GNU bug report logs - #76452
3.11 critical bug: diff -Naur a b does not show a unified diff when one of the file is empty

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From: Collin Funk <collin.funk1 <at> gmail.com>
To: Kate Deplaix <kit-ty-kate <at> outlook.com>, Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 76452 <at> debbugs.gnu.org
Subject: bug#76452: [bug-diffutils] bug#76452: 3.11 critical bug: diff -Naur a b does not show a unified diff when one of the file is empty
Date: Thu, 20 Feb 2025 22:10:44 -0800

[Message part 1 (text/plain, inline)]

Hi Katie,

Kate Deplaix <kit-ty-kate <at> outlook.com> writes:

> With diffutils 3.10, "diff -Naur a b" returns:
>
> diff -Naur a/test b/test
> --- a/test   2025-02-20 14:50:07.870258052 +0000
> +++ b/test   2025-02-20 14:50:18.957287641 +0000
> @@ -0,0 +1 @@
> +content
>
> but with diffutils 3.11, the same command returns:
>
> File a/test is a regular empty file while file b/test is a regular file

Good catch.

CC'ing Paul Eggert since I think that I found the cause and it appears
unintentional to me.

Commit e016d12581ac4ea6be1ded88279527ceface74e1 has the following
ChangeLog:

    diff: improve symlink handling, avoiding a race
    [...]
    (compare_prepped_files): Use filetype and stat macros, not detype.
    [...]

Where filetype refers to a constant string returned by Gnulib's
c_file_type function. The relevant lines are here:

  if (S_ISREG (st->st_mode))
    return st->st_size == 0 ? N_("regular empty file") : N_("regular file");

So empty files will be treated as different file types because of that
change.

I have attached a patch to check for this condition.

Collin

[0001-diff-don-t-treat-empty-files-as-a-different-file-typ.patch (text/x-patch, attachment)]

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