[bitcoin-dev] Libconsensus phase 2

Eric Voskuil eric at voskuil.org
Tue Jan 12 19:17:20 UTC 2016

Jorge, first, thanks again for your work on this.

Without creating and using a public blockchain interface in phase 2, how
will you isolate the database dependency from consensus critical code?
Is it that the interface will exist but you will recommend against its use?

This work presumes that the users of the library reject the argument
that the database implementation is consensus critical code. Faithful
reproduction of stored data is a prerequisite for a validity. But a
common store implementation is only slightly more reasonable for this
library than a common RAM implementation.


On 01/12/2016 09:53 AM, Jorge Timón wrote:
> After talking to some people about libconsensus in the last Hong Kong
> conference I realized that my initial plan of exposing one more thing
> at a time would actually probably slow things down.
> There's still a promised pdf with pictures that will be released, and
> actually drafting the UML pictures helped realize that the whole
> explanation could be much simpler if #7091 was merged first as the
> last step in phase 1 (that phase has so many contributors that I will
> probably never get finished documenting it). Matt Corallo's idea of
> exposing VerifyScript() through a C API certainly helped a lot in
> cementing the more-minimal-than-earlier dependencies (thanks to Cory
> Fields among many other people before him) that are not part of the
> incomplete but existing libbitcoinconsensus library.
> Given this success in protecting encapsulation by exposing things in a
> new library, my instinct was to expose more things: VerifyHeader(),
> VerifyTx() and VerifyBlock() [in that order].
> But all those three new functions depend on storage in one way or
> another. That was part of my reasoning to expose VerifyHeader() first,
> because I believe there will be less discussion on a common interface
> for the stored longest chain than for the utxo view (which may depend
> on other transactions spent within the same block).
> In any case, I realized we should finish putting all the consensus
> critical code in the libconsensus lib and then worry about its "final"
> API.
> Therefore I changed the goal of the phase 2 in my libconsensus
> encapsulation planning from "expose VerifyHeader() in the existing
> libconsensus library" to "build all the consensus critical code within
> the existing libconsensus library, even if we don't expose anything
> else". I believe this is much feasible for a single Bitcoin Core
> release cycle and also more of a priority. Other implementations
> experimenting with libconsensus like
> https://github.com/libbitcoin/libbitcoin-consensus will have the
> chance to compare their reimplementations with the future complete
> libbitcoinconsensus without having to worry about the C API, which
> ideally they will help to define.
> I repeat, the goal of phase 2 in my upcoming libconsensus
> encapsulation plan is to fully decouple libconsensus from Bitcoin
> Core.
> In phase 3, we can refine the storage interfaces and focus on a
> quasi-final C API.
> In phase 4, we can refine and take out code that doesn't belong in
> libconsensus like CTxOut::GetDustThreshold() in
> primitives/transaction.h and move all those consensus files to the
> consensus folder before creating a separate sub-repository like for
> libsecp256k1. Note that most of the file-moving work can be in
> parallel to phases 2 and 3 and, in fact, by any new developer that is
> willing to exchange rebase-patience for meaningless github stats (I'll
> do it if nobody else wants, but I'm more than happy to delegate there:
> I have more than enough github meaningless stats already).
> As said, the document with pictures and the update to #6714 are still
> promised, but until they're ready, merging/reviewing #7091, #7287,
> #7310 and #7311 could do a great deal to make later steps in
> libconsensus phase 2 more readable.
> Most reviewers probably don't need to see any "big picture" to tell
> whether certain functions on Bitcoin Core are consensus-critical or
> not, or whether consensus critical code needs to depend on util.o or
> not.
> But I wouldn't be writing to the mailing list without a plan with
> further words nor pictures if I didn't had what I believe is a
> complete implementation of what I just defined as "libconsensus phase
> 2".
> Phase 3 should finish long pending discussions like "should
> libconsensus be C++14 or plain C" which should NOT delay phase 2.
> Phase 4 should be mostly trivial: rename files to the target dir and
> move the remaining unused code out of libconsensus.
> Phase 5 should make Bitcoin Core eat its own dog food and use
> libbitcoinconsensus oonly by its generic C API (I'm sorry if this
> looks too far away for me to even think about detailing it).
> The work in progress branch (but hopefully being finished, nit and
> merged within the 0.12.99 cycle) can be found in:
> https://github.com/jtimon/bitcoin/commits/libconsensus-f2
> Before sipa asks, signing code may make it into a new library but
> functions will return true or false and an error string. It is based
> on last-0.12.99 3cd836c1 but by popular demand I can open it as a
> "DEPENDENT-tagged" PR linking to smaller steps and keeping track of
> steps done. Analogous to the about to be replaced (for a simpler and
> more maintainable example of testchain) #6382. If people like
> Wladimir, Cory and Pieter cannot see that I've been able to reduce my
> overall cry-for-review noise thanks to github adoption of emacs'
> org-mode's [ ] vs [X] I can alwways leave those "big picture" branches
> as "private" branches out of the pull request count.
> I expect to publish a phase 3 branch very shortly. But as said I
> expect a lot of discussion on the API part, so I don't expect big
> movements in phase 3 until phase 2 is done (as said phase 4 is
> orthogonal to anything, this time git will say "verified MOVEONLY" for
> us).
> To finish this long mail, if you are new to free software and would
> like to get familiarized with Bitcoin Core development in particular,
> moving one file is a simple task that you can always besure you can do
> right.
> The way I plan to hand this to you, you won't need to convince anyone
> to publicly confirm that your "MOVEONLY" commit being legit, because
> all your remaining work will be to build on one platform (ideally you
> should do a gitian build, but embarrassingly enough for someone
> touching consensus code I just trust travis ) and trust travis (as
> said, that's what I do from my laptop, but I plan to buy my own
> building machine [and maybe outsource it for free in some protocol
> that hasn't been invented, sorry again for the distraction]) and fix
> the includes that have stopped working.
> I intend to create an issue to move all the files in this list one by one:
> https://github.com/bitcoin/bitcoin/pull/7091/files#diff-480477e89f9b6ddafb30c4383dcdd705R250
> But don't hesitate to contact me if are eager for moving some files,
> because I believe we can save a few lines of total diff if we chose
> the order of the movements properly.
> Sorry, I forgot many people read this list again.
> Happy to answer any question.
> Specially about https://github.com/jtimon/bitcoin/commits/libconsensus-f2

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