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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/>.
/**
* Postgres advisory 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();
/**
* Postgres advisory locking factory.
*
* Postgres locking implementation using advisory locks. Some important points. Postgres has
* 2 different forms of lock functions, some accepting a single int, and some accepting 2 ints. This implementation
* uses the 2 int version so that it uses a separate namespace from the session locking. The second note,
* is because postgres uses integer keys for locks, we first need to map strings to a unique integer. This is done
* using a prefix of a sha1 hash converted to an integer.
*
* @package core
* @category lock
* @copyright Damyon Wiese 2013
* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
*/
class postgres_lock_factory implements lock_factory {
/** @var int $dblockid - used as a namespace for these types of locks (separate from session locks) */
protected $dblockid = -1;
/** @var array $lockidcache - static cache for string -> int conversions required for pg advisory locks. */
protected static $lockidcache = array();
/** @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();
/**
* Calculate a unique instance id based on the database name and prefix.
* @return int.
*/
protected function get_unique_db_instance_id() {
global $CFG;
$strkey = $CFG->dbname . ':' . $CFG->prefix;
$intkey = crc32($strkey);
// Normalize between 64 bit unsigned int and 32 bit signed ints. Php could return either from crc32.
if (PHP_INT_SIZE == 8) {
if ($intkey > 0x7FFFFFFF) {
$intkey -= 0x100000000;
}
}
return $intkey;
}
/**
* Almighty constructor.
* @param string $type - Used to prefix lock keys.
*/
public function __construct($type) {
global $DB;
$this->type = $type;
$this->dblockid = $this->get_unique_db_instance_id();
// 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'));
}
/**
* Is available.
* @return boolean - True if this lock type is available in this environment.
*/
public function is_available() {
return $this->db->get_dbfamily() === 'postgres';
}
/**
* 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 be held by a single process.
* @return boolean - Defer to the DB driver.
*/
public function supports_recursion() {
return true;
}
/**
* This function generates the unique index for a specific lock key using
* a sha1 prefix converted to decimal.
*
* @param string $key
* @return int
* @throws \moodle_exception
*/
protected function get_index_from_key($key) {
// A prefix of 7 hex chars is chosen as fffffff is the largest hex code
// which when converted to decimal (268435455) fits inside a 4 byte int
// which is the second param to pg_try_advisory_lock().
$hash = substr(sha1($key), 0, 7);
$index = hexdec($hash);
return $index;
}
/**
* 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) {
$giveuptime = time() + $timeout;
$token = $this->get_index_from_key($this->type . '_' . $resource);
$params = array('locktype' => $this->dblockid,
'token' => $token);
$locked = false;
do {
$result = $this->db->get_record_sql('SELECT pg_try_advisory_lock(:locktype, :token) AS locked', $params);
$locked = $result->locked === 't';
if (!$locked && $timeout > 0) {
usleep(rand(10000, 250000)); // Sleep between 10 and 250 milliseconds.
}
// Try until the giveup time.
} while (!$locked && time() < $giveuptime);
if ($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('locktype' => $this->dblockid,
'token' => $lock->get_key());
$result = $this->db->get_record_sql('SELECT pg_advisory_unlock(:locktype, :token) AS unlocked', $params);
$result = $result->unlocked === 't';
if ($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();
}
}
}