How I use Mercurial (and the MQ extension).

I started working for Mozilla Messaging a while ago, and since David Wolever asked me how I used Mercurial and the MQ extension, I thought I would put up some notes on how I’m currently using them in my day-to-day work. Of course, the stuff I’m doing now is a little different than what I’ve done in any of my previous jobs, so I’m not sure how useful any of the following will be to anyone who isn’t contributing to an open source project.

First, let’s talk a little bit about how I have my Work directory set up. The first thing I did when I started working on the Thunderbird source code was to pull down a clean copy of the source into a directory named “src-base”. The purpose of that directory is to always contain a clean copy of the upstream source code so that when I want to update the various branches I have (five, at last count), I only need to download the changes from the Mozilla repo once, and I can then propagate them from src-base to the other branches. I got the idea to do that from the Bazaar-NG developers, and I think it has helped to keep my bandwidth usage down. It might cause a problem if I was sharing my branches, but since Thunderbird seems to revolve around submitting patches to bugzilla, it works out pretty well.

The next thing I did was to clone src-base into a directory named “add-reply-list-button” (because I was writing a patch to add a Reply-To-List button :), go into that directory, and type hg qinit -c to create an mq repository, and put the mq repository itself under version control. (I didn’t actually do that the first time, and was quite annoyed that I couldn’t revert changes I had made in my patch queue.) The other part of that is that I’ve aliased mq to hg -R $(hg root)/.hg/patches. This lets me type mq commit to commit the changes to the patch.

So, now we’re at my day-to-day work. If I’m working on a bug that I’ve already got a patch started for, I cd to the appropriate branch, type hg qseries to see where I am, and hq qpush or hg qpop to get to the patch I want to work on. Then I make my changes, and when I’m happy with the results of hg diff, I type hg qrefresh to put the changes into the patch. After that, I use hg qdiff > ../branch-name-bugnum-description.diff to get a patch that I can upload to bugzilla. At this point, I usually load the patch into Vim, and search for some of the mistakes I’ve made in the past. (/^+.*[[:space:]]+$, and /dump caught a lot of my initial mistakes. Now I’ve moved on to things that are tougher to check for, like putting open-parens on a new line instead of on the previous line.) I usually go through a couple of cycles of hg qrefresh/hg qdiff … before I’m happy with the patch. Once I am, I type mq commit -m "Updated patch to fix foo and bar." to save the state of the patch, and then I upload it.

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