mmdebstrap/mmdebstrap

#!/usr/bin/perl
#
# Copyright: 2018 Johannes Schauer <josch@mister-muffin.de>
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to
# deal in the Software without restriction, including without limitation the
# rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
# sell copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.

use strict;
use warnings;

use English;
use Getopt::Long;
use Pod::Usage;
use File::Copy;
use File::Path qw(make_path remove_tree);
use File::Temp qw(tempfile tempdir);
use Cwd qw(abs_path);
use Dpkg::Index;
require "syscall.ph";
use Fcntl qw(S_IFCHR S_IFBLK);
use List::Util qw(any none);

# from sched.h
use constant {
    CLONE_NEWNS   => 0x20000,
    CLONE_NEWUTS  => 0x4000000,
    CLONE_NEWIPC  => 0x8000000,
    CLONE_NEWUSER => 0x10000000,
    CLONE_NEWPID  => 0x20000000,
    CLONE_NEWNET  => 0x40000000,
};

# type codes:
#   0 -> normal file
#   1 -> hardlink
#   2 -> symlink
#   3 -> character special
#   4 -> block special
#   5 -> directory
my @devfiles = (
    # filename         mode  type link target        major  minor
    [ "./dev/",        0755, 5,   '',                undef, undef ],
    [ "./dev/console", 0666, 3,   '',                5,     1 ],
    [ "./dev/fd",      0777, 2,   '/proc/self/fd',   undef, undef ],
    [ "./dev/full",    0666, 3,   '',                1,     7 ],
    [ "./dev/null",    0666, 3,   '',                1,     3 ],
    [ "./dev/ptmx",    0666, 3,   '',                5,     2 ],
    [ "./dev/pts/",    0755, 5,   '',                undef, undef ],
    [ "./dev/random",  0666, 3,   '',                1,     8 ],
    [ "./dev/shm/",    0755, 5,   '',                undef, undef ],
    [ "./dev/stderr",  0777, 2,   '/proc/self/fd/2', undef, undef ],
    [ "./dev/stdin",   0777, 2,   '/proc/self/fd/0', undef, undef ],
    [ "./dev/stdout",  0777, 2,   '/proc/self/fd/1', undef, undef ],
    [ "./dev/tty",     0666, 3,   '',                5,     0 ],
    [ "./dev/urandom", 0666, 3,   '',                1,     9 ],
    [ "./dev/zero",    0666, 3,   '',                1,     5 ],
);

# tar cannot figure out the decompression program when receiving data on
# standard input, thus we do it ourselves. This is copied from tar's
# src/suffix.c
sub get_tar_compress_options($) {
    my $filename = shift;
    if ($filename =~ /\.(gz|tgz|taz)$/) {
	return ('--gzip');
    } elsif ($filename =~ /\.(Z|taZ)$/) {
	return ('--compress');
    } elsif ($filename =~ /\.(bz2|tbz|tbz2|tz2)$/) {
	return ('--bzip2');
    } elsif ($filename =~ /\.lz$/) {
	return ('--lzip');
    } elsif ($filename =~ /\.(lzma|tlz)$/) {
	return ('--lzma');
    } elsif ($filename =~ /\.lzo$/) {
	return ('--lzop');
    } elsif ($filename =~ /\.lz4$/) {
	return ('--use-compress-program', 'lz4');
    } elsif ($filename =~ /\.(xz|txz)$/) {
	return ('--xz');
    }
    return ();
}

sub test_unshare() {
    # arguments to syscalls have to be stored in their own variable or
    # otherwise we will get "Modification of a read-only value attempted"
    my $unshare_flags = CLONE_NEWUSER;
    # we spawn a new per process because if unshare succeeds, we would
    # otherwise have unshared the sbuild process itself which we don't want
    my $pid = fork() // die "fork() failed: $!";
    if ($pid == 0) {
	my $ret = syscall &SYS_unshare, $unshare_flags;
	if (($ret >> 8) == 0) {
	    exit 0;
	} else {
	    exit 1;
	}
    }
    waitpid($pid, 0);
    if (($? >> 8) != 0) {
	return 0;
    }
    return 1;
}

sub read_subuid_subgid() {
    my $username = getpwuid $<;
    my ($subid, $num_subid, $fh, $n);
    my @result = ();

    if (! -e "/etc/subuid") {
	printf STDERR "/etc/subuid doesn't exist\n";
	return;
    }
    if (! -r "/etc/subuid") {
	printf STDERR "/etc/subuid is not readable\n";
	return;
    }

    open $fh, "<", "/etc/subuid" or die "cannot open /etc/subuid for reading: $!";
    while (my $line = <$fh>) {
	($n, $subid, $num_subid) = split(/:/, $line, 3);
	last if ($n eq $username);
    }
    close $fh;
    push @result, ["u", 0, $subid, $num_subid];

    if (scalar(@result) < 1) {
	printf STDERR "/etc/subuid does not contain an entry for $username\n";
	return;
    }
    if (scalar(@result) > 1) {
	printf STDERR "/etc/subuid contains multiple entries for $username\n";
	return;
    }

    open $fh, "<", "/etc/subgid" or die "cannot open /etc/subgid for reading: $!";
    while (my $line = <$fh>) {
	($n, $subid, $num_subid) = split(/:/, $line, 3);
	last if ($n eq $username);
    }
    close $fh;
    push @result, ["g", 0, $subid, $num_subid];

    if (scalar(@result) < 2) {
	printf STDERR "/etc/subgid does not contain an entry for $username\n";
	return;
    }
    if (scalar(@result) > 2) {
	printf STDERR "/etc/subgid contains multiple entries for $username\n";
	return;
    }

    return @result;
}

# This function spawns two child processes forming the following process tree
#
#   A
#   |
#  fork()
#   |  \
#   B   C
#   |   |
#   |  fork()
#   |   |  \
#   |   D   E
#   |   |   |
#   |unshare()
#   | close()
#   |   |   |
#   |   | read()
#   |   | newuidmap(D)
#   |   | newgidmap(D)
#   |   |  /
#   | waitpid()
#   |   |
#   |  fork()
#   |   |  \
#   |   F   G
#   |   |   |
#   |   |   exec()
#   |   |  /
#   | waitpid()
#   |  /
# waitpid()
#
# To better refer to each individual part, we give each process a new
# identifier after calling fork(). Process A is the main process. After
# executing fork() we call the parent and child B and C, respectively. This
# first fork() is done because we do not want to modify A. B then remains
# waiting for its child C to finish. C calls fork() again, splitting into
# the parent D and its child E. In the parent D we call unshare() and close a
# pipe shared by D and E to signal to E that D is done with calling unshare().
# E notices this by using read() and follows up with executing the tools
# new[ug]idmap on D. E finishes and D continues with doing another fork().
# This is because when unsharing the PID namespace, we need a PID 1 to be kept
# alive or otherwise any child processes cannot fork() anymore themselves. So
# we keep F as PID 1 and finally call exec() in G.
sub get_unshare_cmd(&$) {
    my $cmd = shift;
    my $idmap = shift;

    my $unshare_flags = CLONE_NEWUSER | CLONE_NEWNS | CLONE_NEWPID | CLONE_NEWUTS | CLONE_NEWIPC;

    if (0) {
	$unshare_flags |= CLONE_NEWNET;
    }

    # fork a new process and let the child get unshare()ed
    # we don't want to unshare the parent process
    my $gcpid = fork() // die "fork() failed: $!";
    if ($gcpid == 0) {
	# Create a pipe for the parent process to signal the child process that it is
	# done with calling unshare() so that the child can go ahead setting up
	# uid_map and gid_map.
	pipe my $rfh, my $wfh;

	# We have to do this dance with forking a process and then modifying the
	# parent from the child because:
	#  - new[ug]idmap can only be called on a process id after that process has
	#    unshared the user namespace
	#  - a process looses its capabilities if it performs an execve() with nonzero
	#    user ids see the capabilities(7) man page for details.
	#  - a process that unshared the user namespace by default does not have the
	#    privileges to call new[ug]idmap on itself
	#
	# this also works the other way around (the child setting up a user namespace
	# and being modified from the parent) but that way, the parent would have to
	# stay around until the child exited (so a pid would be wasted). Additionally,
	# that variant would require an additional pipe to let the parent signal the
	# child that it is done with calling new[ug]idmap. The way it is done here,
	# this signaling can instead be done by wait()-ing for the exit of the child.
	my $ppid = $$;
	my $cpid = fork() // die "fork() failed: $!";
	if ($cpid == 0) {
	    # child

