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[mediawiki.git] / includes / jobqueue / JobQueueDB.php

<?php
/**
 * Database-backed job queue code.
 *
 * 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
 * @author Aaron Schulz
 */

/**
 * Class to handle job queues stored in the DB
 *
 * @ingroup JobQueue
 * @since 1.21
 */
class JobQueueDB extends JobQueue {
        const CACHE_TTL_SHORT = 30; // integer; seconds to cache info without re-validating
        const CACHE_TTL_LONG = 300; // integer; seconds to cache info that is kept up to date
        const MAX_AGE_PRUNE = 604800; // integer; seconds a job can live once claimed
        const MAX_JOB_RANDOM = 2147483647; // integer; 2^31 - 1, used for job_random
        const MAX_OFFSET = 255; // integer; maximum number of rows to skip

        /** @var BagOStuff */
        protected $cache;

        /** @var bool|string Name of an external DB cluster. False if not set */
        protected $cluster = false;

        /**
         * Additional parameters include:
         *   - cluster : The name of an external cluster registered via LBFactory.
         *               If not specified, the primary DB cluster for the wiki will be used.
         *               This can be overridden with a custom cluster so that DB handles will
         *               be retrieved via LBFactory::getExternalLB() and getConnection().
         * @param array $params
         */
        protected function __construct( array $params ) {
                global $wgMemc;

                parent::__construct( $params );

                $this->cluster = isset( $params['cluster'] ) ? $params['cluster'] : false;
                // Make sure that we don't use the SQL cache, which would be harmful
                $this->cache = ( $wgMemc instanceof SqlBagOStuff ) ? new EmptyBagOStuff() : $wgMemc;
        }

        protected function supportedOrders() {
                return array( 'random', 'timestamp', 'fifo' );
        }

        protected function optimalOrder() {
                return 'random';
        }

        /**
         * @see JobQueue::doIsEmpty()
         * @return bool
         */
        protected function doIsEmpty() {
                $key = $this->getCacheKey( 'empty' );

                $isEmpty = $this->cache->get( $key );
                if ( $isEmpty === 'true' ) {
                        return true;
                } elseif ( $isEmpty === 'false' ) {
                        return false;
                }

                $dbr = $this->getSlaveDB();
                try {
                        $found = $dbr->selectField( // unclaimed job
                                'job', '1', array( 'job_cmd' => $this->type, 'job_token' => '' ), __METHOD__
                        );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }
                $this->cache->add( $key, $found ? 'false' : 'true', self::CACHE_TTL_LONG );

                return !$found;
        }

        /**
         * @see JobQueue::doGetSize()
         * @return int
         */
        protected function doGetSize() {
                $key = $this->getCacheKey( 'size' );

                $size = $this->cache->get( $key );
                if ( is_int( $size ) ) {
                        return $size;
                }

                try {
                        $dbr = $this->getSlaveDB();
                        $size = (int)$dbr->selectField( 'job', 'COUNT(*)',
                                array( 'job_cmd' => $this->type, 'job_token' => '' ),
                                __METHOD__
                        );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }
                $this->cache->set( $key, $size, self::CACHE_TTL_SHORT );

                return $size;
        }

        /**
         * @see JobQueue::doGetAcquiredCount()
         * @return int
         */
        protected function doGetAcquiredCount() {
                if ( $this->claimTTL <= 0 ) {
                        return 0; // no acknowledgements
                }

                $key = $this->getCacheKey( 'acquiredcount' );

                $count = $this->cache->get( $key );
                if ( is_int( $count ) ) {
                        return $count;
                }

                $dbr = $this->getSlaveDB();
                try {
                        $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
                                array( 'job_cmd' => $this->type, "job_token != {$dbr->addQuotes( '' )}" ),
                                __METHOD__
                        );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }
                $this->cache->set( $key, $count, self::CACHE_TTL_SHORT );

                return $count;
        }

        /**
         * @see JobQueue::doGetAbandonedCount()
         * @return int
         * @throws MWException
         */
        protected function doGetAbandonedCount() {
                global $wgMemc;

                if ( $this->claimTTL <= 0 ) {
                        return 0; // no acknowledgements
                }

