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Re: Using git svn rebase at quarantine environment or a feasible alternative to synchronize git and svn repositories

On Mon, Oct 1, 2018 at 4:17 PM Jose Gisbert <jose.gisbert@xxxxxxxx> wrote:
> > > Dear members of the Git community,
> > >
> > > The enterprise I work for is planning to switch from svn to git.
> > >
> > > Before the complete switch to git we have decided to implement a scenario
> > > where the two SCMs live together, being the svn repository the reference.
> > > We also want this scenario to be transparent for both SCM users.
> > >
> > > I read the articles referenced at the end of the email and I come to the
> > > following solution.
> > >
> > > My proposal consists to import the svn repository to git using git svn and
> > > set receive.denyCurrentBranch to updateInstead. Then install pre-receive
> > > and post-receive hooks and set that repository as the central repository
> > > for git users.
> > >
> > > The pre-receive hook does git svn rebase and, if there is an update at the
> > > svn repository, rejects the push and instructs the user to do git pull.
> > > The post-receive hook does git svn dcommit to update the state of the svn
> > > repository, then instructs the user to do git pull too.
> > >
> > > Both scripts check the changes pushed are made at master before doing
> > > anything and exit after performing these tasks. branches.master.rebase is
> > > set to merges at the user repository to avoid the histories of the central
> > > and the user repositories diverge after doing git svn dcommit.
> > >
> > > However I'm stuck at this point because the pre-receive hook it's not
> > > allowed to do git svn rebase because update refs are not allowed at the
> > > quarantine environment. I was sure that I tried this solution with a past
> > > version of git and it worked, but now I doubt this because the restriction
> > > to update refs at quarantine environment was delivered at version 2.13,
> > > that dates from April 2017, if I'm not wrong.
> > >
> > > I don't know if this solution could be implemented or is there a better
> > > way to accomplish this kind of synchronization (I tried Tmate SubGit, but
> > > it didn't work for me and I don't know if we will be willing to purchase a
> > > license). Could you help me with this question?
> > >
> > > I come here asking for help because I think this is the appropriate place
> > > to do so. I apologise if this is not the case. Any help is welcome. If
> > > anything needs to be clarified, please, ask me to do so. I can share with
> > > you the source code of the hook scripts, if necessary.
> >
> > A very long time ago I had a similar setup where some clients were using
> > git-svn. This was for the first attempt to migrate the Wikimedia
> > repositories away from SVN.
> >
> > There I had a setup where users could fetch my git-svn clone, which was
> > hosted on github, and through some magic (I forgot the details) "catch up"
> > with their local client. I.e. there was some mapping data that wasn't sent
> > over.
> >
> > But users would always push to svn, not git. I think if you can live with
> > that you'd have a much easier time, having this setup where you push to git
> > and you then have to carry that push forward to svn is a lot more complex
> > than just having the clients do that.
> >
> > GitHub also has a SVN gateway, that has no open source equivalent that I
> > know of: https://help.github.com/articles/support-for-subversion-clients/
> >
> > Maybe that's something you'd like to consider, i.e. fully migrate to git
> > sooner than later, and for any leftover SVN clients have them push to a
> > private repo on GitHub. Even if you only keep that GitHub repo as a bride
> > during the migration and host Git in-house it'll be a lot easier with git as
> > a DVCS to continually merge in those changes than pulling the same trick
> > with a centralized system like SVN.
> Hi Ævar,
> First of all, thank you very much for your early response.
> I don't think making users always commit to svn is necessary. In fact, from my
> point of view, updating the svn repository with the changes committed to the
> git central repository is easy because there is no obstacle preventing to run
> git svn dcommit at the post-update hook.
> What I haven't managed to accomplish is to pull diffs from the svn repository
> into the git central repository without manual intervention. I suppose that in
> the setup you describe you manually pulled changes from the svn repository
> into your git-svn repository at GitHub. If don't, it would be very useful for
> me if you could remember how did you managed to do it automatically.
> I guess GitHub svn bridge (thank you for telling me about it, I didn't know
> about its existence) could be the solution if it was not for the fact that we
> want to keep our svn repository. Our whole CD infrastructure feeds from that
> repository and we'd like to figure out if everybody is comfortable using git
> and what is the actual value of using it as a team before making the effort of
> changing everything.

Makes sense. It's certainly not impossible to have some magic "push to
git". I only wanted to point out that it's extra complexity, so if you
could do away with that aspect of it you'd save yourself some
complexity. I was going to elaborate a bit on how that can go wrong,
but I see Jeff sent a mail just now that was better than what I had :)

I'll only add that I think you're somewhat fooling yourself if you
think you can run Subversion and Git side-by-side and evaluate both on
their merits, even if you solve the technical aspects of doing that.
Such a system will always need to cater to the lowest common
denominator of Subversion's very centralized workflow.

The big advantage you get out of DVCSs is being able to be more
flexible, and e.g. using hosting sites (in-house or external) like
GitHub or GitLab which are built around that flexibility. So
ultimately any decision about switching SCMs needs to be a
forward-looking management decision for the project, not based on how
well Git can emulate a SVN-based workflow, which is ultimately not
what you're interested in if you do make the switch.