[bitcoin-dev] Libconsensus separated repository (was Bitcoin Core and hard forks)

Jorge Timón jtimon at jtimon.cc
Tue Jul 28 09:58:33 UTC 2015

Ok, I'm going to separate terms: current-libconsensus from theoretical
future-libconsensus (implementing ALL consensus rules).

On Tue, Jul 28, 2015 at 8:40 AM, Eric Voskuil <eric at voskuil.org> wrote:
> libsecp256k1 has it's own repository, libbitcoinconsensus doesn't.  A
> separate repository was what I considered as a requirement for us to use it.

We want to complete future-libconsensus (decouple all the consensus
rules from the rest of the bitcoin core code) first.
Then we can move future-libconsensus to a subrepository/subtree like
libsecp256k1 and I believe everybody wants this to eventually happen.
Separating current-libconsensus now may make completing
future-libconsensus harder.

>> I'm not sure if that would ever be accepted, but in any case we're
>> certainly far away from that goal.
> If it's not certain whether this would even be accepted, the commitment
> to a community consensus library is too weak to take a strong dependency
> on. But for us it's moot, as we have made the already accomplished that
> goal.

What I mean is that once it is separated to a subtree, there's one more step:

Make Bitcoin Core use future-libconsensus' API instead of a subtree.
Decoupling future-libconsensus from Bitcoin Core is one thing, and
Decoupling Bitcoin Core from future-libconsensus is another thing: you
need to decouple Bitcoin Core from all future-libconsensus
implementation internals. For example, script/sign (part of Bitcoin
Core) depends on individual non-API-exposed classes in
Moving from a subtree to a completely separated library is what I
don't know will ever happen, but I don't think this is "unfairly
advantageous" for Bitcoin Core or anything like that: other
implementations can also use future-libcosensus as a subtree instead
of the C API as well.

You have accomplished the goal of separating curren-libconsensus, not
In fact, if you complete the equivalent of future-libconsensus in
libbitcoin and separate that, maybe that's a better place to start
drafting a full API.

>> 2) Finish libconsensus's API: expose more things than VerifyScript, at
>> the very least, also expose VerifyTx, VerifyHeader and VerifyBlock.
>> Feedback from alternative implementations like libbitcoin is extremely
>> valuable here. Some related closed-for-now PRs:
> In our earlier discussion I believe you said that the library would not
> be undergoing significant change or feature creep. If this is the very
> least that's projected it would seem that constraint will not hold.

future-libconsensus will not have significant changes *once it is
completed*. Currently future-libconsensus is spread around many places
inclusing src/main, so that obviously needs to change before it can be
separated to an independent repo.

>> 3) Move libconsensus to a separate repository as a
>> subtree/subrepository of Bitcoin Core.
>> Only after all that we can discuss whether Bitcoin Core itself should
>> include libconsensus' code or just use its API directly.
> I don't think it's a question of whether it *should* use its own library
> as it is published for others - this is a practically self-evident
> conclusion.

Well, Bitcoin Core is "currently the only user of future-libconsensus"
since bitcoin core and future-libconsensus are currently mutually
Bitcoin Core will always keep using future-libconsensus. The only
question is whether it will use it through the C API or as a
subtree/subrepository (both options are also available to other
implementations). I don't know if decoupling Bitcoin Core from
future-libconsensus' implementation details enough to be able to
directly use the API is worth it or if anyone will be interested in
doing so. For me this last step is not all that interesting: if we
have an independent repo with a full API that other implementations
can use, I don't really care about Bitcoin Core not going through the
API and using including all the code directly instead.

>> I hope that after all this, libbitcoin also reconsiders whether to
>> reimplement its own libconsensus or use the "official" one directly
>> instead.
> We use a fork of libsecp256k1 and would probably do the same with the
> consensus library if it was cleanly isolated.


