July 24, 2008

Converting a Shared Subversion Repository to Git

At Alien Camel, we've always run our Rails projects from a central Subversion repository. We've recently become Git converts, and love its distributed development capabilities, but the centralised repository model suits Rails deployment and we want to stick with it.

There’s plenty of documentation out there on how to convert a Subversion repo to Git, and there’s some stuff on setting up a shared Git repo, but its not obvious how to combine the two.

We've worked out how to do it, and successfully converted a couple of projects. This is our recipe.

Create a git user and group

First of all, you need a git user and group on the server where you host your repositories. On Ubuntu, this will do the job:

sudo adduser git

Now edit /etc/group, and add any users who’ll need to access the Git repository to the git group:

...
git:x:1005:pete,fred
...

Make a directory for Git repositories

We put ours in /usr/local/gitroot, but pick somewhere that suits your setup.

sudo mkdir /usr/local/gitroot
sudo chown git.git /usr/local/gitroot
sudo chmod 2770 /usr/local/gitroot

It’s common practice (although I don’t recommend it) to have multiple projects in a single Subversion repository. You definitely shouldn’t do this with Git. Each project should have its own Git repository within the /usr/local/gitroot directory.

Convert the Subversion repository

First you need to tell Git how to convert your Subversion user names to Git user names. For this you create an authors file, which looks something like this:

pete = Pete Yandell <pete@notahat.com>
fred = Fred Bloggs <fred@bloggs.com>

On the left are the Subversion user names; on the right are the Git equivalents.

We save this in /usr/local/gitroot, just in case we want to use it for converting other projects later on.

Now do the conversion and put the result into a temporary repository (you’re going to clean it up and clone it again a bit later), passing in the authors file and the Subversion repo.

cd /tmp
git svn clone --stdlayout --authors-file=/usr/local/gitroot/authors file:///usr/local/svnroot/my_project

This will probably take a while if your repo is large.

The —stdlayout flag tells Git to expect trunk, tags, and branches directories inside your Subversion project and deal with them correctly.

Some people also recommend the —no-metadata flag. Normally git svn clone puts information about the Subversion revision into each converted Subversion commit. The —no-metadata flag prevents this. I prefer to leave this information in, just in case I need to refer back to the old Subversion repo at some point.

Make the shared Git repository

Now you need to create the git repo in the right place, and with the right permissions.

Do it all as the git user:

sudo -u git bash

Clone the temporary repo into its final location:

umask 007
cd /usr/local/gitroot
git clone --bare /tmp/my_project my_project.git

Finally, make sure all the permissions are correct, and will stay that way:

cd my_project.git
git config core.sharedrepository 1
git config receive.denyNonFastforwards true
find objects -type d -exec chmod 02770 {} \;

The core.sharedrepository flag tells git to keep everything group readable and writable.

The receive.denyNonFastforwards flag makes sure that merges can’t happen when you push to the repo. You have to do the merges on your local machine, and then push the result.

If you’re creating an empty shared repo, you can use the —shared option to git init to set these two flags automatically. A word of warning though: the —shared option to git clone does something completely different.

Try a clone

Now you should switch to your local machine and try cloning the repo:

git clone ssh://myhost.com/usr/local/gitroot/my_project.git

You should be able to commit to this repo, and use git push to push changes back up to the central repo.

Clean up

Once you’re happy that everything is working, delete the temporary repo that you created in /tmp, and you’re done!