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Re: Announcing reposurgeon, and requesting fast-import support.

From: Ramkumar Ramachandra <artagnon_at_gmail.com>
Date: Wed, 10 Nov 2010 04:28:51 +0530

Hi,

[+CC: Daniel, for making me notice this email in the first place]

Eric Raymond writes:
> My interest in tools for repository surgery has continued, and I recently
> spotted an opportunity in the increasing use of git-fast-import streams
> as a history-interchange format. I have written what I believe is the first
> *native* application for fast-import streams, a repository editor I
> call reposurgeon.

Cool tool!

> Which brings me to my feature request. Please add native support for
> fast-export and fast-import to svndump. This would be a good idea
> in general, but my specific reason for wanting it is to enable
> reposurgeon to edit Subversion repositories.
>
> The export side is, of course, almost trivial. Proof of concept under
> MIT license is here: <http://c133.org/code/svn-fast-export.c>. It
> needs a bit of extension work around tags and branches; I won't
> belabor the obvious (and easily solvable) issues with those. There are
> two more substantive ones:

With svnrdump (merged into Subversion trunk in subversion/svnrdump)
and svn-fe (merged into git.git in contrib/svn-fe + vcs-svn/), it's
possible to produce a fast-import stream from a remote repository
without the need for any local mirroring. Unforutunately, svnrdump can
only produce a deltified dumpfile v3, and the patch series that adds
dumpfile v3 support to svn-fe hasn't been merged into git.git yet- you
can pick up the branch `dumpfilev3` from David's repository
<http://github.com/barrbrain/git> though.

> 1) Whatever merge-tracking hair you represent internally should be dumped
> 'as 'merge' commit properties.
>
> 2) User commit properties (e.g. those not in the svn: namespace)
> should be exported using the bzr properties extension, which
> reposurgeon handles now and which seems likely to make it into git core at
> some point. Syntax:
>
> property <space> NAME <space> VALUE-LENGTH <space> VALUE LF
>
> or, if the value is empty:
>
> property <space> NAME LF
>
> NAME and VALUE are utf8-encoded. The properties for each commit are sorted
> by the property name.
>
> Also note that an import stream actually containing commit-property declarations
> should have a line reading "feature commit-properties" before the first commit.

Actually, the objective of svn-fe is to produce a conformant
fast-import stream (so Git can import it into its object store): some
information is lost in the process. Does reposurgeon require all the
information, or can it operate on the stream that svn-fe produces?

That brings us to another point: a fast-import stream is probably not
the most faithful representation of a Subversion repository, and I
think a dumpfile v3 fits this bill. Subversion already supports native
export/ import of this format: svnadmin (dump|load) when mirrored
locally and svnrdump (dump|load) when it's not :)

> The import side is less trivial, but given that you've already got internal
> representations for merge-tracking it shouldn't be too difficult either.
> <http://www.kernel.org/pub/software/scm/git/docs/git-fast-import.html>.

svn-fe already supports converting a dumpfile v3 to a fast-import
stream. Getting it to do the reverse shouldn't be too hard- we are
already working on it :)

> Finally, I will note that I think this feature could be significant
> for Subversion's competitive posture. Because exporters are easy while
> importers are more difficult, supporting import streams only with
> exporters and only through sketchy third-party tools tends to
> encourage migration to git while discouraging migration away from it.

Are you happy with having a combination of svnrdump and svn-fe for
this, or do you think Subversion should natively support fast-import?
I don't think it'll be very difficult to support natively, but it's
kind of a hack because Subversion already has so much infrastructure
to deal with dumpfile v3.

p.s- I'm one of the students who did a GSoC project with Git this
summer. If you recall, you even commented on the proposal I posted to
the Git list :) svnrdump and svn-fe are the products of that same
project.

-- Ram
Received on 2010-11-09 23:59:25 CET

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