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diff --git a/includes/poolcounter/PoolCounter.php b/includes/poolcounter/PoolCounter.php
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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.
+ *
+ * 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;
+
+	/**
+	 * @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;
+	}
+
+	/**
+	 * 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 $this->key;
+	}
+
+	/**
+	 * I want to do this task and I need to do it myself.
+	 *
+	 * @return Status Value is one of Locked/Error
+	 */
+	abstract public function acquireForMe();
+
+	/**
+	 * I want to do this task, but if anyone else does it
+	 * instead, it's also fine for me. I will read its cached data.
+	 *
+	 * @return Status Value is one of Locked/Done/Error
+	 */
+	abstract public function acquireForAnyone();
+
+	/**
+	 * I have successfully finished my task.
+	 * Lets another one grab the lock, and returns the workers
+	 * waiting on acquireForAnyone()
+	 *
+	 * @return Status Value is one of Released/NotLocked/Error
+	 */
+	abstract public function release();
+
+	/**
+	 * Given a key (any string) and the number of lots, returns a slot number (an integer from the [0..($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 );
+	}
+}