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

<?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
 */
use MediaWiki\MediaWikiServices;
use Wikimedia\Rdbms\DBConnectionError;
use Wikimedia\Rdbms\DBError;
use Wikimedia\Rdbms\IDatabase;
use Wikimedia\Rdbms\IMaintainableDatabase;
use Wikimedia\Rdbms\IReadableDatabase;
use Wikimedia\Rdbms\RawSQLValue;
use Wikimedia\Rdbms\SelectQueryBuilder;
use Wikimedia\Rdbms\ServerInfo;
use Wikimedia\ScopedCallback;

/**
 * Database-backed job queue storage.
 *
 * @since 1.21
 * @ingroup JobQueue
 */
class JobQueueDB extends JobQueue {
        /* seconds to cache info without re-validating */
        private const CACHE_TTL_SHORT = 30;
        /* seconds a job can live once claimed */
        private const MAX_AGE_PRUNE = 7 * 24 * 3600;
        /**
         * Used for job_random, the highest safe 32-bit signed integer.
         * Equivalent to `( 2 ** 31 ) - 1` on 64-bit.
         */
        private const MAX_JOB_RANDOM = 2_147_483_647;
        /* maximum number of rows to skip */
        private const MAX_OFFSET = 255;

        /** @var IMaintainableDatabase|DBError|null */
        protected $conn;

        /** @var array|null Server configuration array */
        protected $server;
        /** @var string|null Name of an external DB cluster or null for the local DB cluster */
        protected $cluster;

        /**
         * Additional parameters include:
         *   - server  : Server configuration array for Database::factory. Overrides "cluster".
         *   - 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 ) {
                parent::__construct( $params );

                if ( isset( $params['server'] ) ) {
                        $this->server = $params['server'];
                        // Always use autocommit mode, even if DBO_TRX is configured
                        $this->server['flags'] ??= 0;
                        $this->server['flags'] &= ~( IDatabase::DBO_TRX | IDatabase::DBO_DEFAULT );
                } elseif ( isset( $params['cluster'] ) && is_string( $params['cluster'] ) ) {
                        $this->cluster = $params['cluster'];
                }
        }

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

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

        /**
         * @see JobQueue::doIsEmpty()
         * @return bool
         */
        protected function doIsEmpty() {
                $dbr = $this->getReplicaDB();
                try {
                        // unclaimed job
                        $found = (bool)$dbr->newSelectQueryBuilder()
                                ->select( '1' )
                                ->from( 'job' )
                                ->where( [ 'job_cmd' => $this->type, 'job_token' => '' ] )
                                ->caller( __METHOD__ )->fetchField();
                } catch ( DBError $e ) {
                        throw $this->getDBException( $e );
                }

                return !$found;
        }

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

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

                $dbr = $this->getReplicaDB();
                try {
                        $size = $dbr->newSelectQueryBuilder()
                                ->from( 'job' )
                                ->where( [ 'job_cmd' => $this->type, 'job_token' => '' ] )
                                ->caller( __METHOD__ )->fetchRowCount();
                } catch ( DBError $e ) {
                        throw $this->getDBException( $e );
                }
                $this->wanCache->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->wanCache->get( $key );
                if ( is_int( $count ) ) {
                        return $count;
                }

                $dbr = $this->getReplicaDB();
                try {
                        $count = $dbr->newSelectQueryBuilder()
                                ->from( 'job' )
                                ->where( [
                                        'job_cmd' => $this->type,
                                        $dbr->expr( 'job_token', '!=', '' ),
                                ] )
                                ->caller( __METHOD__ )->fetchRowCount();
                } catch ( DBError $e ) {
                        throw $this->getDBException( $e );
                }
                $this->wanCache->set( $key, $count, self::CACHE_TTL_SHORT );

                return $count;
        }

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

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

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

                $dbr = $this->getReplicaDB();
                try {
                        $count = $dbr->newSelectQueryBuilder()
                                ->from( 'job' )
                                ->where(
                                        [
                                                'job_cmd' => $this->type,
                                                $dbr->expr( 'job_token', '!=', '' ),
                                                $dbr->expr( 'job_attempts', '>=', $this->maxTries ),
                                        ]
                                )
                                ->caller( __METHOD__ )->fetchRowCount();
                } catch ( DBError $e ) {
                        throw $this->getDBException( $e );
                }

