Merge "Add cleanup for handlers cache."

[lhc/web/wiklou.git] / includes / Storage / RevisionStore.php

<?php
/**
 * Service for looking up page revisions.
 *
 * 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
 *
 * Attribution notice: when this file was created, much of its content was taken
 * from the Revision.php file as present in release 1.30. Refer to the history
 * of that file for original authorship.
 *
 * @file
 */

namespace MediaWiki\Storage;

use ActorMigration;
use CommentStore;
use CommentStoreComment;
use Content;
use ContentHandler;
use DBAccessObjectUtils;
use Hooks;
use IDBAccessObject;
use InvalidArgumentException;
use IP;
use LogicException;
use MediaWiki\Linker\LinkTarget;
use MediaWiki\User\UserIdentity;
use MediaWiki\User\UserIdentityValue;
use Message;
use MWException;
use MWUnknownContentModelException;
use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use RecentChange;
use stdClass;
use Title;
use User;
use WANObjectCache;
use Wikimedia\Assert\Assert;
use Wikimedia\Rdbms\Database;
use Wikimedia\Rdbms\DBConnRef;
use Wikimedia\Rdbms\IDatabase;
use Wikimedia\Rdbms\LoadBalancer;

/**
 * Service for looking up page revisions.
 *
 * @since 1.31
 *
 * @note This was written to act as a drop-in replacement for the corresponding
 *       static methods in Revision.
 */
class RevisionStore
        implements IDBAccessObject, RevisionFactory, RevisionLookup, LoggerAwareInterface {

        /**
         * @var SqlBlobStore
         */
        private $blobStore;

        /**
         * @var bool|string
         */
        private $wikiId;

        /**
         * @var boolean
         */
        private $contentHandlerUseDB = true;

        /**
         * @var LoadBalancer
         */
        private $loadBalancer;

        /**
         * @var WANObjectCache
         */
        private $cache;

        /**
         * @var CommentStore
         */
        private $commentStore;

        /**
         * @var ActorMigration
         */
        private $actorMigration;

        /**
         * @var LoggerInterface
         */
        private $logger;

        /**
         * @todo $blobStore should be allowed to be any BlobStore!
         *
         * @param LoadBalancer $loadBalancer
         * @param SqlBlobStore $blobStore
         * @param WANObjectCache $cache
         * @param CommentStore $commentStore
         * @param ActorMigration $actorMigration
         * @param bool|string $wikiId
         */
        public function __construct(
                LoadBalancer $loadBalancer,
                SqlBlobStore $blobStore,
                WANObjectCache $cache,
                CommentStore $commentStore,
                ActorMigration $actorMigration,
                $wikiId = false
        ) {
                Assert::parameterType( 'string|boolean', $wikiId, '$wikiId' );

                $this->loadBalancer = $loadBalancer;
                $this->blobStore = $blobStore;
                $this->cache = $cache;
                $this->commentStore = $commentStore;
                $this->actorMigration = $actorMigration;
                $this->wikiId = $wikiId;
                $this->logger = new NullLogger();
        }

        public function setLogger( LoggerInterface $logger ) {
                $this->logger = $logger;
        }

        /**
         * @return bool Whether the store is read-only
         */
        public function isReadOnly() {
                return $this->blobStore->isReadOnly();
        }

        /**
         * @return bool
         */
        public function getContentHandlerUseDB() {
                return $this->contentHandlerUseDB;
        }

        /**
         * @param bool $contentHandlerUseDB
         */
        public function setContentHandlerUseDB( $contentHandlerUseDB ) {
                $this->contentHandlerUseDB = $contentHandlerUseDB;
        }

        /**
         * @return LoadBalancer
         */
        private function getDBLoadBalancer() {
                return $this->loadBalancer;
        }

        /**
         * @param int $mode DB_MASTER or DB_REPLICA
         *
         * @return IDatabase
         */
        private function getDBConnection( $mode ) {
                $lb = $this->getDBLoadBalancer();
                return $lb->getConnection( $mode, [], $this->wikiId );
        }

        /**
         * @param IDatabase $connection
         */
        private function releaseDBConnection( IDatabase $connection ) {
                $lb = $this->getDBLoadBalancer();
                $lb->reuseConnection( $connection );
        }

        /**
         * @param int $mode DB_MASTER or DB_REPLICA
         *
         * @return DBConnRef
         */
        private function getDBConnectionRef( $mode ) {
                $lb = $this->getDBLoadBalancer();
                return $lb->getConnectionRef( $mode, [], $this->wikiId );
        }

        /**
         * Determines the page Title based on the available information.
         *
         * MCR migration note: this corresponds to Revision::getTitle
         *
         * @note this method should be private, external use should be avoided!
         *
         * @param int|null $pageId
         * @param int|null $revId
         * @param int $queryFlags
         *
         * @return Title
         * @throws RevisionAccessException
         */
        public function getTitle( $pageId, $revId, $queryFlags = self::READ_NORMAL ) {
                if ( !$pageId && !$revId ) {
                        throw new InvalidArgumentException( '$pageId and $revId cannot both be 0 or null' );
                }

                // This method recalls itself with READ_LATEST if READ_NORMAL doesn't get us a Title
                // So ignore READ_LATEST_IMMUTABLE flags and handle the fallback logic in this method
                if ( DBAccessObjectUtils::hasFlags( $queryFlags, self::READ_LATEST_IMMUTABLE ) ) {
                        $queryFlags = self::READ_NORMAL;
                }

                $canUseTitleNewFromId = ( $pageId !== null && $pageId > 0 && $this->wikiId === false );
                list( $dbMode, $dbOptions ) = DBAccessObjectUtils::getDBOptions( $queryFlags );
                $titleFlags = ( $dbMode == DB_MASTER ? Title::GAID_FOR_UPDATE : 0 );

                // Loading by ID is best, but Title::newFromID does not support that for foreign IDs.
                if ( $canUseTitleNewFromId ) {
                        // TODO: better foreign title handling (introduce TitleFactory)
                        $title = Title::newFromID( $pageId, $titleFlags );
                        if ( $title ) {
                                return $title;
                        }
                }

                // rev_id is defined as NOT NULL, but this revision may not yet have been inserted.
                $canUseRevId = ( $revId !== null && $revId > 0 );

                if ( $canUseRevId ) {
                        $dbr = $this->getDBConnectionRef( $dbMode );
                        // @todo: Title::getSelectFields(), or Title::getQueryInfo(), or something like that
                        $row = $dbr->selectRow(
                                [ 'revision', 'page' ],
                                [
                                        'page_namespace',
                                        'page_title',
                                        'page_id',
                                        'page_latest',
                                        'page_is_redirect',
                                        'page_len',
                                ],
                                [ 'rev_id' => $revId ],
                                __METHOD__,
                                $dbOptions,
                                [ 'page' => [ 'JOIN', 'page_id=rev_page' ] ]
                        );
                        if ( $row ) {
                                // TODO: better foreign title handling (introduce TitleFactory)
                                return Title::newFromRow( $row );
                        }
                }

                // If we still don't have a title, fallback to master if that wasn't already happening.
                if ( $dbMode !== DB_MASTER ) {
                        $title = $this->getTitle( $pageId, $revId, self::READ_LATEST );
                        if ( $title ) {
                                $this->logger->info(
                                        __METHOD__ . ' fell back to READ_LATEST and got a Title.',
                                        [ 'trace' => wfBacktrace() ]
                                );
                                return $title;
                        }
                }

                throw new RevisionAccessException(
                        "Could not determine title for page ID $pageId and revision ID $revId"
                );
        }

