From: Zygo Blaxell <zblaxell@gibbs.hungrycats.org>
Date: Fri, 12 Feb 2010 01:23:14 GMT
Subject: Re: Bug#569505: git-core: 'git add' corrupts repository if the working directory is modified as it runs
Message-ID: <20100212012314.GC24809@gibbs.hungrycats.org>
In-Reply-To: <20100212002741.GB9883@progeny.tock>

On Thu, Feb 11, 2010 at 06:27:41PM -0600, Jonathan Nieder wrote:
> Zygo Blaxell reported through http://bugs.debian.org/569505 that ???git
> update-index??? has some issues when the files it is adding change under
> its feet:
> 
> My thoughts:
> 
>  - Low-hanging fruit: it should be possible for update-index to check
>    the stat information to see if the file has changed between when it
>    first opens it and when it finishes.

I don't think this is a good idea.  stat() is very coarse-grained, and
provides accuracy of only a second on a lot of file systems where git
working directories might be found.  If you run the test script on an
ext3 filesystem on a modern machine the stat() data won't change at all
even though the file contents change completely many times.

What would be a good idea is to make sure that the code that copies a
file into the index and calculates its hash does both in a single pass
over the same input data.  That might require replacing a simple mmap()
of the input file with a read-hash-copy loop.