                $this->wanCache->set( $key, $count, self::CACHE_TTL_SHORT );

                return $count;
        }

        /**
         * @see JobQueue::doBatchPush()
         * @param IJobSpecification[] $jobs
         * @param int $flags
         * @throws DBError|Exception
         * @return void
         */
        protected function doBatchPush( array $jobs, $flags ) {
                // Silence expectations related to getting a primary DB, as we have to get a primary DB to insert the job.
                $transactionProfiler = Profiler::instance()->getTransactionProfiler();
                $scope = $transactionProfiler->silenceForScope();
                $dbw = $this->getPrimaryDB();
                ScopedCallback::consume( $scope );
                // In general, there will be two cases here:
                // a) sqlite; DB connection is probably a regular round-aware handle.
                // If the connection is busy with a transaction, then defer the job writes
                // until right before the main round commit step. Any errors that bubble
                // up will rollback the main commit round.
                // b) mysql/postgres; DB connection is generally a separate CONN_TRX_AUTOCOMMIT handle.
                // No transaction is active nor will be started by writes, so enqueue the jobs
                // now so that any errors will show up immediately as the interface expects. Any
                // errors that bubble up will rollback the main commit round.
                $fname = __METHOD__;
                $dbw->onTransactionPreCommitOrIdle(
                        function ( IDatabase $dbw ) use ( $jobs, $flags, $fname ) {
                                $this->doBatchPushInternal( $dbw, $jobs, $flags, $fname );
                        },
                        $fname
                );
        }

        /**
         * This function should *not* be called outside of JobQueueDB
         *
         * @suppress SecurityCheck-SQLInjection Bug in phan-taint-check handling bulk inserts
         * @param IDatabase $dbw
         * @param IJobSpecification[] $jobs
         * @param int $flags
         * @param string $method
         * @throws DBError
         * @return void
         */
        public function doBatchPushInternal( IDatabase $dbw, array $jobs, $flags, $method ) {
                if ( $jobs === [] ) {
                        return;
                }

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

                if ( $flags & self::QOS_ATOMIC ) {
                        $dbw->startAtomic( $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->newSelectQueryBuilder()
                                        ->select( 'job_sha1' )
                                        ->from( 'job' )
                                        ->where(
                                                [
                                                        // No job_type condition since it's part of the job_sha1 hash
                                                        'job_sha1' => array_map( 'strval', array_keys( $rowSet ) ),
                                                        'job_token' => '' // unclaimed
                                                ]
                                        )
                                        ->caller( $method )->fetchResultSet();
                                foreach ( $res as $row ) {
                                        wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate." );
                                        unset( $rowSet[$row->job_sha1] ); // already enqueued
                                }
                        }
                        // Build the full list of job rows to insert
                        $rows = array_merge( $rowList, array_values( $rowSet ) );
                        // Silence expectations related to inserting to the job table, because we have to perform the inserts to
                        // track the job.
                        $transactionProfiler = Profiler::instance()->getTransactionProfiler();
                        $scope = $transactionProfiler->silenceForScope();
                        // Insert the job rows in chunks to avoid replica DB lag...
                        foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
                                $dbw->newInsertQueryBuilder()
                                        ->insertInto( 'job' )
                                        ->rows( $rowBatch )
                                        ->caller( $method )->execute();
                        }
                        ScopedCallback::consume( $scope );
                        $this->incrStats( 'inserts', $this->type, count( $rows ) );
                        $this->incrStats( 'dupe_inserts', $this->type,
                                count( $rowSet ) + count( $rowList ) - count( $rows )
                        );
                } catch ( DBError $e ) {
                        throw $this->getDBException( $e );
                }
                if ( $flags & self::QOS_ATOMIC ) {
                        $dbw->endAtomic( $method );
                }
        }

