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[mediawiki.git] / maintenance / storage / recompressTracked.php

<?php
/**
 * Moves blobs indexed by trackBlobs.php to a specified list of destination
 * clusters, and recompresses them in the process.
 *
 * 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
 * @ingroup Maintenance ExternalStorage
 */

use MediaWiki\Logger\LegacyLogger;
use MediaWiki\MediaWikiServices;
use MediaWiki\Shell\Shell;
use MediaWiki\Storage\SqlBlobStore;
use MediaWiki\Title\Title;
use MediaWiki\WikiMap\WikiMap;
use Wikimedia\AtEase\AtEase;

$optionsWithArgs = RecompressTracked::getOptionsWithArgs();
require __DIR__ . '/../CommandLineInc.php';

if ( count( $args ) < 1 ) {
        echo "Usage: php recompressTracked.php [options] <cluster> [... <cluster>...]
Moves blobs indexed by trackBlobs.php to a specified list of destination clusters,
and recompresses them in the process. Restartable.

Options:
        --procs <procs>       Set the number of child processes (default 1)
        --copy-only           Copy only, do not update the text table. Restart
                          without this option to complete.
        --debug-log <file>    Log debugging data to the specified file
        --info-log <file>     Log progress messages to the specified file
        --critical-log <file> Log error messages to the specified file
";
        exit( 1 );
}

$job = RecompressTracked::newFromCommandLine( $args, $options );
$job->execute();

/**
 * Maintenance script that moves blobs indexed by trackBlobs.php to a specified
 * list of destination clusters, and recompresses them in the process.
 *
 * @ingroup Maintenance ExternalStorage
 */
class RecompressTracked {
        /** @var string[] */
        public $destClusters;
        /** @var int */
        public $batchSize = 1000;
        /** @var int */
        public $orphanBatchSize = 1000;
        /** @var int */
        public $reportingInterval = 10;
        /** @var int */
        public $numProcs = 1;
        /** @var int */
        public $numBatches = 0;
        /** @var string */
        public $pageBlobClass;
        /** @var string */
        public $orphanBlobClass;
        /** @var resource[] */
        public $childPipes;
        /** @var resource[] */
        public $childProcs;
        /** @var int */
        public $prevChildId;
        /** @var bool */
        public $copyOnly = false;
        /** @var bool */
        public $isChild = false;
        /** @var int|false */
        public $childId = false;
        /** @var bool */
        public $noCount = false;
        public ?string $debugLog = null;
        public ?string $infoLog = null;
        public ?string $criticalLog = null;
        /** @var ExternalStoreDB */
        public $store;
        /** @var SqlBlobStore */
        private $blobStore;

        /** @var string[] */
        private static $optionsWithArgs = [
                'procs',
                'child-id',
                'debug-log',
                'info-log',
                'critical-log'
        ];

        /** @var string[] */
        private static $cmdLineOptionMap = [
                'no-count' => 'noCount',
                'procs' => 'numProcs',
                'copy-only' => 'copyOnly',
                'child' => 'isChild',
                'child-id' => 'childId',
                'debug-log' => 'debugLog',
                'info-log' => 'infoLog',
                'critical-log' => 'criticalLog',
        ];

        public static function getOptionsWithArgs() {
                return self::$optionsWithArgs;
        }

        public static function newFromCommandLine( $args, $options ) {
                $jobOptions = [ 'destClusters' => $args ];
                foreach ( self::$cmdLineOptionMap as $cmdOption => $classOption ) {
                        if ( isset( $options[$cmdOption] ) ) {
                                $jobOptions[$classOption] = $options[$cmdOption];
                        }
                }

                return new self( $jobOptions );
        }

        public function __construct( $options ) {
                foreach ( $options as $name => $value ) {
                        $this->$name = $value;
                }
                $esFactory = MediaWikiServices::getInstance()->getExternalStoreFactory();
                $this->store = $esFactory->getStore( 'DB' );
                if ( !$this->isChild ) {
                        $GLOBALS['wgDebugLogPrefix'] = "RCT M: ";
                } elseif ( $this->childId !== false ) {
                        $GLOBALS['wgDebugLogPrefix'] = "RCT {$this->childId}: ";
                }
                $this->pageBlobClass = function_exists( 'xdiff_string_bdiff' ) ?
                        DiffHistoryBlob::class : ConcatenatedGzipHistoryBlob::class;
                $this->orphanBlobClass = ConcatenatedGzipHistoryBlob::class;

