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name : gzip.pm
 #
# Copyright 2014 Andrew Ayer
# Copyright 2016 Chris Lamb <lamby@debian.org>
#
# This file is part of strip-nondeterminism.
#
# strip-nondeterminism is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# strip-nondeterminism is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with strip-nondeterminism.  If not, see <http://www.gnu.org/licenses/>.
#
package File::StripNondeterminism::handlers::gzip;

use strict;
use warnings;

use File::StripNondeterminism;
use File::StripNondeterminism::Common qw(copy_data);
use File::Temp;
use File::Basename;

use constant {
	FTEXT    => 1 << 0,
	FHCRC    => 1 << 1,
	FEXTRA   => 1 << 2,
	FNAME    => 1 << 3,
	FCOMMENT => 1 << 4,
};

=head1 DEPRECATION PLAN

Given that so many libraries and utilities generate gzip archives themselves
(rather than delegating to one or two central libraries) it is infeasible to
get this upstreamed everywhere.

=cut

sub normalize {
	my ($filename) = @_;

	open(my $fh, '<', $filename)
	  or die "Unable to open $filename for reading: $!";

	# See RFC 1952

	# 0   1   2   3   4   5   6   7   8   9   10
	# +---+---+---+---+---+---+---+---+---+---+
	# |ID1|ID2|CM |FLG|     MTIME     |XFL|OS |
	# +---+---+---+---+---+---+---+---+---+---+

	# Read the current header
	my $hdr;
	my $bytes_read = read($fh, $hdr, 10);
	return 0 unless $bytes_read == 10;
	my ($id1, $id2, $cm, $flg, $mtime, $xfl, $os) = unpack('CCCCl<CC', $hdr);
	return 0 unless $id1 == 31 and $id2 == 139;

	my $new_flg = $flg;
	$new_flg &= ~FNAME;	# Don't include filename
	$new_flg &= ~FHCRC
	  ;	# Don't include header CRC (not all implementations support it)
	unless ($mtime == 0)
	{	# Don't set a deterministic timestamp if there wasn't already a timestamp
		if (defined $File::StripNondeterminism::canonical_time) {
			if (  !$File::StripNondeterminism::clamp_time
				|| $mtime > $File::StripNondeterminism::canonical_time) {
				$mtime = $File::StripNondeterminism::canonical_time;
			}
		} else {
			$mtime = 0; # gzip treats 0 as "no timestamp"
		}
	}
	# TODO: question: normalize some of the other fields, such as OS?

	my $tempfile = File::Temp->new(DIR => dirname($filename));

	# Write a new header
	print $tempfile
	  pack('CCCCl<CC', $id1, $id2, $cm, $new_flg, $mtime, $xfl, $os);

	if ($flg & FEXTRA) {	# Copy through
		# 0   1   2
		# +---+---+=================================+
		# | XLEN  |...XLEN bytes of "extra field"...|
		# +---+---+=================================+
		my $buf;
		read($fh, $buf, 2) == 2 or die "$filename: Malformed gzip file";
		my ($xlen) = unpack('v', $buf);
		read($fh, $buf, $xlen) == $xlen
		  or die "$filename: Malformed gzip file";
		print $tempfile pack('vA*', $xlen, $buf);
	}
	if ($flg & FNAME) {	# Read but do not copy through
		# 0
		# +=========================================+
		# |...original file name, zero-terminated...|
		# +=========================================+
		while (1) {
			my $buf;
			read($fh, $buf, 1) == 1 or die "$filename: Malformed gzip file";
			last if ord($buf) == 0;
		}
	}
	if ($flg & FCOMMENT) {	# Copy through
		# 0
		# +===================================+
		# |...file comment, zero-terminated...|
		# +===================================+
		while (1) {
			my $buf;
			read($fh, $buf, 1) == 1 or die "$filename: Malformed gzip file";
			print $tempfile $buf;
			last if ord($buf) == 0;
		}
	}
	if ($flg & FHCRC) {	# Read but do not copy through
		# 0   1   2
		# +---+---+
		# | CRC16 |
		# +---+---+
		my $buf;
		read($fh, $buf, 2) == 2 or die "$filename: Malformed gzip file";
	}

	# Copy through the rest of the file.
	# TODO: also normalize concatenated gzip files.  This will require
	# reading and understanding each DEFLATE block (see RFC 1951), since
	# gzip doesn't include lengths anywhere.
	while (1) {
		my $buf;
		my $bytes_read = read($fh, $buf, 4096);
		defined($bytes_read) or die "$filename: read failed: $!";
		print $tempfile $buf;
		last if $bytes_read == 0;
	}

	$tempfile->close;
	copy_data($tempfile->filename, $filename)
	  or die "$filename: unable to overwrite: copy_data: $!";

	return 1;
}

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