wikimedia/mediawiki-core

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includes/language/MessageCache.php

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<?php
/**
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 */

use MediaWiki\Config\ServiceOptions;
use MediaWiki\Context\RequestContext;
use MediaWiki\Deferred\DeferredUpdates;
use MediaWiki\Deferred\MessageCacheUpdate;
use MediaWiki\HookContainer\HookContainer;
use MediaWiki\HookContainer\HookRunner;
use MediaWiki\Languages\LanguageConverterFactory;
use MediaWiki\Languages\LanguageFactory;
use MediaWiki\Languages\LanguageFallback;
use MediaWiki\Languages\LanguageNameUtils;
use MediaWiki\Linker\LinkTarget;
use MediaWiki\Logger\LoggerFactory;
use MediaWiki\MainConfigNames;
use MediaWiki\MediaWikiServices;
use MediaWiki\Page\PageReference;
use MediaWiki\Page\PageReferenceValue;
use MediaWiki\Parser\Parser;
use MediaWiki\Parser\ParserOutput;
use MediaWiki\Revision\SlotRecord;
use MediaWiki\StubObject\StubObject;
use MediaWiki\StubObject\StubUserLang;
use MediaWiki\Title\Title;
use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Wikimedia\LightweightObjectStore\ExpirationAwareness;
use Wikimedia\Rdbms\Database;
use Wikimedia\Rdbms\IExpression;
use Wikimedia\Rdbms\IResultWrapper;
use Wikimedia\Rdbms\LikeValue;
use Wikimedia\RequestTimeout\TimeoutException;
use Wikimedia\ScopedCallback;

/**
 * MediaWiki message cache structure version.
 * Bump this whenever the message cache format has changed.
 */
define( 'MSG_CACHE_VERSION', 2 );

/**
 * Cache messages that are defined by MediaWiki-namespace pages or by hooks.
 *
 * @ingroup Language
 */
class MessageCache implements LoggerAwareInterface {
    /**
     * Options to be included in the ServiceOptions
     */
    public const CONSTRUCTOR_OPTIONS = [
        MainConfigNames::UseDatabaseMessages,
        MainConfigNames::MaxMsgCacheEntrySize,
        MainConfigNames::AdaptiveMessageCache,
        MainConfigNames::UseXssLanguage,
        MainConfigNames::RawHtmlMessages,
    ];

    /**
     * The size of the MapCacheLRU which stores message data. The maximum
     * number of languages which can be efficiently loaded in a given request.
     */
    public const MAX_REQUEST_LANGUAGES = 10;

    private const FOR_UPDATE = 1; // force message reload

    /** How long to wait for memcached locks */
    private const WAIT_SEC = 15;
    /** How long memcached locks last */
    private const LOCK_TTL = 30;

    /**
     * Lifetime for cache, for keys stored in $wanCache, in seconds.
     * @var int
     */
    private const WAN_TTL = ExpirationAwareness::TTL_DAY;

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

    /**
     * Process cache of loaded messages that are defined in MediaWiki namespace
     *
     * @var MapCacheLRU Map of (language code => key => " <MESSAGE>" or "!TOO BIG" or "!ERROR")
     */
    private $cache;

    /**
     * Map of (lowercase message key => unused) for all software-defined messages
     *
     * @var array
     */
    private $systemMessageNames;

    /**
     * @var bool[] Map of (language code => boolean)
     */
    private $cacheVolatile = [];

    /**
     * Should mean that database cannot be used, but check
     * @var bool
     */
    private $disable;

    /** @var int Maximum entry size in bytes */
    private $maxEntrySize;

    /** @var bool */
    private $adaptive;

    /** @var bool */
    private $useXssLanguage;

    /** @var string[] */
    private $rawHtmlMessages;

    /**
     * Message cache has its own parser which it uses to transform messages
     * @var ParserOptions
     */
    private $parserOptions;

    /** @var ?Parser Lazy-created via self::getParser() */
    private $parser = null;

    /**
     * @var bool
     */
    private $inParser = false;

    /** @var WANObjectCache */
    private $wanCache;
    /** @var BagOStuff */
    private $clusterCache;
    /** @var BagOStuff */
    private $srvCache;
    /** @var Language */
    private $contLang;
    /** @var string */
    private $contLangCode;
    /** @var ILanguageConverter */
    private $contLangConverter;
    /** @var LanguageFactory */
    private $langFactory;
    /** @var LocalisationCache */
    private $localisationCache;
    /** @var LanguageNameUtils */
    private $languageNameUtils;
    /** @var LanguageFallback */
    private $languageFallback;
    /** @var HookRunner */
    private $hookRunner;
    /** @var ParserFactory */
    private $parserFactory;

    /** @var (string|callable)[]|null */
    private $messageKeyOverrides;

