> On 10 May 2018, at 3:27 AM, Peter Todd <p...@petertodd.org> wrote:
> On Thu, May 10, 2018 at 01:56:46AM +0800, Johnson Lau via bitcoin-dev 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.
> I don't think that will work, as a zero-fee tx won't get relayed even with
> CPFP, due to the fact that we haven't yet implemented package-based tx
> relaying.
> -- 
> https://petertodd.org 'peter'[:-1]@petertodd.org

My only concern is UTXO pollution. There could be a “CPFP anchor” softfork that 
outputs with empty scriptPubKey and 0 value are spendable only in the same 
block. If not spent immediately, they become invalid and are removed from UTXO. 
But I still think the best solution is a more flexible SIGHASH system, which 
doesn’t need CPFP at all.
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