Subject: Re: [Bitcoin-development] BIP70: PaymentACK semantics

From: Peter Todd <[email-masked]>
Date: Tue, 28 Jan 2014 12:23:49 -0500

On Tue, Jan 28, 2014 at 07:53:14AM -0500, Gavin Andresen wrote:
> On Tue, Jan 28, 2014 at 6:42 AM, Mike Hearn <[email-masked]> wrote:
> 
> > Yeah, that's the interpretation I think we should go with for now. There
> > was a reason why this isn't specified and I forgot what it was - some
> > inability to come to agreement on when to broadcast vs when to submit via
> > HTTP, I think.
> >
> 
> If the wallet software is doing automatic CoinJoin (for example), then
> typically one or several of the other participants will broadcast the
> transaction as soon as it is complete.
> 
> If the spec said that wallets must not broadcast until they receive a
> PaymentACK (if a payment_url is specified), then you'd have to violate the
> spec to do CoinJoin.
> 
> And even if you don't care about CoinJoin, not broadcasting the transaction
> as soon as the inputs are signed adds implementation complexity (should you
> retry if payment_url is unavailable? how many times? if you eventually
> unlock the probably-not-quite-spent-yet inputs, should you double-spend
> them to yourself just in case the merchant eventually gets around to
> broadcasting the transaction, or should you just unlock them and squirrel
> away the failed Payment so if the merchant does eventually broadcast you
> have a record of why the coins were spent).

Also users don't have infinite unspent txouts in their wallets - if they
need to make two payments in a row and run out their wallet software is
(currently) going to spend the change txout and either be forced to
broadcast both transactions anyway, or the second payment-protocol-using
recipient will do so on their behalf. (in the future they might also do
a replacement tx replacing the first with a single tx paying both to
save on fees, again with the same problem)

Anyway what you want is payment atomicity: the customer losing control
of the funds must be atomic with respect to the payment going through.
From that point of view it's unfortunate that Payment message contains
refund_to, memo, etc. That information should have been provided to the
merchant prior to them providing the list of addresses to pay.

-- 
'peter'[:-1]@petertodd.org
000000000000000085c725a905444d271c56fdee4e4ec7f27bdb2e777c872925