#!/usr/bin/env perl
# vim:ts=4:sw=4:expandtab
# © 2013 Michael Stapelberg <stapelberg@debian.org>
#
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are met:
#
#     * Redistributions of source code must retain the above copyright
#       notice, this list of conditions and the following disclaimer.
#
#     * Redistributions in binary form must reproduce the above copyright
#       notice, this list of conditions and the following disclaimer in the
#       documentation and/or other materials provided with the distribution.
#
#     * Neither the name of Michael Stapelberg nor the
#       names of contributors may be used to endorse or promote products
#       derived from this software without specific prior written permission.
# .
# THIS SOFTWARE IS PROVIDED BY Michael Stapelberg ''AS IS'' AND ANY
# EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
# DISCLAIMED. IN NO EVENT SHALL Michael Stapelberg BE LIABLE FOR ANY
# DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
# (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
# SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

=head1 NAME

deb-systemd-helper - subset of systemctl for machines not running systemd

=head1 SYNOPSIS

B<deb-systemd-helper> enable|disable|mask|unmask|is-enabled|was-enabled|debian-installed|update-state|reenable S<I<unit file> ...>

=head1 DESCRIPTION

B<deb-systemd-helper> is a Debian-specific helper script which re-implements
the enable, disable, is-enabled and reenable commands from systemctl.

The "enable" action will only be performed once (when first installing the
package). On the first "enable", an state file is created which will be deleted
upon "disable", but only when _DEB_SYSTEMD_HELPER_PURGE=1 to distinguish purge
from remove.

The "mask" action will keep state on whether the service was enabled/disabled
before and will properly return to that state on "unmask".

The "was-enabled" action is not present in systemctl, but is required in Debian
so that we can figure out whether a service was enabled before we installed an
updated service file. See http://bugs.debian.org/717603 for details.

The "debian-installed" action is also not present in systemctl. It returns 0 if
the state file of at least one of the given units is present.

The "update-state" action is also not present in systemctl. It updates
B<deb-systemd-helper>'s state file, removing obsolete entries (e.g. service
files that are no longer shipped by the package) and adding new entries (e.g.
new service files shipped by the package) without enabling them.

B<deb-systemd-helper> is intended to be used from maintscripts to enable
systemd unit files. It is specifically NOT intended to be used interactively by
users. Instead, users should run systemd and use systemctl, or not bother about
the systemd enabled state in case they are not running systemd.

=head1 ENVIRONMENT

=over 4

=item B<_DEB_SYSTEMD_HELPER_DEBUG>

If you export _DEB_SYSTEMD_HELPER_DEBUG=1, deb-systemd-helper will print debug
messages to stderr (thus visible in dpkg runs). Please include these when
filing a bugreport.

=back

=cut

use strict;
use warnings;
use File::Path qw(make_path); # in core since Perl 5.001
use File::Basename; # in core since Perl 5
use File::Find; # in core since Perl 5
use File::Temp qw(tempfile); # in core since Perl 5.6.1
use Text::ParseWords qw(shellwords); # in core since Perl 5
use Getopt::Long; # in core since Perl 5
use Data::Dumper;

my $quiet = 0;
my $enabled_state_dir = '/var/lib/systemd/deb-systemd-helper-enabled';
my $masked_state_dir = '/var/lib/systemd/deb-systemd-helper-masked';

# Globals are bad, but in this specific case, it really makes things much
# easier to write and understand.
my $changed_sth;

sub error {
    print STDERR "$0: error: @_\n";
    exit (1);
}

sub debug {
    my ($msg) = @_;
    return if !defined($ENV{_DEB_SYSTEMD_HELPER_DEBUG}) || $ENV{_DEB_SYSTEMD_HELPER_DEBUG} != 1;
    print STDERR "(deb-systemd-helper DEBUG) $msg\n";
}

sub is_purge {
    return (defined($ENV{_DEB_SYSTEMD_HELPER_PURGE}) && $ENV{_DEB_SYSTEMD_HELPER_PURGE} == 1)
}

sub find_unit {
    my ($scriptname) = @_;

    my $service_path = $scriptname;
    if (-f "/etc/systemd/system/$scriptname") {
        $service_path = "/etc/systemd/system/$scriptname";
    } elsif (-f "/lib/systemd/system/$scriptname") {
        $service_path = "/lib/systemd/system/$scriptname";
    }
    return $service_path;
}

sub dsh_state_path {
    my ($scriptname) = @_;
    return $enabled_state_dir . '/' . basename($scriptname) . '.dsh-also';
}

sub state_file_entries {
    my ($dsh_state) = @_;
    debug "Reading state file $dsh_state";
    my @entries;
    if (open(my $fh, '<', $dsh_state)) {
        @entries = map { chomp; $_ } <$fh>;
        close($fh);
    }
    return @entries;
}