                $key = $this->getCacheKey( 'abandonedcount' );

                $count = $wgMemc->get( $key );
                if ( is_int( $count ) ) {
                        return $count;
                }

                $dbr = $this->getSlaveDB();
                try {
                        $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
                                array(
                                        'job_cmd' => $this->type,
                                        "job_token != {$dbr->addQuotes( '' )}",
                                        "job_attempts >= " . $dbr->addQuotes( $this->maxTries )
                                ),
                                __METHOD__
                        );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }
                $wgMemc->set( $key, $count, self::CACHE_TTL_SHORT );

                return $count;
        }

        /**
         * @see JobQueue::doBatchPush()
         * @param array $jobs
         * @param int $flags
         * @throws DBError|Exception
         * @return void
         */
        protected function doBatchPush( array $jobs, $flags ) {
                $dbw = $this->getMasterDB();

                $that = $this;
                $method = __METHOD__;
                $dbw->onTransactionIdle(
                        function () use ( $dbw, $that, $jobs, $flags, $method ) {
                                $that->doBatchPushInternal( $dbw, $jobs, $flags, $method );
                        }
                );
        }

        /**
         * This function should *not* be called outside of JobQueueDB
         *
         * @param IDatabase $dbw
         * @param array $jobs
         * @param int $flags
         * @param string $method
         * @throws DBError
         * @return void
         */
        public function doBatchPushInternal( IDatabase $dbw, array $jobs, $flags, $method ) {
                if ( !count( $jobs ) ) {
                        return;
                }

                $rowSet = array(); // (sha1 => job) map for jobs that are de-duplicated
                $rowList = array(); // list of jobs for jobs that are not de-duplicated
                foreach ( $jobs as $job ) {
                        $row = $this->insertFields( $job );
                        if ( $job->ignoreDuplicates() ) {
                                $rowSet[$row['job_sha1']] = $row;
                        } else {
                                $rowList[] = $row;
                        }
                }

                if ( $flags & self::QOS_ATOMIC ) {
                        $dbw->begin( $method ); // wrap all the job additions in one transaction
                }
                try {
                        // Strip out any duplicate jobs that are already in the queue...
                        if ( count( $rowSet ) ) {
                                $res = $dbw->select( 'job', 'job_sha1',
                                        array(
                                                // No job_type condition since it's part of the job_sha1 hash
                                                'job_sha1' => array_keys( $rowSet ),
                                                'job_token' => '' // unclaimed
                                        ),
                                        $method
                                );
                                foreach ( $res as $row ) {
                                        wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate.\n" );
                                        unset( $rowSet[$row->job_sha1] ); // already enqueued
                                }
                        }
                        // Build the full list of job rows to insert
                        $rows = array_merge( $rowList, array_values( $rowSet ) );
                        // Insert the job rows in chunks to avoid slave lag...
                        foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
                                $dbw->insert( 'job', $rowBatch, $method );
                        }
                        JobQueue::incrStats( 'job-insert', $this->type, count( $rows ), $this->wiki );
                        JobQueue::incrStats(
                                'job-insert-duplicate',
                                $this->type,
                                count( $rowSet ) + count( $rowList ) - count( $rows ),
                                $this->wiki
                        );
                } catch ( DBError $e ) {
                        if ( $flags & self::QOS_ATOMIC ) {
                                $dbw->rollback( $method );
                        }
                        throw $e;
                }
                if ( $flags & self::QOS_ATOMIC ) {
                        $dbw->commit( $method );
                }

                $this->cache->set( $this->getCacheKey( 'empty' ), 'false', JobQueueDB::CACHE_TTL_LONG );

                return;
        }

        /**
         * @see JobQueue::doPop()
         * @return Job|bool
         */
        protected function doPop() {
                if ( $this->cache->get( $this->getCacheKey( 'empty' ) ) === 'true' ) {
                        return false; // queue is empty
                }

