is it kosher for pre-commit to change what's staged?

I've noticed that if I make a pre-commit hook change the files that are staged, those changes are not reflected in the commit message. For example, if a pre-commit hook git add's somefile, the commit message won't reflect that. I guess prepare-commit-msg is being run before pre-commit for some reason?

Joey Hess <joey@kitenet.net> writes:

My intention was that Documentation/githooks.txt would document things that are allowed (e.g. "applypatch-msg" explicitly says "The hook is allowed to edit the message"), and anything that is not specifically allowed is not.

"Is it kosher" is a difficult question to answer, as something may not be allowed but there may not be an enforcement mechanism to deny it, iow, it may happen to work by accident.

In general, pre-anything is about checking and denying and is supposed to be free of side effects.

Junio C Hamano wrote:

I'm guessing it is to allow cancelling a commit before a costly pre-commit hook runs.

In this case, isn't it only a half accident? For example, I think v1.5.4-rc0~78^2~12 (builtin-commit: fix partial-commit support, 2007-11-18) taught git to support this a little better.

That said, I would be interested to hear the use case, since modifying staged content on the fly for a commit sounds a little crazy. :)

Jonathan Nieder wrote:

Partial commits modified by pre-commit still don't work entirely right IME. After the commit, the index is such that git wants to revert any changes added to the commit by the pre-commit hook.

Well, with git-annex (which I should probably properly announce here sometime), large files are not committed to git, but instead a symlink pointing at the content is committed. That symlink can temporarily be replaced with a copy of the file when the user wants to modify it, and if the user then commits, it's nice if it can move away the large file's content, and arrange for the commit to contain an updated symlink pointing at the new content.

That said, since git commit stages the whole large file into the index, which is the kind of expensive operation git-annex exists to avoid, it's still not very practical to intercept the commit like that. What I ideally need is a hook that is run before git commit stages anything.

BTW, githooks says pre-commit is "invoked before obtaining the proposed commit log message", which is a bit confusing. I actually see prepare-commit-msg and commit-msg both called after pre-commit.

On Wed, Nov 10, 2010 at 11:45 PM, Joey Hess <joey@kitenet.net> wrote:

Then maybe you should look at git smudge&clean filters. They perform conversation between the working tree and the index. Though I am not sure how well they work with big files.

Dmitry

On Thu, Nov 11, 2010 at 01:17:04 +0300, Dmitry Potapov wrote:

The problem point is not big files, but whether they can change the file mode associated with the entry, since in this case the tree contains a file, but index should contain a symlink. Looking at the documentation, there does not seem to be a way to do it.

It might be worth adding support for it now we have a use-case.

Jan Hudec wrote:

I suppose that if I were using smudge filters, I'd probably not need to use symlinks at all. I'll think about it.

It belatedly occured to me that I already had something else modifying the index in pre-commit, and smudge filters cannot handle its use case.

That is, etckeeper (http://kitenet.net/~joey/code/etckeeper). The pre-commit script there stores some metadata about files that git usually doesn't track, by putting it in a dotfile, and stages that dotfile for commit. Which currently works fine, except for the minor problem that the user doesn't see the dotfile listed in the commit message.

Joey Hess <joey@kitenet.net> writes:

I am hearing "list of files in the commit message", and am getting confused. As far as I know we don't store such a list in the commit message.

Puzzled.

On Nov 11, 2010, at 2:03 PM, Junio C Hamano wrote:

I believe he means in the # comments at the bottom of the editor. They contain the output of `git status` as a reminder of what you're committing.

-Kevin Ballard