Re: GSOC Proposal draft: git-remote-svn

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Apr 3, 2012, 18:48 UTC
Message-ID: <20120403184803.GH15589@burratino>
In-Reply-To: <2576556.3d3popQR3z@flomedio>

Hi,

Florian Achleitner wrote:

> You know I'm rather new to this topic. I've used svn and git, I know what git 
> plumbing is about, but I haven't used plumbing commands to write something 
> into git yet. So I can't tell from experience if it would be good or not, 
> compared to fast-import.

Yes, no problem. I think the question of using fast-import or other commands is not a fundamental one.

> So please explain what's the advantage/disadvantage of which design decision.
> That makes it easier to get the point.

The main advantages of using fast-import are:

 - it's faster (assuming it works correctly) :)
 - there are backends for version control systems other than git
 - remote helpers can declare the export/import capabilities to support other
   version control systems, instead of declaring fetch/push and supporting
   only git

However, whatever tools you use, the immediate idea is to transfer data between a Subversion repository and a Git repository, and the problems to be solved are the same.

[...]

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> I'm also not yet familiar with svn's internals and what properties they use
> for what.
> So there are several questions I simply don't have an answer for.
> I know that you have discussed several issues in a huge lot of mails on this
> list. I'm watching and learning currently.

The svnbook at http://svnbook.red-bean.com/, the Subversion lists at <http://subversion.apache.org/mailing-lists.html>, and the #svn-dev IRC channel on freenode <http://colabti.org/irclogger/irclogger_logs/svn-dev> are the best resources I know for questions in that vein.

I also learned a lot from looking at the dump format that "svnadmin dump" spits out, since it matches Subversion concepts pretty well. It is documented at

  https://svn.apache.org/repos/asf/subversion/trunk/notes/dump-load-format.txt

Some basic design questions are covered in the thread starting at

  http://thread.gmane.org/gmane.comp.version-control.git/159054

> Jonathan wrote about a script "floating around". What's that?

I think you mean the marks-to-notes converter.  One version is at

  http://thread.gmane.org/gmane.comp.version-control.git/163395/focus=168514

[...]

> On Monday 02 April 2012 16:30:14 Ramkumar Ramachandra wrote:

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>> Are you planning to extend svn-fe to do the mapping, write it as a
>> separate program, or write it into the remote helper? I personally
>> don't mind if the mapping is done in Perl (like in git-svn or SBL) as
>> opposed to C; mapping is just parse-intensive.
>
> I personally don't like Perl. :p (I would use python if i need a scripting 
> language).
> As far as I've seen, svn-fe is a 5-liner calling functions in vcs-svn/. So I 
> thought there is no point of piping something through svn-fe in the remote-
> helper. I thought I would use those functions like svn-fe does.
> I thought about vcs-svn/ being a library for svn interaction that the remote-
> helper, and svn-fe, and svn-fi (?) are using.

Yes, I think when Ram added vcs-svn/ to the main git repository, the intent was to make it a library that some git-remote-svn.c could use directly.

[...]

> On Monday 02 April 2012 15:57:00 Jonathan Nieder wrote:

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>> The word "new" makes me worried that you'd be throwing away whatever
>> work already exists. :)
>
> Probably I missed something. 
> But all I've seen that is directly a remote-helper is a bash script which 
> basically calls a pipeline from svnrdump | svn-fe | fast-import [2]. 
> I'm not planning to write a longer program in bash. (I personally use bash 
> only for things that fit on one terminal height).
>
> Bash and Perl are not my favourites ;)

I think that's fine. It's a prototype, and it has -alpha in its name to make sure people understand there are no compatibility guarantees which avoids constraining us. What I was more worried about is throwing away discoveries made in the previous design and starting over.

Jonathan