                $dbw = $this->getMasterDB();
                try {
                        $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
                        $autoTrx = $dbw->getFlag( DBO_TRX ); // get current setting
                        $dbw->clearFlag( DBO_TRX ); // make each query its own transaction
                        $scopedReset = new ScopedCallback( function () use ( $dbw, $autoTrx ) {
                                $dbw->setFlag( $autoTrx ? DBO_TRX : 0 ); // restore old setting
                        } );

                        $uuid = wfRandomString( 32 ); // pop attempt
                        $job = false; // job popped off
                        do { // retry when our row is invalid or deleted as a duplicate
                                // Try to reserve a row in the DB...
                                if ( in_array( $this->order, array( 'fifo', 'timestamp' ) ) ) {
                                        $row = $this->claimOldest( $uuid );
                                } else { // random first
                                        $rand = mt_rand( 0, self::MAX_JOB_RANDOM ); // encourage concurrent UPDATEs
                                        $gte = (bool)mt_rand( 0, 1 ); // find rows with rand before/after $rand
                                        $row = $this->claimRandom( $uuid, $rand, $gte );
                                }
                                // Check if we found a row to reserve...
                                if ( !$row ) {
                                        $this->cache->set( $this->getCacheKey( 'empty' ), 'true', self::CACHE_TTL_LONG );
                                        break; // nothing to do
                                }
                                JobQueue::incrStats( 'job-pop', $this->type, 1, $this->wiki );
                                // Get the job object from the row...
                                $title = Title::makeTitleSafe( $row->job_namespace, $row->job_title );
                                if ( !$title ) {
                                        $dbw->delete( 'job', array( 'job_id' => $row->job_id ), __METHOD__ );
                                        wfDebug( "Row has invalid title '{$row->job_title}'.\n" );
                                        continue; // try again
                                }
                                $job = Job::factory( $row->job_cmd, $title,
                                        self::extractBlob( $row->job_params ), $row->job_id );
                                $job->metadata['id'] = $row->job_id;
                                break; // done
                        } while ( true );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }

                return $job;
        }

        /**
         * Reserve a row with a single UPDATE without holding row locks over RTTs...
         *
         * @param string $uuid 32 char hex string
         * @param int $rand Random unsigned integer (31 bits)
         * @param bool $gte Search for job_random >= $random (otherwise job_random <= $random)
         * @return stdClass|bool Row|false
         */
        protected function claimRandom( $uuid, $rand, $gte ) {
                $dbw = $this->getMasterDB();
                // Check cache to see if the queue has <= OFFSET items
                $tinyQueue = $this->cache->get( $this->getCacheKey( 'small' ) );

                $row = false; // the row acquired
                $invertedDirection = false; // whether one job_random direction was already scanned
                // This uses a replication safe method for acquiring jobs. One could use UPDATE+LIMIT
                // instead, but that either uses ORDER BY (in which case it deadlocks in MySQL) or is
                // not replication safe. Due to http://bugs.mysql.com/bug.php?id=6980, subqueries cannot
                // be used here with MySQL.
                do {
                        if ( $tinyQueue ) { // queue has <= MAX_OFFSET rows
                                // For small queues, using OFFSET will overshoot and return no rows more often.
                                // Instead, this uses job_random to pick a row (possibly checking both directions).
                                $ineq = $gte ? '>=' : '<=';
                                $dir = $gte ? 'ASC' : 'DESC';
                                $row = $dbw->selectRow( 'job', self::selectFields(), // find a random job
                                        array(
                                                'job_cmd' => $this->type,
                                                'job_token' => '', // unclaimed
                                                "job_random {$ineq} {$dbw->addQuotes( $rand )}" ),
                                        __METHOD__,
                                        array( 'ORDER BY' => "job_random {$dir}" )
                                );
                                if ( !$row && !$invertedDirection ) {
                                        $gte = !$gte;
                                        $invertedDirection = true;
                                        continue; // try the other direction
                                }
                        } else { // table *may* have >= MAX_OFFSET rows
                                // Bug 42614: "ORDER BY job_random" with a job_random inequality causes high CPU
                                // in MySQL if there are many rows for some reason. This uses a small OFFSET
                                // instead of job_random for reducing excess claim retries.
                                $row = $dbw->selectRow( 'job', self::selectFields(), // find a random job
                                        array(
                                                'job_cmd' => $this->type,
                                                'job_token' => '', // unclaimed
                                        ),
                                        __METHOD__,
                                        array( 'OFFSET' => mt_rand( 0, self::MAX_OFFSET ) )
                                );
                                if ( !$row ) {
                                        $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
                                        $this->cache->set( $this->getCacheKey( 'small' ), 1, 30 );
                                        continue; // use job_random
                                }
                        }