# Writes $service_link into $dsh_state unless it’s already in there.
sub record_in_statefile {
    my ($dsh_state, $service_link) = @_;

    # Appending a newline makes the following code simpler; we can skip
    # chomp()ing and appending newlines in every print.
    $service_link .= "\n";

    make_path(dirname($dsh_state));
    my $line_exists;
    my ($outfh, $tmpname) = tempfile('.stateXXXXX',
        DIR => dirname($dsh_state),
        SUFFIX => '.tmp',
        UNLINK => 0);
    chmod(0644, $tmpname);
    if (-e $dsh_state) {
        open(my $infh, '<', $dsh_state) or error("unable to read from $dsh_state");
        while (<$infh>) {
            $line_exists = 1 if $_ eq $service_link;
            print $outfh $_;
        }
        close($infh);
    }
    print $outfh $service_link unless $line_exists;
    close($outfh);

    debug "Renaming temp file $tmpname to state file $dsh_state";
    rename($tmpname, $dsh_state) or
        error("Unable to move $tmpname to $dsh_state");
}

# Gets the transitive closure of links, i.e. all links that need to be created
# when enabling this service file. Not straight-forward because service files
# can refer to other service files using Also=.
sub get_link_closure {
    my ($scriptname, $service_path) = @_;

    my @links;

    open my $fh, '<', $service_path or error("unable to read $service_path");
    while (my $line = <$fh>) {
        chomp($line);
        my $service_link;

        if ($line =~ /^\s*(WantedBy|RequiredBy)=(.+)$/i) {
            for my $value (shellwords($2)) {
                my $wants_dir = "/etc/systemd/system/$value";
                $wants_dir .= '.wants' if $1 eq 'WantedBy';
                $wants_dir .= '.requires' if $1 eq 'RequiredBy';
                push @links, { dest => $service_path, src => "$wants_dir/$scriptname" };
            }
        }

        if ($line =~ /^\s*Also=(.+)$/i) {
            for my $value (shellwords($1)) {
                push @links, get_link_closure($value, find_unit($value));
            }
        }

        if ($line =~ /^\s*Alias=(.+)$/i) {
            for my $value (shellwords($1)) {
                push @links, { dest => $service_path, src => "/etc/systemd/system/$1" };
            }
        }
    }
    close($fh);

    return @links;
}

sub make_systemd_links {
    my ($scriptname, $service_path) = @_;

    my $dsh_state = dsh_state_path($scriptname);

    my @links = get_link_closure($scriptname, $service_path);
    for my $link (@links) {
        my $service_path = $link->{dest};
        my $service_link = $link->{src};

        record_in_statefile($dsh_state, $service_link);

        my $statefile = $service_link;
        $statefile =~ s,^/etc/systemd/system/,$enabled_state_dir/,;
        next if -e $statefile;

        if (! -l $service_link) {
            make_path(dirname($service_link));
            symlink($service_path, $service_link) or
                error("unable to link $service_link to $service_path: $!");
            $changed_sth = 1;
        }

        # Store the fact that we ran enable for this service_path,
        # so that we can skip enable the next time.
        # This allows us to call deb-systemd-helper unconditionally
        # and still only enable unit files on the initial installation
        # of a package.
        make_path(dirname($statefile));
        open(my $fh, '>>', $statefile);
        close($fh);
    }

