> Instead, would you consider to use ANYONECANPAY to sign the tx, so it is
> possible add more inputs for fees? The total tx size is bigger than the
> OP_TRUE approach, but you don’t need to ask for any protocol change.

If one has a "root" commitment with other nested descendent
multi-transaction contracts, then changing the txid of the root commitment
will invalidated all the nested multi tx contracts. In our specific case, we
have pre-signed 2-stage HTLC transaction which rely on a stable txid. As a
result, we can't use the ANYONECANPAY approach atm.

> In long-term, I think the right way is to have a more flexible SIGHASH
> system to allow people to add more inputs and outputs easily.

Agreed, see the recent proposal to introduce SIGHASH_NOINPUT as a new
sighash type. IMO it presents an opportunity to introduce more flexible fine
grained sighash inclusion control.

-- Laolu


On Wed, May 9, 2018 at 11:12 AM Johnson Lau via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> You should make a “0 fee tx with exactly one OP_TRUE output” standard, but
> nothing else. This makes sure CPFP will always be needed, so the OP_TRUE
> output won’t pollute the UTXO set
>
> Instead, would you consider to use ANYONECANPAY to sign the tx, so it is
> possible add more inputs for fees? The total tx size is bigger than the
> OP_TRUE approach, but you don’t need to ask for any protocol change.
>
> In long-term, I think the right way is to have a more flexible SIGHASH
> system to allow people to add more inputs and outputs easily.
>
>
>
> > On 9 May 2018, at 7:57 AM, Rusty Russell via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
> >
> > Hi all,
> >
> >        The largest problem we are having today with the lightning
> > protocol is trying to predict future fees.  Eltoo solves this elegantly,
> > but meanwhile we would like to include a 546 satoshi OP_TRUE output in
> > commitment transactions so that we use minimal fees and then use CPFP
> > (which can't be done at the moment due to CSV delays on outputs).
> >
> > Unfortunately, we'd have to P2SH it at the moment as a raw 'OP_TRUE' is
> > non-standard.  Are there any reasons not to suggest such a policy
> > change?
> >
> > Thanks!
> > Rusty.
> > _______________________________________________
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> > bitcoin-dev@lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>
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