                $this->blobStore = MediaWikiServices::getInstance()
                        ->getBlobStoreFactory()
                        ->newSqlBlobStore();
        }

        public function debug( $msg ) {
                wfDebug( "$msg" );
                if ( $this->debugLog ) {
                        $this->logToFile( $msg, $this->debugLog );
                }
        }

        public function info( $msg ) {
                echo "$msg\n";
                if ( $this->infoLog ) {
                        $this->logToFile( $msg, $this->infoLog );
                }
        }

        public function critical( $msg ) {
                echo "$msg\n";
                if ( $this->criticalLog ) {
                        $this->logToFile( $msg, $this->criticalLog );
                }
        }

        private function logToFile( $msg, $file ) {
                $header = '[' . date( 'd\TH:i:s' ) . '] ' . wfHostname() . ' ' . posix_getpid();
                if ( $this->childId !== false ) {
                        $header .= "({$this->childId})";
                }
                $header .= ' ' . WikiMap::getCurrentWikiDbDomain()->getId();
                LegacyLogger::emit( sprintf( "%-50s %s\n", $header, $msg ), $file );
        }

        /**
         * Wait until the selected replica DB has caught up to the master.
         * This allows us to use the replica DB for things that were committed in a
         * previous part of this batch process.
         */
        private function syncDBs() {
                MediaWikiServices::getInstance()->getDBLoadBalancerFactory()->waitForReplication( [ 'timeout' => 100_000 ] );
        }

        /**
         * Execute parent or child depending on the isChild option
         */
        public function execute() {
                if ( $this->isChild ) {
                        $this->executeChild();
                } else {
                        $this->executeParent();
                }
        }

        /**
         * Execute the parent process
         */
        public function executeParent() {
                if ( !$this->checkTrackingTable() ) {
                        return;
                }

                $this->syncDBs();
                $this->startChildProcs();
                $this->doAllPages();
                $this->doAllOrphans();
                $this->killChildProcs();
        }

        /**
         * Make sure the tracking table exists and isn't empty
         * @return bool
         */
        private function checkTrackingTable() {
                $row = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase()->newSelectQueryBuilder()
                        ->select( '*' )
                        ->from( 'blob_tracking' )
                        ->caller( __METHOD__ )->fetchRow();
                if ( !$row ) {
                        $this->info( "Warning: blob_tracking table contains no rows, skipping this wiki." );

                        return false;
                }

                return true;
        }

        /**
         * Start the worker processes.
         * These processes will listen on stdin for commands.
         * This necessary because text recompression is slow: loading, compressing and
         * writing are all slow.
         */
        private function startChildProcs() {
                $wiki = WikiMap::getCurrentWikiId();

                $cmd = 'php ' . Shell::escape( __FILE__ );
                foreach ( self::$cmdLineOptionMap as $cmdOption => $classOption ) {
                        if ( $cmdOption == 'child-id' ) {
                                continue;
                        }
                        if ( in_array( $cmdOption, self::$optionsWithArgs ) && isset( $this->$classOption ) ) {
                                // @phan-suppress-next-line PhanTypeMismatchArgument False positive
                                $cmd .= " --$cmdOption " . Shell::escape( $this->$classOption );
                        } elseif ( $this->$classOption ) {
                                $cmd .= " --$cmdOption";
                        }
                }
                $cmd .= ' --child' .
                        ' --wiki ' . Shell::escape( $wiki ) .
                        ' ' . Shell::escape( ...$this->destClusters );