	    # Close the writing descriptor at our end of the pipe so that we
	    # see EOF when parent closes its descriptor.
	    close $wfh;

	    # Wait for the parent process to finish its unshare() call by
	    # waiting for an EOF.
	    0 == sysread $rfh, my $c, 1 or die "read() did not receive EOF";

	    # The program's new[ug]idmap have to be used because they are
	    # setuid root.  These privileges are needed to map the ids from
	    # /etc/sub[ug]id to the user namespace set up by the parent.
	    # Without these privileges, only the id of the user itself can be
	    # mapped into the new namespace.
	    #
	    # Since new[ug]idmap is setuid root we also don't need to write
	    # "deny" to /proc/$$/setgroups beforehand (this is otherwise
	    # required for unprivileged processes trying to write to
	    # /proc/$$/gid_map since kernel version 3.19 for security reasons)
	    # and therefore the parent process keeps its ability to change its
	    # own group here.
	    #
	    # Since /proc/$ppid/[ug]id_map can only be written to once,
	    # respectively, instead of making multiple calls to new[ug]idmap,
	    # we assemble a command line that makes one call each.
	    my $uidmapcmd = "";
	    my $gidmapcmd = "";
	    foreach (@{$idmap}) {
		my ($t, $hostid, $nsid, $range) = @{$_};
		if ($t ne "u" and $t ne "g" and $t ne "b") {
		    die "invalid idmap type: $t";
		}
		if ($t eq "u" or $t eq "b") {
		    $uidmapcmd .= " $hostid $nsid $range";
		}
		if ($t eq "g" or $t eq "b") {
		    $gidmapcmd .= " $hostid $nsid $range";
		}
	    }
	    my $idmapcmd = '';
	    if ($uidmapcmd ne "") {
		0 == system "newuidmap $ppid $uidmapcmd" or die "newuidmap failed: $!";
	    }
	    if ($gidmapcmd ne "") {
		0 == system "newgidmap $ppid $gidmapcmd" or die "newgidmap failed: $!";
	    }
	    exit 0;
	}

	# parent

	# After fork()-ing, the parent immediately calls unshare...
	0 == syscall &SYS_unshare, $unshare_flags or die "unshare() failed: $!";

	# .. and then signals the child process that we are done with the
	# unshare() call by sending an EOF.
	close $wfh;

	# Wait for the child process to finish its setup by waiting for its
	# exit.
	$cpid == waitpid $cpid, 0 or die "waitpid() failed: $!";
	my $exit = $? >> 8;
	if ($exit != 0) {
	    die "child had a non-zero exit status: $exit";
	}

	# Currently we are nobody (uid and gid are 65534). So we become root
	# user and group instead.
	#
	# We are using direct syscalls instead of setting $(, $), $< and $>
	# because then perl would do additional stuff which we don't need or
	# want here, like checking /proc/sys/kernel/ngroups_max (which might
	# not exist). It would also also call setgroups() in a way that makes
	# the root user be part of the group unknown.
	0 == syscall &SYS_setgid, 0 or die "setgid failed: $!";
	0 == syscall &SYS_setuid, 0 or die "setuid failed: $!";
	0 == syscall &SYS_setgroups, 0, 0 or die "setgroups failed: $!";

	if (1) {
	    # When the pid namespace is also unshared, then processes expect a
	    # master pid to always be alive within the namespace. To achieve
	    # this, we fork() here instead of exec() to always have one dummy
	    # process running as pid 1 inside the namespace. This is also what
	    # the unshare tool does when used with the --fork option.
	    #
	    # Otherwise, without a pid 1, new processes cannot be forked
	    # anymore after pid 1 finished.
	    my $cpid = fork() // die "fork() failed: $!";
	    if ($cpid != 0) {
		# The parent process will stay alive as pid 1 in this
		# namespace until the child finishes executing. This is
		# important because pid 1 must never die or otherwise nothing
		# new can be forked.
		$cpid == waitpid $cpid, 0 or die "waitpid() failed: $!";
		exit ($? >> 8);
	    }
	}

	&{$cmd}();

	exit 0;
    }

    # parent
    return $gcpid;
}

sub havemknod($) {
    my $root = shift;
    my $havemknod = 0;
    if (-e "$root/test-dev-null") {
	die "/test-dev-null already exists";
    }
    while ($havemknod == 0) {
	# we fork so that we can read STDERR
	my $pid = open my $fh, '-|' // die "failed to fork(): $!";
	if ($pid == 0) {
	    open(STDERR, '>&', STDOUT);
	    # we use mknod(1) instead of the system call because creating the
	    # right dev_t argument requires makedev(3)
	    exec 'mknod', "$root/test-dev-null", 'c', '1', '3';
	}
	chomp (my $content = do { local $/; <$fh> });
	close $fh;
	{
	    last unless $? == 0 and $content eq '';
	    last unless -c "$root/test-dev-null";
	    last unless open my $fh, '>', "$root/test-dev-null";
	    last unless print $fh 'test';
	}
	$havemknod = 1;
    }
    if (-e "$root/test-dev-null") {
	unlink "$root/test-dev-null" or die "cannot unlink /test-dev-null";
    }
    return $havemknod;
}

sub setup {
    my $options = shift;

    foreach my $key (sort keys %{$options}) {
	my $value = $options->{$key};
	if (!defined $value) {
	    next;
	}
	if (ref $value eq '') {
	    print STDERR "I: $key: $options->{$key}\n";
	} elsif (ref $value eq 'ARRAY') {
	    print STDERR "I: $key: [" . (join ', ', @{$value}) . "]\n";
	} else {
	    die "unknown type";
	}
    }

    my ($conf, $tmpfile) = tempfile(UNLINK => 1) or die "cannot open apt.conf: $!";
    print $conf "Apt::Architecture \"$options->{nativearch}\";\n";
    # the host system might have configured additional architectures
    # force only the native architecture
    if (scalar @{$options->{foreignarchs}} > 0) {
	print $conf "Apt::Architectures \"$options->{nativearch} @{$options->{foreignarchs}}\";\n";
    } else {
	print $conf "Apt::Architectures \"$options->{nativearch}\";\n";
    }
    print $conf "Dir::Etc \"$options->{root}/etc/apt\";\n";
    print $conf "Dir::State \"$options->{root}/var/lib/apt\";\n";
    print $conf "Dir::Cache \"$options->{root}/var/cache/apt\";\n";
    print $conf "Apt::Install-Recommends false;\n";
    # for retrieving the essential packages set, only download
    print $conf "Apt::Get::Download-Only true;\n";
    # for authentication, use the keyrings from the host
    print $conf "Dir::Etc::Trusted \"/etc/apt/trusted.gpg\";\n";
    print $conf "Dir::Etc::TrustedParts \"/etc/apt/trusted.gpg.d\";\n";
    print $conf "Acquire::Languages \"none\";\n";
    close $conf;

    foreach my $dir ('/etc/apt/apt.conf.d', '/etc/apt/sources.list.d',
	'/etc/apt/preferences.d', '/var/cache/apt',
	'/var/lib/apt/lists/partial', '/var/lib/dpkg',
	'/etc/dpkg/dpkg.cfg.d/') {
	make_path("$options->{root}/$dir") or die "failed to create $dir: $!";
    }

