Re: Issue when changing staged files in a pre-commit hook

Duy Nguyen [off-list ref] writes:

On Wed, Jan 20, 2016 at 1:44 AM, Junio C Hamano [off-list ref] wrote:

quoted

Duy Nguyen [off-list ref] writes:

quoted

I think it's the intended behavior.

Yeah, pre-commit was designed for inspecting and rejecting, not for
tweaking and munging.  Perhaps "git commit" can be tightened to make
sure that pre-commit that returns successfully did not muck with the
working tree and the index?

That was my impression from the docs, but then I saw this comment,

/*
* Re-read the index as pre-commit hook could have updated it,
* and write it out as a tree.  We must do this before we invoke
* the editor and after we invoke run_status above.
*/

which comes from 2888605 (builtin-commit: fix partial-commit support -
2007-11-18) that admits "the hook can modify it (the index)". And I
was about to update the docs, but the other way around, about updating
index and side effects.

I do not think the comment is wrong per-se, but the code we have,
either back then or even today, is insufficient to allow pre-commit
hook that mucks with the fake index that is shown to it.

Re-reading the in-core index at that point may help creating a
commit whose tree matches what the hook did, but the extra change
made by the hook is not ported forward in the real index that the
user will use after a partial commit (and there is no easy way to do
so cleanly--the change the hook makes may even overlap the change in
the real index that are added but left uncommitted, and you would
end up needing to run a threeway merge).

The only sensible thing we can do at that point in the code after
re-reading the index is to make sure that hasn't been changed by the
pre-commit hook and complain loudly to die if we find it modified, I
think.