                        if ( $row ) { // claim the job
                                $dbw->update( 'job', // update by PK
                                        array(
                                                'job_token' => $uuid,
                                                'job_token_timestamp' => $dbw->timestamp(),
                                                'job_attempts = job_attempts+1' ),
                                        array( 'job_cmd' => $this->type, 'job_id' => $row->job_id, 'job_token' => '' ),
                                        __METHOD__
                                );
                                // This might get raced out by another runner when claiming the previously
                                // selected row. The use of job_random should minimize this problem, however.
                                if ( !$dbw->affectedRows() ) {
                                        $row = false; // raced out
                                }
                        } else {
                                break; // nothing to do
                        }
                } while ( !$row );

                return $row;
        }

        /**
         * Reserve a row with a single UPDATE without holding row locks over RTTs...
         *
         * @param string $uuid 32 char hex string
         * @return stdClass|bool Row|false
         */
        protected function claimOldest( $uuid ) {
                $dbw = $this->getMasterDB();

                $row = false; // the row acquired
                do {
                        if ( $dbw->getType() === 'mysql' ) {
                                // Per http://bugs.mysql.com/bug.php?id=6980, we can't use subqueries on the
                                // same table being changed in an UPDATE query in MySQL (gives Error: 1093).
                                // Oracle and Postgre have no such limitation. However, MySQL offers an
                                // alternative here by supporting ORDER BY + LIMIT for UPDATE queries.
                                $dbw->query( "UPDATE {$dbw->tableName( 'job' )} " .
                                        "SET " .
                                                "job_token = {$dbw->addQuotes( $uuid ) }, " .
                                                "job_token_timestamp = {$dbw->addQuotes( $dbw->timestamp() )}, " .
                                                "job_attempts = job_attempts+1 " .
                                        "WHERE ( " .
                                                "job_cmd = {$dbw->addQuotes( $this->type )} " .
                                                "AND job_token = {$dbw->addQuotes( '' )} " .
                                        ") ORDER BY job_id ASC LIMIT 1",
                                        __METHOD__
                                );
                        } else {
                                // Use a subquery to find the job, within an UPDATE to claim it.
                                // This uses as much of the DB wrapper functions as possible.
                                $dbw->update( 'job',
                                        array(
                                                'job_token' => $uuid,
                                                'job_token_timestamp' => $dbw->timestamp(),
                                                'job_attempts = job_attempts+1' ),
                                        array( 'job_id = (' .
                                                $dbw->selectSQLText( 'job', 'job_id',
                                                        array( 'job_cmd' => $this->type, 'job_token' => '' ),
                                                        __METHOD__,
                                                        array( 'ORDER BY' => 'job_id ASC', 'LIMIT' => 1 ) ) .
                                                ')'
                                        ),
                                        __METHOD__
                                );
                        }
                        // Fetch any row that we just reserved...
                        if ( $dbw->affectedRows() ) {
                                $row = $dbw->selectRow( 'job', self::selectFields(),
                                        array( 'job_cmd' => $this->type, 'job_token' => $uuid ), __METHOD__
                                );
                                if ( !$row ) { // raced out by duplicate job removal
                                        wfDebug( "Row deleted as duplicate by another process.\n" );
                                }
                        } else {
                                break; // nothing to do
                        }
                } while ( !$row );

                return $row;
        }

        /**
         * @see JobQueue::doAck()
         * @param Job $job
         * @throws MWException
         * @return Job|bool
         */
        protected function doAck( Job $job ) {
                if ( !isset( $job->metadata['id'] ) ) {
                        throw new MWException( "Job of type '{$job->getType()}' has no ID." );
                }