                $this->childPipes = $this->childProcs = [];
                for ( $i = 0; $i < $this->numProcs; $i++ ) {
                        $pipes = [];
                        $spec = [
                                [ 'pipe', 'r' ],
                                [ 'file', 'php://stdout', 'w' ],
                                [ 'file', 'php://stderr', 'w' ]
                        ];
                        AtEase::suppressWarnings();
                        $proc = proc_open( "$cmd --child-id $i", $spec, $pipes );
                        AtEase::restoreWarnings();
                        if ( !$proc ) {
                                $this->critical( "Error opening child process: $cmd" );
                                exit( 1 );
                        }
                        $this->childProcs[$i] = $proc;
                        $this->childPipes[$i] = $pipes[0];
                }
                $this->prevChildId = -1;
        }

        /**
         * Gracefully terminate the child processes
         */
        private function killChildProcs() {
                $this->info( "Waiting for child processes to finish..." );
                for ( $i = 0; $i < $this->numProcs; $i++ ) {
                        $this->dispatchToChild( $i, 'quit' );
                }
                for ( $i = 0; $i < $this->numProcs; $i++ ) {
                        $status = proc_close( $this->childProcs[$i] );
                        if ( $status ) {
                                $this->critical( "Warning: child #$i exited with status $status" );
                        }
                }
                $this->info( "Done." );
        }

        /**
         * Dispatch a command to the next available child process.
         * This may block until a child process finishes its work and becomes available.
         * @param array|string ...$args
         */
        private function dispatch( ...$args ) {
                $pipes = $this->childPipes;
                $x = [];
                $y = [];
                $numPipes = stream_select( $x, $pipes, $y, 3600 );
                if ( !$numPipes ) {
                        $this->critical( "Error waiting to write to child process. Aborting" );
                        exit( 1 );
                }
                for ( $i = 0; $i < $this->numProcs; $i++ ) {
                        $childId = ( $i + $this->prevChildId + 1 ) % $this->numProcs;
                        if ( isset( $pipes[$childId] ) ) {
                                $this->prevChildId = $childId;
                                $this->dispatchToChild( $childId, $args );

                                return;
                        }
                }
                $this->critical( "Unreachable" );
                exit( 1 );
        }

        /**
         * Dispatch a command to a specified child process
         * @param int $childId
         * @param array|string $args
         */
        private function dispatchToChild( $childId, $args ) {
                $args = (array)$args;
                $cmd = implode( ' ', $args );
                fwrite( $this->childPipes[$childId], "$cmd\n" );
        }

        /**
         * Move all tracked pages to the new clusters
         */
        private function doAllPages() {
                $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
                $i = 0;
                $startId = 0;
                if ( $this->noCount ) {
                        $numPages = '[unknown]';
                } else {
                        $numPages = $dbr->newSelectQueryBuilder()
                                ->select( 'COUNT(DISTINCT bt_page)' )
                                ->from( 'blob_tracking' )
                                ->where( [ 'bt_moved' => 0 ] )
                                ->caller( __METHOD__ )->fetchField();
                }
                if ( $this->copyOnly ) {
                        $this->info( "Copying pages..." );
                } else {
                        $this->info( "Moving pages..." );
                }
                while ( true ) {
                        $res = $dbr->newSelectQueryBuilder()
                                ->select( [ 'bt_page' ] )
                                ->distinct()
                                ->from( 'blob_tracking' )
                                ->where( [ 'bt_moved' => 0, $dbr->expr( 'bt_page', '>', $startId ) ] )
                                ->orderBy( 'bt_page' )
                                ->limit( $this->batchSize )
                                ->caller( __METHOD__ )->fetchResultSet();
                        if ( !$res->numRows() ) {
                                break;
                        }
                        foreach ( $res as $row ) {
                                $startId = $row->bt_page;
                                $this->dispatch( 'doPage', $row->bt_page );
                                $i++;
                        }
                        $this->report( 'pages', $i, $numPages );
                }
                $this->report( 'pages', $i, $numPages );
                if ( $this->copyOnly ) {
                        $this->info( "All page copies queued." );
                } else {
                        $this->info( "All page moves queued." );
                }
        }

