#!/usr/bin/perl
# vim: shiftwidth=4 tabstop=4
#
# Copyright 2014 by Marco d'Itri <md@Linux.IT>
#
# 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.

use warnings;
use strict;
use autodie;
use v5.16;

use File::Find::Rule;
use Cwd qw(abs_path);
use Errno;

my $Debug = 0;
my $Program_RC = 0;

check_free_space();

go_faster();

# print the long error message if something fails due to autodie
$SIG{__DIE__} = sub { fatal($_[0]); };

{
	foreach my $name (early_conversion_files()) {
		next if $name =~ m#^/usr/#;	# already converted
		convert_file($name);
	}

	my @dirs = directories_to_merge();

	# create any directory which is in / but not in /usr
	foreach my $dir (@dirs) {
		mkdir("/usr$dir") if not -e "/usr$dir";
	}

	my @later;
	my $rule = File::Find::Rule->mindepth(1)->maxdepth(1)->start(@dirs);
	while (defined (my $name = $rule->match)) {
		convert_file($name, \@later);
	}

	# symlinks must be converted after the rest to avoid races, because
	# they may point to binaries which have not been converted yet
	convert_file($_) foreach @later;

	verify_links_only($_) foreach @dirs;

	convert_directory($_) foreach @dirs;

	exit($Program_RC) if $Program_RC;
	print "The system has been successfully converted.\n";
	exit;
}

##############################################################################
sub convert_file {
	my ($n, $later) = @_;
	print "==== $n ====\n" if $Debug;

	# the source is a broken link
	if (-l $n and not -e $n and -e "/usr$n") {
		warn "WARNING: $n is a broken symlink and has been renamed!\n";
		rename($n, "$n.usrmerge-broken");
		return;
	}

	# the destination is a broken link
	if (-l "/usr$n" and not -e "/usr$n") {
		warn "WARNING: /usr$n is a broken symlink and has been renamed!\n";
		rename("/usr$n", "/usr$n.usrmerge-broken");
		# continue and move the source as usual
	}

	# is a directory and the destination does not exist
	if (-d $n and not -e "/usr$n") {
		mv($n, "/usr$n");					# XXX race
		symlink("/usr$n", $n);
		return;
	}

	# is a link and the destination does not exist, but defer the conversion
	if (-l $n and not -e "/usr$n" and $later) {
		push(@$later, $n);
		return;
	}

	# is a file or link and the destination does not exist
	if (not -e "/usr$n") {
		cp($n, "/usr$n");
		symlink("/usr$n", "$n~~tmp~usrmerge~~");
		rename("$n~~tmp~usrmerge~~", $n);
		# XXX alternative implementation:
		#mv($n, "/usr$n");					# XXX race
		#symlink("/usr$n", $n);
		return;
	}

	# both source and dest are directories
	if (-d $n and -d "/usr$n") {
		# so they have to be merged recursively
		my $rule = File::Find::Rule->mindepth(1)->maxdepth(1)->start($n);
		while (defined (my $name = $rule->match)) {
			convert_file($name, $later);
		}
		return;
	}

	# The other cases are more complex and there are some corner cases that
	# we do not try to resolve automatically.

	# both source and dest are links
	if (-l $n and -l "/usr$n") {
		my $l1 = readlink($n);
		my $l2 = readlink("/usr$n");
		my ($basedir) = $n =~ m#^(.+)/[^/]+$#;
		return if $l1 eq $l2;				# and they point to the same file
		return if "$basedir/$l1" eq $l2;	# same (the / link is relative)
		return if $l1 eq "/usr$basedir/$l2";# same (the /usr link is relative)
		return if $l1 eq "/usr$n";	# and the / link points to the /usr link
		if ($l2 eq $n) {			# and the /usr link points to the / link
			# the target of the new link will be an absolute path, so it
			# may be different from the original one even if both point to
			# the same file
			symlink(abs_path($n), "/usr$n~~tmp~usrmerge~~");
			rename("/usr$n~~tmp~usrmerge~~", "/usr$n");
			# convert the /bin link too to make the program idempotent
			symlink(abs_path($n), "$n~~tmp~usrmerge~~");
			rename("$n~~tmp~usrmerge~~", "$n");
			return;
		}
		fatal("Both $n and /usr$n exist");
	}

