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Date:   Fri, 2 Mar 2018 05:46:08 -0500
From:   Jeff King <peff@peff.net>
To:     Jonathan Nieder <jrnieder@gmail.com>
Cc:     Sam Kuper <sam.kuper@uclmail.net>, git@vger.kernel.org
Subject: Re: Bug report: "Use of uninitialized value $_ in print"
Message-ID: <20180302104608.GB11074@sigill.intra.peff.net>
References: <CAD-Jur+6m1SjqHVWBxW5HnTjutSVrkP+dEXdYmFHzoUf0FGdNA@mail.gmail.com>
 <20180302070434.GG238112@aiede.svl.corp.google.com>
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On Thu, Mar 01, 2018 at 11:04:34PM -0800, Jonathan Nieder wrote:

> > Use of uninitialized value $_ in print at
> > /usr/lib/git-core/git-add--interactive line 1371, <STDIN> line 75.
> [...]
> 
> Strange.  The relevant line, for reference:
> 
> $ dpkg-deb --fsys-tarfile git_2.11.0-3+deb9u2_amd64.deb |
>   tar Oxf - ./usr/lib/git-core/git-add--interactive |
>   sed -n '1370,1372 p'
> 
> 		for (@{$hunk[$ix]{DISPLAY}}) {
> 			print; <---- this one
> 		}
> 
> This is a foreach loop, so it's supposed to have set $_ to each value
> in the list @{$hunk[$ix]{DISPLAY}).  So why is Perl considering it
> uninitialized?

Because the array is full of "undef". See parse_diff(), which does this:

  my @diff = run_cmd_pipe("git", @diff_cmd, "--", $path);
  ...
  @colored = run_cmd_pipe(@display_cmd);
  ...
  for (my $i = 0; $i < @diff; $i++) {
          ...
          push @{$hunk[-1]{TEXT}}, $diff[$i];
          push @{$hunk[-1]{DISPLAY}},
               (@colored ? $colored[$i] : $diff[$i]);
  }

If the @colored output is shorter than the normal @diff output, we'll
push a bunch of "undef" onto the DISPLAY array (the ternary there is
because sometimes @colored is completely empty if the user did not ask
for color).

-Peff