        /**
         * @see JobQueue::doPop()
         * @return RunnableJob|false
         */
        protected function doPop() {
                $job = false; // job popped off
                try {
                        $uuid = wfRandomString( 32 ); // pop attempt
                        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, [ '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 ) {
                                        break; // nothing to do
                                }
                                $this->incrStats( 'pops', $this->type );

                                // Get the job object from the row...
                                $job = $this->jobFromRow( $row );
                                break; // done
                        } while ( true );

                        if ( !$job || mt_rand( 0, 9 ) == 0 ) {
                                // Handled jobs that need to be recycled/deleted;
                                // any recycled jobs will be picked up next attempt
                                $this->recycleAndDeleteStaleJobs();
                        }
                } catch ( DBError $e ) {
                        throw $this->getDBException( $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|false Row|false
         */
        protected function claimRandom( $uuid, $rand, $gte ) {
                $dbw = $this->getPrimaryDB();
                // Check cache to see if the queue has <= OFFSET items
                $tinyQueue = $this->wanCache->get( $this->getCacheKey( 'small' ) );

                $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 https://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).
                                $row = $dbw->newSelectQueryBuilder()
                                        ->select( self::selectFields() )
                                        ->from( 'job' )
                                        ->where(
                                                [
                                                        'job_cmd' => $this->type,
                                                        'job_token' => '', // unclaimed
                                                        $dbw->expr( 'job_random', $gte ? '>=' : '<=', $rand )
                                                ]
                                        )
                                        ->orderBy(
                                                'job_random',
                                                $gte ? SelectQueryBuilder::SORT_ASC : SelectQueryBuilder::SORT_DESC
                                        )
                                        ->caller( __METHOD__ )->fetchRow();
                                if ( !$row && !$invertedDirection ) {
                                        $gte = !$gte;
                                        $invertedDirection = true;
                                        continue; // try the other direction
                                }
                        } else { // table *may* have >= MAX_OFFSET rows
                                // T44614: "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->newSelectQueryBuilder()
                                        ->select( self::selectFields() )
                                        ->from( 'job' )
                                        ->where(
                                                [
                                                        'job_cmd' => $this->type,
                                                        'job_token' => '', // unclaimed
                                                ]
                                        )
                                        ->offset( mt_rand( 0, self::MAX_OFFSET ) )
                                        ->caller( __METHOD__ )->fetchRow();
                                if ( !$row ) {
                                        $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
                                        $this->wanCache->set( $this->getCacheKey( 'small' ), 1, 30 );
                                        continue; // use job_random
                                }
                        }

                        if ( !$row ) {
                                break;
                        }

                        $dbw->newUpdateQueryBuilder()
                                ->update( 'job' ) // update by PK
                                ->set( [
                                        'job_token' => $uuid,
                                        'job_token_timestamp' => $dbw->timestamp(),
                                        'job_attempts' => new RawSQLValue( 'job_attempts+1' ),
                                ] )
                                ->where( [
                                        'job_cmd' => $this->type,
                                        'job_id' => $row->job_id,
                                        'job_token' => ''
                                ] )
                                ->caller( __METHOD__ )->execute();

                        // 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() ) {
                                $row = false; // raced out
                        }
                } while ( !$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|false Row|false
         */
        protected function claimOldest( $uuid ) {
                $dbw = $this->getPrimaryDB();

                $row = false; // the row acquired
                do {
                        if ( $dbw->getType() === 'mysql' ) {
                                // Per https://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).
                                // Postgres has 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.
                                $qb = $dbw->newSelectQueryBuilder()
                                        ->select( 'job_id' )
                                        ->from( 'job' )
                                        ->where( [ 'job_cmd' => $this->type, 'job_token' => '' ] )
                                        ->orderBy( 'job_id', SelectQueryBuilder::SORT_ASC )
                                        ->limit( 1 );

                                $dbw->newUpdateQueryBuilder()
                                        ->update( 'job' )
                                        ->set( [
                                                'job_token' => $uuid,
                                                'job_token_timestamp' => $dbw->timestamp(),
                                                'job_attempts' => new RawSQLValue( 'job_attempts+1' ),
                                        ] )
                                        ->where( [ 'job_id' => new RawSQLValue( '(' . $qb->getSQL() . ')' ) ] )
                                        ->caller( __METHOD__ )->execute();
                        }