}

# In contrary to make_systemd_links(), which only modifies the state file in an
# append-only fashion, update_state() can also remove entries from the state
# file.
#
# The distinction is important because update_state() should only be called
# when the unit file(s) are guaranteed to be on-disk, e.g. on package updates,
# but not on package removals.
sub update_state {
    my ($scriptname, $service_path) = @_;

    my $dsh_state = dsh_state_path($scriptname);
    my @links = get_link_closure($scriptname, $service_path);

    debug "Old state file contents: " .
        Dumper([ state_file_entries($dsh_state) ]);

    make_path(dirname($dsh_state));
    my ($outfh, $tmpname) = tempfile('.stateXXXXX',
        DIR => dirname($dsh_state),
        SUFFIX => '.tmp',
        UNLINK => 0);
    chmod(0644, $tmpname);
    for my $link (@links) {
        print $outfh $link->{src} . "\n";
    }
    close($outfh);

    debug "Renaming temp file $tmpname to state file $dsh_state";
    rename($tmpname, $dsh_state) or
        error("Unable to move $tmpname to $dsh_state");

    debug "New state file contents: " .
        Dumper([ state_file_entries($dsh_state) ]);
}

sub was_enabled {
    my ($scriptname) = @_;

    my @entries = state_file_entries(dsh_state_path($scriptname));
    debug "Contents: " . Dumper(\@entries);

    for my $link (@entries) {
        if (! -l $link) {
            debug "Link $link is missing, considering $scriptname was-disabled.";
            return 0;
        }
    }

    debug "All links present, considering $scriptname was-enabled.";
    return 1;
}

sub debian_installed {
    my ($scriptname) = @_;
    return -f dsh_state_path($scriptname);
}

sub remove_links {
    my ($service_path) = @_;

    my $dsh_state = dsh_state_path($service_path);
    my @entries = state_file_entries($dsh_state);
    debug "Contents: " . Dumper(\@entries);

    if (is_purge()) {
        unlink($dsh_state) if -e $dsh_state;
    }

    # Also disable all the units which were enabled when this one was enabled.
    for my $link (@entries) {
        # Delete the corresponding state file:
        # • Always when purging
        # • If the user did not disable (= link still exists) the service.
        #   If we don’t do this, the link will be deleted a few lines down,
        #   but not re-created when re-installing the package.
        if (is_purge() || -l $link) {
            my $link_state = $link;
            $link_state =~ s,^/etc/systemd/system/,$enabled_state_dir/,;
            unlink($link_state);
        }

        next unless -l $link;
        unlink($link) or
            print STDERR "$0: unable to remove '$link': $!\n";

        $changed_sth = 1;
    }

    # Read $service_path, recurse for all Also= units.
    # This might not work when $service_path was already deleted,
    # i.e. after apt-get remove. In this case we just return
    # silently in order to not confuse the user about whether
    # disabling actually worked or not — the case is handled by
    # dh_installsystemd generating an appropriate disable
    # command by parsing the service file at debhelper-time.
    open(my $fh, '<', $service_path) or return;
    while (my $line = <$fh>) {
        chomp($line);
        my $service_link;

        if ($line =~ /^\s*Also=(.+)$/i) {
            remove_links(find_unit($1));
        }
    }
    close($fh);
}

# Recursively deletes a directory structure, if all (!) components are empty,
# e.g. to clean up after purging.
sub rmdir_if_empty {
    my ($dir) = @_;

    debug "rmdir_if_empty $dir";

    rmdir_if_empty($_) for (grep { -d } <$dir/*>);

    if (!rmdir($dir)) {
        debug "rmdir($dir) failed ($!)";
    }
}

sub mask_service {
    my ($scriptname, $service_path) = @_;

    my $mask_link = '/etc/systemd/system/' . basename($service_path);

    if (-e $mask_link) {
        # If the link already exists, don’t do anything.
        return if -l $mask_link && readlink($mask_link) eq '/dev/null';

        # Otherwise, error out.
        error("$mask_link already exists");
    }

    make_path(dirname($mask_link));
    symlink('/dev/null', $mask_link) or
        error("unable to link $mask_link to /dev/null: $!");
    $changed_sth = 1;

    my $statefile = $mask_link;
    $statefile =~ s,^/etc/systemd/system/,$masked_state_dir/,;

    # Store the fact that we masked this service, so that we can unmask it on
    # installation time. We cannot unconditionally unmask because that would
    # interfere with the user’s decision to mask a service.
    make_path(dirname($statefile));
    open(my $fh, '>>', $statefile);
    close($fh);
}

sub unmask_service {
    my ($scriptname, $service_path) = @_;

    my $mask_link = '/etc/systemd/system/' . basename($service_path);

