Or, how to migrate to git using only three programming languages

Another aspect of Bugzilla has been dragged, kicking & screaming, into the future! On March 11, 2014, the Bugzilla source moved to git.mozilla.org. We’re still mirroring to bzr.mozilla.org (more on that later), but the repository of record is now git, meaning it is the only place we accept new code.

Getting over there was no small feat, so I want to record the adventure in the hopes that it can benefit someone else, and so I can look back some day and wonder why I put myself through these things.

Background

The rationale isn’t the focus of this post, but suffice it to say that Bazaar isn’t very widely used, and many projects are abandoning it. Eric S. Raymond wrote a good post on the Emacs dev list about why they need to move from Bazaar to git. The same rationale applies to Bugzilla: “Sticking to a moribund version-control system will compound and exacerbate the project’s difficulty in attracting new talent.”

So, moving on, I started off scouring the Internet to find the best way to perform this migration. One major complication is the fact that we want to keep mirroring (one-way) to Bazaar for at least a while, since the main suggested way to upgrade Bugzilla is from bzr.mozilla.org. It was deemed unreasonable to require existing installations to switch to git to obtain a small security fix, so we’ll continue to mirror changes to Bazaar for some time.

Initial migration

I found a few posts here and there about people who had done migrations like this, but the most useful was a post by David Roth from last year that detailed how to preserve Bazaar’s commit metadata, specifically bug-tracker metadata, which Bugzilla has used on virtually every commit since switching from CVS. It involves using the --no-plain option with bzr fast-export and then translating the output to something git understands.

Interestingly, Roth’s translation script was written in C#, not my personal first choice for such a task (or any, really, since I don’t generally develop for Windows). However it compiled fine under Mono, so I could run it on a Linux box. Something I learned, though, is to not try this kind of thing on OS X, where, by default, the filesystem is case-insensitive.

As much as I’d prefer to deal with a language with which I am more comfortable, I dislike duplicated effort even more. I used Roth’s C# script as a basis, modifying it a bit for our needs. The metadata is in the form <bug URL> <resolution> . Rather than editing existing commit messages, I just took that string and pasted it to the bottom of the commit message, but only if the bug number was not already in the commit message. This actually revealed a few typos in the “Bug 123456” strings that generally start commit messages.

There turned out to a few other subtle bugs, like the fact that a file which is both renamed and modified in the same commit shows up, in the output from bzr fast-export , as being modified under the original name. Thus if the delete is processed first, it looks like bzr has modified an nonexistent file. Those were easy to see by comparing the contents of every file before and after migration (admittedly just for the last revision).

Since there are a lot of branches on bzr.mozilla.org, I created a bash script to record them all and make sure none were missed. It output the pre-/postmigration diff of md5 sums as well as doing a git repack for each repo, after all branches were migrated.

One thing I forgot was pushing tags via the --tags option to git push ; I had to do that manually after the migration. That’s also when I discovered that the same tag existed in several related bzr branches which were all combined into one git repo. This is, of course, not allowed in git. It made me think more about how Bugzilla uses certain tags, like current-stable , which are moved after each release. In git this requires the --force option to git push and is a big no-no if the remote repo is shared. I learned that, in fact, this is also the case in bzr, though perhaps it’s regarded as less of a sin than it is in git. Anyway, I’ve since realized that those should be branches, named appropriately (per branch). Despite them not being branches in the standard sense—they’ll always point to somewhere at or behind a version branch and never fork—it’s perfectly acceptable to move them, as opposed to tags, and since they’ll always be fast-forwarded, they won’t take any more space than a lightweight tag.

Mirroring

This was a harder problem. Originally, I tried to use the bzr-git extension, and it failed when I tried to pull in changes from git. I exchanged some emails with bzr-git’s author, Jelmer Vernooij, and he said that to keep an existing bzr branch in sync with a newly formed git repo is impossible at the moment: “This is essentially the ‘roundtripping’ support that we’ve been trying to achieve with bzr-git for a while. It’s a nontrivial problem (since it requires all semantics from bzr to be preserved when pushing to git).” Considering bzr-git hasn’t had a new release in two years, I won’t be holding my breath.

Luckily (and perhaps somewhat unfortunately) Bugzilla has jumped VCSes before, as I hinted above. With the old bzr-to-cvs script as a starting point, I created a git-to-bzr script—in, of course, Perl, as the original.

This script is essentially an automated way of applying individual commits from a git branch to a bzr branch. For each commit, the entire file tree is copied from a local git clone to a local bzr checkout, bzr add and remove are executed where needed, and the changes committed with the original author, contributor, and date preserved. The script also parses out the standard “Bug X:” commit-message format and passes it to bzr’s --fixes commit option. A file called .gitrev in the bzr repo tracks the associated git commit ID for each bzr commit.

To avoid excessive disk activity, since the script polls git and bzr for changes, the script uses bzr cat to check the contents of the .gitrev file and git ls-remote to get the ID of the last git commit. If they are equal, no further actions are performed.

Summing up

And that, folks, is how you can migrate from bzr to git! The initial migration is pretty straightforward, more so if you don’t care about any bzr commit metadata. It was unfortunate that there was no off-the-shelf way to sync the repos afterwards, but the basic idea isn’t too complicated.

For more, there’s the project page on our wiki, and all the scripts used are in a GitHub repo for your perusal. I’m no VCS expert—I’ve never heavily used bzr, and I’m constantly learning new things about git—but feel free to ask me questions if you want our process further clarified.