Dec 10

Mercurial MQ extension extension

I love using Mercurial’s MQ extension for managing patch queues, even though I have a strong suspicion that it’s fundamentally the wrong idea. I’m only going to discuss one part of that wrongness now, though: it forgets things. Lots of things.

Much of the point of using a revision control system is to not forget anything. I should be able to freely try various lines of development, and get back any of my earlier work. Normally, that would just mean being able to revert to earlier revisions of my source tree, although even there I should really be able to revert portions of changesets. But when using additional tools like mq that manage how I got to a particular source tree, I should be able to back up to any previous state with the tool’s assistance. Fundamentally, it’s not about moving back and forth through a history of artifact versions. It’s that I should never lose any work, even if I do something that in retrospect turns out to be dumb. Or especially when I do something dumb, I should say — that’s why I’m using a revision control system instead of a dumb backup system. It’s supposed to understand source code and what perverse things developers do when writing and modifying it.

Here’s a concrete example:

  • Developer edits code
  • hg qnew my-amazing-patch
  • Developer edits code some more
  • hg qrefresh
  • Developer says “oh f#@@#!!!!”

The problem is that when the developer refreshed the patch, Mercurial forgot the original patch. It also forgot the source tree that existed when the original patch was applied. So if those further edits turned out to be a Bad Idea, well, oops!

Yes, there is a way out: mq patch queues can themselves be revision controlled. Then as long as the developer remembers to hg commit --mq after every change to the patch queue, everything is golden.

You could even argue that this is the Right Way to work. After all, you don’t expect — or even want — your revision control system to remember every character you type. You’d never be able to identify the right point in time to back up to amid the mass of older revisions. Leaving the decision to the developer as to when a state is important enough to remember just makes sense.

Except it doesn’t. The developer already decided that the state was important by running qnew or qrefresh. Why burden the poor sap with yet another decision? Especially when making that decision requires typing in another command, which means that the mental threshold for interestingness is higher, which means it’ll pretty much never happen.

See https://bitbucket.org/sfink/mqext/ for the obvious solution. That’s actually a grab bag of mq extensions, all of which should really be submitted upstream. But I haven’t bothered.

The part that’s relevant to what I’m about here is that I added -Q options to all of the patch queue-modifying functions I could think of. Specifying -Q will commit the change to the patch queue repository, with a commit message describing the basic change (or you can set the message with -M).

Or you can go a step further, as I did, and use the [defaults] section in your ~/.hgrc to set the -Q flag automatically for whichever commands. See the help message (or the README) for details on installation and usage. Update: and now it’s easier, because you can set qcommit = auto in your [mqext] section and it’ll add the -Q option to the relevant commands. Which is good, since there are more of them than you think.

If you install this, you may want to try out the ‘qshow’ command, too. It’s my favorite of the other things implemented in that extension (I alias it to just ‘show’ because my left pinky is slow.) I use it constantly to review the various patches in the queue. hg show <n> is the way I usually use it; it prints out patch #n in your queue (the numbers come from hg qseries -v, though you really ought to just put -v in your [defaults] section too. Or alias series=qseries -v as I did.)

Feel free to use it, fork it, complain about it, or whatever. I’m still trying to figure out whether I really like it or not. It slows down qref operations, which kinda sucks. But I guess if I really cared I would turn off the default -Q for that one command, and just specify it manually. And I haven’t done that yet.

Oh right. One crucial thing I should mention: actually using any of this saved state is a dangerous affair. Why? Well, because you probably have a couple of patches in your queue applied at the time you decide to back up to an older state, and modifying applied patches is not very healthy. Especially if you reordered your series file. In fact, I would probably recommend doing these steps before (or just after, it doesn’t matter) reverting to an older revision of your patch queue:

  1. hg update -r qparent -C
  2. rm $(hg root --mq)/stateThat will “unapply” all patches, forcefully. You can then qpush (or better, qgoto) the place you want in your queue. Note that shell $(…) is the modern version of backticks, in case you’re unfamiliar.Finally, here’s a sampler of the sorts of log messages the extension extension produces:
    UPDATE: multipage-test
     js/jsd/jsd_xpc.cpp               |    1 +
     js/jsd/test/Makefile.in          |    3 +-
     js/jsd/test/browser_multipage.js |  428 +++++++++++++++++++++++++++++++++++++++
     js/src/jsapi.cpp                 |    4 +
     js/src/jscntxt.cpp               |    4 +-
     js/src/jscompartment.cpp         |    1 +
     js/src/jswrapper.cpp             |    9 +
     7 files changed, 447 insertions(+), 3 deletions(-)
    NEW: rename-multipage
    RENAME: bug615277-JM-execHook-3 -> bug615277-JM-execHook
    DELETE: bug-612717.diff
    UPDATE: better-note-dump

    Or as the output of hg log --mq (which only shows the 1st line of each commit message):

    changeset:   92:e2ed45b4a8bf
    user:        Steve Fink 
    date:        Tue Dec 07 14:54:46 2010 -0800
    summary:     UPDATE: bug615277-JM-execHook
    changeset:   91:6e36813b7291
    user:        Steve Fink 
    date:        Tue Dec 07 14:51:45 2010 -0800
    summary:     NEW: rename-multipage
    changeset:   90:b66861e98c29
    user:        Steve Fink 
    date:        Tue Dec 07 14:38:15 2010 -0800
    summary:     RENAME: bug615277-JM-execHook-3 -> bug615277-JM-execHook
    changeset:   89:c02111e0d18d
    user:        Steve Fink 
    date:        Tue Dec 07 14:37:27 2010 -0800
    summary:     NEW: bug615277-JM-execHook-3