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From:   Junio C Hamano <gitster@pobox.com>
To:     Elijah Newren <newren@gmail.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Git Mailing List <git@vger.kernel.org>
Subject: Re: Optimizing writes to unchanged files during merges?
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Date:   Sat, 14 Apr 2018 07:27:01 +0900
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        (Elijah Newren's message of "Fri, 13 Apr 2018 13:04:01 -0700")
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Elijah Newren <newren@gmail.com> writes:

> Yes, precisely.  Checking the *current* index is not reliable in the
> presence of renames.
>
> Trying to use the current index as a proxy for what was in the index
> before the merge started is a problem.  But we had a copy of the index
> before the merge started; we just discarded it at the end of
> unpack_trees().  We could keep it around instead.  That would also
> have the benefits of making the was_dirty() checks more accurate too,
> as using the mtime's in the current index as a proxy for what was in
> the original index has the potential for the same kinds of problems.

Yeah, I agree that you are going in the right direction with the
above.

> It's certainly tempting as an interim solution.  I have an alternative
> interim solution that I think explains well why the code here had been
> fragile, and how to just implement what we want to know rather than
> making approximations to it, which I just posted at [2].  But I can
> see the draw of just gutting the code and replacing with simple brute
> force.

Thanks; I'd love to reserve a block of time to think this through
myself, but I am a bit occupied otherwise this weekend, so I'll try
to catch up after that.