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243 lines
8.0 KiB
243 lines
8.0 KiB
<?php
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// This file is part of Moodle - http://moodle.org/
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//
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// Moodle is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// Moodle is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with Moodle. If not, see <http://www.gnu.org/licenses/>.
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/**
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* Postgres advisory locking factory.
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*
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* @package core
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* @category lock
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* @copyright Damyon Wiese 2013
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* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
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*/
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namespace core\lock;
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defined('MOODLE_INTERNAL') || die();
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/**
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* Postgres advisory locking factory.
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*
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* Postgres locking implementation using advisory locks. Some important points. Postgres has
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* 2 different forms of lock functions, some accepting a single int, and some accepting 2 ints. This implementation
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* uses the 2 int version so that it uses a separate namespace from the session locking. The second note,
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* is because postgres uses integer keys for locks, we first need to map strings to a unique integer. This is
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* done by storing the strings in the lock_db table and using the auto-id returned. There is a static cache for
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* id's in this function.
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*
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* @package core
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* @category lock
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* @copyright Damyon Wiese 2013
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* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
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*/
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class postgres_lock_factory implements lock_factory {
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/** @var int $dblockid - used as a namespace for these types of locks (separate from session locks) */
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protected $dblockid = -1;
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/** @var array $lockidcache - static cache for string -> int conversions required for pg advisory locks. */
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protected static $lockidcache = array();
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/** @var \moodle_database $db Hold a reference to the global $DB */
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protected $db;
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/** @var string $type Used to prefix lock keys */
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protected $type;
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/** @var array $openlocks - List of held locks - used by auto-release */
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protected $openlocks = array();
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/**
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* Calculate a unique instance id based on the database name and prefix.
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* @return int.
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*/
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protected function get_unique_db_instance_id() {
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global $CFG;
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$strkey = $CFG->dbname . ':' . $CFG->prefix;
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$intkey = crc32($strkey);
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// Normalize between 64 bit unsigned int and 32 bit signed ints. Php could return either from crc32.
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if (PHP_INT_SIZE == 8) {
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if ($intkey > 0x7FFFFFFF) {
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$intkey -= 0x100000000;
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}
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}
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return $intkey;
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}
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/**
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* Almighty constructor.
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* @param string $type - Used to prefix lock keys.
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*/
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public function __construct($type) {
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global $DB;
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$this->type = $type;
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$this->dblockid = $this->get_unique_db_instance_id();
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// Save a reference to the global $DB so it will not be released while we still have open locks.
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$this->db = $DB;
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\core_shutdown_manager::register_function(array($this, 'auto_release'));
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}
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/**
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* Is available.
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* @return boolean - True if this lock type is available in this environment.
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*/
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public function is_available() {
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return $this->db->get_dbfamily() === 'postgres';
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}
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/**
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* Return information about the blocking behaviour of the lock type on this platform.
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* @return boolean - Defer to the DB driver.
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*/
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public function supports_timeout() {
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return true;
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}
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/**
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* Will this lock type will be automatically released when a process ends.
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*
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* @return boolean - Via shutdown handler.
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*/
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public function supports_auto_release() {
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return true;
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}
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/**
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* Multiple locks for the same resource can be held by a single process.
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* @return boolean - Defer to the DB driver.
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*/
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public function supports_recursion() {
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return true;
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}
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/**
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* This function generates the unique index for a specific lock key.
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* Once an index is assigned to a key, it never changes - so this is
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* statically cached.
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*
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* @param string $key
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* @return int
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* @throws \moodle_exception
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*/
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protected function get_index_from_key($key) {
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if (isset(self::$lockidcache[$key])) {
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return self::$lockidcache[$key];
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}
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$index = 0;
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$record = $this->db->get_record('lock_db', array('resourcekey' => $key));
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if ($record) {
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$index = $record->id;
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}
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if (!$index) {
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$record = new \stdClass();
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$record->resourcekey = $key;
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try {
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$index = $this->db->insert_record('lock_db', $record);
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} catch (\dml_exception $de) {
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// Race condition - never mind - now the value is guaranteed to exist.
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$record = $this->db->get_record('lock_db', array('resourcekey' => $key));
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if ($record) {
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$index = $record->id;
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}
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}
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}
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if (!$index) {
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throw new \moodle_exception('Could not generate unique index for key');
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}
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self::$lockidcache[$key] = $index;
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return $index;
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}
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/**
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* Create and get a lock
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* @param string $resource - The identifier for the lock. Should use frankenstyle prefix.
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* @param int $timeout - The number of seconds to wait for a lock before giving up.
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* @param int $maxlifetime - Unused by this lock type.
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* @return boolean - true if a lock was obtained.
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*/
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public function get_lock($resource, $timeout, $maxlifetime = 86400) {
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$giveuptime = time() + $timeout;
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$token = $this->get_index_from_key($resource);
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$params = array('locktype' => $this->dblockid,
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'token' => $token);
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$locked = false;
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do {
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$result = $this->db->get_record_sql('SELECT pg_try_advisory_lock(:locktype, :token) AS locked', $params);
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$locked = $result->locked === 't';
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if (!$locked && $timeout > 0) {
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usleep(rand(10000, 250000)); // Sleep between 10 and 250 milliseconds.
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}
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// Try until the giveup time.
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} while (!$locked && time() < $giveuptime);
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if ($locked) {
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$this->openlocks[$token] = 1;
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return new lock($token, $this);
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}
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return false;
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}
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/**
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* Release a lock that was previously obtained with @lock.
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* @param lock $lock - a lock obtained from this factory.
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* @return boolean - true if the lock is no longer held (including if it was never held).
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*/
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public function release_lock(lock $lock) {
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$params = array('locktype' => $this->dblockid,
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'token' => $lock->get_key());
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$result = $this->db->get_record_sql('SELECT pg_advisory_unlock(:locktype, :token) AS unlocked', $params);
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$result = $result->unlocked === 't';
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if ($result) {
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unset($this->openlocks[$lock->get_key()]);
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}
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return $result;
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}
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/**
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* Extend a lock that was previously obtained with @lock.
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* @param lock $lock - a lock obtained from this factory.
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* @param int $maxlifetime - the new lifetime for the lock (in seconds).
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* @return boolean - true if the lock was extended.
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*/
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public function extend_lock(lock $lock, $maxlifetime = 86400) {
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// Not supported by this factory.
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return false;
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}
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/**
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* Auto release any open locks on shutdown.
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* This is required, because we may be using persistent DB connections.
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*/
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public function auto_release() {
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// Called from the shutdown handler. Must release all open locks.
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foreach ($this->openlocks as $key => $unused) {
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$lock = new lock($key, $this);
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$lock->release();
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}
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}
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}
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