                        if ( !$dbw->affectedRows() ) {
                                break;
                        }

                        // Fetch any row that we just reserved...
                        $row = $dbw->newSelectQueryBuilder()
                                ->select( self::selectFields() )
                                ->from( 'job' )
                                ->where( [ 'job_cmd' => $this->type, 'job_token' => $uuid ] )
                                ->caller( __METHOD__ )->fetchRow();
                        if ( !$row ) { // raced out by duplicate job removal
                                wfDebug( "Row deleted as duplicate by another process." );
                        }
                } while ( !$row );

                return $row;
        }

        /**
         * @see JobQueue::doAck()
         * @param RunnableJob $job
         * @throws JobQueueConnectionError
         * @throws JobQueueError
         */
        protected function doAck( RunnableJob $job ) {
                $id = $job->getMetadata( 'id' );
                if ( $id === null ) {
                        throw new UnexpectedValueException( "Job of type '{$job->getType()}' has no ID." );
                }

                $dbw = $this->getPrimaryDB();
                try {
                        // Delete a row with a single DELETE without holding row locks over RTTs...
                        $dbw->newDeleteQueryBuilder()
                                ->deleteFrom( 'job' )
                                ->where( [ 'job_cmd' => $this->type, 'job_id' => $id ] )
                                ->caller( __METHOD__ )->execute();

                        $this->incrStats( 'acks', $this->type );
                } catch ( DBError $e ) {
                        throw $this->getDBException( $e );
                }
        }

        /**
         * @see JobQueue::doDeduplicateRootJob()
         * @param IJobSpecification $job
         * @throws JobQueueConnectionError
         * @return bool
         */
        protected function doDeduplicateRootJob( IJobSpecification $job ) {
                // Callers should call JobQueueGroup::push() before this method 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->getPrimaryDB();
                $dbw->onTransactionCommitOrIdle(
                        function () use ( $job ) {
                                parent::doDeduplicateRootJob( $job );
                        },
                        __METHOD__
                );

                return true;
        }

        /**
         * @see JobQueue::doDelete()
         * @return bool
         */
        protected function doDelete() {
                $dbw = $this->getPrimaryDB();
                try {
                        $dbw->newDeleteQueryBuilder()
                                ->deleteFrom( 'job' )
                                ->where( [ 'job_cmd' => $this->type ] )
                                ->caller( __METHOD__ )->execute();
                } catch ( DBError $e ) {
                        throw $this->getDBException( $e );
                }

                return true;
        }

        /**
         * @see JobQueue::doWaitForBackups()
         * @return void
         */
        protected function doWaitForBackups() {
                if ( $this->server ) {
                        return; // not using LBFactory instance
                }

                $lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
                $lbFactory->waitForReplication();
        }

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

        /**
         * @see JobQueue::getAllQueuedJobs()
         * @return Iterator<RunnableJob>
         */
        public function getAllQueuedJobs() {
                return $this->getJobIterator( [ 'job_cmd' => $this->getType(), 'job_token' => '' ] );
        }

        /**
         * @see JobQueue::getAllAcquiredJobs()
         * @return Iterator<RunnableJob>
         */
        public function getAllAcquiredJobs() {
                $dbr = $this->getReplicaDB();
                return $this->getJobIterator( [ 'job_cmd' => $this->getType(), $dbr->expr( 'job_token', '>', '' ) ] );
        }

        /**
         * @see JobQueue::getAllAbandonedJobs()
         * @return Iterator<RunnableJob>
         */
        public function getAllAbandonedJobs() {
                $dbr = $this->getReplicaDB();
                return $this->getJobIterator( [
                        'job_cmd' => $this->getType(),
                        $dbr->expr( 'job_token', '>', '' ),
                        $dbr->expr( 'job_attempts', '>=', intval( $this->maxTries ) ),
                ] );
        }

