Firefox Mercurial Repository with CVS History

May 18, 2015 at 08:40 AM | categories: Mercurial, Mozilla

When Firefox made the switch from CVS to Mercurial in March 2007, the CVS history wasn't imported into Mercurial. There were good reasons for this at the time. But it's a decision that continues to have side-effects. I am surprised how often I hear of engineers wanting to access blame and commit info from commits now more than 9 years old!

When individuals created a Git mirror of the Firefox repository a few years ago, they correctly decided that importing CVS history would be a good idea. They also correctly decided to combine the logically same but physically separate release and integration repositories into a unified Git repository. These are things we can't easily do to the canonical Mercurial repository because it would break SHA-1 hashes, breaking many systems, and it would require significant changes in process, among other reasons.

While Firefox developers do have access to a single Firefox repository with full CVS history (the Git mirror), they still aren't satisfied.

Running git blame (or hg blame for that matter) can be very expensive. For this reason, the blame interface is disabled on many web-based source viewers by default. On GitHub, some blame URLs for the Firefox repository time out and cause GitHub to display an error message. No matter how hard you try, you can't easily get blame results (running a local Git HTTP/HTML interface is still difficult compared to hg serve).

Another reason developers aren't satisfied with the Git mirror is that Git's querying tools pale in comparison to Mercurial's. I've said it before and I'll say it again: Mercurial's revision sets and templates are incredibly useful features that enable advanced repository querying and reporting. Git's offerings come nowhere close. (I really wish Git would steal these awesome features from Mercurial.)

Anyway, enough people were complaining about the lack of a Mercurial Firefox repository with full CVS history that I decided to create one. If you point your browsers or Mercurial clients to, you'll be able to access it.

The process used for the conversion was the simplest possible: I used hg-git to convert the Git mirror back to Mercurial.

Unlike the Git mirror, I didn't include all heads in this new repository. Instead, there is only mozilla-central's head (the current development tip). If I were doing this properly, I'd include all heads, like gecko-aggregate.

I'm well aware there are oddities in the Git mirror and they now exist in this new repository as well. My goal for this conversion was to deliver something: it wasn't a goal to deliver the most correct result possible.

At this time, this repository should be considered an unstable science experiment. By no means should you rely on this repository. But if you find it useful, I'd appreciate hearing about it. If enough people ask, we could probably make this more official.