Mercurial Precommit Isn't Entirely "Pre"

/mercurial-precommit-isnt-entirely-pre 2009-02-03T16:30:41+00:00

I've been battling with Mercurial's precommit hook for a while now trying to get it to do as I wanted, only to find it's not as "pre" as it or the documentation would lead you to believe.

I've been using Mercurial now for some time for all my version control needs, including all my web development. One of the things I like to do is ensure the version number in the application/web page is correct and accurate (as I use hg code in production). So I whipped up a quick shell script that took the current repo revision (hg id -n | tr -d '+'), added 1 and then replaced all instances of my version strings with the repo tag (hg tags -q | grep -v tip | head -1) and revision in the form "{tag}r{rev}", eg 1.0r34.

The shell script works a treat, but I was finding after running a commit, hg stat still reported several files had been modified, which they had as the precommit script modified them. These were always files I hadn't modified prior to the commit, but were modified by the precommit script.

The Mercurial book describes the precommit hook as follows in ch10:

10.9.6 precommit—before starting to commit a changeset

This hook is run before Mercurial begins to commit a new changeset. It is run before Mercurial has any of the metadata for the commit, such as the files to be committed, the commit message, or the commit date.

So, this would lead you to believe that any modifications you may make to files using a script run by the precommit hook would be registered by the commit and included, right?

Well, sadly that's NOT the case as I discovered when reading Mercurial Issue 1030 which is closed off with this little comment:

By the time the precommit hook is run, we've already figured out what files
will be included in the next commit (the book is wrong)...

So that explains perfectly why my precommit hook wasn't working as expected. The script was working as expected, however the Mercurial functionality wasn't. Bummer!!!

I know the method I'm using isn't ideal, but automatically updating the version number is such a time saver and useful "feature". Unfortunately, I can't think of any other method of getting the behaviour I want short of creating a wrapper script that calls my script BEFORE "hg commit".

Update: Beautiful!!! It looks like I don't read too good or things have changed without me knowing. kuk commented that I could use the "pre-commit" hook to do this. I couldn't find this in the Hg Book so didn't know it existed. I've since gone back to ticket 1030 mentioned above and in the same comment I quote it actually mentions...

Using the "pre-commit" hook might do what you want, but I'm not sure it's a good idea, since you're quite likely to accidentally commit some garbage files.

(pre- are generic hooks that run before the command. E.g. pre-log runs before "hg log", pre-pull before "hg pull", pre-commit before "hg commit", etc. OTOH, "precommit" runs before the internal commit operation.)

I've now tested this and it's EXACTLY what I needed and works a treat. Thanks kuk.

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