    {
	open my $fh, '>', "$options->{root}/var/lib/dpkg/status" or die "failed to open(): $!";
	close $fh;
    }

    if (scalar @{$options->{foreignarchs}} > 0) {
	open my $fh, '>', "$options->{root}/var/lib/dpkg/arch" or die "cannot open /var/lib/dpkg/arch: $!";
	print $fh "$options->{nativearch}\n";
	foreach my $arch ($options->{foreignarchs}) {
	    print $fh "$arch\n";
	}
	close $fh;
    }

    if (scalar @{$options->{aptopts}} > 0) {
	open my $fh, '>', "$options->{root}/etc/apt/apt.conf.d/00mmdebstrap" or die "cannot open /etc/apt/apt.conf.d/00mmdebstrap: $!";
	foreach my $opt (@{$options->{aptopts}}) {
	    if (-r $opt) {
		copy $opt, $fh or die "cannot copy $opt: $!";
	    } else {
		print $fh $opt;
		if ($opt !~ /;$/) {
		    print $fh ';';
		}
		if ($opt !~ /\n$/) {
		    print $fh "\n";
		}
	    }
	}
	close $fh;
    }

    if (scalar @{$options->{dpkgopts}} > 0) {
	open my $fh, '>', "$options->{root}/etc/dpkg/dpkg.cfg.d/00mmdebstrap" or die "cannot open /etc/dpkg/dpkg.cfg.d/00mmdebstrap: $!";
	foreach my $opt (@{$options->{dpkgopts}}) {
	    if (-r $opt) {
		copy $opt, $fh or die "cannot copy $opt: $!";
	    } else {
		print $fh $opt;
		if ($opt !~ /\n$/) {
		    print $fh "\n";
		}
	    }
	}
	close $fh;
    }

    # setup merged usr
    my @amd64_dirs = ('lib32', 'lib64', 'libx32'); # only amd64 for now
    foreach my $dir ("bin", "sbin", "lib", @amd64_dirs) {
	symlink "usr/$dir", "$options->{root}/$dir" or die "cannot create symlink: $!";
	make_path("$options->{root}/usr/$dir") or die "cannot create /usr/$dir: $!";
    }

    {
	open my $fh, '>', "$options->{root}/etc/fstab" or die "cannot open fstab: $!";
	print $fh "# UNCONFIGURED FSTAB FOR BASE SYSTEM\n";
	close $fh;
	chmod 0644, "$options->{root}/etc/fstab" or die "cannot chmod fstab: $!";
    }

    {
	my $archopt = '';
	if (scalar @{$options->{foreignarchs}} > 0) {
	    $archopt = " [arch=$options->{nativearch}]";
	}
	open my $fh, '>', "$options->{root}/etc/apt/sources.list" or die "cannot open /etc/apt/sources.list: $!";
	if (scalar(@{$options->{mirrors}}) > 0) {
	    if((grep /^-$/, @{$options->{mirrors}}) > 1 ) {
		die "can only read from stdin once";
	    }
	    for my $arg (@{$options->{mirrors}}) {
		if ($arg eq '-') {
		    # read from stdin
		    print STDERR "I: Reading sources.list from standard input...\n";
		    copy *STDIN, $fh or die "cannot copy stdin: $!";
		} elsif ($arg =~ /:\/\//) {
		    print $fh "deb$archopt $arg $options->{suite} $options->{components}\n";
		} elsif ($arg =~ /^deb(-src)? /) {
		    print $fh "$arg\n";
		} elsif (-f $arg) {
		    copy($arg, $fh) or die "cannot copy $arg: $!";
		} else {
		    die "invalid mirror: $arg";
		}
	    }
	} else {
	    print $fh "deb$archopt http://deb.debian.org/debian $options->{suite} $options->{components}\n";
	    if (any { $_ eq $options->{suite} } ('stable', 'oldstable', 'stretch') ) {
		print $fh "deb$archopt http://deb.debian.org/debian $options->{suite}-updates $options->{components}\n";
		print $fh "deb$archopt http://security.debian.org/debian-security $options->{suite}/updates $options->{components}\n";
	    }
	}
	close $fh;
    }

    # we tell apt about the configuration via a config file passed via the
    # APT_CONFIG environment variable instead of using the --option command
    # line arguments because configuration settings like Dir::Etc have already
    # been evaluated at the time that apt takes its command line arguments
    # into account.
    $ENV{"APT_CONFIG"} = "$tmpfile";

    # apt-get doesn't report a non-zero exit if the update failed. Thus, we
    # have to parse its output. See #778357, #776152, #696335 and #745735
    {
	my $pid = open(my $pipe_apt, '-|') // die "failed to fork(): $!";
	if ($pid == 0) {
	    # redirect apt's stderr to stdout so that we can capture it
	    open(STDERR, '>&', STDOUT);
	    exec('apt-get', 'update');
	    die "cannot exec apt-get update: $!";
	}
	my $has_error = 0;
	while (my $line = <$pipe_apt>) {
	    if ($line =~ '^W: ') {
		$has_error = 1;
	    } elsif ($line =~ '^Err:') {
		$has_error = 1;
	    }
	    # forward captured apt output
	    print STDERR $line;
	}
	close($pipe_apt);
	if ($? != 0 or $has_error) {
	    die "apt-get update failed to download some indexes";
	}
    }

    # check if anything was downloaded at all
    {
	open my $fh, '-|', 'apt-get', 'indextargets' // die "failed to fork(): $!";
	chomp (my $indextargets = do { local $/; <$fh> });
	close $fh;
	if ($indextargets eq '') {
	    die "apt-get update didn't download anything";
	}
    }

    # setting PATH for chroot, ldconfig, start-stop-daemon...
    $ENV{"PATH"} = "/usr/sbin:/usr/bin:/sbin:/bin";

    my %pkgs_to_install;
    if (defined $options->{include}) {
	for my $pkg (split /,/, $options->{include}) {
	    $pkgs_to_install{$pkg} = ();
	}
    }
    if ($options->{variant} eq 'buildd') {
	$pkgs_to_install{'build-essential'} = ();
    }
    # To figure out the right package set for the apt variant we can use:
    #     $ apt-get dist-upgrade -o dir::state::status=/dev/null
    # This is because that variants only contain essential packages and
    # apt and libapt treats apt as essential. If we want to install less
    # (essential variant) then we have to compute the package set ourselves.
    # Same if we want to install priority based variants.
    if ($options->{variant} ne 'apt') {
	my %ess_pkgs;
	open(my $pipe_apt, '-|', 'apt-get', 'indextargets', '--format', '$(FILENAME)', 'Created-By: Packages', "Architecture: $options->{nativearch}") or die "cannot start apt-get indextargets: $!";
	while (my $fname = <$pipe_apt>) {
	    chomp $fname;
	    open (my $pipe_cat, '-|', '/usr/lib/apt/apt-helper', 'cat-file', $fname) or die "cannot start apt-helper cat-file: $!";

	    my $key_func = sub {
		return $_[0]->{Package} . ' ' . $_[0]->{Version} . ' ' . $_[0]->{Architecture};
	    };

	    my $index = Dpkg::Index->new(get_key_func=>$key_func);

	    $index->parse($pipe_cat, 'apt-helper cat-file') or die "failed to parse";

	    foreach my $key ($index->get_keys()) {
		my $cdata = $index->get_by_key($key);
		my $pkgname = $cdata->{Package};
		my $arch = $cdata->{Architecture} // '';
		my $ess = $cdata->{Essential} // '';
		if ($ess eq 'yes') {
		    $ess_pkgs{$pkgname} = ();
		}
		my $prio = $cdata->{Priority} // 'optional';
		if ($options->{variant} eq 'essential') {
		    # for this variant we are only interested in the
		    # essential packages
		    next;
		} elsif (any { $_ eq $options->{variant} } ('standard', 'important', 'required', 'buildd', 'minbase')) {
		    if ($prio eq 'optional' or $prio eq 'extra') {
			# always ignore packages of priority optional and extra
			next;
		    } elsif ($prio eq 'standard') {
			if (any { $_ eq $options->{variant} } ('important', 'required', 'buildd', 'minbase')) {
			    next;
			}
		    } elsif ($prio eq 'important') {
			if (any { $_ eq $options->{variant} } ('required', 'buildd', 'minbase')) {
			    next;
			}
		    } elsif ($prio eq 'required') {
			# required packages are part of all sets except
			# essential and apt
		    } else {
			die "unknown priority: $prio";
		    }
		} else {
		    die "unknown variant: $options->{variant}";
		}
		$pkgs_to_install{$pkgname} = ();
	    }

	    close $pipe_cat;
	    $? == 0 or die "apt-helper cat-file failed: $?";
	}
	close $pipe_apt;
	$? == 0 or die "apt-get indextargets failed: $?";