    /**
     * Normalize message key input
     *
     * @param string $key Input message key to be normalized
     * @return string Normalized message key
     */
    public static function normalizeKey( $key ) {
        $lckey = strtr( $key, ' ', '_' );
        if ( $lckey === '' ) {
            // T300792
            return $lckey;
        }

        if ( ord( $lckey ) < 128 ) {
            $lckey[0] = strtolower( $lckey[0] );
        } else {
            $lckey = MediaWikiServices::getInstance()->getContentLanguage()->lcfirst( $lckey );
        }

        return $lckey;
    }

    /**
     * @internal For use by ServiceWiring
     * @param WANObjectCache $wanCache
     * @param BagOStuff $clusterCache
     * @param BagOStuff $serverCache
     * @param Language $contLang Content language of site
     * @param LanguageConverterFactory $langConverterFactory
     * @param LoggerInterface $logger
     * @param ServiceOptions $options
     * @param LanguageFactory $langFactory
     * @param LocalisationCache $localisationCache
     * @param LanguageNameUtils $languageNameUtils
     * @param LanguageFallback $languageFallback
     * @param HookContainer $hookContainer
     * @param ParserFactory $parserFactory
     */
    public function __construct(
        WANObjectCache $wanCache,
        BagOStuff $clusterCache,
        BagOStuff $serverCache,
        Language $contLang,
        LanguageConverterFactory $langConverterFactory,
        LoggerInterface $logger,
        ServiceOptions $options,
        LanguageFactory $langFactory,
        LocalisationCache $localisationCache,
        LanguageNameUtils $languageNameUtils,
        LanguageFallback $languageFallback,
        HookContainer $hookContainer,
        ParserFactory $parserFactory
    ) {
        $this->wanCache = $wanCache;
        $this->clusterCache = $clusterCache;
        $this->srvCache = $serverCache;
        $this->contLang = $contLang;
        $this->contLangConverter = $langConverterFactory->getLanguageConverter( $contLang );
        $this->contLangCode = $contLang->getCode();
        $this->logger = $logger;
        $this->langFactory = $langFactory;
        $this->localisationCache = $localisationCache;
        $this->languageNameUtils = $languageNameUtils;
        $this->languageFallback = $languageFallback;
        $this->hookRunner = new HookRunner( $hookContainer );
        $this->parserFactory = $parserFactory;

        // limit size
        $this->cache = new MapCacheLRU( self::MAX_REQUEST_LANGUAGES );

        $options->assertRequiredOptions( self::CONSTRUCTOR_OPTIONS );
        $this->disable = !$options->get( MainConfigNames::UseDatabaseMessages );
        $this->maxEntrySize = $options->get( MainConfigNames::MaxMsgCacheEntrySize );
        $this->adaptive = $options->get( MainConfigNames::AdaptiveMessageCache );
        $this->useXssLanguage = $options->get( MainConfigNames::UseXssLanguage );
        $this->rawHtmlMessages = $options->get( MainConfigNames::RawHtmlMessages );
    }

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

    /**
     * ParserOptions is lazily initialised.
     *
     * @return ParserOptions
     */
    private function getParserOptions() {
        if ( !$this->parserOptions ) {
            $context = RequestContext::getMain();
            $user = $context->getUser();
            if ( !$user->isSafeToLoad() ) {
                // It isn't safe to use the context user yet, so don't try to get a
                // ParserOptions for it. And don't cache this ParserOptions
                // either.
                $po = ParserOptions::newFromAnon();
                $po->setAllowUnsafeRawHtml( false );
                return $po;
            }

            $this->parserOptions = ParserOptions::newFromContext( $context );
            // Messages may take parameters that could come
            // from malicious sources. As a precaution, disable
            // the <html> parser tag when parsing messages.
            $this->parserOptions->setAllowUnsafeRawHtml( false );
        }

        return $this->parserOptions;
    }

    /**
     * Try to load the cache from APC.
     *
     * @param string $code Optional language code, see documentation of load().
     * @return array|false The cache array, or false if not in cache.
     */
    private function getLocalCache( $code ) {
        $cacheKey = $this->srvCache->makeKey( __CLASS__, $code );

        return $this->srvCache->get( $cacheKey );
    }

    /**
     * Save the cache to APC.
     *
     * @param string $code
     * @param array $cache The cache array
     */
    private function saveToLocalCache( $code, $cache ) {
        $cacheKey = $this->srvCache->makeKey( __CLASS__, $code );
        $this->srvCache->set( $cacheKey, $cache );
    }

    /**
     * Loads messages from caches or from database in this order:
     * (1) local message cache (if $wgUseLocalMessageCache is enabled)
     * (2) memcached
     * (3) from the database.
     *
     * When successfully loading from (2) or (3), all higher level caches are
     * updated for the newest version.
     *
     * Nothing is loaded if member variable mDisable is true, either manually
     * set by calling code or if message loading fails (is this possible?).
     *
     * Returns true if cache is already populated, or it was successfully populated,
     * or false if populating empty cache fails. Also returns true if MessageCache
     * is disabled.
     *
     * @param string $code Which language to load messages for
     * @param int|null $mode Use MessageCache::FOR_UPDATE to skip process cache [optional]
     * @return bool
     */
    private function load( string $code, $mode = null ) {
        // Don't do double loading...
        if ( $this->isLanguageLoaded( $code ) && $mode !== self::FOR_UPDATE ) {
            return true;
        }