>>> In any case I agree with your stated need for this isolation (if not the
>>> means) for the reasons you state. The community needs to move beyond a
>>> largely singular and monolithic codebase that is holding that position
>>> in part due to fear about consensus bug forks.
>> I completely agree. That's the goal of libconsensus (and an
>> alternative implementation like libbitcoin being able to use it
>> without sacrificing any of its current or future design differences
>> from Bitcoin Core would be a sign of success in this reward).
> It's a performance sacrifice, and then there's the OpenSSL dependency,
> but these are both optional within our stack - so the application
> developer has the option. So the only downside is that we are
> maintaining the conditional compilation.

As I told you before the reason why current-libconsensus is using
OpenSSL instead of libsecp256k1 is that the very authors of
libsecp256k1 warned that using libsecp256k1 for validation was
consensus risky. As Wladimir said, Pieter Wuille will make an
announcement about this soon.
In any case, as I told you in previous conversations, the plan is to
move from OpenSSL to libsecp256k1 for validation too (so libconsensus
wil drop the OpenSSL dependency and this is just a temporary concern).

> I think it's worthwhile work, especially if you are passionate about the
> longer term objectives. I haven't been involved in these reviews as I
> spend very little time with the satoshi client

Well, the questions about the API are just in english, no need to
deeply know Bitcoin Core's (satoshi client) internals.
But maybe we should have an independent mailing list for
consensus-only things. Not all future-libconsensus users will be
interested in Bitcoin Core-specific discussions, and making them
subscribe and filter seems like an unnecessary burden to

> Well a cynic might observe that fear of consensus bugs is what keeps
> people on the satoshi client, and therefore accelerating the development
> of a clean and independent consensus library would be a very low priority.

I think that's precisely what makes it a high priority in the eyes of
all the people working on it or reviewing related changes.
But, yes, I guess "evil-thinking", maybe that's what make it a low
priority for someone evil that wants Bitcoin Core's implementation
have more importance than it shold forever. I prefer not to evil-think
and just attribute it to having other priorities or just apathy about

>> Another unfortunate fact is that although a script-only libconsensus
>> allows you to avoid a big part of all possible consensus fork bugs,
>> there cannot be users of a finished libconsensus to ask things to until
>> a finished libconsensus actually exists.
> Software is never finished, but this exists and we are using it.

By "finished" I mean a future-libconsensus that implements ALL
consensus rules. We don't have that yet.

>> At the same time, the future
>> users (alternative implementations, since bitcoin core is already
>> "using libconsensus")
> It is using the same source files, but AFAICT not the library.

Bitcoin Core is the ONLY "user" of future-libconsensus (which actually
only exists inside Bitcoin Core and it's not exposed).
Current-libconsensus is used by Bitcoin Core and also exposed as an
independent build (not a separated repository yet).

Once future-Bitcoin's API is completed and the code in a different
repo, how is Bitcoin Core using the API instead of the sources
directly of any importance to other implementations?
That's really the part that I cannot understand. It will be a problem
Bitcoin Core, but if other implementations want to have (and maybe
solve later) the same problem they can use a subtree too and start
coupling their code with implementations details from
Why would they want to do that? Again, I have no idea. I don't
understand what the complain is here.

>> are the most relevant people to listen when it
>> comes to the C API. That's why I beg you to comment on [2], even if
>> #5995 is currently closed. Your input on [1] would be very appreciated
>> as well (maybe you think it's better to expose verifyTx before
>> exposing verifyHeader, even if exposing verifyHeader is something that
>> could be done faster).
> I haven't looked at any of these commits, but I'll make some time to at
> least give a cursory review.

Great. I mean, I wasn't asking about reviewing the commits themselves
(which is also great if you do), but rather on answering the questions
I'm making there, ie: what to expose next (ie VerifyTx or
VerifyHeader)? would this be an acceptable way to expose VerifyHeader
? Which of he step-checks functions is worth exposing too (Bitcoin
Core is currently using some to prevent DoS attacks, for example)?

