Alternate Mercurial Server for Firefox Development

October 17, 2013 at 07:30 AM | categories: Mercurial, Mozilla

I have long opined about the sad state of Mercurial at Mozilla. The short version is Mozilla has failed to use Mercurial optimally, at least for Firefox development. It's easy to see why so many Mozillians are quick to discredit Mercurial when compared to Git!

I have a history attempting to address the deficiencies. Up to this point, I've been able to make things better through local tooling. But, for my next set of tricks, I reached an impasse with the Mercurial server at So, I stood up my own Mercurial server at!

This server is running Mercurial 2.7 and has a few nice features the official Mercurial server at does not.

The repositories is a read-only unified Mercurial repository containing the commits for the major Firefox/Gecko repositories. If you look at its bookmarks, you'll see something special: the heads of all the separate Mercurial repos it is aggregating are being stored as bookmarks! (Bookmarks are effectively Git branches.) The tip of mozilla-central is at the bookmark central/default. The tip of Beta is at beta/default. You get the idea. Once you clone this repo, you can easily switch between project branches by running e.g. hg up central/default. When you pull the repo, you get changesets for all repos by connecting to one server, not several (this reduces load on Mozilla's servers and is faster for clients).

This repository shares the same changesets/SHA-1's as the official repositories. It just has everything under one roof. You can work out of this repository and push to the official repositories. Although, you may want to use the pushtree command from my custom extension to make your life easier (hg push with no arguments will attempt to push all changesets, which you definitely don't want when pushing to e.g. mozilla-central). is an offshoot of the gecko repo that you can push to. Changesets from the gecko repo are pulled into it automatically.

What makes the gecko-collab repository special is that it has obsolescence enabled. That is the core Mercurial feature enabling changeset evolution. More on that feature and why it is amazing in a future blog post. Stay tuned.


If you would like to clone one of these unified repos, please do my paltry EC2 server a favor and bootstrap your clone from an existing clone. e.g. if you have a copy of mozilla-central sitting around but don't want my repo's changesets to pollute it, do the following:

hg clone mozilla-central gecko
cd gecko
hg pull

Or, if you are OK with your clone accumulating the extra changesets from all the project branches, just run:

hg pull

Don't forget to update the [paths] section in your .hg/hgrc file to point to! e.g.

gecko =
collab =

Setting up push support and SSH keys

If you would like to push to the gecko-collab repository, you'll need to give me your SSH public key. But don't give me your key - give an automated process your key!

Head on over to and log in (look for the Persona button). Once you've logged in, go to your settings by clicking the wrench icon in the top right. Then look for SSH Public Keys to add your key(s). If you can't find it, just go to

Once your SSH public key is added, it will take up to a minute for it to be added to my system. It's all automatic. You don't need to wait for any manual action.

To connect to my server over SSH, you'll need to log in as the hgssh user. e.g. in your hgrc file, add:

gecko = ssh://
collab = ssh://

Then, you should be able to pull and push over SSH!

Other Notes

This server is running on an EC2 instance that isn't as powerful as I'd like. Expect some operations to be slower than desired.

I don't guarantee an SLA for this service. It could go down at any moment. However, Mercurial being a distributed version control system, there should be little to no data loss assuming people pull frequently. I know I have a backup on all my machines now.

I'm running this server for two main reasons.

First, I want to demonstrate the utility of a unified Mercurial server for Firefox development in hopes we can run one officially. I've been running a unified repo locally for a few months and I have little doubt I'm more productive because of it. I want others to realize the awesomeness.

Second, I needed a server that supported changeset evolution so I could play around with it. I asked the powers at be to enable it on and didn't get a response that met my timeline. So, I figured setting up my own server was easier.

Please let me know if you have any questions or issues with this server. I'd also love to hear whether people like the unified repo approach!