Re: [bitcoin-dev] libconsensus and bitcoin development process

From: Eric Lombrozo <elombrozo@gmail•com>
To: Jeff Garzik <jgarzik@gmail•com>,
	Jeff Garzik via bitcoin-dev
	<bitcoin-dev@lists•linuxfoundation.org>,
	Btc Drak <btcdrak@gmail•com>
Cc: Bitcoin development mailing list <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] libconsensus and bitcoin development process
Date: Tue, 15 Sep 2015 12:00:11 -0400	[thread overview]
Message-ID: <6F3F39FB-10A4-469B-8B90-4113A822AB1A@gmail.com> (raw)
In-Reply-To: <CADm_WcafXgg5xpQjE8uSfp-2y59XRaCUr9w5yNEoVo6GygYzZw@mail.gmail.com>

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I basically agree with what has been said here.

Refactoring efforts should be well-coordinated. Their short-term impact can be quite disruptive, although if done correctly, longer-term they make it even easier for downstream developers to add and merge changes.

By scheduling move-only changes, others can avoid making PRs immediately prior to or during these changes (which ironically involve considerable disruption to PRs while changing nothing for endusers). Furthermore, it would be useful to document the changes in ways that help other developers rebase properly.

On September 15, 2015 11:26:50 AM EDT, Jeff Garzik via bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org> wrote:
>Drak,
>
>I would say that the refactoring does actually fulfill some conditions
>you
>mention:
>- move-only is almost always clearly separated out
>- the refactoring is not controversial _in minimis_ - meaning, the
>individual pull request is not controversial.
>
>The problem comes with the impact of an unfocused stream of refactors
>to
>key code.
>
>For example, there is much less long term developer impact if
>refactoring
>were _accelerated_, scheduled to be performed in a one-week sprint. 
>There
>is a lot of breakage, yes, but after that week the average level of
>downstream patch breakage is significantly lower.  A "rip the band-aid
>off
>quickly rather than slowly" approach.
>
>
>
>
>On Tue, Sep 15, 2015 at 5:55 AM, Btc Drak <btcdrak@gmail•com> wrote:
>
>> I also share a lot of Jeff's concerns about refactoring and have
>voiced
>> them several times on IRC and in private to Jorge, Wladamir and Greg.
>I
>> meant to do a write up but never got around to it. Jeff has quite
>> eloquently stated the various problems. I would like to share my
>thoughts
>> on the matter because we really do need to come up with a plan on how
>this
>> issue is dealt with.
>>
>> Obviously, Bitcoin Core is quite tightly coupled at the moment and
>> definitely needs extensive modularisation. Such work will inevitably
>> require lots of bulk code moves and then finer refactoring. However,
>it
>> requires proper planning because there are lots of effects and
>consequences
>> for other people contributing to Core and also downstream projects
>relying
>> on Core:
>>
>> 1. Refactoring often causes other pull requests to diverge and
>require
>> rebasing. Continual refactoring can put PRs in "rebase hell" and puts
>a big
>> stress on contributors (many of whom are part time).
>>
>> 2. Version to version, Bitcoin Core changes significantly in
>structure.
>> 0.9 to 0.10 is unrecognisable. 0.10 to 0.11 is even more so. This
>makes
>> makes it hard to follow release to release and the net result is less
>> people upgrade (especially think of miners trying to keep their patch
>sets
>> working while trying not to disrupt or risk their mining operations).
>>
>> 3. Continual refactoring increases risk: we're human, and mistakes
>will
>> slip through peer review. This is especially concerning with
>consensus
>> critical code and this makes it difficult to merge such refactoring
>often,
>> which of course exacerbates the problem.
>>
>> The net negative consequence is it is harder to contribute to Core,
>harder
>> for the Core maintainers to merge and harder for downstream/dependent
>> projects/implementations to keep up.
>>
>> Suggested Way Forward
>> ---------------------------------
>>
>> With the understanding that refactored code by definition must not
>change
>> behaviour. There are three major kinds of refactoring:
>>
>> 1. code moves (e.g. separating concerns into different files);
>> 2. code style;
>> 3. structural optimisation and consolidation (reducing LOC,
>separating
>> concerns, encapsulation etc).
>>
>> Code moves(1) and CS(2) are easy to peer review and merge quickly.
>The
>> third kind(3) requires deeper analysis to ensure that while the code
>> changed, the behaviour (including any bugs) did not.
>>
>> We must resist all temptation to fix bugs or tack on minor fixes and
>> tweaks during refactoring: pull requests should only be refactoring
>only,
>> with no net change to behaviour. Keeping discipline makes it much
>easier to
>> verify and peer review and this faster to merge.
>>
>> With respect to Code moves and CS, I believe we should have a
>"refactoring
