Subject: Re: [bitcoin-dev] On the regularity of soft forks

From: Keagan McClelland <[email-masked]>
Date: Thu, 30 Dec 2021 20:10:48 -0700

>  But whether or not it is a basic principle of general software
engineering kind of misses the point. Security critical software clearly
isn't engineered in the same way as a new social media app. Bugs are easily
reverted in a new social media app.On top of that we aren't just dealing
with security critical software. One of the most important objectives is to
keep all the nodes on the network in consensus. Introducing a consensus
change before we are comfortable there is community consensus for it is a
massive effective bug in itself. The network can split in multiple ways
e.g. part of the network disagrees on whether to activate the consensus
change, part of the network disagrees on how to resist that consensus
change, part of the network disagrees on how to activate that consensus
change etc

>  A consensus change is extremely hard to revert and probably requires a
hard fork, a level of central coordination we generally attempt to avoid
and a speed of deployment that we also attempt to avoid.

This seems to assert the idea that soft forks are all the same: they are
not. For instance a soft fork, lowering the block subsidy is completely
different than changing the semantics of an OP_NOP to have semantics that
may reject a subset of the witnesses that attest to the transactions
permissibility. As a result, reversion means two entirely different things
in these contexts. While a strict reversion of both soft forks is by
definition a hard fork, the requirement of reversion as a result of
undesired behavior is not the same. In the case of opcodes, there is almost
never a requirement to revert it. If you don't like the way the opcodes
behave, then you just don't use them. If you don't like the reduction of
the block subsidy, well that's a much bigger problem.

I make this point to elucidate the idea that we cannot treat SoftForks™ as
a single monolithic idea. Perhaps we need to come up with better
terminology to be specific about what each fork actually is. The soft vs.
hard distinction is a critical one but it is not enough and treating soft
forks that are noninvasive such as OP_NOP tightenings. This has been
proposed before [1], and while I do not necessarily think the terms cited
are necessarily complete, they admit the low resolution of our current
terminology.

> Soft fork features can (and should) obviously be tested thoroughly on
testnet, signet, custom signets, sidechains etc on a standalone basis and a
bundled basis.

I vehemently disagree that any consensus changes should be bundled,
especially when it comes to activation parameters. When we start to bundle
things, we amplify the community resources needed to do review, not reduce
them. I suspect your opinion here is largely informed by your frustration
with the Taproot Activation procedure that you underwent earlier this year.
This is understandable. However, let me present the alternative case. If we
start to bundle features, the review of the features gets significantly
harder. As the Bitcoin project scales, the ability of any one developer to
understand the entire codebase declines. Bundling changes reduces the
number of people who are qualified to review a particular proposal, and
even worse, intimidates people who may be willing and able to review
logically distinct portions of the proposal, resulting in lower amounts of
review overall. This will likely have the opposite effect of what you seem
to desire. BIP8 and BIP9 give us the ability to have multiple independent
soft forks in flight at once. Choosing to bundle them instead makes little
sense when we do not have to. Bundling them will inevitably degenerate into
political horse trading and everyone will be worse off for it.

> part of the network disagrees on whether to activate the consensus
change, part of the network disagrees on how to resist that consensus
change, part of the network disagrees on how to activate that consensus
change etc

Disagreements, and by extension, forks are a part of Bitcoin. What is
important is that they are well defined and clean. This is the reason why
the mandatory signaling period exists in BIP8/9, so that clients that
intend to reject the soft fork change have a very easy means of doing so in
a clean break where consensus is clearly divergent. In accordance with
this, consensus changes should be sequenced so that people can decide which
sides of the forks they want to follow and that the economic reality can
reorganize around that. If choose to bundle them, you have one of two
outcomes: either consensus atomizes into a mist where people have different
ideas of which subsets of a soft fork bundle they want to adopt, or what
likely comes after is a reconvergence on the old client with none of the
soft fork rules in place. This will lead to significantly more confusion as
well given that with sufficient miner consensus some of the rules may stick
anyway even if the rest of the user base reconverges on the old client.

It is quite likely less damaging to consensus to have frequent but strictly
sequenced soft forks so that if one of the new rules is contentious the
break can happen cleanly. That said, if Core or any other client wishes to
cut a release of the software with the parameters bundled into a single
release, that is a significantly more palatable state of affairs, as you
can still pipeline signaling and activation. However, the protocol itself
adopting a tendency to activate unrelated proposals in bundles is a recipe
for disaster.