        /**
         * Display a progress report
         * @param string $label
         * @param int $current
         * @param int $end
         */
        private function report( $label, $current, $end ) {
                $this->numBatches++;
                if ( $current == $end || $this->numBatches >= $this->reportingInterval ) {
                        $this->numBatches = 0;
                        $this->info( "$label: $current / $end" );
                        MediaWikiServices::getInstance()->getDBLoadBalancerFactory()->waitForReplication();
                }
        }

        /**
         * Move all orphan text to the new clusters
         */
        private function doAllOrphans() {
                $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
                $startId = 0;
                $i = 0;
                if ( $this->noCount ) {
                        $numOrphans = '[unknown]';
                } else {
                        $numOrphans = $dbr->newSelectQueryBuilder()
                                ->select( 'COUNT(DISTINCT bt_text_id)' )
                                ->from( 'blob_tracking' )
                                ->where( [ 'bt_moved' => 0, 'bt_page' => 0 ] )
                                ->caller( __METHOD__ )->fetchField();
                        if ( !$numOrphans ) {
                                return;
                        }
                }
                if ( $this->copyOnly ) {
                        $this->info( "Copying orphans..." );
                } else {
                        $this->info( "Moving orphans..." );
                }

                while ( true ) {
                        $res = $dbr->newSelectQueryBuilder()
                                ->select( [ 'bt_text_id' ] )
                                ->distinct()
                                ->from( 'blob_tracking' )
                                ->where( [ 'bt_moved' => 0, 'bt_page' => 0, $dbr->expr( 'bt_text_id', '>', $startId ) ] )
                                ->orderBy( 'bt_text_id' )
                                ->limit( $this->batchSize )
                                ->caller( __METHOD__ )->fetchResultSet();
                        if ( !$res->numRows() ) {
                                break;
                        }
                        $ids = [];
                        foreach ( $res as $row ) {
                                $startId = $row->bt_text_id;
                                $ids[] = $row->bt_text_id;
                                $i++;
                        }
                        // Need to send enough orphan IDs to the child at a time to fill a blob,
                        // so orphanBatchSize needs to be at least ~100.
                        // batchSize can be smaller or larger.
                        while ( count( $ids ) > $this->orphanBatchSize ) {
                                $args = array_slice( $ids, 0, $this->orphanBatchSize );
                                $ids = array_slice( $ids, $this->orphanBatchSize );
                                array_unshift( $args, 'doOrphanList' );
                                $this->dispatch( ...$args );
                        }
                        if ( count( $ids ) ) {
                                $args = $ids;
                                array_unshift( $args, 'doOrphanList' );
                                $this->dispatch( ...$args );
                        }

                        $this->report( 'orphans', $i, $numOrphans );
                }
                $this->report( 'orphans', $i, $numOrphans );
                $this->info( "All orphans queued." );
        }

        /**
         * Main entry point for worker processes
         */
        public function executeChild() {
                $this->debug( 'starting' );
                $this->syncDBs();

                while ( !feof( STDIN ) ) {
                        $line = rtrim( fgets( STDIN ) );
                        if ( $line == '' ) {
                                continue;
                        }
                        $this->debug( $line );
                        $args = explode( ' ', $line );
                        $cmd = array_shift( $args );
                        switch ( $cmd ) {
                                case 'doPage':
                                        $this->doPage( intval( $args[0] ) );
                                        break;
                                case 'doOrphanList':
                                        $this->doOrphanList( array_map( 'intval', $args ) );
                                        break;
                                case 'quit':
                                        return;
                        }
                        MediaWikiServices::getInstance()->getDBLoadBalancerFactory()->waitForReplication();
                }
        }

