Subject: bug#21942: closed (Re: [bug-diffutils] bug#21942: Files with incorrect file sizes)

From: [email-masked] (GNU bug Tracking System)
Date: Sun, 06 Dec 2015 19:41:02 +0000

Your bug report

#21942: Files with incorrect file sizes

which was filed against the diffutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to [email-masked].

-- 
21942: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=21942
GNU Bug Tracking System
Contact [email-masked] with problems

Subject: Re: [bug-diffutils] bug#21942: Files with incorrect file sizes
From: Jim Meyering <[email-masked]>
To: Stephan Müller <[email-masked]>
Cc: [email-masked]
Date: Sun, 6 Dec 2015 11:39:44 -0800
List-Id: <debbugs-submit.debbugs.gnu.org>

On Sun, Dec 6, 2015 at 11:27 AM, Jim Meyering <[email-masked]> wrote:
...
> Thank you, but I don't want to have to specify some new option to
> avoid this misbehavior, so will push the attached patch shortly.
> If someone finds a system for which a falsely reported stat.st_size
> is nonzero, we can revisit this.

Pushed with commit log changes and one fewer blank line in NEWS.

Subject: Files with incorrect file sizes
From: Stephan Müller <[email-masked]>
To: [email-masked]
Date: Tue, 17 Nov 2015 12:44:08 +0100
List-Id: <debbugs-submit.debbugs.gnu.org>

Hi there,

recently I had to debug weird problem. Finally I figured it out.

Virtual file systems like /sys or /proc usually don't care about file
sizes. All files have a size of 0. This leads to difficulties as diff
sometimes looks for file sizes.

Say you do:

> $ cp /proc/cmdline my_cmdline
> $ diff /proc/cmdline my_cmdline ; echo $?
> 0      // ok, files don't differ
> $ diff --brief /proc/cmdline my_cmdline
> Files /proc/cmdline and mycmdline differ  

The --brief option triggers a binary compare, as we aren't interested
in the actual differences this makes sense. As a first step, file sizes
are compared (0 vs ~150) and the files are reported as different.

I am not sure how to fix this. I.e. are the files different or not? What
is the correct behavior. In any case --brief should have no influence on
the overall result! I see the following options (I prefer the 2nd):

1: Don't use file sizes in comparing. This is rather expensive but
would certainly fix it.

2: add a --use-file-sizes flag which enables decisions
based on file sizes. Without this flag, always look into the files.
And --brief should report, if the files differ because of their data or
size. That is 

> $ diff /proc/cmdline my_cmdline ; echo $?
> 0      // ok, files don't differ  

> $ diff --brief /proc/cmdline my_cmdline ; echo $0
> 0  

> $ diff --brief --use-file-sizes /proc/cmdline my_cmdline
> Files /proc/cmdline and mycmdline differ in file size  

3: Leave it at it is and add a warning in the man page. However that
would be very inconvenient.

   best, stephan