        /**
         * @param mixed $value
         * @param string $name
         *
         * @throw IncompleteRevisionException if $value is null
         * @return mixed $value, if $value is not null
         */
        private function failOnNull( $value, $name ) {
                if ( $value === null ) {
                        throw new IncompleteRevisionException(
                                "$name must not be " . var_export( $value, true ) . "!"
                        );
                }

                return $value;
        }

        /**
         * @param mixed $value
         * @param string $name
         *
         * @throw IncompleteRevisionException if $value is empty
         * @return mixed $value, if $value is not null
         */
        private function failOnEmpty( $value, $name ) {
                if ( $value === null || $value === 0 || $value === '' ) {
                        throw new IncompleteRevisionException(
                                "$name must not be " . var_export( $value, true ) . "!"
                        );
                }

                return $value;
        }

        /**
         * Insert a new revision into the database, returning the new revision record
         * on success and dies horribly on failure.
         *
         * MCR migration note: this replaces Revision::insertOn
         *
         * @param RevisionRecord $rev
         * @param IDatabase $dbw (master connection)
         *
         * @throws InvalidArgumentException
         * @return RevisionRecord the new revision record.
         */
        public function insertRevisionOn( RevisionRecord $rev, IDatabase $dbw ) {
                // TODO: pass in a DBTransactionContext instead of a database connection.
                $this->checkDatabaseWikiId( $dbw );

                if ( !$rev->getSlotRoles() ) {
                        throw new InvalidArgumentException( 'At least one slot needs to be defined!' );
                }

                if ( $rev->getSlotRoles() !== [ 'main' ] ) {
                        throw new InvalidArgumentException( 'Only the main slot is supported for now!' );
                }

                // TODO: we shouldn't need an actual Title here.
                $title = Title::newFromLinkTarget( $rev->getPageAsLinkTarget() );
                $pageId = $this->failOnEmpty( $rev->getPageId(), 'rev_page field' ); // check this early

                $parentId = $rev->getParentId() === null
                        ? $this->getPreviousRevisionId( $dbw, $rev )
                        : $rev->getParentId();

                // Record the text (or external storage URL) to the blob store
                $slot = $rev->getSlot( 'main', RevisionRecord::RAW );

                $size = $this->failOnNull( $rev->getSize(), 'size field' );
                $sha1 = $this->failOnEmpty( $rev->getSha1(), 'sha1 field' );

                if ( !$slot->hasAddress() ) {
                        $content = $slot->getContent();
                        $format = $content->getDefaultFormat();
                        $model = $content->getModel();

                        $this->checkContentModel( $content, $title );

                        $data = $content->serialize( $format );

                        // Hints allow the blob store to optimize by "leaking" application level information to it.
                        // TODO: with the new MCR storage schema, we rev_id have this before storing the blobs.
                        // When we have it, add rev_id as a hint. Can be used with rev_parent_id for
                        // differential storage or compression of subsequent revisions.
                        $blobHints = [
                                BlobStore::DESIGNATION_HINT => 'page-content', // BlobStore may be used for other things too.
                                BlobStore::PAGE_HINT => $pageId,
                                BlobStore::ROLE_HINT => $slot->getRole(),
                                BlobStore::PARENT_HINT => $parentId,
                                BlobStore::SHA1_HINT => $slot->getSha1(),
                                BlobStore::MODEL_HINT => $model,
                                BlobStore::FORMAT_HINT => $format,
                        ];

                        $blobAddress = $this->blobStore->storeBlob( $data, $blobHints );
                } else {
                        $blobAddress = $slot->getAddress();
                        $model = $slot->getModel();
                        $format = $slot->getFormat();
                }

                $textId = $this->blobStore->getTextIdFromAddress( $blobAddress );

                if ( !$textId ) {
                        throw new LogicException(
                                'Blob address not supported in 1.29 database schema: ' . $blobAddress
                        );
                }

                // getTextIdFromAddress() is free to insert something into the text table, so $textId
                // may be a new value, not anything already contained in $blobAddress.
                $blobAddress = 'tt:' . $textId;

                $comment = $this->failOnNull( $rev->getComment( RevisionRecord::RAW ), 'comment' );
                $user = $this->failOnNull( $rev->getUser( RevisionRecord::RAW ), 'user' );
                $timestamp = $this->failOnEmpty( $rev->getTimestamp(), 'timestamp field' );

                // Checks.
                $this->failOnNull( $user->getId(), 'user field' );
                $this->failOnEmpty( $user->getName(), 'user_text field' );

                # Record the edit in revisions
                $row = [
                        'rev_page'       => $pageId,
                        'rev_parent_id'  => $parentId,
                        'rev_text_id'    => $textId,
                        'rev_minor_edit' => $rev->isMinor() ? 1 : 0,
                        'rev_timestamp'  => $dbw->timestamp( $timestamp ),
                        'rev_deleted'    => $rev->getVisibility(),
                        'rev_len'        => $size,
                        'rev_sha1'       => $sha1,
                ];

                if ( $rev->getId() !== null ) {
                        // Needed to restore revisions with their original ID
                        $row['rev_id'] = $rev->getId();
                }

                list( $commentFields, $commentCallback ) =
                        $this->commentStore->insertWithTempTable( $dbw, 'rev_comment', $comment );
                $row += $commentFields;

                list( $actorFields, $actorCallback ) =
                        $this->actorMigration->getInsertValuesWithTempTable( $dbw, 'rev_user', $user );
                $row += $actorFields;

                if ( $this->contentHandlerUseDB ) {
                        // MCR migration note: rev_content_model and rev_content_format will go away

                        $defaultModel = ContentHandler::getDefaultModelFor( $title );
                        $defaultFormat = ContentHandler::getForModelID( $defaultModel )->getDefaultFormat();

                        $row['rev_content_model'] = ( $model === $defaultModel ) ? null : $model;
                        $row['rev_content_format'] = ( $format === $defaultFormat ) ? null : $format;
                }

                $dbw->insert( 'revision', $row, __METHOD__ );

                if ( !isset( $row['rev_id'] ) ) {
                        // only if auto-increment was used
                        $row['rev_id'] = intval( $dbw->insertId() );
                }
                $commentCallback( $row['rev_id'] );
                $actorCallback( $row['rev_id'], $row );

                // Insert IP revision into ip_changes for use when querying for a range.
                if ( $user->getId() === 0 && IP::isValid( $user->getName() ) ) {
                        $ipcRow = [
                                'ipc_rev_id'        => $row['rev_id'],
                                'ipc_rev_timestamp' => $row['rev_timestamp'],
                                'ipc_hex'           => IP::toHex( $user->getName() ),
                        ];
                        $dbw->insert( 'ip_changes', $ipcRow, __METHOD__ );
                }

                $newSlot = SlotRecord::newSaved( $row['rev_id'], $textId, $blobAddress, $slot );
                $slots = new RevisionSlots( [ 'main' => $newSlot ] );

                $rev = new RevisionStoreRecord(
                        $title,
                        $user,
                        $comment,
                        (object)$row,
                        $slots,
                        $this->wikiId
                );

                $newSlot = $rev->getSlot( 'main', RevisionRecord::RAW );

                // sanity checks
                Assert::postcondition( $rev->getId() > 0, 'revision must have an ID' );
                Assert::postcondition( $rev->getPageId() > 0, 'revision must have a page ID' );
                Assert::postcondition(
                        $rev->getComment( RevisionRecord::RAW ) !== null,
                        'revision must have a comment'
                );
                Assert::postcondition(
                        $rev->getUser( RevisionRecord::RAW ) !== null,
                        'revision must have a user'
                );

                Assert::postcondition( $newSlot !== null, 'revision must have a main slot' );
                Assert::postcondition(
                        $newSlot->getAddress() !== null,
                        'main slot must have an addess'
                );

                Hooks::run( 'RevisionRecordInserted', [ $rev ] );

                return $rev;
        }

        /**
         * MCR migration note: this corresponds to Revision::checkContentModel
         *
         * @param Content $content
         * @param Title $title
         *
         * @throws MWException
         * @throws MWUnknownContentModelException
         */
        private function checkContentModel( Content $content, Title $title ) {
                // Note: may return null for revisions that have not yet been inserted

                $model = $content->getModel();
                $format = $content->getDefaultFormat();
                $handler = $content->getContentHandler();

                $name = "$title";

                if ( !$handler->isSupportedFormat( $format ) ) {
                        throw new MWException( "Can't use format $format with content model $model on $name" );
                }

                if ( !$this->contentHandlerUseDB ) {
                        // if $wgContentHandlerUseDB is not set,
                        // all revisions must use the default content model and format.