>> Would you agree that asking people to fork an independent libconsensus
>> project instead of having to fork the full Bitcoin-qt is much more
>> reasonable?
> Yes, of course. We've already done it. For each release of the satoshi
> client since we made libbitcoin-consensus I've copied the sources. It's
> pretty much automated and easy to visually verify that the sources
> match. That would be quite a bit more difficult if there wasn't an
> independent build.

Well, neither libbitcoincosnensus nor libbitcoin-consensus implements
all the consensus rules.
That's what makes them different from future-libconsensus.
But great, we're confirming more views that we share.

> Useful specifications often have two reference implementations. It's the
> idea that there can be only one legitimate implementation that's
> problematic.

Well, this is where I fear we will never agree. I think "Bitcoin is
different" in this reward and you disagree.
Maybe Pieter's explanation is more convincing to you:
Otherwise, I think I'll stop trying convincing you.
But one last try:

If there's 2 "specification implementations":

1) widely deployed but containing 1 bug
2) not deployed anywhere but more readable and without the bug.

When the bug is found, is it a consensus rule or not?
If it is, it turns out the second implementation wasn't an specification at all.
If it's not, nobody has been ever following consensus rules!!

>> I believe that's the only point where we fundamentally disagree, but
>> it shouldn't be a barrier in our common goal of taking "power" away
>> from Bitcoin Core development. If we're successful Bitcoin Core won't
>> have any privileged position with regards to, say, libbitcoin when it
>> comes to deciding consensus rules changes.
> I don't think we disagree on anything fundamental. My issues with the
> library were (1) the lack of isolation, (2) the fact that the satoshi
> client wouldn't actually use the library, and (3) backtracking to use
> OpenSSL, which we had recently removed from libbitcoin.

1) Working on it
2) The Satohi client has been using all along and it will use it
forever (maybe not through the API, but I don't get what the problem
with that is).
3) There will be an announce about this soon.

>> Libbitcoin cannot be forced to accept the next, controversial version
>> of bitcoin/libconsensus, so you guys fork libbitcoin/libconsensus out
>> of the last ok version.
> This is already done.

Once future-libconsensus exists as a separated repository, I don't
think you want to preemptively fork it unless you're actually changing

> I don't disagree, but you previously argued that *everyone* had to agree
> on consensus. Above you are making the argument that people can choose
> to disagree. Yes, this is important. Yet it's unrealistic for any
> alternative consensus to overcome inertia unless there are widely
> deployed independent implementations.

Ideally schism consensus-rules changes will never happen, since they
effectively divide the currency in 2 and force the users to chose
(apart from other complications).
But if someone is trying to impose a schism fork (from Bitcoin Core,
from the new future-lbiconsensus repo or from anywhere else) it
becomes trivial to protect your implementation against forced changes.
Ideally all consensus changes will be uncontroversial or not happen at all.
But I think about non-ideal cases too.

> I'm maintain our fork of the consensus sources until they are (1)
> properly isolated from the satoshi client and (2) the satoshi client
> uses the actual library.

1) Reasonable.
2) You mean use it through the API? Seriously, why do you care?

> I don't see it as important or even productive to try and wedge the
> consensus library into the repository/build for bitcoind and Bitcoin-QT.
> It's straightforward to maintain the consensus library independently.

To be clear, Bitcoin Core's using future-libconsensus through a
subtree instead of the API is not a preference or a goal: it is just
how things will be just after completing and separating
future-libconsensus. Making it use the API instead of the subtree will
be additional work. I'm not sure I will want/have time to do it and I
don't know of anyone planning to do that (which seems very reasonably
given that a separated future-libconsensus is a dependency for such a

> Always willing to work with you on it, although we're all busy, and this
> isn't my top priority presently.

Is it because "fear of consensus bugs is what keeps people on the
satoshi client" and you want to keep things this way?
Sorry, just joking about your previous "cynic" comment. See? Don't
attribute to malice what can be attributed to lack of time.

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