        /**
         * @param array $conds Query conditions
         * @return Iterator<RunnableJob>
         */
        protected function getJobIterator( array $conds ) {
                $dbr = $this->getReplicaDB();
                $qb = $dbr->newSelectQueryBuilder()
                        ->select( self::selectFields() )
                        ->from( 'job' )
                        ->where( $conds );
                try {
                        return new MappedIterator(
                                $qb->caller( __METHOD__ )->fetchResultSet(),
                                function ( $row ) {
                                        return $this->jobFromRow( $row );
                                }
                        );
                } catch ( DBError $e ) {
                        throw $this->getDBException( $e );
                }
        }

        public function getCoalesceLocationInternal() {
                if ( $this->server ) {
                        return null; // not using the LBFactory instance
                }

                return is_string( $this->cluster )
                        ? "DBCluster:{$this->cluster}:{$this->domain}"
                        : "LBFactory:{$this->domain}";
        }

        protected function doGetSiblingQueuesWithJobs( array $types ) {
                $dbr = $this->getReplicaDB();
                // @note: this does not check whether the jobs are claimed or not.
                // This is useful so JobQueueGroup::pop() also sees queues that only
                // have stale jobs. This lets recycleAndDeleteStaleJobs() re-enqueue
                // failed jobs so that they can be popped again for that edge case.
                $res = $dbr->newSelectQueryBuilder()
                        ->select( 'job_cmd' )
                        ->distinct()
                        ->from( 'job' )
                        ->where( [ 'job_cmd' => $types ] )
                        ->caller( __METHOD__ )->fetchResultSet();

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

                return $types;
        }

        protected function doGetSiblingQueueSizes( array $types ) {
                $dbr = $this->getReplicaDB();

                $res = $dbr->newSelectQueryBuilder()
                        ->select( [ 'job_cmd', 'count' => 'COUNT(*)' ] )
                        ->from( 'job' )
                        ->where( [ 'job_cmd' => $types ] )
                        ->groupBy( 'job_cmd' )
                        ->caller( __METHOD__ )->fetchResultSet();

                $sizes = [];
                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->getPrimaryDB();

                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->newSelectQueryBuilder()
                                        ->select( 'job_id' )
                                        ->from( 'job' )
                                        ->where(
                                                [
                                                        'job_cmd' => $this->type,
                                                        $dbw->expr( 'job_token', '!=', '' ), // was acquired
                                                        $dbw->expr( 'job_token_timestamp', '<', $claimCutoff ), // stale
                                                        $dbw->expr( 'job_attempts', '<', $this->maxTries ), // retries left
                                                ]
                                        )
                                        ->caller( __METHOD__ )->fetchResultSet();
                                $ids = array_map(
                                        static 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->newUpdateQueryBuilder()
                                                ->update( 'job' )
                                                ->set( [
                                                        'job_token' => '',
                                                        'job_token_timestamp' => $dbw->timestamp( $now ) // time of release
                                                ] )
                                                ->where( [
                                                        'job_id' => $ids,
                                                        $dbw->expr( 'job_token', '!=', '' ),
                                                ] )
                                                ->caller( __METHOD__ )->execute();

                                        $affected = $dbw->affectedRows();
                                        $count += $affected;
                                        $this->incrStats( 'recycles', $this->type, $affected );
                                }
                        }

                        // Just destroy any stale jobs...
                        $pruneCutoff = $dbw->timestamp( $now - self::MAX_AGE_PRUNE );
                        $qb = $dbw->newSelectQueryBuilder()
                                ->select( 'job_id' )
                                ->from( 'job' )
                                ->where(
                                        [
                                                'job_cmd' => $this->type,
                                                $dbw->expr( 'job_token', '!=', '' ), // was acquired
                                                $dbw->expr( 'job_token_timestamp', '<', $pruneCutoff ) // stale
                                        ]
                                );
                        if ( $this->claimTTL > 0 ) { // only prune jobs attempted too many times...
                                $qb->andWhere( $dbw->expr( 'job_attempts', '>=', $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 = $qb->caller( __METHOD__ )->fetchResultSet();
                        $ids = array_map(
                                static function ( $o ) {
                                        return $o->job_id;
                                }, iterator_to_array( $res )
                        );
                        if ( count( $ids ) ) {
                                $dbw->newDeleteQueryBuilder()
                                        ->deleteFrom( 'job' )
                                        ->where( [ 'job_id' => $ids ] )
                                        ->caller( __METHOD__ )->execute();
                                $affected = $dbw->affectedRows();
                                $count += $affected;
                                $this->incrStats( 'abandons', $this->type, $affected );
                        }