                        $defaultModel = ContentHandler::getDefaultModelFor( $title );
                        $defaultHandler = ContentHandler::getForModelID( $defaultModel );
                        $defaultFormat = $defaultHandler->getDefaultFormat();

                        if ( $model != $defaultModel ) {
                                throw new MWException( "Can't save non-default content model with "
                                        . "\$wgContentHandlerUseDB disabled: model is $model, "
                                        . "default for $name is $defaultModel"
                                );
                        }

                        if ( $format != $defaultFormat ) {
                                throw new MWException( "Can't use non-default content format with "
                                        . "\$wgContentHandlerUseDB disabled: format is $format, "
                                        . "default for $name is $defaultFormat"
                                );
                        }
                }

                if ( !$content->isValid() ) {
                        throw new MWException(
                                "New content for $name is not valid! Content model is $model"
                        );
                }
        }

        /**
         * Create a new null-revision for insertion into a page's
         * history. This will not re-save the text, but simply refer
         * to the text from the previous version.
         *
         * Such revisions can for instance identify page rename
         * operations and other such meta-modifications.
         *
         * MCR migration note: this replaces Revision::newNullRevision
         *
         * @todo Introduce newFromParentRevision(). newNullRevision can then be based on that
         * (or go away).
         *
         * @param IDatabase $dbw
         * @param Title $title Title of the page to read from
         * @param CommentStoreComment $comment RevisionRecord's summary
         * @param bool $minor Whether the revision should be considered as minor
         * @param User $user The user to attribute the revision to
         * @return RevisionRecord|null RevisionRecord or null on error
         */
        public function newNullRevision(
                IDatabase $dbw,
                Title $title,
                CommentStoreComment $comment,
                $minor,
                User $user
        ) {
                $this->checkDatabaseWikiId( $dbw );

                $fields = [ 'page_latest', 'page_namespace', 'page_title',
                        'rev_id', 'rev_text_id', 'rev_len', 'rev_sha1' ];

                if ( $this->contentHandlerUseDB ) {
                        $fields[] = 'rev_content_model';
                        $fields[] = 'rev_content_format';
                }

                $current = $dbw->selectRow(
                        [ 'page', 'revision' ],
                        $fields,
                        [
                                'page_id' => $title->getArticleID(),
                                'page_latest=rev_id',
                        ],
                        __METHOD__,
                        [ 'FOR UPDATE' ] // T51581
                );

                if ( $current ) {
                        $fields = [
                                'page'       => $title->getArticleID(),
                                'user_text'  => $user->getName(),
                                'user'       => $user->getId(),
                                'actor'      => $user->getActorId(),
                                'comment'    => $comment,
                                'minor_edit' => $minor,
                                'text_id'    => $current->rev_text_id,
                                'parent_id'  => $current->page_latest,
                                'len'        => $current->rev_len,
                                'sha1'       => $current->rev_sha1
                        ];

                        if ( $this->contentHandlerUseDB ) {
                                $fields['content_model'] = $current->rev_content_model;
                                $fields['content_format'] = $current->rev_content_format;
                        }

                        $fields['title'] = Title::makeTitle( $current->page_namespace, $current->page_title );

                        $mainSlot = $this->emulateMainSlot_1_29( $fields, self::READ_LATEST, $title );
                        $revision = new MutableRevisionRecord( $title, $this->wikiId );
                        $this->initializeMutableRevisionFromArray( $revision, $fields );
                        $revision->setSlot( $mainSlot );
                } else {
                        $revision = null;
                }

                return $revision;
        }

        /**
         * MCR migration note: this replaces Revision::isUnpatrolled
         *
         * @todo This is overly specific, so move or kill this method.
         *
         * @param RevisionRecord $rev
         *
         * @return int Rcid of the unpatrolled row, zero if there isn't one
         */
        public function getRcIdIfUnpatrolled( RevisionRecord $rev ) {
                $rc = $this->getRecentChange( $rev );
                if ( $rc && $rc->getAttribute( 'rc_patrolled' ) == 0 ) {
                        return $rc->getAttribute( 'rc_id' );
                } else {
                        return 0;
                }
        }

        /**
         * Get the RC object belonging to the current revision, if there's one
         *
         * MCR migration note: this replaces Revision::getRecentChange
         *
         * @todo move this somewhere else?
         *
         * @param RevisionRecord $rev
         * @param int $flags (optional) $flags include:
         *      IDBAccessObject::READ_LATEST: Select the data from the master
         *
         * @return null|RecentChange
         */
        public function getRecentChange( RevisionRecord $rev, $flags = 0 ) {
                $dbr = $this->getDBConnection( DB_REPLICA );

                list( $dbType, ) = DBAccessObjectUtils::getDBOptions( $flags );

                $userIdentity = $rev->getUser( RevisionRecord::RAW );

                if ( !$userIdentity ) {
                        // If the revision has no user identity, chances are it never went
                        // into the database, and doesn't have an RC entry.
                        return null;
                }

                // TODO: Select by rc_this_oldid alone - but as of Nov 2017, there is no index on that!
                $actorWhere = $this->actorMigration->getWhere( $dbr, 'rc_user', $rev->getUser(), false );
                $rc = RecentChange::newFromConds(
                        [
                                $actorWhere['conds'],
                                'rc_timestamp' => $dbr->timestamp( $rev->getTimestamp() ),
                                'rc_this_oldid' => $rev->getId()
                        ],
                        __METHOD__,
                        $dbType
                );

                $this->releaseDBConnection( $dbr );

                // XXX: cache this locally? Glue it to the RevisionRecord?
                return $rc;
        }

        /**
         * Maps fields of the archive row to corresponding revision rows.
         *
         * @param object $archiveRow
         *
         * @return object a revision row object, corresponding to $archiveRow.
         */
        private static function mapArchiveFields( $archiveRow ) {
                $fieldMap = [
                        // keep with ar prefix:
                        'ar_id'        => 'ar_id',

                        // not the same suffix:
                        'ar_page_id'        => 'rev_page',
                        'ar_rev_id'         => 'rev_id',

                        // same suffix:
                        'ar_text_id'        => 'rev_text_id',
                        'ar_timestamp'      => 'rev_timestamp',
                        'ar_user_text'      => 'rev_user_text',
                        'ar_user'           => 'rev_user',
                        'ar_actor'          => 'rev_actor',
                        'ar_minor_edit'     => 'rev_minor_edit',
                        'ar_deleted'        => 'rev_deleted',
                        'ar_len'            => 'rev_len',
                        'ar_parent_id'      => 'rev_parent_id',
                        'ar_sha1'           => 'rev_sha1',
                        'ar_comment'        => 'rev_comment',
                        'ar_comment_cid'    => 'rev_comment_cid',
                        'ar_comment_id'     => 'rev_comment_id',
                        'ar_comment_text'   => 'rev_comment_text',
                        'ar_comment_data'   => 'rev_comment_data',
                        'ar_comment_old'    => 'rev_comment_old',
                        'ar_content_format' => 'rev_content_format',
                        'ar_content_model'  => 'rev_content_model',
                ];

                if ( empty( $archiveRow->ar_text_id ) ) {
                        $fieldMap['ar_text'] = 'old_text';
                        $fieldMap['ar_flags'] = 'old_flags';
                }

                $revRow = new stdClass();
                foreach ( $fieldMap as $arKey => $revKey ) {
                        if ( property_exists( $archiveRow, $arKey ) ) {
                                $revRow->$revKey = $archiveRow->$arKey;
                        }
                }

                return $revRow;
        }

        /**
         * Constructs a RevisionRecord for the revisions main slot, based on the MW1.29 schema.
         *
         * @param object|array $row Either a database row or an array
         * @param int $queryFlags for callbacks
         * @param Title $title
         *
         * @return SlotRecord The main slot, extracted from the MW 1.29 style row.
         * @throws MWException
         */
        private function emulateMainSlot_1_29( $row, $queryFlags, Title $title ) {
                $mainSlotRow = new stdClass();
                $mainSlotRow->role_name = 'main';
                $mainSlotRow->model_name = null;
                $mainSlotRow->slot_revision_id = null;
                $mainSlotRow->content_address = null;
                $mainSlotRow->slot_content_id = null;

                $content = null;
                $blobData = null;
                $blobFlags = null;

                if ( is_object( $row ) ) {
                        // archive row
                        if ( !isset( $row->rev_id ) && ( isset( $row->ar_user ) || isset( $row->ar_actor ) ) ) {
                                $row = $this->mapArchiveFields( $row );
                        }

                        if ( isset( $row->rev_text_id ) && $row->rev_text_id > 0 ) {
                                $mainSlotRow->slot_content_id = $row->rev_text_id;
                                $mainSlotRow->content_address = 'tt:' . $row->rev_text_id;
                        }

