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July 01, 2005

CPAN vs. dot-underscore

I was so keen on the idea of pushing a module to CPAN that I almost didn't catch a fatal flaw in my scheme. Mac OS X (motto: It Just Works) was doing something odd to my tarball as it was created: it was adding a whole host of files that weren't there originally. Not just the .DS_Store files we all know and love (and learn to remove), but odd shadow files like ._foo.txt for foo.txt and ._Makefile.PL for Makefile.PL.

Yep. Now you might see the problem.

At first I wasn't worried about them, because tar made the files vanish when I extracted the tarball to another location on my Mac. Assuming this was some quirk in tar, I posted the tarball to CPAN and went on my merry way. As it turned out, extracting the tarball on any non-Mac system retained all the shadow files, including ._Makefile.PL. That one caused the install process to fail and made the module essentially useless.

The only mention of tar's new behavior I found was in a blog post about the same problem I was having. Turns out that it's a relatively new twist on an old problem, new because tar itself had been updated to create and consume the dot-underscore files. There were other notes about these files (aka AppleDouble files) around the Web, but the proposed remedy was always the same: delete the files once they're on the target system. Unfortunately, that's not good enough when the target is CPAN.

tar's own ability to --exclude input files was no use, either, because it creates the files after the exclude filter is processed. There is a --delete option, but it requires a specific filename so I'd have to remove the files one at a time. Now that the task had moved from impossibility to drudgery, however, the solution was obvious: a Perl script!

#!/usr/bin/perl

use Archive::Tar;
use IO::Zlib;
use File::Copy;

my $USAGE = "  Usage: $0 FILENAME\n";

my $tar_file = $ARGV[0] 
  or die $USAGE;

my $tar = Archive::Tar->new($tar_file);

my @files_to_remove;
foreach my $file ( sort $tar->list_files( ['name'] ) )
{
  print "  $file";
  if ($file =~ '/\._' or $file =~ '.DS_Store')
  {
    print "  ...will be removed.";
    push @files_to_remove, $file;
  }
  print "\n";
}

unless ( scalar(@files_to_remove) )
{
  print "No files were found to remove.  The archive is unchanged.\n";
  exit;
}

my $backup_file = $tar_file . ".bak";
copy($tar_file, $backup_file);
print "Backup file written to $backup_file.\n";

$tar->remove(@files_to_remove);

print "Clean archive:\n";
foreach my $file ( sort $tar->list_files( ['name'] ) )
{
  print "  $file";
  print "\n";
}

$tar->write($tar_file, 1);

print "Clean file has been written.\n\n";

I'll probably clean that up and submit it to CPAN as well, but hopefully Apple will add a --dont-be-evil option to tar before I get the chance.

Posted by Chris at July 1, 2005 09:33 AM

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