	# the source is a link
	if (-l $n) {
		my $l = readlink($n);
		return if $l eq "/usr$n";			# and it points to dest
		fatal("Both $n and /usr$n exist");
	}

	# the destination is a link
	if (-l "/usr$n") {
		my $l = readlink("/usr$n");
		if ($l eq $n) {						# and it points to source
			cp($n, "/usr$n~~tmp~usrmerge~~");
			rename("/usr$n~~tmp~usrmerge~~", "/usr$n");
			symlink("/usr$n", "$n~~tmp~usrmerge~~");
			rename("$n~~tmp~usrmerge~~", $n);
			# XXX alternative implementation:
			#mv($n, "/usr$n");				# XXX race
			#symlink("/usr$n", $n);
			return;
		}
		fatal("Both $n and /usr$n exist");
	}

	fatal("$n is a directory and /usr$n is not")
		if -d $n and -e "/usr$n";
	fatal("/usr$n is a directory and $n is not")
		if -d "/usr$n";
	fatal("Both $n and /usr$n exist")
		if -e "/usr$n";

	fatal("The status of $n and /usr$n is really unexpected");
}

##############################################################################
# To prevent a failure later, the regular files of the libraries used by
# cp and mv must be converted before of the symlinks that point to them.
sub early_conversion_files {
	open(my $fh, '-|', 'ldd /bin/cp');
	my @ldd = <$fh>;
	close $fh;

	# the libraries
	my @list = grep { $_ } map { /^\s+\S+ => (\S+) / and $1 } @ldd;
	# the dynamic linker
	push(@list, grep { $_ } map { /^\s+(\/\S+) \(/ and $1 } @ldd);

	# this is the equivalent of readlink --canonicalize
	my @newlist;
	foreach my $name (@list) {
		my $newname = -l $name ? readlink($name) : $name;
		if ($newname !~ m#^/#) {
			my $dir = $name;
			$dir =~ s#[^/]+$##;
			$newname = $dir . $newname;
		}
		my $topdir = $newname;
		$topdir =~ s#^(/[^/]+).*#$1#;
		# this is needed to be idempotent after a complete conversion
		next if -l $topdir;
		push(@newlist, $newname);
	}

	return @newlist;
}

##############################################################################
# Safety check: verify that there are no regular files in the directories
# that will be deleted by the final pass.
sub verify_links_only {
	my ($dir) = @_;

	my $link_or_dir = File::Find::Rule->or(
		File::Find::Rule->symlink,
		File::Find::Rule->directory,
	);

	my $rule = File::Find::Rule->mindepth(1)->not($link_or_dir)->start($dir);
	while (defined (my $name = $rule->match)) {
		print STDERR "$name is not a symlink!\n";
		print STDERR "\nSafety check: the conversion has failed!\n";
		exit 1;
	}
}

sub convert_directory {
	my ($dir) = @_;

	return if -l $dir;

	if (not -d $dir) {
		symlink("usr$dir", $dir);
		restore_context($dir);
		return;
	}

	no autodie;
	if (not rename($dir, "$dir~~delete~usrmerge~~")) {	# XXX race
		if ($!{EBUSY}) {
			handle_ebusy($dir);
			return;
		}
		die "Can't rename $dir: $!";
	}
	use autodie;
	symlink("usr$dir", $dir);
	restore_context($dir);

	rm('-rf', "$dir~~delete~usrmerge~~");
}

sub restore_context {
	my ($file) = @_;

	return if not -x '/sbin/restorecon';

	safe_system('restorecon', $file);
}

sub handle_ebusy {
	my ($dir) = @_;

	print STDERR <<END;

