Proyectos de Subversion Moodle

<?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/>.

namespace core\lock;

use coding_exception;

/**

 * 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;

    }

    /**

     * 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;

    }

    /**

     * 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();

        }

    }

}