Respectfully,
Keagan

[1] https://www.truthcoin.info/blog/protocol-upgrade-terminology

On Sat, Oct 16, 2021 at 12:57 PM Michael Folkson via bitcoin-dev <
[email-masked]> wrote:

> > Interesting discussion. Correct me if I'm wrong: but putting too many
> features together in one shot just can't make things harder to debug in
> production if something very unexpected happens. It's a basic principle
> of software engineering.
>
> Soft fork features can (and should) obviously be tested thoroughly on
> testnet, signet, custom signets, sidechains etc on a standalone basis and a
> bundled basis. But whether or not it is a basic principle of general
> software engineering kind of misses the point. Security critical software
> clearly isn't engineered in the same way as a new social media app. Bugs
> are easily reverted in a new social media app. A consensus change is
> extremely hard to revert and probably requires a hard fork, a level of
> central coordination we generally attempt to avoid and a speed of
> deployment that we also attempt to avoid. On top of that we aren't just
> dealing with security critical software. One of the most important
> objectives is to keep all the nodes on the network in consensus.
> Introducing a consensus change before we are comfortable there is community
> consensus for it is a massive effective bug in itself. The network can
> split in multiple ways e.g. part of the network disagrees on whether to
> activate the consensus change, part of the network disagrees on how to
> resist that consensus change, part of the network disagrees on how to
> activate that consensus change etc
>
> In addition, a social media app can experiment in production whether
> Feature A works, whether Feature B works or whether Feature A and B work
> best together. In Bitcoin if we activate consensus Feature A, later decide
> we want consensus Feature B but find out that by previously activating
> Feature A we can't have Feature B (it is now unsafe to activate it) or its
> design now has to be suboptimal because we have to ensure it can safely
> work in the presence of Feature A we have made a mistake by activating
> Feature A in the first place. Decentralized security critical consensus
> changes are an emerging field in itself and really can't be treated like
> any other software project. This will become universally understood I'm
> sure over time.
>
> --Michael Folkson
> Email: michaelfolkson at protonmail.com
> Keybase: michaelfolkson
> PGP: 43ED C999 9F85 1D40 EAF4 9835 92D6 0159 214C FEE3
>
>
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Friday, October 15th, 2021 at 1:43 AM, Felipe Micaroni Lalli via
> bitcoin-dev <[email-masked]> wrote:
>
> Interesting discussion. Correct me if I'm wrong: but putting too many
> features together in one shot just can't make things harder to debug in
> production if something very unexpected happens. It's a basic principle
> of software engineering.
>
> Change. Deploy. Nothing bad happened? Change it a little more. Deployment.
> Or: Change, change, change. Deploy. Did something bad happen? What change
> caused the problem?
>
> On Thu, Oct 14, 2021 at 8:53 PM Anthony Towns via bitcoin-dev <
> [email-masked]> wrote:
>
>> On Mon, Oct 11, 2021 at 12:12:58PM -0700, Jeremy via bitcoin-dev wrote:
>> > > ... in this post I will argue against frequent soft forks with a
>> single or
>> > minimal
>> > > set of features and instead argue for infrequent soft forks with
>> batches
>> > > of features.
>> > I think this type of development has been discussed in the past and has
>> been
>> > rejected.
>>
>> > AJ: - improvements: changes might not make everyone better off, but we
>> >    don't want changes to screw anyone over either -- pareto
>> >    improvements in economics, "first, do no harm", etc. (if we get this
>> >    right, there's no need to make compromises and bundle multiple
>> >    flawed proposals so that everyone's an equal mix of happy and
>> >    miserable)
>>
>> I don't think your conclusion above matches my opinion, for what it's
>> worth.
>>
>> If you've got two features, A and B, where the game theory is:
>>
>>  If A happens, I'm +100, You're -50
>>  If B happens, I'm -50, You're +100
>>
>> then even though A+B is +50, +50, then I do think the answer should
>> generally be "think harder and come up with better proposals" rather than
>> "implement A+B as a bundle that makes us both +50".
>>
>> _But_ if the two features are more like:
>>
>>   If C happens, I'm +100, You're +/- 0
>>   If D happens, I'm +/- 0, You're +100
>>
>> then I don't have a problem with bundling them together as a single
>> simultaneous activation of both C and D.
>>
>> Also, you can have situations where things are better together,
>> that is:
>>
>>   If E happens, we're both at +100
>>   If F happens, we're both at +50
>>   If E+F both happen, we're both at +9000
>>
>> In general, I think combining proposals when the combination is better
>> than the individual proposals were is obviously good; and combining
>> related proposals into a single activation can be good if it is easier
>> to think about the ideas as a set.
>>
>> It's only when you'd be rejecting the proposal on its own merits that
>> I think combining it with others is a bad idea in principle.
>>
>> For specific examples, we bundled schnorr, Taproot, MAST, OP_SUCCESSx
>> and CHECKSIGADD together because they do have synergies like that; we
>> didn't bundle ANYPREVOUT and graftroot despite the potential synergies
>> because those features needed substantially more study.
>>
>> The nulldummy soft-fork (bip 147) was deployed concurrently with
>> the segwit soft-fork (bip 141, 143), but I don't think there was any
>> particular synergy or need for those things to be combined, it just
>> reduced the overhead of two sets of activation signalling to one.
>>
>> Note that the implementation code for nulldummy had already been merged
>> and were applied as relay policy well before activation parameters were
>> defined (May 2014 via PR#3843 vs Sep 2016 for PR#8636) let alone becoming
>> an active soft fork.
>>
>> Cheers,
>> aj
>>
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