#!/usr/bin/perl

use DBI;
use Data::Dumper;

use warnings;
use strict;

my $mountpoint = "/var/backups/bytemyback/mnt/";
my $snapshot_name = "byteback_data_snap";
my $snapshot_size = "20G";
my $dbh;

die "Must specify either pre or post\n" unless @ARGV;

if ($ARGV[0] eq 'pre') {
    # Check snapshot doesn't already exist
    die "Snapshot still exists\n" if `lvs 2>&1` =~ /$snapshot_name/;
    my $lock_tables = 1; # This will be configurable. Whether or not it runs FLUSH TABLES WITH READ LOCK before taking the snapshot.
    my $defaults_file = '/etc/mysql/debian.cnf';
    my $dsn = "DBI:mysql:;mysql_read_default_file=$defaults_file";
    $dbh = DBI->connect(
        $dsn, 
        undef, 
        undef, 
        {RaiseError => 1}
    ) or die "DBI::errstr: $DBI::errstr";

    my $data_dir = ask_mysql_for_var('datadir');
    my $lvm_dir = get_LVM_dir();
    die "MySQL doesn't seem to be running from LVM\n" if !$lvm_dir;
    $dbh->do("FLUSH TABLES WITH READ LOCK") if $lock_tables;
    # Create snapshot
    my $mounted_lvm = `df $lvm_dir | tail -n1 | cut -f1 -d" "`;
    if (!`lvcreate -n $snapshot_name -L $snapshot_size --snapshot $mounted_lvm`) {
        my $error = $!;
        $dbh->do("UNLOCK TABLES") if $lock_tables;
        die("Unable to create snapshot: $!\n");
    }
    else {
        $dbh->do("UNLOCK TABLES") if $lock_tables;
        chomp (my $mapped_snapshot = "/dev/mapper/" . `dmsetup ls | grep $snapshot_name | grep -v cow | cut -f1 -d "	"`);
        `mkdir -p $mountpoint`;
        `mount $mapped_snapshot $mountpoint`;
        `touch ${mountpoint}.bytebacklvm`;
    }
}
elsif ($ARGV[0] eq 'post') {
    `umount $mountpoint`;
    chomp (my $mapped_snapshot = "/dev/mapper/" . `dmsetup ls | grep $snapshot_name | grep -v cow | cut -f1 -d "	"`);
    `lvremove -f $mapped_snapshot`;
}

sub ask_mysql_for_var {
    my $var = '@@' . shift;
    my $query = $dbh->prepare("SELECT ${var}");
    $query->execute;
    return $query->fetchrow_hashref()->{$var};
}

sub get_LVM_dir {
    # This is a bit hacky, it checks if lvs returns,
    # then checks if the mountpoint is /dev/mapper/$firstbit-$secondbit
    # Returns true only if above matches and lvs output has $secondbit $firstbit somewhere
    my $datadir = ask_mysql_for_var('datadir');
    my $lvs = `lvs 2>&1`;
    if (($?) || ($lvs =~ /No volume groups found/)) {
        return 0;
    }
    my $output = `df $datadir | tail -n1`;
    $output =~ s/--/:/g;
    if ($output =~ m#/dev/mapper/([\w:]+)-([\w:]+)\s#) {
        my ($vg, $lv) = ($1, $2);
        $vg =~ s/:/-/; $lv =~ s/:/-/;
        if ($lvs =~ /\s+$lv\s+$vg\s/) {
            return $datadir;
        }
    }
    return 0;
}