        // Show a log message (once) if loading is disabled
        if ( $this->disable ) {
            static $shownDisabled = false;
            if ( !$shownDisabled ) {
                $this->logger->debug( __METHOD__ . ': disabled' );
                $shownDisabled = true;
            }

            return true;
        }

        try {
            return $this->loadUnguarded( $code, $mode );
        } catch ( Throwable $e ) {
            // Don't try to load again during the exception handler
            $this->disable = true;
            throw $e;
        }
    }

    /**
     * Load messages from the cache or database, without exception guarding.
     *
     * @param string $code Which language to load messages for
     * @param int|null $mode Use MessageCache::FOR_UPDATE to skip process cache [optional]
     * @return bool
     */
    private function loadUnguarded( $code, $mode ) {
        $success = false; // Keep track of success
        $staleCache = false; // a cache array with expired data, or false if none has been loaded
        $where = []; // Debug info, delayed to avoid spamming debug log too much

        // A hash of the expected content is stored in a WAN cache key, providing a way
        // to invalid the local cache on every server whenever a message page changes.
        [ $hash, $hashVolatile ] = $this->getValidationHash( $code );
        $this->cacheVolatile[$code] = $hashVolatile;
        $volatilityOnlyStaleness = false;

        // Try the local cache and check against the cluster hash key...
        $cache = $this->getLocalCache( $code );
        if ( !$cache ) {
            $where[] = 'local cache is empty';
        } elseif ( !isset( $cache['HASH'] ) || $cache['HASH'] !== $hash ) {
            $where[] = 'local cache has the wrong hash';
            $staleCache = $cache;
        } elseif ( $this->isCacheExpired( $cache ) ) {
            $where[] = 'local cache is expired';
            $staleCache = $cache;
        } elseif ( $hashVolatile ) {
            // Some recent message page changes might not show due to DB lag
            $where[] = 'local cache validation key is expired/volatile';
            $staleCache = $cache;
            $volatilityOnlyStaleness = true;
        } else {
            $where[] = 'got from local cache';
            $this->cache->set( $code, $cache );
            $success = true;
        }

        if ( !$success ) {
            // Try the cluster cache, using a lock for regeneration...
            $cacheKey = $this->clusterCache->makeKey( 'messages', $code );
            for ( $failedAttempts = 0; $failedAttempts <= 1; $failedAttempts++ ) {
                if ( $volatilityOnlyStaleness && $staleCache ) {
                    // While the cluster cache *might* be more up-to-date, we do not want
                    // the I/O strain of every application server fetching the key here during
                    // the volatility period. Either this thread wins the lock and regenerates
                    // the cache or the stale local cache value gets reused.
                    $where[] = 'global cache is presumed expired';
                } else {
                    $cache = $this->clusterCache->get( $cacheKey );
                    if ( !$cache ) {
                        $where[] = 'global cache is empty';
                    } elseif ( $this->isCacheExpired( $cache ) ) {
                        $where[] = 'global cache is expired';
                        $staleCache = $cache;
                    } elseif ( $hashVolatile ) {
                        // Some recent message page changes might not show due to DB lag
                        $where[] = 'global cache is expired/volatile';
                        $staleCache = $cache;
                    } else {
                        $where[] = 'got from global cache';
                        $this->cache->set( $code, $cache );
                        $this->saveToCaches( $cache, 'local-only', $code );
                        $success = true;
                        break;
                    }
                }

                // We need to call loadFromDB(). Limit the concurrency to one thread.
                // This prevents the site from going down when the cache expires.
                // Note that the DB slam protection lock here is non-blocking.
                $loadStatus = $this->loadFromDBWithMainLock( $code, $where, $mode );
                if ( $loadStatus === true ) {
                    $success = true;
                    break;
                } elseif ( $staleCache ) {
                    // Use the stale cache while some other thread constructs the new one
                    $where[] = 'using stale cache';
                    $this->cache->set( $code, $staleCache );
                    $success = true;
                    break;
                } elseif ( $failedAttempts > 0 ) {
                    $where[] = 'failed to find cache after waiting';
                    // Already blocked once, so avoid another lock/unlock cycle.
                    // This case will typically be hit if memcached is down, or if
                    // loadFromDB() takes longer than LOCK_WAIT.
                    break;
                } elseif ( $loadStatus === 'cantacquire' ) {
                    // Wait for the other thread to finish, then retry. Normally,
                    // the memcached get() will then yield the other thread's result.
                    $where[] = 'waiting for other thread to complete';
                    [ , $ioError ] = $this->getReentrantScopedLock( $code );
                    if ( $ioError ) {
                        $where[] = 'failed waiting';
                        // Call loadFromDB() with concurrency limited to one thread per server.
                        // It should be rare for all servers to lack even a stale local cache.
                        $success = $this->loadFromDBWithLocalLock( $code, $where, $mode );
                        break;
                    }
                } else {
                    // Disable cache; $loadStatus is 'disabled'
                    break;
                }
            }
        }

        if ( !$success ) {
            $where[] = 'loading FAILED - cache is disabled';
            $this->disable = true;
            $this->cache->set( $code, [] );
            $this->logger->error( __METHOD__ . ": Failed to load $code" );
            // This used to throw an exception, but that led to nasty side effects like
            // the whole wiki being instantly down if the memcached server died
        }

        if ( !$this->isLanguageLoaded( $code ) ) {
            throw new LogicException( "Process cache for '$code' should be set by now." );
        }