                        if ( isset( $row->old_text ) ) {
                                // this happens when the text-table gets joined directly, in the pre-1.30 schema
                                $blobData = isset( $row->old_text ) ? strval( $row->old_text ) : null;
                                // Check against selects that might have not included old_flags
                                if ( !property_exists( $row, 'old_flags' ) ) {
                                        throw new InvalidArgumentException( 'old_flags was not set in $row' );
                                }
                                $blobFlags = ( $row->old_flags === null ) ? '' : $row->old_flags;
                        }

                        $mainSlotRow->slot_revision_id = intval( $row->rev_id );

                        $mainSlotRow->content_size = isset( $row->rev_len ) ? intval( $row->rev_len ) : null;
                        $mainSlotRow->content_sha1 = isset( $row->rev_sha1 ) ? strval( $row->rev_sha1 ) : null;
                        $mainSlotRow->model_name = isset( $row->rev_content_model )
                                ? strval( $row->rev_content_model )
                                : null;
                        // XXX: in the future, we'll probably always use the default format, and drop content_format
                        $mainSlotRow->format_name = isset( $row->rev_content_format )
                                ? strval( $row->rev_content_format )
                                : null;
                } elseif ( is_array( $row ) ) {
                        $mainSlotRow->slot_revision_id = isset( $row['id'] ) ? intval( $row['id'] ) : null;

                        $mainSlotRow->slot_content_id = isset( $row['text_id'] )
                                ? intval( $row['text_id'] )
                                : null;
                        $mainSlotRow->content_address = isset( $row['text_id'] )
                                ? 'tt:' . intval( $row['text_id'] )
                                : null;
                        $mainSlotRow->content_size = isset( $row['len'] ) ? intval( $row['len'] ) : null;
                        $mainSlotRow->content_sha1 = isset( $row['sha1'] ) ? strval( $row['sha1'] ) : null;

                        $mainSlotRow->model_name = isset( $row['content_model'] )
                                ? strval( $row['content_model'] ) : null;  // XXX: must be a string!
                        // XXX: in the future, we'll probably always use the default format, and drop content_format
                        $mainSlotRow->format_name = isset( $row['content_format'] )
                                ? strval( $row['content_format'] ) : null;
                        $blobData = isset( $row['text'] ) ? rtrim( strval( $row['text'] ) ) : null;
                        // XXX: If the flags field is not set then $blobFlags should be null so that no
                        // decoding will happen. An empty string will result in default decodings.
                        $blobFlags = isset( $row['flags'] ) ? trim( strval( $row['flags'] ) ) : null;

                        // if we have a Content object, override mText and mContentModel
                        if ( !empty( $row['content'] ) ) {
                                if ( !( $row['content'] instanceof Content ) ) {
                                        throw new MWException( 'content field must contain a Content object.' );
                                }

                                /** @var Content $content */
                                $content = $row['content'];
                                $handler = $content->getContentHandler();

                                $mainSlotRow->model_name = $content->getModel();

                                // XXX: in the future, we'll probably always use the default format.
                                if ( $mainSlotRow->format_name === null ) {
                                        $mainSlotRow->format_name = $handler->getDefaultFormat();
                                }
                        }
                } else {
                        throw new MWException( 'Revision constructor passed invalid row format.' );
                }

                // With the old schema, the content changes with every revision.
                // ...except for null-revisions. Would be nice if we could detect them.
                $mainSlotRow->slot_inherited = 0;

                if ( $mainSlotRow->model_name === null ) {
                        $mainSlotRow->model_name = function ( SlotRecord $slot ) use ( $title ) {
                                // TODO: MCR: consider slot role in getDefaultModelFor()! Use LinkTarget!
                                // TODO: MCR: deprecate $title->getModel().
                                return ContentHandler::getDefaultModelFor( $title );
                        };
                }

                if ( !$content ) {
                        $content = function ( SlotRecord $slot )
                                use ( $blobData, $blobFlags, $queryFlags, $mainSlotRow )
                        {
                                return $this->loadSlotContent(
                                        $slot,
                                        $blobData,
                                        $blobFlags,
                                        $mainSlotRow->format_name,
                                        $queryFlags
                                );
                        };
                }

                $mainSlotRow->slot_id = $mainSlotRow->slot_revision_id;
                return new SlotRecord( $mainSlotRow, $content );
        }

        /**
         * Loads a Content object based on a slot row.
         *
         * This method does not call $slot->getContent(), and may be used as a callback
         * called by $slot->getContent().
         *
         * MCR migration note: this roughly corresponds to Revision::getContentInternal
         *
         * @param SlotRecord $slot The SlotRecord to load content for
         * @param string|null $blobData The content blob, in the form indicated by $blobFlags
         * @param string|null $blobFlags Flags indicating how $blobData needs to be processed.
         *        Use null if no processing should happen. That is in constrast to the empty string,
         *        which causes the blob to be decoded according to the configured legacy encoding.
         * @param string|null $blobFormat MIME type indicating how $dataBlob is encoded
         * @param int $queryFlags
         *
         * @throw RevisionAccessException
         * @return Content
         */
        private function loadSlotContent(
                SlotRecord $slot,
                $blobData = null,
                $blobFlags = null,
                $blobFormat = null,
                $queryFlags = 0
        ) {
                if ( $blobData !== null ) {
                        Assert::parameterType( 'string', $blobData, '$blobData' );
                        Assert::parameterType( 'string|null', $blobFlags, '$blobFlags' );

                        $cacheKey = $slot->hasAddress() ? $slot->getAddress() : null;

                        if ( $blobFlags === null ) {
                                // No blob flags, so use the blob verbatim.
                                $data = $blobData;
                        } else {
                                $data = $this->blobStore->expandBlob( $blobData, $blobFlags, $cacheKey );
                                if ( $data === false ) {
                                        throw new RevisionAccessException(
                                                "Failed to expand blob data using flags $blobFlags (key: $cacheKey)"
                                        );
                                }
                        }

                } else {
                        $address = $slot->getAddress();
                        try {
                                $data = $this->blobStore->getBlob( $address, $queryFlags );
                        } catch ( BlobAccessException $e ) {
                                throw new RevisionAccessException(
                                        "Failed to load data blob from $address: " . $e->getMessage(), 0, $e
                                );
                        }
                }

                // Unserialize content
                $handler = ContentHandler::getForModelID( $slot->getModel() );

                $content = $handler->unserializeContent( $data, $blobFormat );
                return $content;
        }

        /**
         * Load a page revision from a given revision ID number.
         * Returns null if no such revision can be found.
         *
         * MCR migration note: this replaces Revision::newFromId
         *
         * $flags include:
         *      IDBAccessObject::READ_LATEST: Select the data from the master
         *      IDBAccessObject::READ_LOCKING : Select & lock the data from the master
         *
         * @param int $id
         * @param int $flags (optional)
         * @return RevisionRecord|null
         */
        public function getRevisionById( $id, $flags = 0 ) {
                return $this->newRevisionFromConds( [ 'rev_id' => intval( $id ) ], $flags );
        }

        /**
         * Load either the current, or a specified, revision
         * that's attached to a given link target. If not attached
         * to that link target, will return null.
         *
         * MCR migration note: this replaces Revision::newFromTitle
         *
         * $flags include:
         *      IDBAccessObject::READ_LATEST: Select the data from the master
         *      IDBAccessObject::READ_LOCKING : Select & lock the data from the master
         *
         * @param LinkTarget $linkTarget
         * @param int $revId (optional)
         * @param int $flags Bitfield (optional)
         * @return RevisionRecord|null
         */
        public function getRevisionByTitle( LinkTarget $linkTarget, $revId = 0, $flags = 0 ) {
                $conds = [
                        'page_namespace' => $linkTarget->getNamespace(),
                        'page_title' => $linkTarget->getDBkey()
                ];
                if ( $revId ) {
                        // Use the specified revision ID.
                        // Note that we use newRevisionFromConds here because we want to retry
                        // and fall back to master if the page is not found on a replica.
                        // Since the caller supplied a revision ID, we are pretty sure the revision is
                        // supposed to exist, so we should try hard to find it.
                        $conds['rev_id'] = $revId;
                        return $this->newRevisionFromConds( $conds, $flags );
                } else {
                        // Use a join to get the latest revision.
                        // Note that we don't use newRevisionFromConds here because we don't want to retry
                        // and fall back to master. The assumption is that we only want to force the fallback
                        // if we are quite sure the revision exists because the caller supplied a revision ID.
                        // If the page isn't found at all on a replica, it probably simply does not exist.
                        $db = $this->getDBConnection( ( $flags & self::READ_LATEST ) ? DB_MASTER : DB_REPLICA );