                $dbw = $this->getMasterDB();
                try {
                        $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
                        $autoTrx = $dbw->getFlag( DBO_TRX ); // get current setting
                        $dbw->clearFlag( DBO_TRX ); // make each query its own transaction
                        $scopedReset = new ScopedCallback( function () use ( $dbw, $autoTrx ) {
                                $dbw->setFlag( $autoTrx ? DBO_TRX : 0 ); // restore old setting
                        } );

                        // Delete a row with a single DELETE without holding row locks over RTTs...
                        $dbw->delete( 'job',
                                array( 'job_cmd' => $this->type, 'job_id' => $job->metadata['id'] ), __METHOD__ );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }

                return true;
        }

        /**
         * @see JobQueue::doDeduplicateRootJob()
         * @param Job $job
         * @throws MWException
         * @return bool
         */
        protected function doDeduplicateRootJob( Job $job ) {
                $params = $job->getParams();
                if ( !isset( $params['rootJobSignature'] ) ) {
                        throw new MWException( "Cannot register root job; missing 'rootJobSignature'." );
                } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
                        throw new MWException( "Cannot register root job; missing 'rootJobTimestamp'." );
                }
                $key = $this->getRootJobCacheKey( $params['rootJobSignature'] );
                // Callers should call batchInsert() and then this function so that if the insert
                // fails, the de-duplication registration will be aborted. Since the insert is
                // deferred till "transaction idle", do the same here, so that the ordering is
                // maintained. Having only the de-duplication registration succeed would cause
                // jobs to become no-ops without any actual jobs that made them redundant.
                $dbw = $this->getMasterDB();
                $cache = $this->dupCache;
                $dbw->onTransactionIdle( function () use ( $cache, $params, $key, $dbw ) {
                        $timestamp = $cache->get( $key ); // current last timestamp of this job
                        if ( $timestamp && $timestamp >= $params['rootJobTimestamp'] ) {
                                return true; // a newer version of this root job was enqueued
                        }

                        // Update the timestamp of the last root job started at the location...
                        return $cache->set( $key, $params['rootJobTimestamp'], JobQueueDB::ROOTJOB_TTL );
                } );

                return true;
        }

        /**
         * @see JobQueue::doDelete()
         * @return bool
         */
        protected function doDelete() {
                $dbw = $this->getMasterDB();
                try {
                        $dbw->delete( 'job', array( 'job_cmd' => $this->type ) );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }

                return true;
        }

        /**
         * @see JobQueue::doWaitForBackups()
         * @return void
         */
        protected function doWaitForBackups() {
                wfWaitForSlaves( false, $this->wiki, $this->cluster ?: false );
        }

        /**
         * @return array
         */
        protected function doGetPeriodicTasks() {
                return array(
                        'recycleAndDeleteStaleJobs' => array(
                                'callback' => array( $this, 'recycleAndDeleteStaleJobs' ),
                                'period' => ceil( $this->claimTTL / 2 )
                        )
                );
        }

        /**
         * @return void
         */
        protected function doFlushCaches() {
                foreach ( array( 'empty', 'size', 'acquiredcount' ) as $type ) {
                        $this->cache->delete( $this->getCacheKey( $type ) );
                }
        }

        /**
         * @see JobQueue::getAllQueuedJobs()
         * @return Iterator
         */
        public function getAllQueuedJobs() {
                $dbr = $this->getSlaveDB();
                try {
                        return new MappedIterator(
                                $dbr->select( 'job', self::selectFields(),
                                        array( 'job_cmd' => $this->getType(), 'job_token' => '' ) ),
                                function ( $row ) use ( $dbr ) {
                                        $job = Job::factory(
                                                $row->job_cmd,
                                                Title::makeTitle( $row->job_namespace, $row->job_title ),
                                                strlen( $row->job_params ) ? unserialize( $row->job_params ) : false
                                        );
                                        $job->metadata['id'] = $row->job_id;
                                        return $job;
                                }
                        );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }
        }

        public function getCoalesceLocationInternal() {
                return $this->cluster
                        ? "DBCluster:{$this->cluster}:{$this->wiki}"
                        : "LBFactory:{$this->wiki}";
        }

        protected function doGetSiblingQueuesWithJobs( array $types ) {
                $dbr = $this->getSlaveDB();
                $res = $dbr->select( 'job', 'DISTINCT job_cmd',
                        array( 'job_cmd' => $types ), __METHOD__ );