        $info = implode( ', ', $where );
        $this->logger->debug( __METHOD__ . ": Loading $code... $info" );

        return $success;
    }

    /**
     * @param string $code
     * @param string[] &$where List of debug comments
     * @param int|null $mode Use MessageCache::FOR_UPDATE to use DB_PRIMARY
     * @return true|string One (true, "cantacquire", "disabled")
     */
    private function loadFromDBWithMainLock( $code, array &$where, $mode = null ) {
        // If cache updates on all levels fail, give up on message overrides.
        // This is to avoid easy site outages; see $saveSuccess comments below.
        $statusKey = $this->clusterCache->makeKey( 'messages', $code, 'status' );
        $status = $this->clusterCache->get( $statusKey );
        if ( $status === 'error' ) {
            $where[] = "could not load; method is still globally disabled";
            return 'disabled';
        }

        // Now let's regenerate
        $where[] = 'loading from DB';

        // Lock the cache to prevent conflicting writes.
        // This lock is non-blocking so stale cache can quickly be used.
        // Note that load() will call a blocking getReentrantScopedLock()
        // after this if it really needs to wait for any current thread.
        [ $scopedLock ] = $this->getReentrantScopedLock( $code, 0 );
        if ( !$scopedLock ) {
            $where[] = 'could not acquire main lock';
            return 'cantacquire';
        }

        $cache = $this->loadFromDB( $code, $mode );
        $this->cache->set( $code, $cache );
        $saveSuccess = $this->saveToCaches( $cache, 'all', $code );

        if ( !$saveSuccess ) {
            /**
             * Cache save has failed.
             *
             * There are two main scenarios where this could be a problem:
             * - The cache is more than the maximum size (typically 1MB compressed).
             * - Memcached has no space remaining in the relevant slab class. This is
             *   unlikely with recent versions of memcached.
             *
             * Either way, if there is a local cache, nothing bad will happen. If there
             * is no local cache, disabling the message cache for all requests avoids
             * incurring a loadFromDB() overhead on every request, and thus saves the
             * wiki from complete downtime under moderate traffic conditions.
             */
            if ( $this->srvCache instanceof EmptyBagOStuff ) {
                $this->clusterCache->set( $statusKey, 'error', 60 * 5 );
                $where[] = 'could not save cache, disabled globally for 5 minutes';
            } else {
                $where[] = "could not save global cache";
            }
        }

        return true;
    }

    /**
     * @param string $code
     * @param string[] &$where List of debug comments
     * @param int|null $mode Use MessageCache::FOR_UPDATE to use DB_PRIMARY
     * @return bool Success
     */
    private function loadFromDBWithLocalLock( $code, array &$where, $mode = null ) {
        $success = false;
        $where[] = 'loading from DB using local lock';

        $scopedLock = $this->srvCache->getScopedLock(
            $this->srvCache->makeKey( 'messages', $code ),
            self::WAIT_SEC,
            self::LOCK_TTL,
            __METHOD__
        );
        if ( $scopedLock ) {
            $cache = $this->loadFromDB( $code, $mode );
            $this->cache->set( $code, $cache );
            $this->saveToCaches( $cache, 'local-only', $code );
            $success = true;
        }

        return $success;
    }

    /**
     * Loads cacheable messages from the database. Messages bigger than
     * $wgMaxMsgCacheEntrySize are assigned a special value, and are loaded
     * on-demand from the database later.
     *
     * @param string $code Language code
     * @param int|null $mode Use MessageCache::FOR_UPDATE to skip process cache
     * @return array Loaded messages for storing in caches
     */
    private function loadFromDB( $code, $mode = null ) {
        $icp = MediaWikiServices::getInstance()->getConnectionProvider();

        $dbr = ( $mode === self::FOR_UPDATE ) ? $icp->getPrimaryDatabase() : $icp->getReplicaDatabase();

        $cache = [];

        $mostused = []; // list of "<cased message key>/<code>"
        if ( $this->adaptive && $code !== $this->contLangCode ) {
            if ( !$this->cache->has( $this->contLangCode ) ) {
                $this->load( $this->contLangCode );
            }
            $mostused = array_keys( $this->cache->get( $this->contLangCode ) );
            foreach ( $mostused as $key => $value ) {
                $mostused[$key] = "$value/$code";
            }
        }

        // Common conditions
        $conds = [
            'page_is_redirect' => 0,
            'page_namespace' => NS_MEDIAWIKI,
        ];
        if ( count( $mostused ) ) {
            $conds['page_title'] = $mostused;
        } elseif ( $code !== $this->contLangCode ) {
            $conds[] = $dbr->expr(
                'page_title',
                IExpression::LIKE,
                new LikeValue( $dbr->anyString(), '/', $code )
            );
        } else {
            // Effectively disallows use of '/' character in NS_MEDIAWIKI for uses
            // other than language code.
            $conds[] = $dbr->expr(
                'page_title',
                IExpression::NOT_LIKE,
                new LikeValue( $dbr->anyString(), '/', $dbr->anyString() )
            );
        }

