the hsmd needs to know is the total amount, right?

We can calculate by the current funding tx amount + remote one.

I think we need to specify two amounts in setup_channel that the splice affects:

• the total channel amount, which is the size of the funding outpoint
• the push_val, which expresses how much of the channel value should be on the non-originating side.

I think this should be the current channel balance of the fundee (non-originator) + amount of additional sats added (signed, possible negative) by the fundee.

Does this seem correct?

That sounds good to me, just to rephrase in other words, the signer with the setup_channel needs to know the current total amount of the channel and then also
a relative amount (push_val) that we are adding with the splice.

This looks good to me.

I also noted that I made a typo in my previous message, the formula that I was suggesting is current funding tx amount + relative amount

actually, push_val is the counterparty's value in the channel. sorry, it's a bit confusing:

• holder (local) value is channel_val - push_val (let's call that holder_val for clarity)
• counterparty value is push_val (let's call that counterparty_val for clarify)
• these are absolute values
• to convert to deltas from previous values (before the current splice), you have to compute the difference for each side:
  • e.g. holder_delta = holder_val_current - holder_val_previous