Subject: Re: Git and Google Summer of Code 2012

From: Jakub Narebski <[email-masked]>
Date: Tue, 20 Mar 2012 16:37:10 +0100

On Tue, 20 Mar 2012, André Walker wrote:

> Hello Jakub, I tweeted to you, but as your last tweet was from January, I'm  
> guessing here [personal email via Google Profile] is probably a better
> place to talk.

Actually the discussion of GSoC project application ideas with mentoring
organization[1] should take place in the open, on git mailing list,
[email-masked].  (You don't need to be subscribed to send email to
it, and there is custom on this mailing list of Cc-ing all people
participating in discussion; you can read git mailing list via other
interfaces e.g. via GMane.)

Perhaps this should be stated more clearly, for example in application
template[2].

[1]: http://www.google-melange.com/gsoc/events/google/gsoc2012
[2]: https://github.com/peff/git/wiki/SoC-2012-Template

> I'm a Computer Science student from a Brazilian university, and I
> have participated in GSoC last year with The Perl Foundation.
>
> I reworked the Catalyst MVC framework to use an Inversion of Control
> framework called Bread::Board instead of it's home grown component
> loading system. I was supposed to continue my work on it this year,
> we were going to release Catalyst v6.0 with it, but TPF was not   
> accepted in GSoC. I guess we'll have to ship it without GSoC then.
>
> Anyhow, I'd like to participate in it again this year, even if not  
> continuing what I began last year. Preferably using Perl, but I guess I  
> could try something different. And looking in the accepted orgs for GSoC,  
> Git seemed a very cool option, I'd be very honored to work on it. I use git  
> on most of my projects, but I have never delved in it's code.
>
> From the ideas page, I got interested in two items: "Modernizing and
> expanding Git.pm", and "Linus's ultimate content tracking tool".
> To be honest, the latter sounded awesome! But I don't know if I could
> be able to pull it off, as I don't know how complex it would be.

From what I understand "Linus's ultimate content tracking tool" is
rather a sketch of an idea, rather than concrete project proposal.
You would have to carve a project from this proposal yourself.

> I'd be willing to try though, if I could talk to the relevant people,
> understand how the implementation would work, etc.

People on git mailing list could help there, perhaps starting with
person that suggested it.

> And, well, in the first one (Modernizing Git.pm) they pointed   
> you as a possible mentor, so I'd like to know. First, how much of Git is  
> actually in Perl? In other words, how much of it is implemented in Git.pm  
> and related modules, or who would use it exactly? 

There are quite a few git commands implemented in Perl, and there were
even more before "builtin-ification" of git code (moving to C).  Those
include the interactive part of git-add (git-add--interactive helper),
git-cvsimport, git-cvsserver and git-svn, git-send-email, git-difftool
and gitweb.  Not all of those use Git.pm module (git-cvsimport, 
git-cvsserver and gitweb do not); changing this might be part of GSoC
project.

Git.pm currently does mainly cover safe and portable (ActiveState Perl)
invoking of git commands, and a bit of converting / translating
output to Perl (e.g. config_bool() method).

It is by no means complete; some of code could be refactored and moved
from individual commands to Git.pm module.

> Also, why is it not on CPAN? Wouldn't it be useful to other people to
> write interfaces in Perl for Git?

It is not on CPAN probably because Git Development Community lack(s|ed)
a Perl hacker, having only Perl dabblers ;-P

More seriously, putting Git.pm on CPAN might be a part of this GSoC
project.  Not that CPAN lacks git modules: Git::Class, Git::PurePerl,
Git::Repository, Git::Wrapper, Git::XS (libgit2 based)...

Note however that Git.pm must (in my opinion) remain "dual lived" module,
i.e. reside in git.git repository and be installable alongside git
with nothing but git sources.  This also means that any extra non-core
(or even not installed by default with "perl" package) modules that
Git.pm requires to work need to have copy in git.git repository just
like private-Error.pm (should be 'inc/Error.pm') does currently.

Git.pm might ultimately be put in separate repository, and subtree-merged
into git.git like git-gui and gitk subsystems (or as submodule), but that
would require having real maintainer for this module.

> It mentions replacing Error and Error::Simple for Try::Tiny and   
> Exception::Class. What else should be modernized? And where else is there  
> room for expansion?

You could borrow from IPC::Run, Capture::Tiny and similar modules to make
it possible to capture stderr of git commands to separate string or
separate filehandle, or just silencing stderr completely.  Perhaps even
allowing creating pipelines.

You could polish and modernize Git::Object, Git::Commit, Git::Tag,
Git::Repo and Git::RepoRoot from GSoC 2008 project[2], and add similar
modules for other concepts: diff (tree level and patchset level), tree
(directory), refs and refspec, etc.  All those with tests.

[2]: git://repo.or.cz/git/gitweb-caching.git
     http://repo.or.cz/w/git/gitweb-caching.git (gitweb)

You could separate somehow the idea of git commands that do not
require repository like "git version" or "git config --file", or
"git init" (Git::Cmd?), those that require repository but not working
area like "git log" or "git show" (Git::Repo?), and those that require
working area like "git status" or "git pull" (Git::Repo::Workdir?).

You could, for example based on existing gitweb code, create Git::Config
module that would read all configuration at once with `git config -l`,
and not use one git command for one variable like current $git->config()
does.

You could create interfaces to persistent "git cat-file --batch",
"git cat-file --batch-check" and "git diff-tree --stdin".  IIRC gsoc2008
project includes something like that.

And of course borrow^W steal interesting parts of Git::* modules
available on CPAN.

> Do you think we could unite both projects, or it would   
> be too much work? And if you're not the person I should be asking about the  
> content tracking tool, could you point me to that person?

I think it might be too much work, though prototyping "Linus's ultimate
content tracking tool" in Perl might be good idea...

HTH
-- 
Jakub Narebski
Poland