WARNING: renaming $dir/ (for the purpose of replacing it with a symlink
to /usr$dir/) has failed with the EBUSY error.
This is probably caused by a systemd service started with the
ProtectSystem option. Before running again this program you will need to
stop the relevant daemon(s) or reboot the system.
Do not install or update other Debian packages until the program
has been run successfully. (Removing packages is not harmful.)
END

	# continue, but have the program eventually return an error
	$Program_RC = 1;

	# if systemd is running...
	return if -d '/run/systemd/system';

	# list the services with ProtectSystem enabled
	my $cmd = q{
	for service in $(systemctl --no-legend --full list-units \
		    --state=active --type=service | cut -d ' ' -f 1); do
	  if systemctl show $service | egrep -q "^ProtectSystem=(yes|full)"; then
		echo $service
	  fi
	done
	};

	my $cmd_output = qx{$cmd} || return;
	print STDERR <<END;

The following services are using the ProtectSystem feature:
$cmd_output
END
}

##############################################################################
sub check_free_space {
	local $ENV{LC_ALL} = 'C';
	my $fh;

	open($fh, '-|', 'stat --dereference --file-system --format="%i" /');
	my $root_id = <$fh>;
	die "stat / failed" if not defined $root_id;
	chomp $root_id;

	# beware: df(1) reports the value of %a, not of %f
	open($fh, '-|',
		'stat --dereference --file-system --format="%f %S %i" /usr/');
	my $stat_output = <$fh>;
	die "stat /usr failed" if not defined $stat_output;
	chomp $stat_output;
	my ($free_blocks, $bs, $usr_id) = split(/ /, $stat_output);

	return if $root_id eq $usr_id;

	my $free = $free_blocks * ($bs / 1024);
	my @dirs = grep { -e $_ } directories_to_merge();

	open($fh, '-|',
		"du --summarize --no-dereference --total --block-size=1K @dirs");
	my $needed;
	while (<$fh>) {
		($needed) = /^(\d+)\s+total$/;
	}
	close $fh;
	die "df @dirs failed" if not defined $needed;

	say "Free space in /usr: $free KB." if $Debug;
	say "The origin directories (@dirs) require $needed KB." if $Debug;

	die "$free KB in /usr but $needed KB are required!\n" if $needed > $free;
}

##############################################################################
# Try to avoid the inherent races by being as fast as possible.
sub go_faster {
	system('/usr/bin/ionice', '--class=realtime', "--pid=$$")
		if -x '/usr/bin/ionice';
	system('/usr/bin/chrt', '--rr', '-p', '99', '--pid', $$)
		if -x '/usr/bin/chrt';
}

# We use cp from coreutils because it preserves extended attributes and
# handles sparse files.
sub cp {
	my ($source, $dest) = @_;

	safe_system('cp', '--no-dereference', '--preserve=all',
		'--reflink=auto', '--sparse=always', $source, $dest);
}

# We use mv from coreutils because it supports moving directories across
# filesystems, which would be inconvenient to implement here.
sub mv {
	my ($source, $dest) = @_;

	safe_system('mv', '--no-clobber', $source, $dest);
}

# We use rm from coreutils because I cannot be bothered to implement -r.
sub rm {
	safe_system('rm', @_);
}

sub safe_system {
	my (@cmd) = @_;

	my $rc = system(@cmd);
	die "Failed to execute @cmd: $!\n" if $rc == -1;
	die "@cmd: rc=" . ($? >> 8) . "\n" if $rc;
}

sub fatal {
	my ($msg) = @_;

	$msg .= ".\n" if $msg !~ /\n$/;

	print STDERR <<END;

FATAL ERROR:
$msg
You can try correcting the errors reported and running again
$0 until it will complete without errors.
Do not install or update other Debian packages until the program
has been run successfully.