                        $conds[] = 'rev_id=page_latest';
                        $rev = $this->loadRevisionFromConds( $db, $conds, $flags );

                        $this->releaseDBConnection( $db );
                        return $rev;
                }
        }

        /**
         * Load either the current, or a specified, revision
         * that's attached to a given page ID.
         * Returns null if no such revision can be found.
         *
         * MCR migration note: this replaces Revision::newFromPageId
         *
         * $flags include:
         *      IDBAccessObject::READ_LATEST: Select the data from the master (since 1.20)
         *      IDBAccessObject::READ_LOCKING : Select & lock the data from the master
         *
         * @param int $pageId
         * @param int $revId (optional)
         * @param int $flags Bitfield (optional)
         * @return RevisionRecord|null
         */
        public function getRevisionByPageId( $pageId, $revId = 0, $flags = 0 ) {
                $conds = [ 'page_id' => $pageId ];
                if ( $revId ) {
                        // Use the specified revision ID.
                        // Note that we use newRevisionFromConds here because we want to retry
                        // and fall back to master if the page is not found on a replica.
                        // Since the caller supplied a revision ID, we are pretty sure the revision is
                        // supposed to exist, so we should try hard to find it.
                        $conds['rev_id'] = $revId;
                        return $this->newRevisionFromConds( $conds, $flags );
                } else {
                        // Use a join to get the latest revision.
                        // Note that we don't use newRevisionFromConds here because we don't want to retry
                        // and fall back to master. The assumption is that we only want to force the fallback
                        // if we are quite sure the revision exists because the caller supplied a revision ID.
                        // If the page isn't found at all on a replica, it probably simply does not exist.
                        $db = $this->getDBConnection( ( $flags & self::READ_LATEST ) ? DB_MASTER : DB_REPLICA );

                        $conds[] = 'rev_id=page_latest';
                        $rev = $this->loadRevisionFromConds( $db, $conds, $flags );

                        $this->releaseDBConnection( $db );
                        return $rev;
                }
        }

        /**
         * Load the revision for the given title with the given timestamp.
         * WARNING: Timestamps may in some circumstances not be unique,
         * so this isn't the best key to use.
         *
         * MCR migration note: this replaces Revision::loadFromTimestamp
         *
         * @param Title $title
         * @param string $timestamp
         * @return RevisionRecord|null
         */
        public function getRevisionByTimestamp( $title, $timestamp ) {
                return $this->newRevisionFromConds(
                        [
                                'rev_timestamp' => $timestamp,
                                'page_namespace' => $title->getNamespace(),
                                'page_title' => $title->getDBkey()
                        ],
                        0,
                        $title
                );
        }

        /**
         * Make a fake revision object from an archive table row. This is queried
         * for permissions or even inserted (as in Special:Undelete)
         *
         * MCR migration note: this replaces Revision::newFromArchiveRow
         *
         * @param object $row
         * @param int $queryFlags
         * @param Title|null $title
         * @param array $overrides associative array with fields of $row to override. This may be
         *   used e.g. to force the parent revision ID or page ID. Keys in the array are fields
         *   names from the archive table without the 'ar_' prefix, i.e. use 'parent_id' to
         *   override ar_parent_id.
         *
         * @return RevisionRecord
         * @throws MWException
         */
        public function newRevisionFromArchiveRow(
                $row,
                $queryFlags = 0,
                Title $title = null,
                array $overrides = []
        ) {
                Assert::parameterType( 'object', $row, '$row' );

                // check second argument, since Revision::newFromArchiveRow had $overrides in that spot.
                Assert::parameterType( 'integer', $queryFlags, '$queryFlags' );

                if ( !$title && isset( $overrides['title'] ) ) {
                        if ( !( $overrides['title'] instanceof Title ) ) {
                                throw new MWException( 'title field override must contain a Title object.' );
                        }

                        $title = $overrides['title'];
                }

                if ( !isset( $title ) ) {
                        if ( isset( $row->ar_namespace ) && isset( $row->ar_title ) ) {
                                $title = Title::makeTitle( $row->ar_namespace, $row->ar_title );
                        } else {
                                throw new InvalidArgumentException(
                                        'A Title or ar_namespace and ar_title must be given'
                                );
                        }
                }

                foreach ( $overrides as $key => $value ) {
                        $field = "ar_$key";
                        $row->$field = $value;
                }

                try {
                        $user = User::newFromAnyId(
                                isset( $row->ar_user ) ? $row->ar_user : null,
                                isset( $row->ar_user_text ) ? $row->ar_user_text : null,
                                isset( $row->ar_actor ) ? $row->ar_actor : null
                        );
                } catch ( InvalidArgumentException $ex ) {
                        wfWarn( __METHOD__ . ': ' . $ex->getMessage() );
                        $user = new UserIdentityValue( 0, '', 0 );
                }

                $comment = $this->commentStore
                        // Legacy because $row may have come from self::selectFields()
                        ->getCommentLegacy( $this->getDBConnection( DB_REPLICA ), 'ar_comment', $row, true );

                $mainSlot = $this->emulateMainSlot_1_29( $row, $queryFlags, $title );
                $slots = new RevisionSlots( [ 'main' => $mainSlot ] );

                return new RevisionArchiveRecord( $title, $user, $comment, $row, $slots, $this->wikiId );
        }

        /**
         * @see RevisionFactory::newRevisionFromRow_1_29
         *
         * MCR migration note: this replaces Revision::newFromRow
         *
         * @param object $row
         * @param int $queryFlags
         * @param Title|null $title
         *
         * @return RevisionRecord
         * @throws MWException
         * @throws RevisionAccessException
         */
        private function newRevisionFromRow_1_29( $row, $queryFlags = 0, Title $title = null ) {
                Assert::parameterType( 'object', $row, '$row' );

                if ( !$title ) {
                        $pageId = isset( $row->rev_page ) ? $row->rev_page : 0; // XXX: also check page_id?
                        $revId = isset( $row->rev_id ) ? $row->rev_id : 0;

                        $title = $this->getTitle( $pageId, $revId, $queryFlags );
                }

                if ( !isset( $row->page_latest ) ) {
                        $row->page_latest = $title->getLatestRevID();
                        if ( $row->page_latest === 0 && $title->exists() ) {
                                wfWarn( 'Encountered title object in limbo: ID ' . $title->getArticleID() );
                        }
                }

                try {
                        $user = User::newFromAnyId(
                                isset( $row->rev_user ) ? $row->rev_user : null,
                                isset( $row->rev_user_text ) ? $row->rev_user_text : null,
                                isset( $row->rev_actor ) ? $row->rev_actor : null
                        );
                } catch ( InvalidArgumentException $ex ) {
                        wfWarn( __METHOD__ . ': ' . $ex->getMessage() );
                        $user = new UserIdentityValue( 0, '', 0 );
                }

                $comment = $this->commentStore
                        // Legacy because $row may have come from self::selectFields()
                        ->getCommentLegacy( $this->getDBConnection( DB_REPLICA ), 'rev_comment', $row, true );

                $mainSlot = $this->emulateMainSlot_1_29( $row, $queryFlags, $title );
                $slots = new RevisionSlots( [ 'main' => $mainSlot ] );

                return new RevisionStoreRecord( $title, $user, $comment, $row, $slots, $this->wikiId );
        }

        /**
         * @see RevisionFactory::newRevisionFromRow
         *
         * MCR migration note: this replaces Revision::newFromRow
         *
         * @param object $row
         * @param int $queryFlags
         * @param Title|null $title
         *
         * @return RevisionRecord
         */
        public function newRevisionFromRow( $row, $queryFlags = 0, Title $title = null ) {
                return $this->newRevisionFromRow_1_29( $row, $queryFlags, $title );
        }

        /**
         * Constructs a new MutableRevisionRecord based on the given associative array following
         * the MW1.29 convention for the Revision constructor.
         *
         * MCR migration note: this replaces Revision::newFromRow
         *
         * @param array $fields
         * @param int $queryFlags
         * @param Title|null $title
         *
         * @return MutableRevisionRecord
         * @throws MWException
         * @throws RevisionAccessException
         */
        public function newMutableRevisionFromArray(
                array $fields,
                $queryFlags = 0,
                Title $title = null
        ) {
                if ( !$title && isset( $fields['title'] ) ) {
                        if ( !( $fields['title'] instanceof Title ) ) {
                                throw new MWException( 'title field must contain a Title object.' );
                        }

