Probably merits a more thorough response, but, I wanted to respond on the framework above:

 1a) can you make transactions using the new feature with bitcoin-cli,
     eg createrawtransaction etc? (YES)

since ~Feb 2020, this has existed: https://github.com/JeremyRubin/bitcoin/tree/checktemplateverify-feb1-workshop

CTV hasn't changed so this code should work un-rebased. The transaction outputs may need to be manually submitted to the network, but the covenant is enforced. This covers congestion control and vaults.

 1b) can you make transactions using the new feature with some other
     library? (YES)

Sapio, Test Framework, also https://min.sc/nextc/ produced independently by Shesek

 1c) can you make transactions using the new feature with most common
     libraries? (YES, kinda)

Yes, https://crates.io/crates/sapio-miniscript and https://crates.io/crates/sapio-bitcoin have been maintained for about 1 year, and are now taproot compatible.

Sapio's use of these libraries has even helped find bugs in the release process of Taproot for rust-bitcoin.

kinda: It's not _most_ common libraries, it's _a_ common library. it's also not upstreamed, because the patches would not be accepted were it to be.

 2) has anyone done a usable prototype of the major use cases of the new
    feature? (YES)

In addition to https://github.com/jamesob/simple-ctv-vault, there is also https://github.com/kanzure/python-vaults, although it has an interesting bug.

There's also a myriad of uses shown in https://github.com/sapio-lang/sapio/tree/master/sapio-contrib/src/contracts and in https://github.com/sapio-lang/sapio/tree/master/plugin-example. While these aren't quite "usable" as an end-to-end application, e.g., something you'd want to put real money on, they are a part of a *massive* infrastructure investment in general purpose smart contract tooling for covenant design with CTV. That CTV can be targeted with a compiler to generate a wide variety of composable use cases *is* one of the use cases for CTV, since it enables people to design many different types of thing relatively easily. That is a feature of CTV! It's not just for one use case.

The suite of Sapio apps are less "production ready" than they could be for a few reasons:

1) I've been working hard at pushing the limits of what is possible & the theory of it v.s. making it production ready

2) I prioritized supporting Taproot v.s. legacy script, and much of the taproot tooling isn't production ready

3) Sapio is really ambitious undertaking, and it will take time to make it production

That said, https://rubin.io/bitcoin/2022/03/22/sapio-studio-btc-dev-mtg-6/ tutorial was completed by people who weren't me, and at the pleb.fi/miami2022 one of the projects was able to use sapio congestion control transactions as well, so it does "work". As it matures, we'll get a number of implemented use cases people have been excited about like DLCs, which are implemented here https://github.com/sapio-lang/sapio/blob/master/sapio-contrib/src/contracts/derivatives/dlc.rs. You can see the test case shows how to construct one.

Why did I not focus on production grade? Well, production grade can always happen later, and I don't think it takes as much imagination. But the main critique I'd heard of CTV was that no one could see it being used for anything but one or two use cases. So I built Sapio, in part, to show how CTV could be used for an incredibly wide and diverse set of applications, as opposed to the polish on them.

If I knew the bar to surpass was to be polish, I probably could have taken a less ambitious approach with Sapio and shown like 1-2 applications working end-to-end. But because the main feedback I got was that CTV wasn't powerful enough, I opted to build a very general framework for covenants and demonstrate how CTV fits that.

--
@JeremyRubin

On Thu, Apr 21, 2022 at 12:05 AM Anthony Towns via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

On Wed, Apr 20, 2022 at 05:13:19PM +0000, Buck O Perley via bitcoin-dev wrote:
> All merits (or lack thereof depending on your view) of CTV aside, I find this topic around decision making both interesting and important. While I think I sympathize with the high level concern about making sure there are use cases, interest, and sufficient testing of a particular proposal before soft forking it into consensus code, it does feel like the attempt to attribute hard numbers in this way is somewhat arbitrary.

Sure. I included the numbers for falsifiability mostly -- so people
could easily check if my analysis was way off the mark.

> For example, I think it could be reasonable to paint the list of examples you provided where CTV has been used on signet in a positive light. 317 CTV spends “out in the wild” before there’s a known activation date is quite a lot

Not really? Once you can make one transaction, it's trivial to make
hundreds. It's more interesting to see if there's multiple wallets or
similar that support it; or if one wallet has a particularly compelling
use case.

> (more than taproot had afaik).

Yes; as I've said a few times now, I think we should have had more
real life demos before locking taproot's activation in. I think that
would have helped avoid bugs like Neutrino's [0] and made it easier for
hardware wallets etc to have support for taproot as soon as it was active,
without having to rush around adding library support at the last minute.