                $types = array();
                foreach ( $res as $row ) {
                        $types[] = $row->job_cmd;
                }

                return $types;
        }

        protected function doGetSiblingQueueSizes( array $types ) {
                $dbr = $this->getSlaveDB();
                $res = $dbr->select( 'job', array( 'job_cmd', 'COUNT(*) AS count' ),
                        array( 'job_cmd' => $types ), __METHOD__, array( 'GROUP BY' => 'job_cmd' ) );

                $sizes = array();
                foreach ( $res as $row ) {
                        $sizes[$row->job_cmd] = (int)$row->count;
                }

                return $sizes;
        }

        /**
         * Recycle or destroy any jobs that have been claimed for too long
         *
         * @return int Number of jobs recycled/deleted
         */
        public function recycleAndDeleteStaleJobs() {
                $now = time();
                $count = 0; // affected rows
                $dbw = $this->getMasterDB();

                try {
                        if ( !$dbw->lock( "jobqueue-recycle-{$this->type}", __METHOD__, 1 ) ) {
                                return $count; // already in progress
                        }

                        // Remove claims on jobs acquired for too long if enabled...
                        if ( $this->claimTTL > 0 ) {
                                $claimCutoff = $dbw->timestamp( $now - $this->claimTTL );
                                // Get the IDs of jobs that have be claimed but not finished after too long.
                                // These jobs can be recycled into the queue by expiring the claim. Selecting
                                // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
                                $res = $dbw->select( 'job', 'job_id',
                                        array(
                                                'job_cmd' => $this->type,
                                                "job_token != {$dbw->addQuotes( '' )}", // was acquired
                                                "job_token_timestamp < {$dbw->addQuotes( $claimCutoff )}", // stale
                                                "job_attempts < {$dbw->addQuotes( $this->maxTries )}" ), // retries left
                                        __METHOD__
                                );
                                $ids = array_map(
                                        function ( $o ) {
                                                return $o->job_id;
                                        }, iterator_to_array( $res )
                                );
                                if ( count( $ids ) ) {
                                        // Reset job_token for these jobs so that other runners will pick them up.
                                        // Set the timestamp to the current time, as it is useful to now that the job
                                        // was already tried before (the timestamp becomes the "released" time).
                                        $dbw->update( 'job',
                                                array(
                                                        'job_token' => '',
                                                        'job_token_timestamp' => $dbw->timestamp( $now ) ), // time of release
                                                array(
                                                        'job_id' => $ids ),
                                                __METHOD__
                                        );
                                        $affected = $dbw->affectedRows();
                                        $count += $affected;
                                        JobQueue::incrStats( 'job-recycle', $this->type, $affected, $this->wiki );
                                        // The tasks recycled jobs or release delayed jobs into the queue
                                        $this->cache->set( $this->getCacheKey( 'empty' ), 'false', self::CACHE_TTL_LONG );
                                        $this->aggr->notifyQueueNonEmpty( $this->wiki, $this->type );
                                }
                        }

