Re: Schrödinger's diff

From: Daniel Barkalow <barkalow@iabervon.org>
Date: Jul 7, 2009, 17:36 UTC
Message-ID: <alpine.LNX.2.00.0907071316490.2147@iabervon.org>
In-Reply-To: <279b37b20907062353k34bca06erf035458e80933c8d@mail.gmail.com>

On Mon, 6 Jul 2009, Eric Raible wrote:

Show 24 quoted lines

> git version 1.6.3.2.1299.gee46c (msysgit)
> 
> In trying to track down some annoying crlf corruption in a repo
> I have found a Schrödinger's diff.  In other words it's unknown
> whether the diff will produce output or not on any particular run
> of the following script.
> 
> Sometimes it does, and sometimes it doesn't (seems to be about
> 50/50).  But either way in any given repo rerunning the git-diff will
> always give the same result.
> 
> Doing an "git ls-tree HEAD" gives an identical tree in both cases.
> 
> Can anyone explain why the output to this is not deterministic?
> I'm at a complete loss.
> 
> 	# Clean up from last run and start over
> 	rm -rf .git has-crlf
> 	git init
> 	git config core.autocrlf false
> 
> 	# Add a "bad" file
> 	perl -e 'printf( "12%c%c", 0xd, 0xa )' > has-crlf
> 	git add has-crlf

If has-crlf and .git/index have the same timestamp, git does not know whether the file has been modified afterwards or not. If they have different timestamps, git knows the file hasn't been modified after the add. (More precisely, the index contains the mtime of the file, and it will agree with the file system. However, if the timestamp on the index matches a timestamp *in* the index, that means that, when the index was written, the time period represented by that timestamp was not yet over when git looked at the file. Therefore, the file could have changed again after that time and still gotten the same timestamp it already had. This means that git can't be sure that there's nothing new to see in the filesystem.)

Show 10 quoted lines

> 	git commit -m"add crlf"
> 
> 	# I realize that switching is ill-advised, but I'm
> 	# trying to track down a possibly related problem...
> 	git config core.autocrlf true
> 
> 	# This sometimes produces output and sometimes it doesn't.
> 	# Either way rerunning just git-diff always gives the same result
> 	# as the first run in this repo.
> 	git diff

If git knows the file hasn't been modified, it doesn't produce a diff.

If it doesn't know the file hasn't been modified, it looks at the actual contents and it find that the result of reading the disk applying autocrlf now doesn't match the contents of the index.

	-Daniel
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