[0] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-November/019589.html

Lightning's "two independent implementations" rule might be worth aspiring
too, eg.

> If we don’t think it is enough, then what number of unique spends and use cases should we expect to see of a new proposal before it’s been sufficiently tested?

I don't really think that's the metric. I'd go for something more like:

 1a) can you make transactions using the new feature with bitcoin-cli,
     eg createrawtransaction etc?
 1b) can you make transactions using the new feature with some other
     library?
 1c) can you make transactions using the new feature with most common
     libraries?

 2) has anyone done a usable prototype of the major use cases of the new
    feature?

I think the answers for CTV are:

 1a) no
 1b) yes, core's python test suite, sapio
 1c) no
 2) no

Though presumably jamesob's simple ctv vault is close to being an answer
for (2)?

For taproot, we had,

 1a) yes, with difficulty [1]
 1b) yes, core's python test suite; kalle's btcdeb sometimes worked too
 1c) no
 2) optech's python notebook [2] from it's taproot workshops had demos for
    musig and degrading multisig via multiple merkle paths, though I
    think they were out of date with the taproot spec for a while

[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-October/019543.html
[2] https://github.com/bitcoinops/taproot-workshop/

To some extent those things are really proxies for:

 3) how well do people actually understand the feature?

 4) are we sure the tradeoffs being made in this implementation of the
    feature, vs other implementations or other features actually make
    sense?

 5) how useful is the feature?

I think we were pretty confident in the answers for those questions
for taproot. At least personally, I'm still not super confident in
the answers for CTV. In particular:

 - is there really any benefit to doing it as a NOP vs a taproot-only
   opcode like TXHASH? Theoretically, sure, that saves some bytes; but as
   was pointed out on #bitcoin-wizards the other day, you can't express
   those outputs as an address, which makes them not very interoperable,
   and if they're not interoperable between, say, an exchange and its
   users trying to do a withdraw, how useful is that really ever going
   to be?

 - the scriptSig commitments seems very kludgy; combining multiple
   inputs likewise seems kludgy

The continual push to rush activation of it certainly doesn't increase my
confidence either. Personally, I suspect it's counterproductive; better
to spend the time answering questions and improving the proposal, rather
than spending time going around in circles about activating something
people aren't (essentially) unanimously confident about.

> In absence of the above, the risk of a constantly moving bar

I'd argue the bar *should* be constantly moving, in the sense that we
should keep raising it.

> To use your meme, miners know precisely what they’re mining for and what a metric of success looks like which makes the risk/costs of attempting the PoW worth it

The difference between mining and R&D is variance: if you're competing for
50k blocks a year, you can get your actual returns to closely match your
expected return, especially if you pool with others so your probability
of success isn't miniscule -- for consensus dev, you can reasonably only
work on a couple of projects a year, so your median return is likely $0,
rather than a close match to your average/expected return.

> We also have new ideas that only started coming up after Taproot activation (TLUV and Taro for example), so there’s also the unknown of what we could have once it becomes clear that it’s worth devoting mental energy and financial resources towards research.

TLUV was an offshoot of SCRIPTREPLACE which was public (though not
really published) since 2019.

> One last wrinkle with regards to using countable metrics to determine a feature’s “worth” is that not all features are the same. Many of the use cases that people are excited to use CTV for ([5], [6]) are very long term in nature and targeted for long term store of value in contrast to medium of exchange.

I mean, if those use cases are so exciting, it really doesn't seem much
to ask to see them demoed live on the CTV signet that already exists?

> You can build a CTV vault in signet, but you’ll only really see a lot of people using it when it’s to store real value on a time scale measured in decades not minutes or days

On the other hand, if the value is really "very long term" and there's no
rush to implement these features and demo them ASAP, then it doesn't seem
like there should be a rush to adapt consensus to these use cases either.
Why not wait until someone does have time to finish sketching out the
use case so they can demo them in public?

> To put another way and leave CTV out of it completely, what should an outside, unbiased observer that doesn’t spend much time on Twitter expect to be able to see to evaluate the readiness or acceptability of ANYPREVOUT, TLUV,

For ANYPREVOUT, I would like to see a toy implementation of eltoo using
it, that can handle fees and layered transactions (or has a good argument
why layered transactions aren't necessary). It's going to take a while
even to update LN to taproot and PTLCs though, so eltoo doesn't seem like
it's on the immediate horizon. Besides eltoo, I don't think ANYPREVOUT
is an optimal design for covenants, so if that was the motivation and
not eltoo, maybe some other approach would be better.

TLUV's design parameters don't really seem optimal (the mess with x-only
pubkeys, alternatives like OP_EVICT), so I think it's still on the
whiteboard.

Cheers,
aj

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