249 lines
6.9 KiB
PHP
249 lines
6.9 KiB
PHP
<?php
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/**
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* Provides of semaphore semantics for restricting the number
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* of workers that may be concurrently performing the same task.
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*
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* This program 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 2 of the License, or
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* (at your option) any later version.
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*
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* This program 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 along
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* with this program; if not, write to the Free Software Foundation, Inc.,
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* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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* http://www.gnu.org/copyleft/gpl.html
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*
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* @file
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*/
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/**
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* When you have many workers (threads/servers) giving service, and a
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* cached item expensive to produce expires, you may get several workers
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* doing the job at the same time.
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*
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* Given enough requests and the item expiring fast (non-cacheable,
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* lots of edits...) that single work can end up unfairly using most (all)
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* of the cpu of the pool. This is also known as 'Michael Jackson effect'
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* since this effect triggered on the english wikipedia on the day Michael
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* Jackson died, the biographical article got hit with several edits per
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* minutes and hundreds of read hits.
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*
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* The PoolCounter provides semaphore semantics for restricting the number
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* of workers that may be concurrently performing such single task.
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*
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* By default PoolCounter_Stub is used, which provides no locking. You
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* can get a useful one in the PoolCounter extension.
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*/
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abstract class PoolCounter {
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/* Return codes */
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const LOCKED = 1; /* Lock acquired */
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const RELEASED = 2; /* Lock released */
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const DONE = 3; /* Another worker did the work for you */
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const ERROR = -1; /* Indeterminate error */
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const NOT_LOCKED = -2; /* Called release() with no lock held */
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const QUEUE_FULL = -3; /* There are already maxqueue workers on this lock */
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const TIMEOUT = -4; /* Timeout exceeded */
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const LOCK_HELD = -5; /* Cannot acquire another lock while you have one lock held */
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/** @var string All workers with the same key share the lock */
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protected $key;
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/** @var integer Maximum number of workers doing the task simultaneously */
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protected $workers;
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/** @var integer If this number of workers are already working/waiting, fail instead of wait */
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protected $maxqueue;
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/** @var float Maximum time in seconds to wait for the lock */
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protected $timeout;
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/**
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* @param array $conf
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* @param string $type
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* @param string $key
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*/
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protected function __construct( $conf, $type, $key ) {
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$this->key = $key;
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$this->workers = $conf['workers'];
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$this->maxqueue = $conf['maxqueue'];
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$this->timeout = $conf['timeout'];
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}
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/**
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* Create a Pool counter. This should only be called from the PoolWorks.
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*
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* @param $type
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* @param $key
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*
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* @return PoolCounter
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*/
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public static function factory( $type, $key ) {
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global $wgPoolCounterConf;
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if ( !isset( $wgPoolCounterConf[$type] ) ) {
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return new PoolCounter_Stub;
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}
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$conf = $wgPoolCounterConf[$type];
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$class = $conf['class'];
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return new $class( $conf, $type, $key );
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}
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/**
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* I want to do this task and I need to do it myself.
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*
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* @return Status Value is one of Locked/Error
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*/
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abstract public function acquireForMe();
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/**
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* I want to do this task, but if anyone else does it
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* instead, it's also fine for me. I will read its cached data.
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*
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* @return Status Value is one of Locked/Done/Error
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*/
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abstract public function acquireForAnyone();
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/**
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* I have successfully finished my task.
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* Lets another one grab the lock, and returns the workers
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* waiting on acquireForAnyone()
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*
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* @return Status value is one of Released/NotLocked/Error
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*/
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abstract public function release();
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}
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class PoolCounter_Stub extends PoolCounter {
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public function __construct() {
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/* No parameters needed */
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}
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public function acquireForMe() {
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return Status::newGood( PoolCounter::LOCKED );
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}
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public function acquireForAnyone() {
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return Status::newGood( PoolCounter::LOCKED );
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}
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public function release() {
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return Status::newGood( PoolCounter::RELEASED );
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}
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}
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/**
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* Handy class for dealing with PoolCounters using class members instead of callbacks.
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*/
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abstract class PoolCounterWork {
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protected $cacheable = false; //Does this override getCachedWork() ?
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/**
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* @param string $type The type of PoolCounter to use
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* @param string $key Key that identifies the queue this work is placed on
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*/
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public function __construct( $type, $key ) {
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$this->poolCounter = PoolCounter::factory( $type, $key );
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}
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/**
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* Actually perform the work, caching it if needed
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* @return mixed work result or false
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*/
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abstract public function doWork();
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/**
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* Retrieve the work from cache
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* @return mixed work result or false
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*/
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public function getCachedWork() {
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return false;
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}
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/**
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* A work not so good (eg. expired one) but better than an error
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* message.
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* @return mixed work result or false
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*/
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public function fallback() {
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return false;
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}
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/**
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* Do something with the error, like showing it to the user.
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* @return bool
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*/
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function error( $status ) {
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return false;
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}
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/**
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* Log an error
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*
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* @param $status Status
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* @return void
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*/
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function logError( $status ) {
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wfDebugLog( 'poolcounter', $status->getWikiText() );
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}
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/**
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* Get the result of the work (whatever it is), or false.
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* @param $skipcache bool
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* @return bool|mixed
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*/
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public function execute( $skipcache = false ) {
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if ( $this->cacheable && !$skipcache ) {
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$status = $this->poolCounter->acquireForAnyone();
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} else {
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$status = $this->poolCounter->acquireForMe();
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}
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if ( !$status->isOK() ) {
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// Respond gracefully to complete server breakage: just log it and do the work
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$this->logError( $status );
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return $this->doWork();
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}
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switch ( $status->value ) {
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case PoolCounter::LOCKED:
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$result = $this->doWork();
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$this->poolCounter->release();
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return $result;
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case PoolCounter::DONE:
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$result = $this->getCachedWork();
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if ( $result === false ) {
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/* That someone else work didn't serve us.
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* Acquire the lock for me
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*/
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return $this->execute( true );
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}
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return $result;
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case PoolCounter::QUEUE_FULL:
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case PoolCounter::TIMEOUT:
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$result = $this->fallback();
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if ( $result !== false ) {
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return $result;
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}
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/* no break */
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/* These two cases should never be hit... */
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case PoolCounter::ERROR:
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default:
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$errors = array(
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PoolCounter::QUEUE_FULL => 'pool-queuefull',
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PoolCounter::TIMEOUT => 'pool-timeout' );
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$status = Status::newFatal( isset( $errors[$status->value] )
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? $errors[$status->value]
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: 'pool-errorunknown' );
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$this->logError( $status );
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return $this->error( $status );
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}
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}
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}
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