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

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 19, 2016, 23:41 UTC
Message-ID: <xmqqwpr56ste.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <CACsJy8DxQnGV5JZnHuvA1Zbpf2BuGdmMF+YNiq51HNK+8vW56Q@mail.gmail.com>

Duy Nguyen <pclouds@gmail.com> writes:

Show 22 quoted lines

> On Wed, Jan 20, 2016 at 1:44 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> Duy Nguyen <pclouds@gmail.com> writes:
>>
>>> 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.