From mboxrd@z Thu Jan  1 00:00:00 1970
From: Junio C Hamano <gitster@pobox.com>
Subject: Re: Handling racy entries in the v5 format [Re: [GSoC] Designing a
 faster index format - Progress report week 7]
Date: Wed, 06 Jun 2012 10:31:26 -0700
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References: <20120604200746.GK6449@tgummerer>
 <87aa0gbwon.fsf@thomas.inf.ethz.ch>
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Cc: Thomas Gummerer <t.gummerer@gmail.com>, <git@vger.kernel.org>,
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To: Thomas Rast <trast@student.ethz.ch>
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Thomas Rast <trast@student.ethz.ch> writes:

> (Michael, we have adapted it somewhat this since you left IRC.)
>
>   When writing an entry: check whether ce_mtime >= index.mtime.  If so,
>   write out ce_mtime=0.
>
> The index.mtime here is a lower bound on the mtime of the new index,
> obtained e.g. by touching the index and then stat()ing it immediately
> before writing out the changed entries.

Is this even workable?  I found that "open, read a byte, write it
back in place, then stat" was not giving useful timestamp and that
was the reason the original racy-git code chose not to do this.

You may be able to do "open, read a byte, write it back in place,
fsync, close and then stat", but doing so while holding that file
also as a lock feels somewhat dirty...