	0 == system('apt-get', '--yes', 'install', keys %ess_pkgs) or die "apt-get install failed: $?";
    } else {
	# if we just want to install Essential:yes packages, apt and their
	# dependencies then we can make use of libapt treating apt as
	# implicitly essential. An upgrade with the (currently) empty status
	# file will trigger an installation of the essential packages plus apt.
	#
	# 2018-09-02, #debian-dpkg on OFTC, times in UTC+2
	# 23:39 < josch> I'll just put it in my script and if it starts
	#                breaking some time I just say it's apt's fault. :P
	# 23:42 < DonKult> that is how it usually works, so yes, do that :P (<-
	#                  and please add that line next to it so you can
	#                  remind me in 5+ years that I said that after I wrote
	#                  in the bugreport: "Are you crazy?!? Nobody in his
	#                  right mind would even suggest depending on it!")
	0 == system('apt-get', '--yes', 'dist-upgrade') or die "apt-get dist-upgrade failed: $?";
    }

    # extract the downloaded packages
    my @essential_pkgs;
    {
	my $apt_archives = "/var/cache/apt/archives/";
	opendir my $dh, "$options->{root}/$apt_archives" or die "cannot read $apt_archives";
	while (my $deb = readdir $dh) {
	    if ($deb !~ /\.deb$/) {
		next;
	    }
	    $deb = "$apt_archives/$deb";
	    if (!-f "$options->{root}/$deb") {
		next;
	    }
	    push @essential_pkgs, $deb;
	}
	close $dh;
    }

    if (scalar @essential_pkgs == 0) {
	# check if a file:// URI was used
	open(my $pipe_apt, '-|', 'apt-get', 'indextargets', '--format', '$(URI)', 'Created-By: Packages') or die "cannot start apt-get indextargets: $!";
	while (my $uri = <$pipe_apt>) {
	    if ($uri =~ /^file:\/\//) {
		die "nothing got downloaded -- use copy:// instead of file://";
	    }
	}
	die "nothing got downloaded";
    }

    foreach my $deb (@essential_pkgs) {
	# not using dpkg-deb --extract as that would replace the
	# merged-usr symlinks with plain directories
	pipe my $rfh, my $wfh;
	my $pid1 = fork() // die "fork() failed: $!";
	if ($pid1 == 0) {
	    open(STDOUT, '>&', $wfh);
	    exec 'dpkg-deb', '--fsys-tarfile', "$options->{root}/$deb";
	}
	my $pid2 = fork() // die "fork() failed: $!";
	if ($pid2 == 0) {
	    open(STDIN, '<&', $rfh);
	    exec 'tar', '-C', $options->{root}, '--keep-directory-symlink', '--extract', '--file', '-';
	}
	waitpid($pid1, 0);
	$? == 0 or die "dpkg-deb --fsys-tarfile failed: $?";
	waitpid($pid2, 0);
	$? == 0 or die "tar --extract failed: $?";
    }

    if ($options->{mode} eq 'fakechroot') {
	$ENV{FAKECHROOT_CMD_SUBST} = join ':', (
	    '/bin/mount=/bin/true',
	    '/usr/bin/env=/usr/bin/env.fakechroot',
	    '/usr/bin/ldd=/usr/bin/ldd.fakechroot',
	    '/usr/bin/mkfifo=/bin/true',
	    '/usr/sbin/ldconfig=/bin/true',
	);
    }

    my @chrootcmd = ('/usr/sbin/chroot', $options->{root});
    if ($options->{mode} eq 'proot') {
	# FIXME: proot currently cannot install apt because of https://github.com/proot-me/PRoot/issues/147
	@chrootcmd = ('proot', '--root-id', '--bind=/dev', "--rootfs=$options->{root}", '--cwd=/');
    }

    # copy qemu-user-static binary into chroot or setup proot with --qemu
    if (defined $options->{qemu}) {
	if ($options->{mode} eq 'proot') {
	    push @chrootcmd, "--qemu=qemu-$options->{qemu}";
	} elsif ($options->{mode} eq 'fakechroot') {
	    # The binfmt support on the outside is used, so qemu needs to know
	    # where it has to look for shared libraries
	    $ENV{QEMU_LD_PREFIX} = $options->{root};
	    # Make sure that the fakeroot and fakechroot shared libraries
	    # exist for the right architecture
	    open my $fh, '-|', 'dpkg-architecture', '-a', $options->{nativearch}, '-qDEB_HOST_MULTIARCH' // die "failed to fork(): $!";
	    chomp (my $deb_host_multiarch = do { local $/; <$fh> });
	    close $fh;
	    if ($? != 0 or !$deb_host_multiarch) {
		die "dpkg-architecture failed: $?";
	    }
	    my $fakechrootdir = "/usr/lib/$deb_host_multiarch/fakechroot";
	    if (!-e "$fakechrootdir/libfakechroot.so") {
		die "$fakechrootdir/libfakechroot.so doesn't exist. Install libfakechroot:$options->{nativearch} outside the chroot";
	    }
	    my $fakerootdir = "/usr/lib/$deb_host_multiarch/libfakeroot";
	    if (!-e "$fakerootdir/libfakeroot-sysv.so") {
		die "$fakerootdir/libfakeroot-sysv.so doesn't exist. Install libfakeroot:$options->{nativearch} outside the chroot";
	    }
	    # fakechroot only fills LD_LIBRARY_PATH with the directories of
	    # the host's architecture. We append the directories of the chroot
	    # architecture.
	    $ENV{LD_LIBRARY_PATH} .= ":$fakechrootdir:$fakerootdir";
	} else {
	    # other modes require a static qemu-user binary
	    my $qemubin = "/usr/bin/qemu-$options->{qemu}-static";
	    if (!-e $qemubin) {
		die "cannot find $qemubin";
	    }
	    copy $qemubin, "$options->{root}/$qemubin";
	}
    }

    if ($options->{havemknod}) {
	foreach my $file (@devfiles) {
	    my ($fname, $mode, $type, $linkname, $devmajor, $devminor) = @{$file};
	    next if $fname eq './dev/';
	    if ($type == 0) { # normal file
		die "type 0 not implemented";
	    } elsif ($type == 1) { # hardlink
		die "type 1 not implemented";
	    } elsif ($type == 2) { # symlink
		# nothing to do
	    } elsif ($type == 3) { # character special
		0 == system('mknod', "$options->{root}/$fname", 'c', $devmajor, $devminor) or die "mknod failed: $?";
	    } elsif ($type == 4) { # block special
		0 == system('mknod', "$options->{root}/$fname", 'b', $devmajor, $devminor) or die "mknod failed: $?";
	    } elsif ($type == 5) { # directory
		make_path "$options->{root}/$fname", { error => \my $err };
		if (@$err) {
		    die "cannot create $fname";
		}
	    } else {
		die "unsupported type: $type";
	    }
	    chmod $mode, "$options->{root}/$fname";
	}
    }

    # install the extracted packages properly
    0 == system(@chrootcmd, 'dpkg', '--install', '--force-depends', @essential_pkgs) or die "dpkg --install failed: $?";

    # if the path-excluded option was added to the dpkg config, reinstall all
    # packages
    if (-e "$options->{root}/etc/dpkg/dpkg.cfg.d/00mmdebstrap") {
	open(my $fh, '<', "$options->{root}/etc/dpkg/dpkg.cfg.d/00mmdebstrap") or die "cannot open /etc/dpkg/dpkg.cfg.d/00mmdebstrap: $!";
	my $num_matches = grep /^path-exclude=/, <$fh>;
	close $fh;
	if ($num_matches > 0) {
	    # without --skip-same-version, dpkg will install the given
	    # packages even though they are already installed
	    0 == system(@chrootcmd, 'dpkg', '--install', @essential_pkgs) or die "dpkg --install failed: $?";
	}
    }

    foreach my $deb (@essential_pkgs) {
	unlink "$options->{root}/$deb" or die "cannot unlink $deb";
    }

    if (%pkgs_to_install) {
	# some packages have to be installed from the outside before anything
	# can be installed from the inside.
	my %pkgs_to_install_from_outside;