                        $title = $fields['title'];
                }

                if ( !$title ) {
                        $pageId = isset( $fields['page'] ) ? $fields['page'] : 0;
                        $revId = isset( $fields['id'] ) ? $fields['id'] : 0;

                        $title = $this->getTitle( $pageId, $revId, $queryFlags );
                }

                if ( !isset( $fields['page'] ) ) {
                        $fields['page'] = $title->getArticleID( $queryFlags );
                }

                // if we have a content object, use it to set the model and type
                if ( !empty( $fields['content'] ) ) {
                        if ( !( $fields['content'] instanceof Content ) ) {
                                throw new MWException( 'content field must contain a Content object.' );
                        }

                        if ( !empty( $fields['text_id'] ) ) {
                                throw new MWException(
                                        "Text already stored in external store (id {$fields['text_id']}), " .
                                        "can't serialize content object"
                                );
                        }
                }

                if (
                        isset( $fields['comment'] )
                        && !( $fields['comment'] instanceof CommentStoreComment )
                ) {
                        $commentData = isset( $fields['comment_data'] ) ? $fields['comment_data'] : null;

                        if ( $fields['comment'] instanceof Message ) {
                                $fields['comment'] = CommentStoreComment::newUnsavedComment(
                                        $fields['comment'],
                                        $commentData
                                );
                        } else {
                                $commentText = trim( strval( $fields['comment'] ) );
                                $fields['comment'] = CommentStoreComment::newUnsavedComment(
                                        $commentText,
                                        $commentData
                                );
                        }
                }

                $mainSlot = $this->emulateMainSlot_1_29( $fields, $queryFlags, $title );

                $revision = new MutableRevisionRecord( $title, $this->wikiId );
                $this->initializeMutableRevisionFromArray( $revision, $fields );
                $revision->setSlot( $mainSlot );

                return $revision;
        }

        /**
         * @param MutableRevisionRecord $record
         * @param array $fields
         */
        private function initializeMutableRevisionFromArray(
                MutableRevisionRecord $record,
                array $fields
        ) {
                /** @var UserIdentity $user */
                $user = null;

                if ( isset( $fields['user'] ) && ( $fields['user'] instanceof UserIdentity ) ) {
                        $user = $fields['user'];
                } else {
                        try {
                                $user = User::newFromAnyId(
                                        isset( $fields['user'] ) ? $fields['user'] : null,
                                        isset( $fields['user_text'] ) ? $fields['user_text'] : null,
                                        isset( $fields['actor'] ) ? $fields['actor'] : null
                                );
                        } catch ( InvalidArgumentException $ex ) {
                                $user = null;
                        }
                }

                if ( $user ) {
                        $record->setUser( $user );
                }

                $timestamp = isset( $fields['timestamp'] )
                        ? strval( $fields['timestamp'] )
                        : wfTimestampNow(); // TODO: use a callback, so we can override it for testing.

                $record->setTimestamp( $timestamp );

                if ( isset( $fields['page'] ) ) {
                        $record->setPageId( intval( $fields['page'] ) );
                }

                if ( isset( $fields['id'] ) ) {
                        $record->setId( intval( $fields['id'] ) );
                }
                if ( isset( $fields['parent_id'] ) ) {
                        $record->setParentId( intval( $fields['parent_id'] ) );
                }

                if ( isset( $fields['sha1'] ) ) {
                        $record->setSha1( $fields['sha1'] );
                }
                if ( isset( $fields['size'] ) ) {
                        $record->setSize( intval( $fields['size'] ) );
                }

                if ( isset( $fields['minor_edit'] ) ) {
                        $record->setMinorEdit( intval( $fields['minor_edit'] ) !== 0 );
                }
                if ( isset( $fields['deleted'] ) ) {
                        $record->setVisibility( intval( $fields['deleted'] ) );
                }

                if ( isset( $fields['comment'] ) ) {
                        Assert::parameterType(
                                CommentStoreComment::class,
                                $fields['comment'],
                                '$row[\'comment\']'
                        );
                        $record->setComment( $fields['comment'] );
                }
        }

        /**
         * Load a page revision from a given revision ID number.
         * Returns null if no such revision can be found.
         *
         * MCR migration note: this corresponds to Revision::loadFromId
         *
         * @note direct use is deprecated!
         * @todo remove when unused! there seem to be no callers of Revision::loadFromId
         *
         * @param IDatabase $db
         * @param int $id
         *
         * @return RevisionRecord|null
         */
        public function loadRevisionFromId( IDatabase $db, $id ) {
                return $this->loadRevisionFromConds( $db, [ 'rev_id' => intval( $id ) ] );
        }

        /**
         * Load either the current, or a specified, revision
         * that's attached to a given page. If not attached
         * to that page, will return null.
         *
         * MCR migration note: this replaces Revision::loadFromPageId
         *
         * @note direct use is deprecated!
         * @todo remove when unused!
         *
         * @param IDatabase $db
         * @param int $pageid
         * @param int $id
         * @return RevisionRecord|null
         */
        public function loadRevisionFromPageId( IDatabase $db, $pageid, $id = 0 ) {
                $conds = [ 'rev_page' => intval( $pageid ), 'page_id' => intval( $pageid ) ];
                if ( $id ) {
                        $conds['rev_id'] = intval( $id );
                } else {
                        $conds[] = 'rev_id=page_latest';
                }
                return $this->loadRevisionFromConds( $db, $conds );
        }

        /**
         * Load either the current, or a specified, revision
         * that's attached to a given page. If not attached
         * to that page, will return null.
         *
         * MCR migration note: this replaces Revision::loadFromTitle
         *
         * @note direct use is deprecated!
         * @todo remove when unused!
         *
         * @param IDatabase $db
         * @param Title $title
         * @param int $id
         *
         * @return RevisionRecord|null
         */
        public function loadRevisionFromTitle( IDatabase $db, $title, $id = 0 ) {
                if ( $id ) {
                        $matchId = intval( $id );
                } else {
                        $matchId = 'page_latest';
                }

                return $this->loadRevisionFromConds(
                        $db,
                        [
                                "rev_id=$matchId",
                                'page_namespace' => $title->getNamespace(),
                                'page_title' => $title->getDBkey()
                        ],
                        0,
                        $title
                );
        }

        /**
         * Load the revision for the given title with the given timestamp.
         * WARNING: Timestamps may in some circumstances not be unique,
         * so this isn't the best key to use.
         *
         * MCR migration note: this replaces Revision::loadFromTimestamp
         *
         * @note direct use is deprecated! Use getRevisionFromTimestamp instead!
         * @todo remove when unused!
         *
         * @param IDatabase $db
         * @param Title $title
         * @param string $timestamp
         * @return RevisionRecord|null
         */
        public function loadRevisionFromTimestamp( IDatabase $db, $title, $timestamp ) {
                return $this->loadRevisionFromConds( $db,
                        [
                                'rev_timestamp' => $db->timestamp( $timestamp ),
                                'page_namespace' => $title->getNamespace(),
                                'page_title' => $title->getDBkey()
                        ],
                        0,
                        $title
                );
        }

        /**
         * Given a set of conditions, fetch a revision
         *
         * This method should be used if we are pretty sure the revision exists.
         * Unless $flags has READ_LATEST set, this method will first try to find the revision
         * on a replica before hitting the master database.
         *
         * MCR migration note: this corresponds to Revision::newFromConds
         *
         * @param array $conditions
         * @param int $flags (optional)
         * @param Title $title
         *
         * @return RevisionRecord|null
         */
        private function newRevisionFromConds( $conditions, $flags = 0, Title $title = null ) {
                $db = $this->getDBConnection( ( $flags & self::READ_LATEST ) ? DB_MASTER : DB_REPLICA );
                $rev = $this->loadRevisionFromConds( $db, $conditions, $flags, $title );
                $this->releaseDBConnection( $db );

                $lb = $this->getDBLoadBalancer();

