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<?php
/**
* Provides of semaphore semantics for restricting the number
* of workers that may be concurrently performing the same task.
*
* This program 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 2 of the License, or
* (at your option) any later version.
*
* This program 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 this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
* http://www.gnu.org/copyleft/gpl.html
*
* @file
*/
/**
* When you have many workers (threads/servers) giving service, and a
* cached item expensive to produce expires, you may get several workers
* doing the job at the same time.
*
* Given enough requests and the item expiring fast (non-cacheable,
* lots of edits...) that single work can end up unfairly using most (all)
* of the cpu of the pool. This is also known as 'Michael Jackson effect'
* since this effect triggered on the english wikipedia on the day Michael
* Jackson died, the biographical article got hit with several edits per
* minutes and hundreds of read hits.
*
* The PoolCounter provides semaphore semantics for restricting the number
* of workers that may be concurrently performing such single task. Only one
* key can be locked by any PoolCounter instance of a process, except for keys
* that start with "nowait:". However, only 0 timeouts (non-blocking requests)
* can be used with "nowait:" keys.
*
* By default PoolCounter_Stub is used, which provides no locking. You
* can get a useful one in the PoolCounter extension.
*/
abstract class PoolCounter {
/* Return codes */
const LOCKED = 1; /* Lock acquired */
const RELEASED = 2; /* Lock released */
const DONE = 3; /* Another worker did the work for you */
const ERROR = -1; /* Indeterminate error */
const NOT_LOCKED = -2; /* Called release() with no lock held */
const QUEUE_FULL = -3; /* There are already maxqueue workers on this lock */
const TIMEOUT = -4; /* Timeout exceeded */
const LOCK_HELD = -5; /* Cannot acquire another lock while you have one lock held */
/** @var string All workers with the same key share the lock */
protected $key;
/** @var int Maximum number of workers working on tasks with the same key simultaneously */
protected $workers;
/**
* Maximum number of workers working on this task type, regardless of key.
* 0 means unlimited. Max allowed value is 65536.
* The way the slot limit is enforced is overzealous - this option should be used with caution.
* @var int
*/
protected $slots = 0;
/** @var int If this number of workers are already working/waiting, fail instead of wait */
protected $maxqueue;
/** @var float Maximum time in seconds to wait for the lock */
protected $timeout;
/**
* @var boolean Whether the key is a "might wait" key
*/
private $isMightWaitKey;
/**
* @var boolean Whether this process holds a "might wait" lock key
*/
private static $acquiredMightWaitKey = 0;
/**
* @param array $conf
* @param string $type
* @param string $key
*/
protected function __construct( $conf, $type, $key ) {
$this->workers = $conf['workers'];
$this->maxqueue = $conf['maxqueue'];
$this->timeout = $conf['timeout'];
if ( isset( $conf['slots'] ) ) {
$this->slots = $conf['slots'];
}
if ( $this->slots ) {
$key = $this->hashKeyIntoSlots( $key, $this->slots );
}
$this->key = $key;
$this->isMightWaitKey = !preg_match( '/^nowait:/', $this->key );
}
/**
* Create a Pool counter. This should only be called from the PoolWorks.
*
* @param string $type
* @param string $key
*
* @return PoolCounter
*/
public static function factory( $type, $key ) {
global $wgPoolCounterConf;
if ( !isset( $wgPoolCounterConf[$type] ) ) {
return new PoolCounter_Stub;
}
$conf = $wgPoolCounterConf[$type];
$class = $conf['class'];
return new $class( $conf, $type, $key );
}
/**
* @return string
*/
public function getKey() {
return $this->key;
}
/**
* I want to do this task and I need to do it myself.
*
* @return Status Value is one of Locked/Error
*/
abstract public function acquireForMe();
/**
* I want to do this task, but if anyone else does it
* instead, it's also fine for me. I will read its cached data.
*
* @return Status Value is one of Locked/Done/Error
*/
abstract public function acquireForAnyone();
/**
* I have successfully finished my task.
* Lets another one grab the lock, and returns the workers
* waiting on acquireForAnyone()
*
* @return Status Value is one of Released/NotLocked/Error
*/
abstract public function release();
/**
* Checks that the lock request is sane.
* @return Status - good for sane requests fatal for insane
* @since 1.25
*/
final protected function precheckAcquire() {
if ( $this->isMightWaitKey ) {
if ( self::$acquiredMightWaitKey ) {
/*
* The poolcounter itself is quite happy to allow you to wait
* on another lock while you have a lock you waited on already
* but we think that it is unlikely to be a good idea. So we
* made it an error. If you are _really_ _really_ sure it is a
* good idea then feel free to implement an unsafe flag or
* something.
*/
return Status::newFatal( 'poolcounter-usage-error',
'You may only aquire a single non-nowait lock.' );
}
} elseif ( $this->timeout !== 0 ) {
return Status::newFatal( 'poolcounter-usage-error',
'Locks starting in nowait: must have 0 timeout.' );
}
return Status::newGood();
}
/**
* Update any lock tracking information when the lock is acquired
* @since 1.25
*/
final protected function onAcquire() {
self::$acquiredMightWaitKey |= $this->isMightWaitKey;
}
/**
* Update any lock tracking information when the lock is released
* @since 1.25
*/
final protected function onRelease() {
self::$acquiredMightWaitKey &= !$this->isMightWaitKey;
}
/**
* Given a key (any string) and the number of lots, returns a slot number (an integer from the [0..($slots-1)] range).
* This is used for a global limit on the number of instances of a given type that can acquire a lock.
* The hashing is deterministic so that PoolCounter::$workers is always an upper limit of how many instances with
* the same key can acquire a lock.
*
* @param string $key PoolCounter instance key (any string)
* @param int $slots The number of slots (max allowed value is 65536)
* @return int
*/
protected function hashKeyIntoSlots( $key, $slots ) {
return hexdec( substr( sha1( $key ), 0, 4 ) ) % $slots;
}
}
// @codingStandardsIgnoreStart Squiz.Classes.ValidClassName.NotCamelCaps
class PoolCounter_Stub extends PoolCounter {
// @codingStandardsIgnoreEnd
public function __construct() {
/* No parameters needed */
}
public function acquireForMe() {
return Status::newGood( PoolCounter::LOCKED );
}
public function acquireForAnyone() {
return Status::newGood( PoolCounter::LOCKED );
}
public function release() {
return Status::newGood( PoolCounter::RELEASED );
}
}
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