	# install apt if necessary
	if ($options->{variant} ne 'apt') {
	    $pkgs_to_install_from_outside{apt} = ();
	}

	# since apt will be run inside the chroot, make sure that
	# apt-transport-https and ca-certificates gets installed first if any
	# mirror is a https URI
	open(my $pipe_apt, '-|', 'apt-get', 'indextargets', '--format', '$(URI)', 'Created-By: Packages') or die "cannot start apt-get indextargets: $!";
	while (my $uri = <$pipe_apt>) {
	    if ($uri =~ /^https:\/\//) {
		# FIXME: support for https is part of apt >= 1.5
		$pkgs_to_install_from_outside{'apt-transport-https'} = ();
		$pkgs_to_install_from_outside{'ca-certificates'} = ();
		last;
	    } elsif ($uri =~ /^tor(\+[a-z]+)*:\/\//) {
		# tor URIs can be tor+http://, tor+https:// or even
		# tor+mirror+file://
		$pkgs_to_install_from_outside{'apt-transport-tor'} = ();
		last;
	    }
	}
	close $pipe_apt;
	$? == 0 or die "apt-get indextargets failed";

	if (%pkgs_to_install_from_outside) {
	    0 == system('apt-get', '--yes', 'install', (keys %pkgs_to_install_from_outside)) or die "apt-get install failed: $?";
	    my @debs_to_install;
	    my $apt_archives = "/var/cache/apt/archives/";
	    opendir my $dh, "$options->{root}/$apt_archives" or die "cannot read $apt_archives";
	    while (my $deb = readdir $dh) {
		if ($deb !~ /\.deb$/) {
		    next;
		}
		$deb = "$apt_archives/$deb";
		if (!-f "$options->{root}/$deb") {
		    next;
		}
		push @debs_to_install, $deb;
	    }
	    close $dh;
	    if (scalar @debs_to_install == 0) {
		die "nothing got downloaded";
	    }
	    # we need --force-depends because dpkg does not take Pre-Depends
	    # into account and thus doesn't install them in the right order
	    0 == system(@chrootcmd, 'dpkg',  '--install', '--force-depends', @debs_to_install) or die "dpkg --install failed: $?";
	    foreach my $deb (@debs_to_install) {
		unlink "$options->{root}/$deb" or die "cannot unlink $deb";
	    }
	}

	# from now on, apt will be executed inside the chroot
	undef $ENV{"APT_CONFIG"};

	# if more than essential should be installed, make the system look
	# more like a real one by creating or bind-mounting the device nodes
	foreach my $file (@devfiles) {
	    my ($fname, $mode, $type, $linkname, $devmajor, $devminor) = @{$file};
	    next if $fname eq './dev/';
	    if ($type == 0) { # normal file
		die "type 0 not implemented";
	    } elsif ($type == 1) { # hardlink
		die "type 1 not implemented";
	    } elsif ($type == 2) { # symlink
		symlink $linkname, "$options->{root}/$fname";
	    } elsif ($type == 3 or $type == 4) { # character/block special
		if (!$options->{havemknod}) {
		    open my $fh, '>', "$options->{root}/$fname" or die "cannot open $options->{root}/$fname: $!";
		    close $fh;
		    0 == system('mount', '-o', 'bind', "/$fname", "$options->{root}/$fname") or die "mount failed: $?";
		}
	    } elsif ($type == 5) { # directory
		if (!$options->{havemknod}) {
		    make_path "$options->{root}/$fname";
		}
		0 == system('mount', '-o', 'bind', "/$fname", "$options->{root}/$fname") or die "mount failed: $?";
	    } else {
		die "unsupported type: $type";
	    }
	    chmod $mode, "$options->{root}/$fname";
	}
	# We can only mount /proc and /sys after extracting the essential
	# set because if we mount it before, then base-files not be able
	# to extract those
	0 == system('mount', '-o', 'rbind', '/sys', "$options->{root}/sys") or die "mount failed: $?";
	0 == system('mount', '-t', 'proc', 'proc', "$options->{root}/proc") or die "mount failed: $?";

	# prevent daemons from starting
	{
	    open my $fh, '>', "$options->{root}/usr/sbin/policy-rc.d" or die "cannot open policy-rc.d: $!";
	    print $fh "#!/bin/sh\n";
	    print $fh "exit 101\n";
	    close $fh;
	    chmod 0755, "$options->{root}/usr/sbin/policy-rc.d" or die "cannot chmod policy-rc.d: $!";
	}

	{
	    move("$options->{root}/sbin/start-stop-daemon", "$options->{root}/sbin/start-stop-daemon.REAL") or die "cannot move start-stop-daemon";
	    open my $fh, '>', "$options->{root}/sbin/start-stop-daemon" or die "cannot open policy-rc.d: $!";
	    print $fh "#!/bin/sh\n";
	    print $fh "echo \"Warning: Fake start-stop-daemon called, doing nothing\">&2\n";
	    close $fh;
	    chmod 0755, "$options->{root}/sbin/start-stop-daemon" or die "cannot chmod start-stop-daemon: $!";
	}

	# allow network access from within
	copy("/etc/resolv.conf", "$options->{root}/etc/resolv.conf");
	copy("/etc/hostname", "$options->{root}/etc/hostname");

	0 == system(@chrootcmd, 'apt-get',  '--yes', 'install',
	    '--no-install-recommends',
	    keys %pkgs_to_install) or die "apt-get install failed: $?";

	# cleanup
	move("$options->{root}/sbin/start-stop-daemon.REAL", "$options->{root}/sbin/start-stop-daemon") or die "cannot move start-stop-daemon";
	unlink "$options->{root}/usr/sbin/policy-rc.d" or die "cannot unlink policy-rc.d";

	foreach my $file (@devfiles) {
	    my ($fname, undef, $type, $linkname, undef, undef) = @{$file};
	    next if $fname eq './dev/';
	    if ($type == 0) { # normal file
		die "type 0 not implemented";
	    } elsif ($type == 1) { # hardlink
		die "type 1 not implemented";
	    } elsif ($type == 2) { # symlink
		unlink "$options->{root}/$fname" or die "cannot unlink $fname: $!";
	    } elsif ($type == 3 or $type == 4) { # character/block special
		if (!$options->{havemknod}) {
		    0 == system('umount', '--no-mtab', "$options->{root}/$fname") or die "umount failed: $?";
		    unlink "$options->{root}/$fname";
		}
	    } elsif ($type == 5) { # directory
		0 == system('umount', '--no-mtab', "$options->{root}/$fname") or die "umount failed: $?";
		if (!$options->{havemknod}) {
		    rmdir "$options->{root}/$fname" or die "cannot rmdir $fname: $!";
		}
	    } else {
		die "unsupported type: $type";
	    }
	}
	# no need to umount if the mount namespace was unshared
	if ($options->{mode} ne 'unshare') {
	    0 == system('umount', '--no-mtab', '--recursive', '--lazy', "$options->{root}/sys") or die "umount failed: $?";
	    0 == system('umount', '--no-mtab', "$options->{root}/proc") or die "umount failed: $?";
	}
    }

    # if there is no apt inside the chroot, clean it from the outside
    if ($options->{variant} eq 'essential') {
	$ENV{"APT_CONFIG"} = "$tmpfile";
	0 == system('apt-get', '--option', 'Dir::Etc::SourceList=/dev/null', 'update') or die "apt-get update failed: $?";
	0 == system('apt-get', 'clean') or die "apt-get clean failed: $?";
	undef $ENV{"APT_CONFIG"};
    } else {
	0 == system(@chrootcmd, 'apt-get', '--option', 'Dir::Etc::SourceList=/dev/null', 'update') or die "apt-get update failed: $?";
	0 == system(@chrootcmd, 'apt-get', 'clean') or die "apt-get clean failed: $?";
    }

    if (defined $options->{qemu} and $options->{mode} ne 'proot' and $options->{mode} ne 'fakechroot') {
	unlink "$options->{root}/usr/bin/qemu-$options->{qemu}-static" or die "cannot unlink /usr/bin/qemu-$options->{qemu}-static";
    }

    # clean up certain files to make output reproducible
    unlink "$options->{root}/var/log/dpkg.log";
    unlink "$options->{root}/var/log/apt/history.log";
    unlink "$options->{root}/var/log/apt/term.log";
    unlink "$options->{root}/var/log/alternatives.log";
    unlink "$options->{root}/var/cache/ldconfig/aux-cache";