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

                return $count;
        }

        /**
         * @param IJobSpecification $job
         * @param IReadableDatabase $db
         * @return array
         */
        protected function insertFields( IJobSpecification $job, IReadableDatabase $db ) {
                return [
                        // Fields that describe the nature of the job
                        'job_cmd' => $job->getType(),
                        'job_namespace' => $job->getParams()['namespace'] ?? NS_SPECIAL,
                        'job_title' => $job->getParams()['title'] ?? '',
                        'job_params' => self::makeBlob( $job->getParams() ),
                        // Additional job metadata
                        'job_timestamp' => $db->timestamp(),
                        'job_sha1' => Wikimedia\base_convert(
                                sha1( serialize( $job->getDeduplicationInfo() ) ),
                                16, 36, 31
                        ),
                        'job_random' => mt_rand( 0, self::MAX_JOB_RANDOM )
                ];
        }

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

        /**
         * @throws JobQueueConnectionError
         * @return IMaintainableDatabase
         * @since 1.37
         */
        protected function getPrimaryDB() {
                try {
                        return $this->getDB( DB_PRIMARY );
                } catch ( DBConnectionError $e ) {
                        throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
                }
        }

        /**
         * @param int $index (DB_REPLICA/DB_PRIMARY)
         * @return IMaintainableDatabase
         */
        protected function getDB( $index ) {
                if ( $this->server ) {
                        if ( $this->conn instanceof IDatabase ) {
                                return $this->conn;
                        } elseif ( $this->conn instanceof DBError ) {
                                throw $this->conn;
                        }

                        try {
                                $this->conn = MediaWikiServices::getInstance()->getDatabaseFactory()->create(
                                        $this->server['type'],
                                        $this->server
                                );
                        } catch ( DBError $e ) {
                                $this->conn = $e;
                                throw $e;
                        }

                        return $this->conn;
                } else {
                        $lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
                        $lb = is_string( $this->cluster )
                                ? $lbFactory->getExternalLB( $this->cluster )
                                : $lbFactory->getMainLB( $this->domain );

                        if ( $lb->getServerType( ServerInfo::WRITER_INDEX ) !== 'sqlite' ) {
                                // Keep a separate connection to avoid contention and deadlocks;
                                // However, SQLite has the opposite behavior due to DB-level locking.
                                $flags = $lb::CONN_TRX_AUTOCOMMIT;
                        } else {
                                // Jobs insertion will be deferred until the PRESEND stage to reduce contention.
                                $flags = 0;
                        }

                        return $lb->getMaintenanceConnectionRef( $index, [], $this->domain, $flags );
                }
        }

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

                return $this->wanCache->makeGlobalKey(
                        'jobqueue',
                        $this->domain,
                        $cluster,
                        $this->type,
                        $property
                );
        }

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

        /**
         * @param stdClass $row
         * @return RunnableJob
         */
        protected function jobFromRow( $row ) {
                $params = ( (string)$row->job_params !== '' ) ? unserialize( $row->job_params ) : [];
                if ( !is_array( $params ) ) { // this shouldn't happen
                        throw new UnexpectedValueException(
                                "Could not unserialize job with ID '{$row->job_id}'." );
                }

                $params += [ 'namespace' => $row->job_namespace, 'title' => $row->job_title ];
                $job = $this->factoryJob( $row->job_cmd, $params );
                $job->setMetadata( 'id', $row->job_id );
                $job->setMetadata( 'timestamp', $row->job_timestamp );

                return $job;
        }

        /**
         * @param DBError $e
         * @return JobQueueError
         */
        protected function getDBException( DBError $e ) {
                return 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 [
                        'job_id',
                        'job_cmd',
                        'job_namespace',
                        'job_title',
                        'job_timestamp',
                        'job_params',
                        'job_random',
                        'job_attempts',
                        'job_token',
                        'job_token_timestamp',
                        'job_sha1',
                ];
        }
}