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PoolCounter.php
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PoolCounter.php
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<?php
/**
* 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
*/
namespace MediaWiki\PoolCounter;
use MediaWiki\Status\Status;
use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
/**
* Semaphore semantics to restrict how many workers may concurrently perform a task.
*
* When you have many workers (threads/servers) in service, and a
* cached item expensive to produce expires, you may get several workers
* computing the same expensive item at the same time.
*
* Given enough incoming requests and the item expiring quickly (non-cacheable,
* or lots of edits or other invalidation events) that single task can end up
* unfairly using most (or all) of the CPUs of the server cluster.
* This is also known as "Michael Jackson effect", as this scenario happened on
* the English Wikipedia in 2009 on the day Michael Jackson died.
* See also <https://wikitech.wikimedia.org/wiki/Michael_Jackson_effect>.
*
* PoolCounter was created to provide semaphore semantics to restrict the number
* of workers that may be concurrently performing a given task. Only one key
* can be locked by any PoolCounter instance of a process, except for keys
* that start with "nowait:". However, only non-blocking requests (timeout=0)
* may be used with a "nowait:" key.
*
* By default PoolCounterNull is used, which provides no locking.
* Install the poolcounterd service from
* <https://gerrit.wikimedia.org/g/mediawiki/services/poolcounter> to
* enable this feature.
*
* @since 1.16
* @stable to extend
*/
abstract class PoolCounter implements LoggerAwareInterface {
/* Return codes */
public const LOCKED = 1; /* Lock acquired */
public const RELEASED = 2; /* Lock released */
public const DONE = 3; /* Another worker did the work for you */
public const ERROR = -1; /* Indeterminate error */
public const NOT_LOCKED = -2; /* Called release() with no lock held */
public const QUEUE_FULL = -3; /* There are already maxqueue workers on this lock */
public const TIMEOUT = -4; /* Timeout exceeded */
public 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 int Maximum time in seconds to wait for the lock */
protected $timeout;
protected LoggerInterface $logger;
/**
* @var bool Whether the key is a "might wait" key
*/
private $isMightWaitKey;
/**
* @var int Whether this process holds a "might wait" lock key
*/
private static $acquiredMightWaitKey = 0;
/**
* @var bool Enable fast stale mode (T250248). This may be overridden by the work class.
*/
private $fastStale;
/**
* @param array $conf
* @param string $type The class of actions to limit concurrency for (task type)
* @param string $key
*/
public function __construct( array $conf, string $type, string $key ) {
$this->workers = $conf['workers'];
$this->maxqueue = $conf['maxqueue'];
$this->timeout = $conf['timeout'];
if ( isset( $conf['slots'] ) ) {
$this->slots = $conf['slots'];
}
$this->fastStale = $conf['fastStale'] ?? false;
$this->logger = new NullLogger();
if ( $this->slots ) {
$key = $this->hashKeyIntoSlots( $type, $key, $this->slots );
}
$this->key = $key;
$this->isMightWaitKey = !preg_match( '/^nowait:/', $this->key );
}
/**
* @return string
*/
public function getKey() {
return $this->key;
}
/**
* I want to do this task and I need to do it myself.
*
* @param int|null $timeout Wait timeout, or null to use value passed to
* the constructor
* @return Status Value is one of Locked/Error
*/
abstract public function acquireForMe( $timeout = null );
/**
* 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.
*
* @param int|null $timeout Wait timeout, or null to use value passed to
* the constructor
* @return Status Value is one of Locked/Done/Error
*/
abstract public function acquireForAnyone( $timeout = null );
/**
* 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 sensible.
* @return Status good for sensible requests, fatal for the not so sensible
* @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 &= !$this->isMightWaitKey;
}
/**
* Given a key (any string) and the number of lots, returns a slot key (a prefix with a suffix
* integer from the [0..($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 $type The class of actions to limit concurrency for (task type)
* @param string $key PoolCounter instance key (any string)
* @param int $slots The number of slots (max allowed value is 65536)
* @return string Slot key with the type and slot number
*/
protected function hashKeyIntoSlots( $type, $key, $slots ) {
return $type . ':' . ( hexdec( substr( sha1( $key ), 0, 4 ) ) % $slots );
}
/**
* Is fast stale mode (T250248) enabled? This may be overridden by the
* PoolCounterWork subclass.
*
* @return bool
*/
public function isFastStaleEnabled() {
return $this->fastStale;
}
/**
* @since 1.42
* @param LoggerInterface $logger
* @return void
*/
public function setLogger( LoggerInterface $logger ) {
$this->logger = $logger;
}
/**
* @internal For use in PoolCounterWork only
* @return LoggerInterface
*/
public function getLogger(): LoggerInterface {
return $this->logger;
}
}
/** @deprecated class alias since 1.42 */
class_alias( PoolCounter::class, 'PoolCounter' );

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