                        // Just destroy any stale jobs...
                        $pruneCutoff = $dbw->timestamp( $now - self::MAX_AGE_PRUNE );
                        $conds = array(
                                'job_cmd' => $this->type,
                                "job_token != {$dbw->addQuotes( '' )}", // was acquired
                                "job_token_timestamp < {$dbw->addQuotes( $pruneCutoff )}" // stale
                        );
                        if ( $this->claimTTL > 0 ) { // only prune jobs attempted too many times...
                                $conds[] = "job_attempts >= {$dbw->addQuotes( $this->maxTries )}";
                        }
                        // Get the IDs of jobs that are considered stale and should be removed. Selecting
                        // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
                        $res = $dbw->select( 'job', 'job_id', $conds, __METHOD__ );
                        $ids = array_map(
                                function ( $o ) {
                                        return $o->job_id;
                                }, iterator_to_array( $res )
                        );
                        if ( count( $ids ) ) {
                                $dbw->delete( 'job', array( 'job_id' => $ids ), __METHOD__ );
                                $affected = $dbw->affectedRows();
                                $count += $affected;
                                JobQueue::incrStats( 'job-abandon', $this->type, $affected, $this->wiki );
                        }

                        $dbw->unlock( "jobqueue-recycle-{$this->type}", __METHOD__ );
                } catch ( DBError $e ) {
                        $this->throwDBException( $e );
                }

                return $count;
        }

        /**
         * @param IJobSpecification $job
         * @return array
         */
        protected function insertFields( IJobSpecification $job ) {
                $dbw = $this->getMasterDB();

                return array(
                        // Fields that describe the nature of the job
                        'job_cmd' => $job->getType(),
                        'job_namespace' => $job->getTitle()->getNamespace(),
                        'job_title' => $job->getTitle()->getDBkey(),
                        'job_params' => self::makeBlob( $job->getParams() ),
                        // Additional job metadata
                        'job_id' => $dbw->nextSequenceValue( 'job_job_id_seq' ),
                        'job_timestamp' => $dbw->timestamp(),
                        'job_sha1' => wfBaseConvert(
                                sha1( serialize( $job->getDeduplicationInfo() ) ),
                                16, 36, 31
                        ),
                        'job_random' => mt_rand( 0, self::MAX_JOB_RANDOM )
                );
        }

        /**
         * @throws JobQueueConnectionError
         * @return DBConnRef
         */
        protected function getSlaveDB() {
                try {
                        return $this->getDB( DB_SLAVE );
                } catch ( DBConnectionError $e ) {
                        throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
                }
        }

        /**
         * @throws JobQueueConnectionError
         * @return DBConnRef
         */
        protected function getMasterDB() {
                try {
                        return $this->getDB( DB_MASTER );
                } catch ( DBConnectionError $e ) {
                        throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
                }
        }

        /**
         * @param int $index (DB_SLAVE/DB_MASTER)
         * @return DBConnRef
         */
        protected function getDB( $index ) {
                $lb = ( $this->cluster !== false )
                        ? wfGetLBFactory()->getExternalLB( $this->cluster, $this->wiki )
                        : wfGetLB( $this->wiki );

                return $lb->getConnectionRef( $index, array(), $this->wiki );
        }

        /**
         * @param string $property
         * @return string
         */
        private function getCacheKey( $property ) {
                list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
                $cluster = is_string( $this->cluster ) ? $this->cluster : 'main';

                return wfForeignMemcKey( $db, $prefix, 'jobqueue', $cluster, $this->type, $property );
        }

        /**
         * @param array|bool $params
         * @return string
         */
        protected static function makeBlob( $params ) {
                if ( $params !== false ) {
                        return serialize( $params );
                } else {
                        return '';
                }
        }

        /**
         * @param string $blob
         * @return bool|mixed
         */
        protected static function extractBlob( $blob ) {
                if ( (string)$blob !== '' ) {
                        return unserialize( $blob );
                } else {
                        return false;
                }
        }

        /**
         * @param DBError $e
         * @throws JobQueueError
         */
        protected function throwDBException( DBError $e ) {
                throw new JobQueueError( get_class( $e ) . ": " . $e->getMessage() );
        }

        /**
         * Return the list of job fields that should be selected.
         * @since 1.23
         * @return array
         */
        public static function selectFields() {
                return array(
                        'job_id',
                        'job_cmd',
                        'job_namespace',
                        'job_title',
                        'job_timestamp',
                        'job_params',
                        'job_random',
                        'job_attempts',
                        'job_token',
                        'job_token_timestamp',
                        'job_sha1',
                );
        }
}