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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/>.
/**
* This is a db record locking factory.
*
* @package core
* @category lock
* @copyright Damyon Wiese 2013
* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
*/
namespace core\lock;
defined('MOODLE_INTERNAL') || die();
/**
* This is a db record locking factory.
*
* This lock factory uses record locks relying on sql of the form "SET XXX where YYY" and checking if the
* value was set. It supports timeouts, autorelease and can work on any DB. The downside - is this
* will always be slower than some shared memory type locking function.
*
* @package core
* @category lock
* @copyright Damyon Wiese 2013
* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
*/
class db_record_lock_factory implements lock_factory {
/** @var \moodle_database $db Hold a reference to the global $DB */
protected $db;
/** @var string $type Used to prefix lock keys */
protected $type;
/** @var array $openlocks - List of held locks - used by auto-release */
protected $openlocks = array();
/**
* Is available.
* @return boolean - True if this lock type is available in this environment.
*/
public function is_available() {
return true;
}
/**
* Almighty constructor.
* @param string $type - Used to prefix lock keys.
*/
public function __construct($type) {
global $DB;
$this->type = $type;
// Save a reference to the global $DB so it will not be released while we still have open locks.
$this->db = $DB;
\core_shutdown_manager::register_function(array($this, 'auto_release'));
}
/**
* Return information about the blocking behaviour of the lock type on this platform.
* @return boolean - True
*/
public function supports_timeout() {
return true;
}
/**
* Will this lock type will be automatically released when a process ends.
*
* @return boolean - True (shutdown handler)
*/
public function supports_auto_release() {
return true;
}
/**
* Multiple locks for the same resource can be held by a single process.
* @return boolean - False - not process specific.
*/
public function supports_recursion() {
return false;
}
/**
* This function generates a unique token for the lock to use.
* It is important that this token is not solely based on time as this could lead
* to duplicates in a clustered environment (especially on VMs due to poor time precision).
*/
protected function generate_unique_token() {
return \core\uuid::generate();
}
/**
* 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) {
$token = $this->generate_unique_token();
$now = time();
$giveuptime = $now + $timeout;
$expires = $now + $maxlifetime;
$resourcekey = $this->type . '_' . $resource;
if (!$this->db->record_exists('lock_db', array('resourcekey' => $resourcekey))) {
$record = new \stdClass();
$record->resourcekey = $resourcekey;
$result = $this->db->insert_record('lock_db', $record);
}
$params = array('expires' => $expires,
'token' => $token,
'resourcekey' => $resourcekey,
'now' => $now);
$sql = 'UPDATE {lock_db}
SET
expires = :expires,
owner = :token
WHERE
resourcekey = :resourcekey AND
(owner IS NULL OR expires < :now)';
do {
$now = time();
$params['now'] = $now;
$this->db->execute($sql, $params);
$countparams = array('owner' => $token, 'resourcekey' => $resourcekey);
$result = $this->db->count_records('lock_db', $countparams);
$locked = $result === 1;
if (!$locked && $timeout > 0) {
usleep(rand(10000, 250000)); // Sleep between 10 and 250 milliseconds.
}
// Try until the giveup time.
} while (!$locked && $now < $giveuptime);
if ($locked) {
$this->openlocks[$token] = 1;
return new lock($token, $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 = array('noexpires' => null,
'token' => $lock->get_key(),
'noowner' => null);
$sql = 'UPDATE {lock_db}
SET
expires = :noexpires,
owner = :noowner
WHERE
owner = :token';
$result = $this->db->execute($sql, $params);
if ($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) {
$now = time();
$expires = $now + $maxlifetime;
$params = array('expires' => $expires,
'token' => $lock->get_key());
$sql = 'UPDATE {lock_db}
SET
expires = :expires,
WHERE
owner = :token';
$this->db->execute($sql, $params);
$countparams = array('owner' => $lock->get_key());
$result = $this->count_records('lock_db', $countparams);
return $result === 0;
}
/**
* Auto release any open locks on shutdown.
* This is required, because we may be using persistent DB connections.
*/
public function auto_release() {
// Called from the shutdown handler. Must release all open locks.
foreach ($this->openlocks as $key => $unused) {
$lock = new lock($key, $this);
$lock->release();
}
}
}