                // Make sure new pending/committed revision are visibile later on
                // within web requests to certain avoid bugs like T93866 and T94407.
                if ( !$rev
                        && !( $flags & self::READ_LATEST )
                        && $lb->getServerCount() > 1
                        && $lb->hasOrMadeRecentMasterChanges()
                ) {
                        $flags = self::READ_LATEST;
                        $db = $this->getDBConnection( DB_MASTER );
                        $rev = $this->loadRevisionFromConds( $db, $conditions, $flags, $title );
                        $this->releaseDBConnection( $db );
                }

                return $rev;
        }

        /**
         * Given a set of conditions, fetch a revision from
         * the given database connection.
         *
         * MCR migration note: this corresponds to Revision::loadFromConds
         *
         * @param IDatabase $db
         * @param array $conditions
         * @param int $flags (optional)
         * @param Title $title
         *
         * @return RevisionRecord|null
         */
        private function loadRevisionFromConds(
                IDatabase $db,
                $conditions,
                $flags = 0,
                Title $title = null
        ) {
                $row = $this->fetchRevisionRowFromConds( $db, $conditions, $flags );
                if ( $row ) {
                        $rev = $this->newRevisionFromRow( $row, $flags, $title );

                        return $rev;
                }

                return null;
        }

        /**
         * Throws an exception if the given database connection does not belong to the wiki this
         * RevisionStore is bound to.
         *
         * @param IDatabase $db
         * @throws MWException
         */
        private function checkDatabaseWikiId( IDatabase $db ) {
                $storeWiki = $this->wikiId;
                $dbWiki = $db->getDomainID();

                if ( $dbWiki === $storeWiki ) {
                        return;
                }

                // XXX: we really want the default database ID...
                $storeWiki = $storeWiki ?: wfWikiID();
                $dbWiki = $dbWiki ?: wfWikiID();

                if ( $dbWiki === $storeWiki ) {
                        return;
                }

                // HACK: counteract encoding imposed by DatabaseDomain
                $storeWiki = str_replace( '?h', '-', $storeWiki );
                $dbWiki = str_replace( '?h', '-', $dbWiki );

                if ( $dbWiki === $storeWiki ) {
                        return;
                }

                throw new MWException( "RevisionStore for $storeWiki "
                        . "cannot be used with a DB connection for $dbWiki" );
        }

        /**
         * Given a set of conditions, return a row with the
         * fields necessary to build RevisionRecord objects.
         *
         * MCR migration note: this corresponds to Revision::fetchFromConds
         *
         * @param IDatabase $db
         * @param array $conditions
         * @param int $flags (optional)
         *
         * @return object|false data row as a raw object
         */
        private function fetchRevisionRowFromConds( IDatabase $db, $conditions, $flags = 0 ) {
                $this->checkDatabaseWikiId( $db );

                $revQuery = self::getQueryInfo( [ 'page', 'user' ] );
                $options = [];
                if ( ( $flags & self::READ_LOCKING ) == self::READ_LOCKING ) {
                        $options[] = 'FOR UPDATE';
                }
                return $db->selectRow(
                        $revQuery['tables'],
                        $revQuery['fields'],
                        $conditions,
                        __METHOD__,
                        $options,
                        $revQuery['joins']
                );
        }

        /**
         * Return the tables, fields, and join conditions to be selected to create
         * a new revision object.
         *
         * MCR migration note: this replaces Revision::getQueryInfo
         *
         * @since 1.31
         *
         * @param array $options Any combination of the following strings
         *  - 'page': Join with the page table, and select fields to identify the page
         *  - 'user': Join with the user table, and select the user name
         *  - 'text': Join with the text table, and select fields to load page text
         *
         * @return array With three keys:
         *  - tables: (string[]) to include in the `$table` to `IDatabase->select()`
         *  - fields: (string[]) to include in the `$vars` to `IDatabase->select()`
         *  - joins: (array) to include in the `$join_conds` to `IDatabase->select()`
         */
        public function getQueryInfo( $options = [] ) {
                $ret = [
                        'tables' => [],
                        'fields' => [],
                        'joins'  => [],
                ];

                $ret['tables'][] = 'revision';
                $ret['fields'] = array_merge( $ret['fields'], [
                        'rev_id',
                        'rev_page',
                        'rev_text_id',
                        'rev_timestamp',
                        'rev_minor_edit',
                        'rev_deleted',
                        'rev_len',
                        'rev_parent_id',
                        'rev_sha1',
                ] );

                $commentQuery = $this->commentStore->getJoin( 'rev_comment' );
                $ret['tables'] = array_merge( $ret['tables'], $commentQuery['tables'] );
                $ret['fields'] = array_merge( $ret['fields'], $commentQuery['fields'] );
                $ret['joins'] = array_merge( $ret['joins'], $commentQuery['joins'] );

                $actorQuery = $this->actorMigration->getJoin( 'rev_user' );
                $ret['tables'] = array_merge( $ret['tables'], $actorQuery['tables'] );
                $ret['fields'] = array_merge( $ret['fields'], $actorQuery['fields'] );
                $ret['joins'] = array_merge( $ret['joins'], $actorQuery['joins'] );

                if ( $this->contentHandlerUseDB ) {
                        $ret['fields'][] = 'rev_content_format';
                        $ret['fields'][] = 'rev_content_model';
                }

                if ( in_array( 'page', $options, true ) ) {
                        $ret['tables'][] = 'page';
                        $ret['fields'] = array_merge( $ret['fields'], [
                                'page_namespace',
                                'page_title',
                                'page_id',
                                'page_latest',
                                'page_is_redirect',
                                'page_len',
                        ] );
                        $ret['joins']['page'] = [ 'INNER JOIN', [ 'page_id = rev_page' ] ];
                }

                if ( in_array( 'user', $options, true ) ) {
                        $ret['tables'][] = 'user';
                        $ret['fields'] = array_merge( $ret['fields'], [
                                'user_name',
                        ] );
                        $u = $actorQuery['fields']['rev_user'];
                        $ret['joins']['user'] = [ 'LEFT JOIN', [ "$u != 0", "user_id = $u" ] ];
                }

                if ( in_array( 'text', $options, true ) ) {
                        $ret['tables'][] = 'text';
                        $ret['fields'] = array_merge( $ret['fields'], [
                                'old_text',
                                'old_flags'
                        ] );
                        $ret['joins']['text'] = [ 'INNER JOIN', [ 'rev_text_id=old_id' ] ];
                }

                return $ret;
        }

        /**
         * Return the tables, fields, and join conditions to be selected to create
         * a new archived revision object.
         *
         * MCR migration note: this replaces Revision::getArchiveQueryInfo
         *
         * @since 1.31
         *
         * @return array With three keys:
         *   - tables: (string[]) to include in the `$table` to `IDatabase->select()`
         *   - fields: (string[]) to include in the `$vars` to `IDatabase->select()`
         *   - joins: (array) to include in the `$join_conds` to `IDatabase->select()`
         */
        public function getArchiveQueryInfo() {
                $commentQuery = $this->commentStore->getJoin( 'ar_comment' );
                $actorQuery = $this->actorMigration->getJoin( 'ar_user' );
                $ret = [
                        'tables' => [ 'archive' ] + $commentQuery['tables'] + $actorQuery['tables'],
                        'fields' => [
                                        'ar_id',
                                        'ar_page_id',
                                        'ar_namespace',
                                        'ar_title',
                                        'ar_rev_id',
                                        'ar_text',
                                        'ar_text_id',
                                        'ar_timestamp',
                                        'ar_minor_edit',
                                        'ar_deleted',
                                        'ar_len',
                                        'ar_parent_id',
                                        'ar_sha1',
                                ] + $commentQuery['fields'] + $actorQuery['fields'],
                        'joins' => $commentQuery['joins'] + $actorQuery['joins'],
                ];

                if ( $this->contentHandlerUseDB ) {
                        $ret['fields'][] = 'ar_content_format';
                        $ret['fields'][] = 'ar_content_model';
                }

                return $ret;
        }

        /**
         * Do a batched query for the sizes of a set of revisions.
         *
         * MCR migration note: this replaces Revision::getParentLengths
         *
         * @param int[] $revIds
         * @return int[] associative array mapping revision IDs from $revIds to the nominal size
         *         of the corresponding revision.
         */
        public function getRevisionSizes( array $revIds ) {
                return $this->listRevisionSizes( $this->getDBConnection( DB_REPLICA ), $revIds );
        }

