Re: Schrödinger's diff

From: Andreas Ericsson <ae@op5.se>
Date: Jul 7, 2009, 07:52 UTC
Message-ID: <4A52FEAE.90705@op5.se>
In-Reply-To: <279b37b20907062353k34bca06erf035458e80933c8d@mail.gmail.com>

Eric Raible wrote:

Show 11 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.
>

I don't get the same result in the same repo, although it only differs in 1-1.5% of the tests.

> Doing an "git ls-tree HEAD" gives an identical tree in both cases.
> 
> Can anyone explain why the output to this is not deterministic?

On Linux with git version 1.6.3.3.354.g3b4cc

Pasting your commands into "repro.sh", but redirecting output from git commit to /dev/null, and then running the following commands has yielded 9 to 15 sample.$i files over 5 tries of the following:

sh repro.sh > correct
for i in $(seq 1 1000); do
  sh repro.sh > sample && cmp sample correct >/dev/null || \
     { echo "fail $i" && cp sample sample.$i; };
done

> I'm at a complete loss.
>

Inserting "sync" between calls as shown below doesn't fix the issue (although it drops from 9-15 to 4-10 fails on Linux; Not a very good improvement and only two test-runs). I have no idea how it can behave so strangely, and I refuse to believe that the ext3 fs driver allows dirty reads.

Show 10 quoted lines

> 	# 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
> 	git commit -m"add crlf"
>

sync

> 	# I realize that switching is ill-advised, but I'm
> 	# trying to track down a possibly related problem...
> 	git config core.autocrlf true
>

sync

Show 5 quoted lines

> 	# 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
>

-- 
Andreas Ericsson                   andreas.ericsson@op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

Considering the successes of the wars on alcohol, poverty, drugs and
terror, I think we should give some serious thought to declaring war
on peace.