END
	exit(1);
}

# Find out where the runtime dynamic linker and the shared libraries
# can be installed on each architecture: native, multilib and multiarch.
#
# This function must be updated to support any new architecture which
# either installs the RTLD in a directory different from /lib or builds
# multilib library packages.
sub directories_to_merge {
	my @dirs;
	push(@dirs, qw(lib32 lib64 libx32))	if running_arch('amd64');
	push(@dirs, qw(      lib64 libx32))	if running_arch('i386');
	push(@dirs, qw(lib32 lib64))		if running_arch('mips');
	push(@dirs, qw(lib32 lib64))		if running_arch('mipsel');
	push(@dirs, qw(lib32 lib64 libo32))	if running_arch('mips64');
	push(@dirs, qw(lib32 lib64 libo32))	if running_arch('mips64el');
	push(@dirs, qw(lib32 lib64 libo32))	if running_arch('mipsn32');
	push(@dirs, qw(lib32 lib64 libo32))	if running_arch('mipsn32el');
	push(@dirs, qw(      lib64))		if running_arch('powerpc');
	push(@dirs, qw(lib32 lib64))		if running_arch('ppc64');
	push(@dirs, qw(      lib64))		if running_arch('ppc64el');
	push(@dirs, qw(lib32))				if running_arch('s390x');
	push(@dirs, qw(      lib64))		if running_arch('sparc');
	push(@dirs, qw(lib32 lib64))		if running_arch('sparc64');
	push(@dirs, qw(lib32 lib64 libx32))	if running_arch('x32');

	# remove the duplicates
	my %seen;
	@dirs = sort grep { not $seen{ $_ }++ } @dirs;

	unshift(@dirs, qw(bin sbin lib));
	return map { "/$_" } @dirs;
}

# check if the argument is one of the architectures enabled on the system
sub running_arch {
	my ($wanted) = @_;

	state @system_arch;
	if (not @system_arch) {
		push(@system_arch, `dpkg --print-architecture`);
		push(@system_arch, `dpkg --print-foreign-architectures`);
		chomp @system_arch;
	}

	return 1 if grep { $_ eq $wanted } @system_arch;
	return 0;
}

##############################################################################
__END__

=head1 NAME

convert-usrmerge - converts the system to everything-in-usr

=head1 SYNOPSIS

convert-usrmerge

=head1 DESCRIPTION

This program will automatically convert the system to the
everything-in-usr directory scheme.

There is no automatic method to restore the precedent configuration, so
there is no going back once the program has been run.

The program is idempotent, unless the system crashes at a really bad time.

The conflicts of all files in the Debian archive can be solved
automatically, but some corner cases of custom setups may require
manual changes.

=head1 CONFLICTS RESOLUTION MATRIX

 s/d F   D   L   B
 F   X   X   S   Rd
 D   X   D   S   Rd
 L   K   K   K?  Rd
 B   Rs  Rs  Rs  Rs

I<Source> is / and I<destination> is /usr.

Types of files:

=over 4

=item F: file

=item D: directory

=item L: symbolic link

=item B: broken symbolic link

=back

Actions:

=over 4

=item X: unresolvable conflict

=item D: recurse and compare the content of the directories

=item K: keep the source (if the link matches the destination)

=item S: swap source and destination (if the link matches the destination)

=item R: rename (source or destination)

=back

=head1 BUGS

Replacing a directory with a symlink is racy unless we do a complex dance
of bind mounts. We should decide if this is really needed.

Conflicting relative symbolic links are not handled automatically.

The libc6-i386 and libc6-x32 packages require to convert the /lib32 and
/libx32 directories as well, otherwise they would only contain the
symlink to the dynamic linker specified by the architecture ABI.

=head1 AUTHOR

The program and this man page have been written by Marco d'Itri
and are licensed under the terms of the GNU General Public License,
version 2 or higher.