    # clean up leftover lock files
    unlink "$options->{root}/var/cache/apt/archives/lock";
    unlink "$options->{root}/var/lib/apt/extended_states";
    unlink "$options->{root}/var/lib/apt/lists/lock";
    unlink "$options->{root}/var/lib/dpkg/lock-frontend";
}

sub main() {
    umask 022;

    my $mtime = time;
    if (exists $ENV{SOURCE_DATE_EPOCH}) {
	$mtime = $ENV{SOURCE_DATE_EPOCH}+0;
    }

    $ENV{DEBIAN_FRONTEND} = 'noninteractive';
    $ENV{DEBCONF_NONINTERACTIVE_SEEN} = 'true';
    $ENV{LC_ALL} = 'C.UTF-8';
    $ENV{LANGUAGE} = 'C.UTF-8';
    $ENV{LANG} = 'C.UTF-8';

    # copy ARGV because getopt modifies it
    my @ARGVORIG = @ARGV;

    my $options = {
	components => "main",
	variant => "important",
	include => undef,
	mode => 'auto',
	dpkgopts => [],
	aptopts => [],
    };
    chomp ($options->{architectures} = `dpkg --print-architecture`);
    Getopt::Long::Configure ("bundling");
    GetOptions(
	'h|help'   => sub { pod2usage(-exitval => 0, -verbose => 2) },
	'components=s' => \$options->{components},
	'variant=s' => \$options->{variant},
	'include=s' => \$options->{include},
	'architectures=s' => \$options->{architectures},
	'mode=s' => \$options->{mode},
	'dpkgopt=s@' => \$options->{dpkgopts},
	'aptopt=s@' => \$options->{aptopts},
    ) or pod2usage(-exitval => 2, -verbose => 1);

    my @valid_variants = ('essential', 'apt', 'required', 'minbase', 'buildd',
	'important', 'debootstrap', '-', 'standard');
    if (none { $_ eq $options->{variant}} @valid_variants) {
	die "invalid variant. Choose from " . (join ', ', @valid_variants);
    }
    # debootstrap and - are an alias for important
    if (any { $_ eq $options->{variant} } ('-', 'debootstrap')) {
	$options->{variant} = 'important';
    }

    # fakeroot is an alias for fakechroot
    if ($options->{mode} eq 'fakeroot') {
	$options->{mode} = 'fakechroot';
    }
    # sudo is an alias for root
    if ($options->{mode} eq 'sudo') {
	$options->{mode} = 'root';
    }
    my @valid_modes = ('auto', 'root', 'unshare', 'fakechroot', 'proot');
    if (none { $_ eq $options->{mode} } @valid_modes) {
	die "invalid mode. Choose from " . (join ', ', @valid_modes);
    }

    if ($options->{variant} eq 'essential' and defined $options->{include}) {
	die "cannot install extra packages in essential and required variants because apt is missing";
    }

    my ($nativearch, @foreignarchs) = split /,/, $options->{architectures};
    $options->{nativearch} = $nativearch;
    $options->{foreignarchs} = \@foreignarchs;

    {
	my $deb2qemu = {
	    alpha => 'alpha',
	    amd64 => 'x86_64',
	    arm => 'arm',
	    arm64 => 'aarch64',
	    armel => 'arm',
	    armhf => 'arm',
	    hppa => 'hppa',
	    i386 => 'i386',
	    m68k => 'm68k',
	    mips => 'mips',
	    mips64 => 'mips64',
	    mips64el => 'mips64el',
	    mipsel => 'mipsel',
	    powerpc => 'ppc',
	    ppc64 => 'ppc64',
	    ppc64el => 'ppc64le',
	    riscv64 => 'riscv64',
	    s390x => 's390x',
	    sh4 => 'sh4',
	    sparc => 'sparc',
	    sparc64 => 'sparc64',
	};
	chomp (my $hostarch = `dpkg --print-architecture`);
	if ($hostarch ne $nativearch) {
	    my $pid = open my $fh, '-|' // die "failed to fork(): $!";
	    if ($pid == 0) {
		{ exec 'arch-test', '-n', $nativearch; }
		# if exec didn't work (for example because the arch-test program is
		# missing) prepare for the worst and assume that the architecture
		# cannot be executed
		print "$nativearch: not supported on this machine/kernel\n";
		exit 1;
	    }
	    chomp (my $content = do { local $/; <$fh> });
	    close $fh;
	    if ($? != 0 or $content ne "$nativearch: ok") {
		print STDERR "I: $nativearch cannot be executed, falling back to qemu-user\n";
		if (!exists $deb2qemu->{$nativearch}) {
		    die "no mapping from $nativearch to qemu-user binary";
		}
		$options->{qemu} = $deb2qemu->{$nativearch};
	    } else {
		print STDERR "I: $nativearch can be executed on this $hostarch machine\n";
	    }
	} else {
	    print STDERR "I: chroot architecture $nativearch is equal to the host's architecture\n";
	}
    }

    if (scalar @ARGV > 0) {
	$options->{suite} = shift @ARGV;
	if (scalar @ARGV > 0) {
	    $options->{target} = shift @ARGV;
	} else {
	    $options->{target} = '-';
	}
	$options->{mirrors} = [@ARGV];
    } else {
	print STDERR "I: No SUITE specified, expecting sources.list on standard input\n";
	$options->{suite} = 'UNDEFINED';
	$options->{target} = '-';
	$options->{mirrors} = ['-'];
    }

    if ($options->{target} ne '-') {
	my $abs_path = abs_path($options->{target});
	if (!defined $abs_path) {
	    die "unable to get absolute path of target directory $options->{target}";
	}
	$options->{target} = $abs_path;
    }

    if ($options->{target} eq '/') {
	die "refusing to use the filesystem root as output directory";
    }

    # figure out the mode to use or test whether the chosen mode is legal
    if ($options->{mode} eq 'auto') {
	if ($EFFECTIVE_USER_ID == 0) {
	    $options->{mode} = 'root';
	} elsif (test_unshare()) {
	    $options->{mode} = 'unshare';
	} elsif (system('proot --version>/dev/null') == 0) {
	    $options->{mode} = 'proot';
	} elsif (system('fakechroot --version>/dev/null') == 0) {
	    $options->{mode} = 'fakechroot';
	} else {
	    die "unable to pick chroot mode automatically";
	}
	print STDERR "I: automatically chosen mode: $options->{mode}\n";
    } elsif ($options->{mode} eq 'root') {
	if ($EFFECTIVE_USER_ID != 0) {
	    die "need to be root";
	}
    } elsif ($options->{mode} eq 'proot') {
	if (system('proot --version>/dev/null') != 0) {
	    die "need working proot binary";
	}
    } elsif ($options->{mode} eq 'fakechroot') {
	# test if we are inside fakechroot already
	# We fork a child process because setting FAKECHROOT_DETECT seems to
	# be an irreversible operation for fakechroot.
	my $pid = open my $rfh, '-|' // die "failed to fork(): $!";
	if ($pid == 0) {
	    # with the FAKECHROOT_DETECT environment variable set, any program
	    # execution will be replaced with the output "fakeroot [version]"
	    $ENV{FAKECHROOT_DETECT} = 0;
	    exec 'echo', 'If fakechroot is running, this will not be printed';
	}
	my $content = do { local $/; <$rfh> };
	waitpid $pid, 0;
	if ($? == 0 and $content =~ /^fakechroot \d\.\d+$/) {
	    # fakechroot is already running
	} elsif (system('fakechroot --version>/dev/null') != 0) {
	    die "need working fakechroot binary";
	} else {
	    # exec ourselves again but within fakechroot
	    exec 'fakechroot', 'fakeroot', $PROGRAM_NAME, @ARGVORIG;
	}
    } elsif ($options->{mode} eq 'unshare') {
	if (!test_unshare()) {
	    my $procfile = '/proc/sys/kernel/unprivileged_userns_clone';
	    open(my $fh, '<', $procfile) or die "failed to open $procfile: $!";
	    chomp(my $content = do { local $/; <$fh> });
	    close($fh);
	    if ($content ne "1") {
		print STDERR "I: /proc/sys/kernel/unprivileged_userns_clone is set to $content\n";
		print STDERR "I: try running: sudo sysctl -w kernel.unprivileged_userns_clone=1\n";
		print STDERR "I: or permanently enable unprivileged usernamespaces by putting the setting into /etc/sysctl.d/\n";
	    }
	    exit 1;
	}
    } else {
	die "unknown mode: $options->{mode}";
    }

    my @tar_compress_opts = get_tar_compress_options($options->{target});