        /**
         * Do a batched query for the sizes of a set of revisions.
         *
         * MCR migration note: this replaces Revision::getParentLengths
         *
         * @deprecated use RevisionStore::getRevisionSizes instead.
         *
         * @param IDatabase $db
         * @param int[] $revIds
         * @return int[] associative array mapping revision IDs from $revIds to the nominal size
         *         of the corresponding revision.
         */
        public function listRevisionSizes( IDatabase $db, array $revIds ) {
                $this->checkDatabaseWikiId( $db );

                $revLens = [];
                if ( !$revIds ) {
                        return $revLens; // empty
                }

                $res = $db->select(
                        'revision',
                        [ 'rev_id', 'rev_len' ],
                        [ 'rev_id' => $revIds ],
                        __METHOD__
                );

                foreach ( $res as $row ) {
                        $revLens[$row->rev_id] = intval( $row->rev_len );
                }

                return $revLens;
        }

        /**
         * Get previous revision for this title
         *
         * MCR migration note: this replaces Revision::getPrevious
         *
         * @param RevisionRecord $rev
         * @param Title $title if known (optional)
         *
         * @return RevisionRecord|null
         */
        public function getPreviousRevision( RevisionRecord $rev, Title $title = null ) {
                if ( $title === null ) {
                        $title = $this->getTitle( $rev->getPageId(), $rev->getId() );
                }
                $prev = $title->getPreviousRevisionID( $rev->getId() );
                if ( $prev ) {
                        return $this->getRevisionByTitle( $title, $prev );
                }
                return null;
        }

        /**
         * Get next revision for this title
         *
         * MCR migration note: this replaces Revision::getNext
         *
         * @param RevisionRecord $rev
         * @param Title $title if known (optional)
         *
         * @return RevisionRecord|null
         */
        public function getNextRevision( RevisionRecord $rev, Title $title = null ) {
                if ( $title === null ) {
                        $title = $this->getTitle( $rev->getPageId(), $rev->getId() );
                }
                $next = $title->getNextRevisionID( $rev->getId() );
                if ( $next ) {
                        return $this->getRevisionByTitle( $title, $next );
                }
                return null;
        }

        /**
         * Get previous revision Id for this page_id
         * This is used to populate rev_parent_id on save
         *
         * MCR migration note: this corresponds to Revision::getPreviousRevisionId
         *
         * @param IDatabase $db
         * @param RevisionRecord $rev
         *
         * @return int
         */
        private function getPreviousRevisionId( IDatabase $db, RevisionRecord $rev ) {
                $this->checkDatabaseWikiId( $db );

                if ( $rev->getPageId() === null ) {
                        return 0;
                }
                # Use page_latest if ID is not given
                if ( !$rev->getId() ) {
                        $prevId = $db->selectField(
                                'page', 'page_latest',
                                [ 'page_id' => $rev->getPageId() ],
                                __METHOD__
                        );
                } else {
                        $prevId = $db->selectField(
                                'revision', 'rev_id',
                                [ 'rev_page' => $rev->getPageId(), 'rev_id < ' . $rev->getId() ],
                                __METHOD__,
                                [ 'ORDER BY' => 'rev_id DESC' ]
                        );
                }
                return intval( $prevId );
        }

        /**
         * Get rev_timestamp from rev_id, without loading the rest of the row
         *
         * MCR migration note: this replaces Revision::getTimestampFromId
         *
         * @param Title $title
         * @param int $id
         * @param int $flags
         * @return string|bool False if not found
         */
        public function getTimestampFromId( $title, $id, $flags = 0 ) {
                $db = $this->getDBConnection(
                        ( $flags & IDBAccessObject::READ_LATEST ) ? DB_MASTER : DB_REPLICA
                );

                $conds = [ 'rev_id' => $id ];
                $conds['rev_page'] = $title->getArticleID();
                $timestamp = $db->selectField( 'revision', 'rev_timestamp', $conds, __METHOD__ );

                $this->releaseDBConnection( $db );
                return ( $timestamp !== false ) ? wfTimestamp( TS_MW, $timestamp ) : false;
        }

        /**
         * Get count of revisions per page...not very efficient
         *
         * MCR migration note: this replaces Revision::countByPageId
         *
         * @param IDatabase $db
         * @param int $id Page id
         * @return int
         */
        public function countRevisionsByPageId( IDatabase $db, $id ) {
                $this->checkDatabaseWikiId( $db );

                $row = $db->selectRow( 'revision',
                        [ 'revCount' => 'COUNT(*)' ],
                        [ 'rev_page' => $id ],
                        __METHOD__
                );
                if ( $row ) {
                        return intval( $row->revCount );
                }
                return 0;
        }

        /**
         * Get count of revisions per page...not very efficient
         *
         * MCR migration note: this replaces Revision::countByTitle
         *
         * @param IDatabase $db
         * @param Title $title
         * @return int
         */
        public function countRevisionsByTitle( IDatabase $db, $title ) {
                $id = $title->getArticleID();
                if ( $id ) {
                        return $this->countRevisionsByPageId( $db, $id );
                }
                return 0;
        }

        /**
         * Check if no edits were made by other users since
         * the time a user started editing the page. Limit to
         * 50 revisions for the sake of performance.
         *
         * MCR migration note: this replaces Revision::userWasLastToEdit
         *
         * @deprecated since 1.31; Can possibly be removed, since the self-conflict suppression
         *       logic in EditPage that uses this seems conceptually dubious. Revision::userWasLastToEdit
         *       has been deprecated since 1.24.
         *
         * @param IDatabase $db The Database to perform the check on.
         * @param int $pageId The ID of the page in question
         * @param int $userId The ID of the user in question
         * @param string $since Look at edits since this time
         *
         * @return bool True if the given user was the only one to edit since the given timestamp
         */
        public function userWasLastToEdit( IDatabase $db, $pageId, $userId, $since ) {
                $this->checkDatabaseWikiId( $db );

                if ( !$userId ) {
                        return false;
                }

                $revQuery = self::getQueryInfo();
                $res = $db->select(
                        $revQuery['tables'],
                        [
                                'rev_user' => $revQuery['fields']['rev_user'],
                        ],
                        [
                                'rev_page' => $pageId,
                                'rev_timestamp > ' . $db->addQuotes( $db->timestamp( $since ) )
                        ],
                        __METHOD__,
                        [ 'ORDER BY' => 'rev_timestamp ASC', 'LIMIT' => 50 ],
                        $revQuery['joins']
                );
                foreach ( $res as $row ) {
                        if ( $row->rev_user != $userId ) {
                                return false;
                        }
                }
                return true;
        }

        /**
         * Load a revision based on a known page ID and current revision ID from the DB
         *
         * This method allows for the use of caching, though accessing anything that normally
         * requires permission checks (aside from the text) will trigger a small DB lookup.
         *
         * MCR migration note: this replaces Revision::newKnownCurrent
         *
         * @param Title $title the associated page title
         * @param int $revId current revision of this page. Defaults to $title->getLatestRevID().
         *
         * @return RevisionRecord|bool Returns false if missing
         */
        public function getKnownCurrentRevision( Title $title, $revId ) {
                $db = $this->getDBConnectionRef( DB_REPLICA );

                $pageId = $title->getArticleID();

                if ( !$pageId ) {
                        return false;
                }

                if ( !$revId ) {
                        $revId = $title->getLatestRevID();
                }

                if ( !$revId ) {
                        wfWarn(
                                'No latest revision known for page ' . $title->getPrefixedDBkey()
                                . ' even though it exists with page ID ' . $pageId
                        );
                        return false;
                }

                $row = $this->cache->getWithSetCallback(
                        // Page/rev IDs passed in from DB to reflect history merges
                        $this->cache->makeGlobalKey( 'revision-row-1.29', $db->getDomainID(), $pageId, $revId ),
                        WANObjectCache::TTL_WEEK,
                        function ( $curValue, &$ttl, array &$setOpts ) use ( $db, $pageId, $revId ) {
                                $setOpts += Database::getCacheSetOptions( $db );

                                $conds = [
                                        'rev_page' => intval( $pageId ),
                                        'page_id' => intval( $pageId ),
                                        'rev_id' => intval( $revId ),
                                ];

                                $row = $this->fetchRevisionRowFromConds( $db, $conds );
                                return $row ?: false; // don't cache negatives
                        }
                );

                // Reflect revision deletion and user renames
                if ( $row ) {
                        return $this->newRevisionFromRow( $row, 0, $title );
                } else {
                        return false;
                }
        }

        // TODO: move relevant methods from Title here, e.g. getFirstRevision, isBigDeletion, etc.

}