Sunday, November 14, 2010

Git-SVN Mirror for multiple branches

This post is part of a series on Git and Subversion. To see all the related posts, screencasts and other resources, please click here

This extends the posts where I explained how to set up a git-svn mirror for a single directory.

NOTE: If you just want to use Git against a SVN repo on your own, stop reading ,now, and stick to the git-svn basics. However, if you want a setup where you can share a Git repository with colleagues and friends while still interfacing with Subversion, keep reading.

I'll show how to set up a git-svn mirror for a standard Subversion project with trunk, branches and tags. It's a bit like the single directory mirror, but in order to keep all branches in sync, it's a bit more fiddling. The good part is that this setup enables us to cherry-pick commits from one branch to the other. This is slightly smoother than using svn merge.

First of all, let's repeat how our Subversion and Git-repositories look physically (roughly the same as in previous posts):
We'll set this up in the following order.

  1. Clone a fresh Git repo from Subversion. This will be our fetching repo.
  2. Set up a bare repo.
  3. Configure pushing from the fetching repo to bare repo
  4. In the shoes of a developer, clone the repo
  5. Set up an SVN remote in the developer's repo


You'll need to have a Subversion repository url that points to a project with trunk and branches like this (we'll skip tags for the purpose of this how-to):

file:///svn-repos/company-repo/websites/trunk
file:///svn-repos/company-repo/websites/branches/yksi
file:///svn-repos/company-repo/websites/branches/kaksi

We'll do our development in trunk, and every time we make a release we branch out (so far we've released yksi and kaksi).

1. Clone the Subversion repo

Do the following:

tfnico@flint:~/git-repos/>git svn clone -s file:///svn-repos/company-repo/websites

This will run for a while (depending on the size of your repo), and create a websites directory. This will be our fecthing repo. The -s flag means "standard layout", and tells git-svn to scan the trunk and branches structure.

We can poll for changes from the SVN repo like this:

tfnico@flint:~/git-repos/websites>git svn fetch

2. Set up a bare repo

Create the bare repo like this:

tfnico@flint:~/git-repos/>git init --bare websites.git

In the next step we'll configure the fetching repo to push changes into this one.

3. Configure fetching repo to push changes

Now there are a lot of different ways to do this, but this is the easiest workflow I've found so far.

First, go into the fetching repo and set up the newly created bare repo as a remote repo with the alias 'origin':

tfnico@flint:~/git-repos/websites/>git remote add origin ../websites.git/

Now, if you look inside the fetching repo's config in : git-repos/websites/.git/config
you'll see the following entry:


[remote "origin"]
    url = ../websites.git/
    fetch = +refs/heads/*:refs/remotes/origin/*

Now modify the above so it looks like the config below:

[remote "origin"]
    url = ../websites.git/
    fetch = +refs/remotes/*:refs/remotes/origin/*
    push = refs/remotes/*:refs/heads/*


I won't explain the details of this, but we've changed the fetch operations to update the remote branches. We've also added a push configuration, saying that all remote branches should be pushed.

We can now push the remote branches into the bare repo:


tfnico@flint:~/git-repos/websites>git push origin


We can update with changes from SVN and push to the repo in one go:


tfnico@flint:~/git-repos/websites>git svn fetch; git push origin


You'll want to automate these steps somewhat, so that they occur regularly in a cron-job, or as an svn commit hook.

Update (21.08.2012): Set trunk as the default branch to be cloned:

tfnico@flint:~/git-repos/websites>cd ../websites.git
tfnico@flint:~/git-repos/websites.git>git symbolic-ref HEAD refs/heads/trunk

We are now done with setting up the infrastructure. Next up: The developer's clone.

Update (25.09.2011): Instead of doing the last two steps as described below, I recommend you rather jump to my newer recipe that makes things a bit easier

4. Clone the bare repo

Pretty straight forward:

tfnico@flint:~/sources/git/>git clone ~/git-repos/websites.git/

We can now track the remote branches, and pull the latest changes as they come in to the bare repo.

tfnico@flint:~/sources/git/websites/>git checkout -t origin/yksi
tfnico@flint:~/sources/git/websites/>git pull --rebase

(Remember to use --rebase to avoid merge commits in case you have local commits.)

When the time comes to push commits back to Subversion, we have to complete the last step.

5. Set up an SVN remote in the developer's repo

We'll do an svn-init with the same parameters as we cloned in the beginning, like this:

tfnico@flint:~/sources/git/websites/>git svn init -s file:///svn-repos/company-repo/websites/

Before we can do this in the yksi branch, we need to update the pointer to git-svn's awareness of Subversion:

tfnico@flint:~/sources/git/websites/>git update-ref refs/remotes/yksi refs/remotes/origin/yksi


The above command is the biggest PITA with this Git-SVN mirror setup:


After each pull/rebase, before you can push/dcommit back to SVN, you have to update-ref on the respective branch/trunk.


If you don't, you'll get a message like this:

tfnico@flint:~/sources/git/websites/>git svn dcommit
Committing to file:///svn-repos/company-repo/websites/branches/yksi ...
Transaction is out of date: File '/websites/branches/yksi/README.txt' is out of date at /opt/local/libexec/git-core/git-svn line 573


So, to correct the above, repeat this:

tfnico@flint:~/sources/git/websites/>git update-ref refs/remotes/yksi refs/remotes/origin/yksi

It's a bit of hassle, but I'm sure you'll figure out a neat little script for doing this in no time.


Update (20/11): I've figured out a script for you! (or rather the friendly folks on #git did):

Put this in your .gitconfig aliases:


upci = !git update-ref refs/remotes/$(git branch | grep '^*' | awk '{print $2}') refs/remotes/origin/$(git branch|grep '^*'|awk '{print $2}') && git svn dcommit


And from now on use that alias for pushing to SVN.


That brings us through the basics. I would've loved to show this off in a screencast, but unfortunately my Camtasia trial has expired, and I'm not so keen on dishing out euros for this little hobby. And there is no good free screencasting software for Mac.