--- /dev/null
+<?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 bool Whether the key is a "might wait" key
+ */
+ private $isMightWaitKey;
+ /**
+ * @var bool Whether this process holds a "might wait" lock key
+ */
+ private static $acquiredMightWaitKey = 0;
+
+ /**
+ * @param array $conf
+ * @param string $type The class of actions to limit concurrency for (task 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( $type, $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 The class of actions to limit concurrency for (task 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 &= !$this->isMightWaitKey;
+ }
+
+ /**
+ * Given a key (any string) and the number of lots, returns a slot key (a prefix with a suffix
+ * integer from the [0..($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 );
+ }
+}
+
+// @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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