    # figure out whether a tarball has to be created in the end
    $options->{maketar} = 0;
    if (scalar @tar_compress_opts > 0 or $options->{target} =~ /\.tar$/ or $options->{target} eq '-') {
	$options->{maketar} = 1;
    }

    if ($options->{maketar}) {
	# since the output is a tarball, we create the rootfs in a temporary
	# directory
	$options->{root} = tempdir();
	# in unshare and root mode, other users than the current user need to
	# access the rootfs, most prominently, the _apt user. Thus, make the
	# temporary directory world readable.
	if (any { $_ eq $options->{mode} } ('unshare', 'root')) {
	    chmod 0755, $options->{root} or die "cannot chmod root: $!";
	}
    } else {
	# user does not seem to have specified a tarball as output, thus work
	# directly in the supplied directory
	$options->{root} = $options->{target};
	if (-e $options->{root}) {
	    if (!-d $options->{root}) {
		die "$options->{root} exists and is not a directory";
	    }
	    opendir(my $dh, $options->{root}) or die "Can't opendir($options->{root}): $!";
	    # Attempt reading the directory thrice. If the third time succeeds,
	    # then it has more entries than just "." and ".." and must thus not
	    # be empty.
	    readdir $dh;
	    readdir $dh;
	    # rationale for requiring an empty directory: https://bugs.debian.org/833525
	    die "$options->{root} is not empty" if (readdir $dh);
	} else {
	    make_path($options->{root}) or die "cannot create root: $!";
	}
    }

    my @idmap;
    # for unshare mode the rootfs directory has to have appropriate
    # permissions
    if ($options->{mode} eq 'unshare') {
	@idmap = read_subuid_subgid;
	# sanity check
	if (scalar(@idmap) != 2 || $idmap[0][0] ne 'u' || $idmap[1][0] ne 'g') {
	    printf STDERR "invalid idmap\n";
	    return 0;
	}

	my $outer_gid = $REAL_GROUP_ID+0;

	my $pid = get_unshare_cmd { chown 1, 1, $options->{root} }
	[
	    ['u', '0', $REAL_USER_ID, '1'],
	    ['g', '0', $outer_gid, '1'],
	    ['u', '1', $idmap[0][2], '1'],
	    ['g', '1', $idmap[1][2], '1']];
	waitpid $pid, 0;
	$? == 0 or die "chown failed";
    }

    # figure out whether we have mknod
    $options->{havemknod} = 0;
    if ($options->{mode} eq 'unshare') {
	my $pid = get_unshare_cmd {
	    $options->{havemknod} = havemknod($options->{root});
	} \@idmap;
	waitpid $pid, 0;
	$? == 0 or die "havemknod failed";
    } else {
	$options->{havemknod} = havemknod($options->{root});
    }

    my $devtar = '';
    # without real device nodes having been created, we manually craft the
    # right entries for the final tarball
    if ($options->{maketar} and not $options->{havemknod}) {
	foreach my $file (@devfiles) {
	    my ($fname, $mode, $type, $linkname, $devmajor, $devminor) = @{$file};
	    my $entry = pack('a100 a8 a8 a8 a12 a12 A8 a1 a100 a8 a32 a32 a8 a8 a155 x12',
		$fname,
		sprintf('%07o', $mode),
		sprintf('%07o', 0), # uid
		sprintf('%07o', 0), # gid
		sprintf('%011o', 0), # size
		sprintf('%011o', $mtime),
		'', # checksum
		$type,
		$linkname,
		"ustar  ",
		'', # username
		'', # groupname
		defined($devmajor) ? sprintf('%07o', $devmajor) : '',
		defined($devminor) ? sprintf('%07o', $devminor) : '',
		'', # prefix
	    );
	    # compute and insert checksum
	    substr($entry,148,7) = sprintf("%06o\0", unpack("%16C*",$entry));
	    $devtar .= $entry;
	}
    }

    my $exitstatus = 0;
    my @taropts = ('--sort=name', "--mtime=\@$mtime", '--clamp-mtime', '--numeric-owner', '--one-file-system', '-c');
    push @taropts, @tar_compress_opts;
    my $pid;
    pipe my $rfh, my $wfh;
    if ($options->{mode} eq 'unshare') {
	$pid = get_unshare_cmd {
	    close $rfh;
	    open(STDOUT, '>&', STDERR);

	    setup($options);

	    if ($options->{maketar}) {
		# redirect tar output to the writing end of the pipe so that the
		# parent process can capture the output
		open(STDOUT, '>&', $wfh);

		# Add ./dev as the first entries of the tar file.
		# We cannot add them after calling tar, because there is no way to
		# prevent tar from writing NULL entries at the end.
		print $devtar;

		# pack everything except ./dev
		0 == system('tar', @taropts, '--exclude=./dev', '-C', $options->{root}, '.') or die "tar failed: $?";

	    }

	    exit 0;
	} \@idmap;
    } else {
	$pid = fork() // die "fork() failed: $!";
	if ($pid == 0) {
	    close $rfh;
	    open(STDOUT, '>&', STDERR);

	    setup($options);

	    if ($options->{maketar}) {
		# redirect tar output to the writing end of the pipe so that the
		# parent process can capture the output
		open(STDOUT, '>&', $wfh);

		# Add ./dev as the first entries of the tar file.
		# We cannot add them after calling tar, because there is no way to
		# prevent tar from writing NULL entries at the end.
		print $devtar;

		if ($options->{mode} eq 'fakechroot') {
		    # Fakechroot requires tar to run inside the chroot or
		    # otherwise absolute symlinks will include the path to the
		    # root directory
		    0 == system('/usr/sbin/chroot', $options->{root}, 'tar', @taropts, '-C', '/', '.') or die "tar failed: $?";
		} elsif ($options->{mode} eq 'proot') {
		    # proot requires tar to run inside proot or otherwise
		    # permissions will be completely off
		    0 == system('proot', '--root-id', "--rootfs=$options->{root}", 'tar', @taropts, '-C', '/', '.') or die "tar failed: $?";
		} else {
		    0 == system('tar', @taropts, '-C', $options->{root}, '.') or die "tar failed: $?";
		}
	    }

	    exit 0;
	}
    }

    close $wfh;
    if ($options->{maketar}) {
	if ($options->{target} ne '-') {
	    open(my $fh, '>', $options->{target});
	    copy($rfh, $fh);
	    close($fh);
	} else {
	    copy($rfh, *STDOUT);
	}
    }
    close($rfh);
    waitpid $pid, 0;
    if ($? != 0) {
	$exitstatus = 1;
    }

    if ($options->{maketar} and -e $options->{root}) {
	if ($options->{mode} eq 'unshare') {
	    # We don't have permissions to remove the directory outside
	    # the unshared namespace, so we remove it here.
	    # Since this is still inside the unshared namespace, there is
	    # no risk of removing anything important.
	    $pid = get_unshare_cmd {
		remove_tree($options->{root}, {error => \my $err});
		if (@$err) {
		    for my $diag (@$err) {
			my ($file, $message) = %$diag;
			if ($file eq '') {
			    print "general error: $message\n";
			}
			else {
			    print "problem unlinking $file: $message\n";
			}
		    }
		}
	    } \@idmap;
	    waitpid $pid, 0;
	    $? == 0 or die "remove_tree failed";
	} else {
	    # without unshare, we use the system's rm to recursively remove the
	    # temporary directory just to make sure that we do not accidentally
	    # remove more than we should by using --one-file-system.
	    0 == system('rm', '--recursive', '--preserve-root', '--one-file-system', $options->{root}) or die "rm failed: $!";
	}
    }

    exit $exitstatus;
}

main();

__END__

=head1 NAME

mmdebstrap - multi-mirror Debian chroot creation

=head1 SYNOPSIS

B<mmdebstrap> [B<OPTION...>] [I<SUITE> [I<TARGET> [I<MIRROR>...]]]