        // Set the stubs for oversized software-defined messages in the main cache map
        $res = $dbr->newSelectQueryBuilder()
            ->select( [ 'page_title', 'page_latest' ] )
            ->from( 'page' )
            ->where( $conds )
            ->andWhere( [ 'page_len > ' . intval( $this->maxEntrySize ) ] )
            ->caller( __METHOD__ . "($code)-big" )->fetchResultSet();
        foreach ( $res as $row ) {
            // Include entries/stubs for all keys in $mostused in adaptive mode
            if ( $this->adaptive || $this->isMainCacheable( $row->page_title ) ) {
                $cache[$row->page_title] = '!TOO BIG';
            }
            // At least include revision ID so page changes are reflected in the hash
            $cache['EXCESSIVE'][$row->page_title] = $row->page_latest;
        }

        // RevisionStore cannot be injected as it would break the installer since
        // it instantiates MessageCache before the DB.
        $revisionStore = MediaWikiServices::getInstance()->getRevisionStore();
        // Set the text for small software-defined messages in the main cache map
        $revQuery = $revisionStore->getQueryInfo( [ 'page' ] );

        // T231196: MySQL/MariaDB (10.1.37) can sometimes irrationally decide that querying `actor` then
        // `revision` then `page` is somehow better than starting with `page`. Tell it not to reorder the
        // query (and also reorder it ourselves because as generated by RevisionStore it'll have
        // `revision` first rather than `page`).
        $revQuery['joins']['revision'] = $revQuery['joins']['page'];
        unset( $revQuery['joins']['page'] );
        // It isn't actually necessary to reorder $revQuery['tables'] as Database does the right thing
        // when join conditions are given for all joins, but Gergő is wary of relying on that so pull
        // `page` to the start.
        $revQuery['tables'] = array_merge(
            [ 'page' ],
            array_diff( $revQuery['tables'], [ 'page' ] )
        );

        $res = $dbr->newSelectQueryBuilder()
            ->queryInfo( $revQuery )
            ->where( $conds )
            ->andWhere( [
                $dbr->expr( 'page_len', '<=', intval( $this->maxEntrySize ) ),
                'page_latest = rev_id' // get the latest revision only
            ] )
            ->caller( __METHOD__ . "($code)-small" )
            ->straightJoinOption()
            ->fetchResultSet();

        // Don't load content from uncacheable rows (T313004)
        [ $cacheableRows, $uncacheableRows ] = $this->separateCacheableRows( $res );
        $result = $revisionStore->newRevisionsFromBatch( $cacheableRows, [
            'slots' => [ SlotRecord::MAIN ],
            'content' => true
        ] );
        $revisions = $result->isOK() ? $result->getValue() : [];

        foreach ( $cacheableRows as $row ) {
            try {
                $rev = $revisions[$row->rev_id] ?? null;
                $content = $rev ? $rev->getContent( SlotRecord::MAIN ) : null;
                $text = $this->getMessageTextFromContent( $content );
            } catch ( TimeoutException $e ) {
                throw $e;
            } catch ( Exception $ex ) {
                $text = false;
            }

            if ( !is_string( $text ) ) {
                $entry = '!ERROR';
                $this->logger->error(
                    __METHOD__
                    . ": failed to load message page text for {$row->page_title} ($code)"
                );
            } else {
                $entry = ' ' . $text;
            }
            $cache[$row->page_title] = $entry;
        }

        foreach ( $uncacheableRows as $row ) {
            // T193271: The cache object gets too big and slow to generate.
            // At least include revision ID, so that page changes are reflected in the hash.
            $cache['EXCESSIVE'][$row->page_title] = $row->page_latest;
        }

        $cache['VERSION'] = MSG_CACHE_VERSION;
        ksort( $cache );

        // Hash for validating local cache (APC). No need to take into account
        // messages larger than $wgMaxMsgCacheEntrySize, since those are only
        // stored and fetched from memcache.
        $cache['HASH'] = md5( serialize( $cache ) );
        $cache['EXPIRY'] = wfTimestamp( TS_MW, time() + self::WAN_TTL );
        unset( $cache['EXCESSIVE'] ); // only needed for hash

        return $cache;
    }

    /**
     * Whether the language was loaded and its data is still in the process cache.
     *
     * @param string $lang
     * @return bool
     */
    private function isLanguageLoaded( $lang ) {
        // It is important that this only returns true if the cache was fully
        // populated by load(), so that callers can assume all cache keys exist.
        // It is possible for $this->cache to be only partially populated through
        // methods like MessageCache::replace(), which must not make this method
        // return true (T208897). And this method must cease to return true
        // if the language was evicted by MapCacheLRU (T230690).
        return $this->cache->hasField( $lang, 'VERSION' );
    }