        /**
         * Move tracked text in a given page
         *
         * @param int $pageId
         */
        private function doPage( $pageId ) {
                $title = Title::newFromID( $pageId );
                if ( $title ) {
                        $titleText = $title->getPrefixedText();
                } else {
                        $titleText = '[deleted]';
                }
                $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();

                // Finish any incomplete transactions
                if ( !$this->copyOnly ) {
                        $this->finishIncompleteMoves( [ 'bt_page' => $pageId ] );
                        $this->syncDBs();
                }

                $startId = 0;
                $trx = new CgzCopyTransaction( $this, $this->pageBlobClass );

                $lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
                while ( true ) {
                        $res = $dbr->newSelectQueryBuilder()
                                ->select( '*' )
                                ->from( 'blob_tracking' )
                                ->join( 'text', null, 'bt_text_id=old_id' )
                                ->where( [
                                        'bt_page' => $pageId,
                                        $dbr->expr( 'bt_text_id', '>', $startId ),
                                        'bt_moved' => 0,
                                        'bt_new_url' => null,
                                ] )
                                ->orderBy( 'bt_text_id' )
                                ->limit( $this->batchSize )
                                ->caller( __METHOD__ )->fetchResultSet();
                        if ( !$res->numRows() ) {
                                break;
                        }

                        $lastTextId = 0;
                        foreach ( $res as $row ) {
                                $startId = $row->bt_text_id;
                                if ( $lastTextId == $row->bt_text_id ) {
                                        // Duplicate (null edit)
                                        continue;
                                }
                                $lastTextId = $row->bt_text_id;
                                // Load the text
                                $text = $this->blobStore->expandBlob( $row->old_text, $row->old_flags );
                                if ( $text === false ) {
                                        $this->critical( "Error loading {$row->bt_rev_id}/{$row->bt_text_id}" );
                                        continue;
                                }

                                // Queue it
                                if ( !$trx->addItem( $text, $row->bt_text_id ) ) {
                                        $this->debug( "$titleText: committing blob with " . $trx->getSize() . " items" );
                                        $trx->commit();
                                        $trx = new CgzCopyTransaction( $this, $this->pageBlobClass );
                                        $lbFactory->waitForReplication();
                                }
                        }
                }

                $this->debug( "$titleText: committing blob with " . $trx->getSize() . " items" );
                $trx->commit();
        }

        /**
         * Atomic move operation.
         *
         * Write the new URL to the text table and set the bt_moved flag.
         *
         * This is done in a single transaction to provide restartable behavior
         * without data loss.
         *
         * The transaction is kept short to reduce locking.
         *
         * @param int $textId
         * @param string $url
         */
        public function moveTextRow( $textId, $url ) {
                if ( $this->copyOnly ) {
                        $this->critical( "Internal error: can't call moveTextRow() in --copy-only mode" );
                        exit( 1 );
                }
                $dbw = MediaWikiServices::getInstance()->getConnectionProvider()->getPrimaryDatabase();

                $dbw->begin( __METHOD__ );
                $dbw->newUpdateQueryBuilder()
                        ->update( 'text' )
                        ->set( [
                                'old_text' => $url,
                                'old_flags' => 'external,utf-8',
                        ] )
                        ->where( [
                                'old_id' => $textId
                        ] )
                        ->caller( __METHOD__ )
                        ->execute();
                $dbw->newUpdateQueryBuilder()
                        ->update( 'blob_tracking' )
                        ->set( [ 'bt_moved' => 1 ] )
                        ->where( [ 'bt_text_id' => $textId ] )
                        ->caller( __METHOD__ )
                        ->execute();
                $dbw->commit( __METHOD__ );
        }

        /**
         * Moves are done in two phases: bt_new_url and then bt_moved.
         *  - bt_new_url indicates that the text has been copied to the new cluster.
         *  - bt_moved indicates that the text table has been updated.
         *
         * This function completes any moves that only have done bt_new_url. This
         * can happen when the script is interrupted, or when --copy-only is used.
         *
         * @param array $conds
         */
        private function finishIncompleteMoves( $conds ) {
                $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
                $lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();