=head1 DESCRIPTION

B<mmdebstrap> creates a Debian chroot of I<SUITE> into I<TARGET> from one or
more I<MIRROR>s. It is meant as an alternative to the debootstrap tool (see
section B<DEBOOTSTRAP>). In contrast to debootstrap it uses apt to resolve
dependencies and is thus able to use more than one mirror and resolve more
complex dependencies.

If no I<MIRROR> option is provided, http://deb.debian.org/debian is used. If
I<SUITE> is a stable release name, then mirrors for updates and security are
automatically added. If a I<MIRROR> option starts with "deb " or "deb-src "
then it is used as a one-line-style format entry for apt's sources.list inside
the chroot.  If a I<MIRROR> option contains a "://" then it is interpreted as
a mirror URI and the apt line inside the chroot is assembled as "deb [arch=A]
B C D" where A is the host's native architecture, B is the I<MIRROR>, C is the
given I<SUITE> and D is the components given via --components (defaults to
"main"). If a I<MIRROR> option happens to be an existing file, then its
contents are pasted into the chroot's sources.list. This can be used to supply
a deb822 style sources.list. If I<MIRROR> is C<-> then standard input is
pasted into the chroot's sources.list. More than one mirror can be specified
and are appended to the chroot's sources.list in the given order. If any
mirror contains a https URI, then the packages apt-transport-https and
ca-certificates will be installed inside the chroot.

The I<TARGET> argument can either be a directory or a tarball filename. If
I<TARGET> is a directory, then it must not exist beforehand. A tarball
filename is detected by the filename extension of I<TARGET>. Choosing a
directory only makes sense with the B<sudo> mode because otherwise the
contents of the chroot will not be owned by the superuser. If no I<TARGET> was
specified or if I<TARGET> is C<->, then a tarball of the chroot is written to
standard output.

The I<SUITE> may be a valid release code name (eg, sid, stretch, jessie) or a
symbolic name (eg, unstable, testing, stable, oldstable). Any suite name that
works with apt on the given mirror will work. If no I<SUITE> was specified,
then a single I<MIRROR> C<-> is added and thus the information of the desired
suite has to come from standard input as part of a valid apt sources.list
file.

=head1 OPTIONS

=over 8

=item B<-h,--help>

Print this help text and exit.

=item B<--variant>

Choose which package set to install. Valid variant names are B<essential>,
B<apt>, B<required>, B<minbase>, B<buildd>, B<important>, B<debootstrap>,
B<->, and B<standard>. The default variant is B<required>. See the section
B<VARIANTS> for more information.

=item B<--mode>

Choose how to perform the chroot operation and create a filesystem with
ownership information different from the current user. Valid modes are B<auto>,
B<sudo>, B<root>, B<unshare>, B<fakeroot>, B<fakechroot> and B<proot>. The
default mode is B<auto>. See the section B<MODES> for more information.

=item B<--aptopt>

Pass arbitrary options to apt. Will be added to
/etc/apt/apt.conf.d/00mmdebstrap inside the chroot.  Can be specified multiple
times. Each option with be appended to 00mmdebstrap. A semicolon will be added
at the end of the option if necessary. If the command line argument is an
existing file, the content of the file will be appended to 00mmdebstrap
verbatim.

Examples:

    --aptopt="Acquire::Check-Valid-Until false"
    --aptopt="Apt::Install-Recommends true"

=item B<--dpkgopt>

Pass arbitrary options to dpkg. Will be added to
/etc/dpkg/dpkg.cfg.d/00mmdebstrap inside the chroot. Can be specified multiple
times. Each option will be appended to 00mmdebstrap. If the command line
argument is an existing file, the content of the file will be appended to
00mmdebstrap verbatim.

Example: --dpkgopt="path-exclude=/usr/share/man/*"

=item B<--include>

Comma separated list of packages which will be installed in addition to the
packages installed by the specified variant. This option is incompatible with
the essential variant because apt inside the chroot is needed to install extra
packages.

=item B<--components>

Comma separated list of components like main, contrib and non-free.

=item B<--architectures>

Comma separated list of architectures. The first architecture is the native
architecture inside the chroot. The remaining architectures will be added to
the foreign architectures. This defaults to the native architecture of the
system running this program.

=back

=head1 MODES

Creating a Debian chroot requires not only permissions for running chroot but
also the ability to create files owned by the superuser. The selected mode
decides which way this is achieved.

=over 8

=item B<auto>

This mode automatically selects a fitting mode. If the effective user id is the
one of the superuser, then the B<sudo> mode is chosen. Otherwise, the
B<unshare> mode is picked if the system has the sysctl
C<kernel.unprivileged_userns_clone> set to C<1>. Should that not be the case
and if the proot binary exists, the B<proot> mode is chosen. Lastly, the
B<fakechroot> is used if the fakechroot binary exists.

=item B<sudo>, B<root>

This mode directly executes chroot and is the same mode of operation as is used
by debootstrap. It is the only mode that can directly create a directory chroot
with the right permissions.

=item B<unshare>

This mode uses Linux user namespaces to allow unpriviliged use of chroot and
creation of files that appear to be owned by the superuser inside the unshared
namespace. A directory chroot created with this mode will end up with wrong
permissions. Choose to create a tarball instead.

=item B<fakeroot>, B<fakechroot>

This mode will exec this program again under C<fakechroot fakeroot>. A
directory chroot created with this mode will end up with wrong permissions.
Choose to create a tarball instead.

=item B<proot>

This mode will carry out all calls to chroot with proot instead. Since
permissions are only retained while proot is still running, this will lead to
wrong permissions in the final directory and tarball. This mode is useful if
you plan to use the chroot with proot.

=back

=head1 VARIANTS

All package sets also include the hard dependencies (but not recommends) of
the selected package sets. The variants B<minbase>, B<buildd> and B<->,
resemble the package sets that debootstrap would install with the same
I<--variant> argument.

=over 8

=item B<essential>

Essential:yes packages.

=item B<apt>

The B<essential> set plus apt.

=item B<required>, B<minbase>

The B<essential> set plus all packages with Priority:required and apt.

=item B<buildd>

The B<minbase> set plus build-essential.

=item B<important>, B<debootstrap>, B<->

The B<required> set plus all packages with Priority:important. This is the default of debootstrap.

=item B<standard>

The B<important> set plus all packages with Priority:standard.

=back

=head1 EXAMPLES

Use like debootstrap:

    sudo mmdebstrap unstable ./unstable-chroot

Without superuser privileges:

    mmdebstrap unstable unstable-chroot.tar

With complex apt options:

    mmdebstrap < /etc/apt/sources.list > unstable-chroot.tar

Drop locales (but not the symlink to the locale name alias database),
translated manual packages (but not the untranslated ones), and documentation
(but not copyright and Debian changelog).

    mmdebstrap --variant=essential \
        --dpkgopt='path-exclude=/usr/share/man/*' \
        --dpkgopt='path-include=/usr/share/man/man[1-9]/*' \
        --dpkgopt='path-exclude=/usr/share/locale/*' \
        --dpkgopt='path-include=/usr/share/locale/locale.alias' \
        --dpkgopt='path-exclude=/usr/share/doc/*' \
        --dpkgopt='path-include=/usr/share/doc/*/copyright' \
        --dpkgopt='path-include=/usr/share/doc/*/changelog.Debian.*' \
        unstable debian-unstable.tar

=head1 DEBOOTSTRAP

This section lists some differences to debootstrap.

=over 8

=item * More than one mirror possible

=item * Default mirrors for stable releases include updates and security mirror

=item * Multiple ways to operate as non-root: fakechroot, proot, unshare

=item * 2-3 times faster

=item * Can create a chroot with only Essential:yes packages and their dependencies

=item * Reproducible output by default if $SOURCE_DATE_EPOCH is set

=item * Can create output on filesystems with nodev set

=item * apt cache and lists are cleaned at the end

=item * foreign architecture chroots using qemu-user

=back

Limitations in comparison to debootstrap:

=over 8

=item * Only runs on systems with apt installed

=item * No I<SCRIPT> argument

=item * No I<--second-stage> option.

=back

=head1 SEE ALSO

    debootstrap(8)

=cut