    /**
     * Can the given DB key be added to the main cache blob? To reduce the
     * abuse impact of the MediaWiki namespace by {{int:}} and CentralNotice,
     * this is only true if the page overrides a predefined message.
     *
     * @param string $name Message name (possibly with /code suffix)
     * @param string|null $code The language code. If this is null, message
     *   presence will be bulk loaded for the content language. Otherwise,
     *   presence will be detected by loading the specified message.
     * @return bool
     */
    private function isMainCacheable( $name, $code = null ) {
        // Convert the first letter to lowercase, and strip /code suffix
        $name = $this->contLang->lcfirst( $name );
        // Include common conversion table pages. This also avoids problems with
        // Installer::parse() bailing out due to disallowed DB queries (T207979).
        if ( strpos( $name, 'conversiontable/' ) === 0 ) {
            return true;
        }
        $msg = preg_replace( '/\/[a-z0-9-]{2,}$/', '', $name );

        if ( $code === null ) {
            // Bulk load
            if ( $this->systemMessageNames === null ) {
                $this->systemMessageNames = array_fill_keys(
                    $this->localisationCache->getSubitemList( $this->contLangCode, 'messages' ),
                    true );
            }
            return isset( $this->systemMessageNames[$msg] );
        } else {
            // Use individual subitem
            return $this->localisationCache->getSubitem( $code, 'messages', $msg ) !== null;
        }
    }

    /**
     * Separate cacheable from uncacheable rows in a page/revsion query result.
     *
     * @param IResultWrapper $res
     * @return array{0:IResultWrapper|stdClass[],1:stdClass[]} An array with the cacheable
     *    rows in the first element and the uncacheable rows in the second.
     */
    private function separateCacheableRows( $res ) {
        if ( $this->adaptive ) {
            // Include entries/stubs for all keys in $mostused in adaptive mode
            return [ $res, [] ];
        }
        $cacheableRows = [];
        $uncacheableRows = [];
        foreach ( $res as $row ) {
            if ( $this->isMainCacheable( $row->page_title ) ) {
                $cacheableRows[] = $row;
            } else {
                $uncacheableRows[] = $row;
            }
        }
        return [ $cacheableRows, $uncacheableRows ];
    }

    /**
     * Updates cache as necessary when message page is changed
     *
     * @param string $title Message cache key with the initial uppercase letter
     * @param string|false $text New contents of the page (false if deleted)
     */
    public function replace( $title, $text ) {
        if ( $this->disable ) {
            return;
        }

        [ $msg, $code ] = $this->figureMessage( $title );
        if ( strpos( $title, '/' ) !== false && $code === $this->contLangCode ) {
            // Content language overrides do not use the /<code> suffix
            return;
        }

        // (a) Update the process cache with the new message text
        if ( $text === false ) {
            // Page deleted
            $this->cache->setField( $code, $title, '!NONEXISTENT' );
        } else {
            // Ignore $wgMaxMsgCacheEntrySize so the process cache is up-to-date
            $this->cache->setField( $code, $title, ' ' . $text );
        }

        // (b) Update the shared caches in a deferred update with a fresh DB snapshot
        DeferredUpdates::addUpdate(
            new MessageCacheUpdate( $code, $title, $msg ),
            DeferredUpdates::PRESEND
        );
    }

    /**
     * @param string $code
     * @param array[] $replacements List of (title, message key) pairs
     */
    public function refreshAndReplaceInternal( string $code, array $replacements ) {
        // Allow one caller at a time to avoid race conditions
        [ $scopedLock ] = $this->getReentrantScopedLock( $code );
        if ( !$scopedLock ) {
            foreach ( $replacements as [ $title ] ) {
                $this->logger->error(
                    __METHOD__ . ': could not acquire lock to update {title} ({code})',
                    [ 'title' => $title, 'code' => $code ] );
            }

            return;
        }

        // Load the existing cache to update it in the local DC cache.
        // The other DCs will see a hash mismatch.
        if ( $this->load( $code, self::FOR_UPDATE ) ) {
            $cache = $this->cache->get( $code );
        } else {
            // Err? Fall back to loading from the database.
            $cache = $this->loadFromDB( $code, self::FOR_UPDATE );
        }
        // Check if individual cache keys should exist and update cache accordingly
        $newTextByTitle = []; // map of (title => content)
        $newBigTitles = []; // map of (title => latest revision ID), like EXCESSIVE in loadFromDB()
        // Can not inject the WikiPageFactory as it would break the installer since
        // it instantiates MessageCache before the DB.
        $wikiPageFactory = MediaWikiServices::getInstance()->getWikiPageFactory();
        foreach ( $replacements as [ $title ] ) {
            $page = $wikiPageFactory->newFromTitle( Title::makeTitle( NS_MEDIAWIKI, $title ) );
            $page->loadPageData( IDBAccessObject::READ_LATEST );
            $text = $this->getMessageTextFromContent( $page->getContent() );
            // Remember the text for the blob store update later on
            $newTextByTitle[$title] = $text ?? '';
            // Note that if $text is false, then $cache should have a !NONEXISTANT entry
            if ( !is_string( $text ) ) {
                $cache[$title] = '!NONEXISTENT';
            } elseif ( strlen( $text ) > $this->maxEntrySize ) {
                $cache[$title] = '!TOO BIG';
                $newBigTitles[$title] = $page->getLatest();
            } else {
                $cache[$title] = ' ' . $text;
            }
        }
        // Update HASH for the new key. Incorporates various administrative keys,
        // including the old HASH (and thereby the EXCESSIVE value from loadFromDB()
        // and previous replace() calls), but that doesn't really matter since we
        // only ever compare it for equality with a copy saved by saveToCaches().
        $cache['HASH'] = md5( serialize( $cache + [ 'EXCESSIVE' => $newBigTitles ] ) );
        // Update the too-big WAN cache entries now that we have the new HASH
        foreach ( $newBigTitles as $title => $id ) {
            // Match logic of loadCachedMessagePageEntry()
            $this->wanCache->set(
                $this->bigMessageCacheKey( $cache['HASH'], $title ),
                ' ' . $newTextByTitle[$title],
                self::WAN_TTL
            );
        }
        // Mark this cache as definitely being "latest" (non-volatile) so
        // load() calls do not try to refresh the cache with replica DB data
        $cache['LATEST'] = time();
        // Update the process cache
        $this->cache->set( $code, $cache );
        // Pre-emptively update the local datacenter cache so things like edit filter and
        // prevented changes are reflected immediately; these often use MediaWiki: pages.
        // The datacenter handling replace() calls should be the same one handling edits
        // as they require HTTP POST.
        $this->saveToCaches( $cache, 'all', $code );
        // Release the lock now that the cache is saved
        ScopedCallback::consume( $scopedLock );

