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Edit: //bin/X11/linux-update-symlinks

#!/usr/bin/perl

# Copyright 2016 Ben Hutchings
#
# This program 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 2 of the License, or
# (at your option) any later version.
#
# This program 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 this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

use strict;
use warnings;
use File::Spec;
use POSIX ();

use DebianLinux qw(image_stem image_list read_kernelimg_conf version_cmp);

sub usage {
    my $fh = shift;
    print $fh (<< "EOT");
Usage: $0 {install|upgrade|remove} VERSION IMAGE-PATH

This command is intended to be called from the postinst and postrm
maintainer scripts of Linux kernel packages.  The postinst script must
pass the first argument 'install' or 'upgrade' depending on whether a
fresh installation or an upgrade has taken place.

The VERSION argument must be the kernel version string as shown by
'uname -r' and used in filenames.

The IMAGE-PATH argument must be the absolute filename of the kernel
image.
EOT
}

sub usage_error {
    usage(*STDERR{IO});
    exit 2;
}

sub update_symlink {
    my ($source, $dest) = @_;

$source = File::Spec->abs2rel($source, (File::Spec->splitpath($dest))[1]);

# Don't make unnecessary changes
    my $old_source = readlink($dest);
    if (defined($old_source) && $old_source eq $source) {
	return;
    }

# Create a symlink with a temporary name
    my $rand;
    while (($rand = int(rand(1000000))) && !symlink($source, "$dest.$rand")) {
	if ($! != POSIX::EEXIST) {
	    die "Failed to create symlink to $source: $!";
	}
    }

# Move it into place atomically
    if (!rename("$dest.$rand", $dest)) {
	my $err = "$!";
	unlink("$dest.$rand");
	die "Failed to create or replace $dest: $err";
    }

print "I: $dest is now a symlink to $source\n";
}

sub promote_default {
    my ($sorted_images, $link) = @_;

# Get the absolute path
    my $image_path = readlink($link);
    if (!defined($image_path)) {
	return;
    }
    $image_path = File::Spec->rel2abs($image_path,
				      (File::Spec->splitpath($link))[1]);

# If it's already on the list, move it to the end.  (If it's not,
    # presumably the symlink is broken.)
    for my $i (0..$#$sorted_images) {
	if ($sorted_images->[$i]->[1] eq $image_path) {
	    my $tmp = $sorted_images->[$i];
	    splice @$sorted_images, $i, 1;
	    push @$sorted_images, $tmp;
	    last;
	}
    }
}

sub update_all {
    my ($change, $version, $image_path) = @_;
    my $conf = read_kernelimg_conf();

# Check whether symlinks are actually wanted
    if (!$conf->{do_symlinks}) {
	exit 0;
    }

my $default_dir = $conf->{image_dest};
    $default_dir =~ s|/*$|/|; # tidy up

my $image_stem = image_stem();

# We build a list of [version, image_path, initrd_path] in
    # ascending priority order, then update the symlinks to match it.

# Start with a list of existing images sorted by version, adding any
    # existing initrds.
    my @sorted_images = sort({version_cmp($a->[0], $b->[0])} image_list());
    for my $i (0..$#sorted_images) {
	my $exist_version = $sorted_images[$i]->[0];
	if (-f "/boot/initrd.img-$exist_version") {
	    $sorted_images[$i]->[2] = "/boot/initrd.img-$exist_version";
	}
    }

# The files for this version may not actually have been
    # installed/removed yet (e.g. on installation the initrd will
    # probably be built later).  Also, on installation this version
    # should have the highest priority, not dependent on version
    # ordering.  So ensure that this version is included (for
    # for upgrade) or excluded (otherwise).
    my $initrd_path = ((exists($ENV{INITRD}) && $ENV{INITRD} eq 'No') ?
		       undef : "/boot/initrd.img-$version");
    for my $i (0..@sorted_images) {
	my $diff = ($i < @sorted_images ?
		    version_cmp($version, $sorted_images[$i]->[0]) : -1);
	if ($diff == 0) {
	    if ($change eq 'upgrade') {
		$sorted_images[$i] = [$version, $image_path, $initrd_path];
	    } else {
		splice @sorted_images, $i, 1;
	    }
	    last;
	} elsif ($diff < 0) {
	    if ($change eq 'upgrade') {
		splice @sorted_images, $i, 0,
		    [$version, $image_path, $initrd_path];
	    }
	    last;
	}
    }

# Any existing non-broken symlinks have higher priority.
    promote_default(\@sorted_images, "${default_dir}${image_stem}.old");
    promote_default(\@sorted_images, "${default_dir}${image_stem}");

# In case of installation, this version has highest priority.
    if ($change eq 'install') {
	push @sorted_images, [$version, $image_path, $initrd_path];
    }

if (@sorted_images) {
	# Take the two highest priority entries on the list.  In case there
	# is only one entry, use it twice!
	my ($old_version, $old_image_path, $old_initrd_path) =
	    @{$sorted_images[@sorted_images >= 2 ? -2 : -1]};
	my ($cur_version, $cur_image_path, $cur_initrd_path) =
	    @{$sorted_images[-1]};

update_symlink($old_image_path, "${default_dir}${image_stem}.old");
	if (defined($old_initrd_path)) {
	    update_symlink($old_initrd_path, "${default_dir}initrd.img.old");
	} else {
	    unlink("${default_dir}initrd.img.old");
	}
	update_symlink($cur_image_path, "${default_dir}${image_stem}");
	if (defined($cur_initrd_path)) {
	    update_symlink($cur_initrd_path, "${default_dir}initrd.img");
	} else {
	    unlink("${default_dir}initrd.img");
	}
    } else {
	print STDERR "W: Last kernel image has been removed, so removing the default symlinks\n";
	unlink("${default_dir}${image_stem}.old");
	unlink("${default_dir}initrd.img.old");
	unlink("${default_dir}${image_stem}");
	unlink("${default_dir}initrd.img");
    }

exit 0;
}

if (@ARGV == 0) {
    usage_error();
}

my $change = $ARGV[0];
if ($change eq 'help' or grep({$_ eq '--help'} @ARGV)) {
    usage(*STDOUT{IO});
    exit 0;
} elsif ($change =~ /^(?:install|upgrade|remove)$/ && @ARGV == 3) {
    update_all(@ARGV);
}
usage_error();

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