#!/usr/bin/perl

use strict;
use Cwd 'abs_path';

my $RSYNC = '/usr/bin/rsync';

my $dest_dir = $ARGV[-1];
usage(1) if $dest_dir eq '' || $dest_dir =~ /^--/;

if (!-d $dest_dir) {
    print STDERR "$dest_dir is not a directory.\n\n";
    usage(1);
}

if (@_ = grep(/^--(link|compare)-dest/, @ARGV)) {
    $_ = join(' or ', @_);
    print STDERR "You may not use $_ as an rsync option.\n\n";
    usage(1);
}

$dest_dir = abs_path($dest_dir);
if ($dest_dir eq '/') {
    print STDERR 'You must not use "/" as the destination directory.', "\n\n";
    usage(1);
}

my $old_dir = "$dest_dir~old~";
my $new_dir = $ARGV[-1] = "$dest_dir~new~";

if (-d $old_dir) {
    rename($old_dir, $new_dir) or die "Unable to rename $old_dir to $new_dir: $!";
}

if (system($RSYNC, "--link-dest=$dest_dir", @ARGV)) {
    if ($? == -1) {
	print "failed to execute $RSYNC: $!\n";
    } elsif ($? & 127) {
	printf "child died with signal %d, %s coredump\n",
	    ($? & 127),  ($? & 128) ? 'with' : 'without';
    } else {
	printf "child exited with value %d\n", $? >> 8;
    }
    exit $?;
}

rename($dest_dir, $old_dir) or die "Unable to rename $new_dir to $old_dir: $!";
rename($new_dir, $dest_dir) or die "Unable to rename $new_dir to $dest_dir: $!";

exit;


sub usage
{
    my($ret) = @_;
    my $fh = $ret ? *STDERR : *STDOUT;
    print $fh <<EOT;
Usage: atomic-rsync [RSYNC-OPTIONS] HOST:SOURCE DEST

This script allows you to pull some files into DEST on the local system
(which must exist) in an atomic manner.  It does this by first pulling
files to DEST~new~ (using hard-links to unchanged files in order to keep
the space requirements down), and then, at the end of the transfer, it
renames DEST to DEST~old~ and renames DEST~new~ to DEST to effect the
atomic update.  The DEST~old~ hierarchy will be preserved until the next
run of this script, at which point it will be renamed to DEST~new~ and
used in the copy.

See the "rsync" command for its list of options.  You may not use the
--link-dest or --compare-dest options (since this script uses --link-dest
to effect the atomic transfer).  Also, DEST cannot be "/".
EOT
    exit $ret;
}