        // Relay the purge. Touching this check key expires cache contents
        // and local cache (APC) validation hash across all datacenters.
        $this->wanCache->touchCheckKey( $this->getCheckKey( $code ) );

        // Purge the messages in the message blob store and fire any hook handlers
        $blobStore = MediaWikiServices::getInstance()->getResourceLoader()->getMessageBlobStore();
        foreach ( $replacements as [ $title, $msg ] ) {
            $blobStore->updateMessage( $this->contLang->lcfirst( $msg ) );
            $this->hookRunner->onMessageCacheReplace( $title, $newTextByTitle[$title] );
        }
    }

    /**
     * Is the given cache array expired due-to-time passing or a version change?
     *
     * @param array $cache
     * @return bool
     */
    private function isCacheExpired( $cache ) {
        return !isset( $cache['VERSION'] ) ||
            !isset( $cache['EXPIRY'] ) ||
            $cache['VERSION'] !== MSG_CACHE_VERSION ||
            $cache['EXPIRY'] <= wfTimestampNow();
    }

    /**
     * Shortcut to update caches.
     *
     * @param array $cache Cached messages with a version.
     * @param string $dest Either "local-only" to save to local caches only
     *   or "all" to save to all caches.
     * @param string|false $code Language code (default: false)
     * @return bool
     */
    private function saveToCaches( array $cache, $dest, $code = false ) {
        if ( $dest === 'all' ) {
            $cacheKey = $this->clusterCache->makeKey( 'messages', $code );
            $success = $this->clusterCache->set( $cacheKey, $cache );
            $this->setValidationHash( $code, $cache );
        } else {
            $success = true;
        }

        $this->saveToLocalCache( $code, $cache );

        return $success;
    }

    /**
     * Get the md5 used to validate the local server cache
     *
     * @param string $code
     * @return array (hash or false, bool expiry/volatility status)
     */
    private function getValidationHash( $code ) {
        $curTTL = null;
        $value = $this->wanCache->get(
            $this->wanCache->makeKey( 'messages', $code, 'hash', 'v1' ),
            $curTTL,
            [ $this->getCheckKey( $code ) ]
        );

        if ( $value ) {
            $hash = $value['hash'];
            if ( ( time() - $value['latest'] ) < WANObjectCache::TTL_MINUTE ) {
                // Cache was recently updated via replace() and should be up-to-date.
                // That method is only called in the primary datacenter and uses FOR_UPDATE.
                $expired = false;
            } else {
                // See if the "check" key was bumped after the hash was generated
                $expired = ( $curTTL < 0 );
            }
        } else {
            // No hash found at all; cache must regenerate to be safe
            $hash = false;
            $expired = true;
        }

        return [ $hash, $expired ];
    }

    /**
     * Set the md5 used to validate the local server cache
     *
     * If $cache has a 'LATEST' UNIX timestamp key, then the hash will not
     * be treated as "volatile" by getValidationHash() for the next few seconds.
     * This is triggered when $cache is generated using FOR_UPDATE mode.
     *
     * @param string $code
     * @param array $cache Cached messages with a version
     */
    private function setValidationHash( $code, array $cache ) {
        $this->wanCache->set(
            $this->wanCache->makeKey( 'messages', $code, 'hash', 'v1' ),
            [
                'hash' => $cache['HASH'],
                'latest' => $cache['LATEST'] ?? 0
            ],
            WANObjectCache::TTL_INDEFINITE
        );
    }

    /**
     * @param string $code Which language to load messages for
     * @param int $timeout Wait timeout in seconds
     * @return array (ScopedCallback or null, whether locking failed due to an I/O error)
     * @phan-return array{0:ScopedCallback|null,1:bool}
     */
    private function getReentrantScopedLock( $code, $timeout = self::WAIT_SEC ) {
        $key = $this->clusterCache->makeKey( 'messages', $code );