                $startId = 0;
                $conds = array_merge( $conds, [
                        'bt_moved' => 0,
                        $dbr->expr( 'bt_new_url', '!=', null ),
                ] );
                while ( true ) {
                        $res = $dbr->newSelectQueryBuilder()
                                ->select( '*' )
                                ->from( 'blob_tracking' )
                                ->where( $conds )
                                ->andWhere( $dbr->expr( 'bt_text_id', '>', $startId ) )
                                ->orderBy( 'bt_text_id' )
                                ->limit( $this->batchSize )
                                ->caller( __METHOD__ )->fetchResultSet();
                        if ( !$res->numRows() ) {
                                break;
                        }
                        $this->debug( 'Incomplete: ' . $res->numRows() . ' rows' );
                        foreach ( $res as $row ) {
                                $startId = $row->bt_text_id;
                                $this->moveTextRow( $row->bt_text_id, $row->bt_new_url );
                                if ( $row->bt_text_id % 10 == 0 ) {
                                        $lbFactory->waitForReplication();
                                }
                        }
                }
        }

        /**
         * Returns the name of the next target cluster
         * @return string
         */
        public function getTargetCluster() {
                $cluster = next( $this->destClusters );
                if ( $cluster === false ) {
                        $cluster = reset( $this->destClusters );
                }

                return $cluster;
        }

        /**
         * Move an orphan text_id to the new cluster
         *
         * @param array $textIds
         */
        private function doOrphanList( $textIds ) {
                // Finish incomplete moves
                if ( !$this->copyOnly ) {
                        $this->finishIncompleteMoves( [ 'bt_text_id' => $textIds ] );
                        $this->syncDBs();
                }

                $trx = new CgzCopyTransaction( $this, $this->orphanBlobClass );

                $lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
                $res = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase()->newSelectQueryBuilder()
                        ->select( [ 'old_id', 'old_text', 'old_flags' ] )
                        ->distinct()
                        ->from( 'text' )
                        ->join( 'blob_tracking', null, 'bt_text_id=old_id' )
                        ->where( [ 'old_id' => $textIds, 'bt_moved' => 0 ] )
                        ->caller( __METHOD__ )->fetchResultSet();

                foreach ( $res as $row ) {
                        $text = $this->blobStore->expandBlob( $row->old_text, $row->old_flags );
                        if ( $text === false ) {
                                $this->critical( "Error: cannot load revision text for old_id={$row->old_id}" );
                                continue;
                        }

                        if ( !$trx->addItem( $text, $row->old_id ) ) {
                                $this->debug( "[orphan]: committing blob with " . $trx->getSize() . " rows" );
                                $trx->commit();
                                $trx = new CgzCopyTransaction( $this, $this->orphanBlobClass );
                                $lbFactory->waitForReplication();
                        }
                }
                $this->debug( "[orphan]: committing blob with " . $trx->getSize() . " rows" );
                $trx->commit();
        }
}

/**
 * Class to represent a recompression operation for a single CGZ blob
 */
class CgzCopyTransaction {
        /** @var RecompressTracked */
        public $parent;
        /** @var string */
        public $blobClass;
        /** @var ConcatenatedGzipHistoryBlob|false */
        public $cgz;
        /** @var string[] */
        public $referrers;
        /** @var array */
        private $texts;

        /**
         * Create a transaction from a RecompressTracked object
         * @param RecompressTracked $parent
         * @param string $blobClass
         */
        public function __construct( $parent, $blobClass ) {
                $this->blobClass = $blobClass;
                $this->cgz = false;
                $this->texts = [];
                $this->parent = $parent;
        }