>> fortnight" where we so the bulk of code move-only refactoring plus CS
>where
>> necessary. This is by fat the most disruptive kind of change because
>it
>> widely affects other PRs mergeability. We should aim to get most of
>this
>> done in one go, so that it's not happening in dribs and drabs over
>months
>> and many releases. Once done, it gives everyone a good idea to the
>overall
>> new structure and where one can expect to find things in the future.
>The
>> idea here is to help orientation and not have to continuously hunt
>for
>> where things have moved to.
>>
>> To be clear, I am strongly suggesting code move-only refactoring PRs
>not
>> be mixed with anything else. Same for CS changes. This makes the PRs
>> extremely easy to vet and thus quick to merge.
>>
>> Towards this end, maybe there should be an IRC meeting to agree the
>> initial moves, then someone who has the stomach for it can get on and
>do it
>> - during that time, we do not merge anything else. We need to bite
>the
>> bullet and break the back out of code moves.
>>
>> With regards to CS, I think we do need to get CS right, because a
>> continual dribble of CS changes also makes diffs between releases
>less easy
>> to follow. Much of CS checking can be automated by the continuous
>> integration so authors can get it right easily. It can be just like a
>> Travis check.
>>
>> With respect to the 3rd kind of refactoring, we need to set some
>standards
>> and goals and aim for some kind of consistency. Refactoring needs to
>fulfil
>> certain goals and criterion otherwise contributors will always find a
>> reason to fiddle over and over forever. Obvious targets here can be
>things
>> like proper encapsulation and separation of concerns.
>>
>> Overall, refactoring should be merged quickly, but only on a schedule
>so
>> it doesn't cause major disruption to others.
>>
>> Obviously the third kind of refactoring more complex and time
>consuming
>> and will need to occur over time, but it should happen in defined
>steps. As
>> Jeff said, one week a month, or maybe one month a release. In any
>case,
>> refactoring changes should be quickly accepted or rejected by the
>project
>> maintainer and not left hanging.
>>
>> Finally, refactoring should *always* be uncontroversial because
>> essentially functionality is not changing. If functionality changes
>(e.g.
>> you try to sneak in a big fix or feature tweak "because it's small")
>the PR
>> should be rejected outright. Additionally, if we break down
>refactoring
>> into the three kinds stated above, peer review will be much more
>> straightforward.
>>
>>
>>
>> On Tue, Sep 15, 2015 at 5:10 AM, Jeff Garzik via bitcoin-dev <
>> bitcoin-dev@lists•linuxfoundation.org> wrote:
>>
>>> [collating a private mail and a github issue comment, moving it to a
>>> better forum]
>>>
>>> On libconsensus
>>> ---------------
>>> In general there exists the reasonable goal to move consensus state
>>> and code to a specific, separate lib.
>>>
>>> To someone not closely reviewing the seemingly endless stream of
>>> libconsensus refactoring PRs, the 10,000 foot view is that there is
>a
>>> rather random stream of refactors that proceed in fits and starts
>>> without apparent plan or end other than a one sentence "isolate
>>> consensus state and code" summary.
>>>
>>> I am hoping that
>>> * There is some plan
>>> * We will not see a five year stream of random consensus code
>movement
>>> patches causing lots of downstream developer headaches.
>>>
>>> I read every code change in every pull request that comes into
>>> github/bitcoin/bitcoin with three exceptions:
>>> * consensus code movement changes - too big, too chaotic, too
>>> frequent, too unfocused, laziness guarantees others will inevitably
>>> ACK it without me.
>>> * some non-code changes (docs)
>>> * ignore 80% of the Qt changes
>>>
>>> As with any sort of refactoring, they are easy to prove correct,
>easy
>>> to reason, and therefore quick and easy to ACK and merge.
>>>
>>> Refactors however have a very real negative impact.
>>> bitcoin/bitcoin.git is not only the source tree in the universe.
>>> Software engineers at home, at startups, and at major companies are
>>> maintaining branches of their own.
>>>
>>> It is very very easy to fall into a trap where a project is merging
>>> lots of cosmetic changes and not seeing the downstream ripple
>effects.
>>> Several people complained to me at the conference about all the code
>>> movement changes breaking their own work, causing them to stay on
>>> older versions of bitcoin due to the effort required to rebase to
>each
>>> new release version - and I share those complaints.
>>>
>>> Complex code changes with longer development cycles than simple code
>>> movement patches keep breaking.  It is very frustrating, and causes
>>> folks to get trapped between a rock and a hard place:
>>> - Trying to push non-trivial changes upstream is difficult, for
>normal
>>> and reasonable reasons (big important changes need review etc.).