    # Not masked? Nothing to do.
    return unless -e $mask_link;

    if (! -l $mask_link || readlink($mask_link) ne '/dev/null') {
        debug "Not unmasking $mask_link because it is not a link to /dev/null";
        return;
    }

    my $statefile = $mask_link;
    $statefile =~ s,^/etc/systemd/system/,$masked_state_dir/,;

    if (! -e $statefile) {
        debug "Not unmasking $mask_link because the state file $statefile does not exist";
        return;
    }

    unlink($mask_link) or
        error("unable to remove $mask_link: $!");
    $changed_sth = 1;
    unlink($statefile);
}

my $result = GetOptions(
    "quiet" => \$quiet,
);

my $action = shift;
if (!defined($action)) {
    # Called without arguments. Explain that this script should not be run interactively.
    print "$0 is a program which should be called by dpkg maintscripts only.\n";
    print "Please do not run it interactively, ever. Also see the manpage deb-systemd-helper(1).\n";
    exit 0;
}

if (!$ENV{DPKG_MAINTSCRIPT_PACKAGE}) {
    print STDERR "$0 was not called from dpkg. Exiting.\n";
    exit 1;
}

debug "is purge = " . (is_purge() ? "yes" : "no");

my $rc = 0;
if ($action eq 'is-enabled' ||
    $action eq 'was-enabled' ||
    $action eq 'debian-installed') {
    $rc = 1;
}
for my $scriptname (@ARGV) {
    my $service_path = find_unit($scriptname);

    debug "action = $action, scriptname = $scriptname, service_path = $service_path";

    if ($action eq 'is-enabled') {
        my @links = get_link_closure($scriptname, $service_path);
        my @missing_links = grep { ! -l $_->{src} } @links;
        my $enabled = (@missing_links == 0);
        print STDERR ($enabled ? "enabled\n" : "disabled\n") unless $quiet;
        $rc = 0 if $enabled;
    }

    # was-enabled is the same as is-enabled, but only considers links recorded
    # in the state file. This is useful after package upgrades, to determine
    # whether the unit file was enabled before upgrading, even if the unit file
    # has changed and is not entirely enabled currently (due to a new Alias=
    # line for example).
    #
    # If all machines were running systemd, this issue would not be present
    # because is-enabled would query systemd, which would not have picked up
    # the new unit file yet.
    if ($action eq 'was-enabled') {
        my $enabled = was_enabled($scriptname);
        print STDERR ($enabled ? "enabled\n" : "disabled\n") unless $quiet;
        $rc = 0 if $enabled;
    }

    if ($action eq 'update-state') {
        update_state($scriptname, $service_path);
    }

    if ($action eq 'debian-installed') {
        $rc = 0 if debian_installed($scriptname);
    }

    if ($action eq 'reenable') {
        remove_links($service_path);
        make_systemd_links($scriptname, $service_path);
    }

    if ($action eq 'disable') {
        remove_links($service_path);
        # Clean up the state dir if it’s empty, or at least clean up all empty
        # subdirectories. Necessary to cleanly pass a piuparts run.
        rmdir_if_empty('/var/lib/systemd/deb-systemd-helper-enabled');

        # Same with /etc/systemd, where we create symlinks. If systemd is not
        # installed (and no other package shipping service files), this would
        # make piuparts fail, too.
        rmdir_if_empty('/etc/systemd');
    }

    if ($action eq 'enable') {
        make_systemd_links($scriptname, $service_path);
    }

    if ($action eq 'mask') {
        mask_service($scriptname, $service_path);
    }

    if ($action eq 'unmask') {
        unmask_service($scriptname, $service_path);
        # Clean up the state dir if it’s empty, or at least clean up all empty
        # subdirectories. Necessary to cleanly pass a piuparts run.
        rmdir_if_empty('/var/lib/systemd/deb-systemd-helper-masked');
    }
}

# If we changed anything and this machine is running systemd, tell
# systemd to reload so that it will immediately pick up our
# changes.
if ($changed_sth && -d "/run/systemd/system") {
    system("systemctl", "daemon-reload");
}

exit $rc;

=head1 AUTHOR

Michael Stapelberg <stapelberg@debian.org>

=cut