        /**
         * Add text.
         * Returns false if it's ready to commit.
         * @param string $text
         * @param int $textId
         * @return bool
         */
        public function addItem( $text, $textId ) {
                if ( !$this->cgz ) {
                        $class = $this->blobClass;
                        $this->cgz = new $class;
                }
                $hash = $this->cgz->addItem( $text );
                $this->referrers[$textId] = $hash;
                $this->texts[$textId] = $text;

                return $this->cgz->isHappy();
        }

        public function getSize() {
                return count( $this->texts );
        }

        /**
         * Recompress text after some aberrant modification
         */
        public function recompress() {
                $class = $this->blobClass;
                $this->cgz = new $class;
                $this->referrers = [];
                foreach ( $this->texts as $textId => $text ) {
                        $hash = $this->cgz->addItem( $text );
                        $this->referrers[$textId] = $hash;
                }
        }

        /**
         * Commit the blob.
         * Does nothing if no text items have been added.
         * May skip the move if --copy-only is set.
         */
        public function commit() {
                $originalCount = count( $this->texts );
                if ( !$originalCount ) {
                        return;
                }

                /* Check to see if the target text_ids have been moved already.
                 *
                 * We originally read from the replica DB, so this can happen when a single
                 * text_id is shared between multiple pages. It's rare, but possible
                 * if a delete/move/undelete cycle splits up a null edit.
                 *
                 * We do a locking read to prevent closer-run race conditions.
                 */
                $dbw = MediaWikiServices::getInstance()->getConnectionProvider()->getPrimaryDatabase();
                $dbw->begin( __METHOD__ );
                $res = $dbw->newSelectQueryBuilder()
                        ->select( [ 'bt_text_id', 'bt_moved' ] )
                        ->forUpdate()
                        ->from( 'blob_tracking' )
                        ->where( [ 'bt_text_id' => array_keys( $this->referrers ) ] )
                        ->caller( __METHOD__ )->fetchResultSet();
                $dirty = false;
                foreach ( $res as $row ) {
                        if ( $row->bt_moved ) {
                                # This row has already been moved, remove it
                                $this->parent->debug( "TRX: conflict detected in old_id={$row->bt_text_id}" );
                                unset( $this->texts[$row->bt_text_id] );
                                $dirty = true;
                        }
                }

                // Recompress the blob if necessary
                if ( $dirty ) {
                        if ( !count( $this->texts ) ) {
                                // All have been moved already
                                if ( $originalCount > 1 ) {
                                        // This is suspcious, make noise
                                        $this->parent->critical(
                                                "Warning: concurrent operation detected, are there two conflicting " .
                                                "processes running, doing the same job?" );
                                }

                                return;
                        }
                        $this->recompress();
                }

                // Insert the data into the destination cluster
                $targetCluster = $this->parent->getTargetCluster();
                $store = $this->parent->store;
                $targetDB = $store->getPrimary( $targetCluster );
                $targetDB->begin( __METHOD__ );
                $baseUrl = $this->parent->store->store( $targetCluster, serialize( $this->cgz ) );

                // Write the new URLs to the blob_tracking table
                foreach ( $this->referrers as $textId => $hash ) {
                        $url = $baseUrl . '/' . $hash;
                        $dbw->newUpdateQueryBuilder()
                                ->update( 'blob_tracking' )
                                ->set( [ 'bt_new_url' => $url ] )
                                ->where( [
                                        'bt_text_id' => $textId,
                                        'bt_moved' => 0, # Check for concurrent conflicting update
                                ] )
                                ->caller( __METHOD__ )
                                ->execute();
                }

                $targetDB->commit( __METHOD__ );
                // Critical section here: interruption at this point causes blob duplication
                // Reversing the order of the commits would cause data loss instead
                $dbw->commit( __METHOD__ );

                // Write the new URLs to the text table and set the moved flag
                if ( !$this->parent->copyOnly ) {
                        foreach ( $this->referrers as $textId => $hash ) {
                                $url = $baseUrl . '/' . $hash;
                                $this->parent->moveTextRow( $textId, $url );
                        }
                }
        }
}