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<?php
// This file is part of Moodle - http://moodle.org/
//
// Moodle 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 3 of the License, or
// (at your option) any later version.
//
// Moodle is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Moodle. If not, see <http://www.gnu.org/licenses/>.
/**
* MySQL / MariaDB locking factory.
*
* @package core
* @category lock
* @copyright Brendan Heywood <brendan@catalyst-au.net>
* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
*/
namespace core\lock;
defined('MOODLE_INTERNAL') || die();
/**
* MySQL / MariaDB locking factory.
*
* @package core
* @category lock
* @copyright Brendan Heywood <brendan@catalyst-au.net>
* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
*/
class mysql_lock_factory implements lock_factory {
/** @var string $dbprefix - used as a namespace for these types of locks */
protected $dbprefix = '';
/** @var \moodle_database $db Hold a reference to the global $DB */
protected $db;
/** @var array $openlocks - List of held locks - used by auto-release */
protected $openlocks = [];
/**
* Return a unique prefix based on the database name and prefix.
* @param string $type - Used to prefix lock keys.
* @return string.
*/
protected function get_unique_db_prefix($type) {
global $CFG;
$prefix = $CFG->dbname . ':';
if (isset($CFG->prefix)) {
$prefix .= $CFG->prefix;
}
$prefix .= '_' . $type . '_';
return $prefix;
}
/**
* Lock constructor.
* @param string $type - Used to prefix lock keys.
*/
public function __construct($type) {
global $DB;
$this->dbprefix = $this->get_unique_db_prefix($type);
// Save a reference to the global $DB so it will not be released while we still have open locks.
$this->db = $DB;
\core_shutdown_manager::register_function([$this, 'auto_release']);
}
/**
* Is available.
* @return boolean - True if this lock type is available in this environment.
*/
public function is_available() {
return $this->db->get_dbfamily() === 'mysql';
}
/**
* Return information about the blocking behaviour of the lock type on this platform.
* @return boolean - Defer to the DB driver.
*/
public function supports_timeout() {
return true;
}
/**
* Will this lock type will be automatically released when a process ends.
*
* @return boolean - Via shutdown handler.
*/
public function supports_auto_release() {
return true;
}
/**
* Multiple locks for the same resource can NOT be held by a single process.
*
* Hard coded to false and workaround inconsistent support in different
* versions of MySQL / MariaDB.
*
* @return boolean - false
*/
public function supports_recursion() {
return false;
}
/**
* Create and get a lock
* @param string $resource - The identifier for the lock. Should use frankenstyle prefix.
* @param int $timeout - The number of seconds to wait for a lock before giving up.
* @param int $maxlifetime - Unused by this lock type.
* @return boolean - true if a lock was obtained.
*/
public function get_lock($resource, $timeout, $maxlifetime = 86400) {
// We sha1 to avoid long key names hitting the mysql str limit.
$resourcekey = sha1($this->dbprefix . $resource);
// Even though some versions of MySQL and MariaDB can support stacked locks
// just never stack them and always fail fast.
if (isset($this->openlocks[$resourcekey])) {
return false;
}
$params = [
'resourcekey' => $resourcekey,
'timeout' => $timeout
];
$result = $this->db->get_record_sql('SELECT GET_LOCK(:resourcekey, :timeout) AS locked', $params);
$locked = $result->locked == 1;
if ($locked) {
$this->openlocks[$resourcekey] = 1;
return new lock($resourcekey, $this);
}
return false;
}
/**
* Release a lock that was previously obtained with @lock.
* @param lock $lock - a lock obtained from this factory.
* @return boolean - true if the lock is no longer held (including if it was never held).
*/
public function release_lock(lock $lock) {
$params = [
'resourcekey' => $lock->get_key()
];
$result = $this->db->get_record_sql('SELECT RELEASE_LOCK(:resourcekey) AS unlocked', $params);
$result = $result->unlocked == 1;
if ($result) {
unset($this->openlocks[$lock->get_key()]);
}
return $result;
}
/**
* Extend a lock that was previously obtained with @lock.
* @param lock $lock - a lock obtained from this factory.
* @param int $maxlifetime - the new lifetime for the lock (in seconds).
* @return boolean - true if the lock was extended.
*/
public function extend_lock(lock $lock, $maxlifetime = 86400) {
// Not supported by this factory.
return false;
}
/**
* Auto release any open locks on shutdown.
* This is required, because we may be using persistent DB connections.
*/
public function auto_release() {
// Called from the shutdown handler. Must release all open locks.
foreach ($this->openlocks as $key => $unused) {
$lock = new lock($key, $this);
$lock->release();
}
}
}