>>> - Maintaining non-trivial changes out of tree is also painful, for
>the
>>> aforementioned reasons.
>>>
>>> Reasonable work languishes in constant-rebase hell, and incentivizes
>>> against keeping up with the latest tree.
>>>
>>>
>>> Aside from the refactor, libconsensus appears to be engineering in
>the
>>> dark.  Where is any sort of plan?  I have low standards - a photo of
>a
>>> whiteboard or youtube clip will do.
>>>
>>> The general goal is good.   But we must not stray into unfocused
>>> engineering for a non-existent future library user.
>>>
>>> The higher priority must be given to having a source code base that
>>> maximizes the collective developers' ability to maintain The Router
>--
>>> the core bitcoin full node P2P engine.
>>>
>>> I recommend time-based bursts of code movement changes.  See below;
>>> for example, just submit & merge code movement changes on the first
>>> week of every 2nd month.  Code movement changes are easy to create
>>> from scratch once a concrete goal is known.  The coding part is
>>> trivial and takes no time.
>>>
>>> As we saw in the Linux kernel - battle lessons hard learned - code
>>> movement and refactors have often unseen negative impact on
>downstream
>>> developers working on more complicated changes that have more
>positive
>>> impact to our developers and users.
>>>
>>>
>>> On Bitcoin development release cycles & process
>>> ------------------------------------------------------------------
>>>
>>> As I've outlined in the past, the Linux kernel maintenance phases
>>> address some of these problems.  The merge window into git master
>>> opens for 1 week, a very chaotic week full of merging (and
>rebasing),
>>> and then the merge window closes.  Several weeks follow as the "dust
>>> settles" -- testing, bug fixing, moving in parallel OOB with
>>> not-yet-ready development.  Release candidates follow, then the
>>> release, then the cycle repeats.
>>>
>>> IMO a merge window approach fixes some of the issues with
>refactoring,
>>> as well as introduces some useful -developer discipline- into the
>>> development process.  Bitcoin Core still needs rapid iteration --
>>> another failing of the current project -- and so something of a more
>>> rapid pace is needed:
>>> - 1st week of each month, merge changes.  Lots of rebasing during
>this
>>> week.
>>> - remaining days of the month, test, bug fix
>>> - release at end of month
>>>
>>> If changes are not ready for merging, then so be it, they wait until
>>> next month's release.  Some releases have major features, some
>>> releases are completely boring and offer little of note.  That is
>the
>>> nature of time-based development iteration.  It's like dollar cost
>>> averaging, a bit.
>>>
>>>
>>> And frankly, I would like to close all github pull requests that are
>>> not ready to merge That Week.  I'm as guilty of this as any, but
>that
>>> stuff just languishes.  Excluding a certain category of
>obvious-crap,
>>> pull requests tend to default to a state of either (a) rapid
>merging,
>>> (b) months-long issues/projects, (c) limbo.
>>>
>>> Under a more time-based approach, a better pull request process
>would be
>>> to
>>> * Only open pull requests if it's a bug fix, or the merge window is
>>> open and the change is ready to be merged in the developer's
>opinion.
>>> * Developers CC bitcoin-dev list to discuss Bitcoin Core-bound
>projects
>>> * Developers maintain and publish projects via their own git trees
>>> * Pull requests should be closed if unmerged after 7 days, unless it
>>> is an important bug fix etc.
>>>
>>> The problem with projects like libconsensus is that they can get
>>> unfocused and open ended.  Code movement changes in particular are
>>> cheap to generate.  It is low developer cost for the developer to
>>> iterate all the way to the end state, see what that looks like, and
>>> see if people like it.  That end state is not something you would
>>> merge all in one go.  I would likely stash that tree, and then start
>>> again, seek the most optimal and least disruptive set of refactors,
>>> and generate and merge those into bitcoin/bitcoin.git in a
>time-based,
>>> paced manner.  Announce the pace ahead of time - "cosmetic stuff
>that
>>> breaks your patches will be merged 1st week of every second month"
>>>
>>> To underscore, the higher priority must be given to having a source
>>> code base and disciplined development process that maximizes the
>>> collective developers' ability to maintain The Router that maintains
>>> most of our network.
>>>
>>> Modularity, refactoring, cleaning up grotty code generates a deep
>>> seated happiness in many engineers.  Field experience however shows
>>> refactoring is a never ending process which sometimes gets in the
>way
>>> of More Important Work.
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev@lists•linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>
>>
>
>
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