        $watchPoint = $this->clusterCache->watchErrors();
        $scopedLock = $this->clusterCache->getScopedLock(
            $key,
            $timeout,
            self::LOCK_TTL,
            __METHOD__
        );
        $error = ( !$scopedLock && $this->clusterCache->getLastError( $watchPoint ) );

        return [ $scopedLock, $error ];
    }

    /**
     * Get a message from either the content language or the user language.
     *
     * First, assemble a list of languages to attempt getting the message from. This
     * chain begins with the requested language and its fallbacks and then continues with
     * the content language and its fallbacks. For each language in the chain, the following
     * process will occur (in this order):
     *  1. If a language-specific override, i.e., [[MW:msg/lang]], is available, use that.
     *     Note: for the content language, there is no /lang subpage.
     *  2. Fetch from the static CDB cache.
     *  3. If available, check the database for fallback language overrides.
     *
     * This process provides a number of guarantees. When changing this code, make sure all
     * of these guarantees are preserved.
     *  * If the requested language is *not* the content language, then the CDB cache for that
     *    specific language will take precedence over the root database page ([[MW:msg]]).
     *  * Fallbacks will be just that: fallbacks. A fallback language will never be reached if
     *    the message is available *anywhere* in the language for which it is a fallback.
     *
     * @param string $key The message key
     * @param bool $useDB If true, look for the message in the DB, false
     *   to use only the compiled l10n cache.
     * @param bool|string|Language $langcode Code of the language to get the message for.
     *   - If string and a valid code, will create a standard language object
     *   - If string but not a valid code, will create a basic language object
     *   - If boolean and false, create object from the current users language
     *   - If boolean and true, create object from the wikis content language
     *   - If language object, use it as given
     * @param string &$usedKey @phan-output-reference If given, will be set to the message key
     *   that the message was fetched from (the requested key may be overridden by hooks).
     *
     * @return string|false False if the message doesn't exist, otherwise the
     *   message (which can be empty)
     */
    public function get( $key, $useDB = true, $langcode = true, &$usedKey = '' ) {
        if ( is_int( $key ) ) {
            // Fix numerical strings that somehow become ints on their way here
            $key = (string)$key;
        } elseif ( !is_string( $key ) ) {
            throw new TypeError( 'Message key must be a string' );
        } elseif ( $key === '' ) {
            // Shortcut: the empty key is always missing
            return false;
        }

        $language = $this->getLanguageObject( $langcode );

        // Normalise title-case input (with some inlining)
        $lckey = self::normalizeKey( $key );

        // Initialize the overrides here to prevent calling the hook too early.
        if ( $this->messageKeyOverrides === null ) {
            $this->messageKeyOverrides = [];
            $this->hookRunner->onMessageCacheFetchOverrides( $this->messageKeyOverrides );
        }

        if ( isset( $this->messageKeyOverrides[$lckey] ) ) {
            $override = $this->messageKeyOverrides[$lckey];

            // Strings are deliberately interpreted as message keys,
            // to prevent ambiguity between message keys and functions.
            if ( is_string( $override ) ) {
                $lckey = $override;
            } else {
                $lckey = $override( $lckey, $this, $language, $useDB );
            }
        }

        $this->hookRunner->onMessageCache__get( $lckey );

        $usedKey = $lckey;

        // Loop through each language in the fallback list until we find something useful
        $message = $this->getMessageFromFallbackChain(
            $language,
            $lckey,
            !$this->disable && $useDB
        );

        // If we still have no message, maybe the key was in fact a full key so try that
        if ( $message === false ) {
            $parts = explode( '/', $lckey );
            // We may get calls for things that are http-urls from sidebar
            // Let's not load nonexistent languages for those
            // They usually have more than one slash.
            if ( count( $parts ) === 2 && $parts[1] !== '' ) {
                $message = $this->localisationCache->getSubitem( $parts[1], 'messages', $parts[0] ) ?? false;
            }
        }

        // Post-processing if the message exists
        if ( $message !== false ) {
            // Fix whitespace
            $message = str_replace(
                [
                    // Fix for trailing whitespace, removed by textarea
                    '&#32;',
                    // Fix for NBSP, converted to space by firefox
                    '&nbsp;',
                    '&#160;',
                    '&shy;'
                ],
                [
                    ' ',
                    "\u{00A0}",
                    "\u{00A0}",
                    "\u{00AD}"
                ],
                $message
            );
        }

        return $message;
    }

    /**
     * Return a Language object from $langcode
     *
     * @param Language|string|bool $langcode Either:
     *                  - a Language object
     *                  - code of the language to get the message for, if it is
     *                    a valid code create a language for that language, if
     *                    it is a string but not a valid code then make a basic
     *                    language object
     *                  - a boolean: if it's false then use the global object for
     *                    the current user's language (as a fallback for the old parameter
     *                    functionality), or if it is true then use global object
     *                    for the wiki's content language.
     * @return Language|StubUserLang
     */
    private function getLanguageObject( $langcode ) {
        # Identify which language to get or create a language object for.
        # Using is_object here due to Stub objects.
        if ( is_object( $langcode ) ) {
            # Great, we already have the object (hopefully)!
            return $langcode;
        }

        if ( $langcode === true || $langcode === $this->contLangCode ) {
            # $langcode is the language code of the wikis content language object.
            # or it is a boolean and value is true
            return $this->contLang;
        }

        global $wgLang;
        if ( $langcode === false || $langcode === $wgLang->getCode() ) {
            # $langcode is the language code of user language object.
            # or it was a boolean and value is false
            return $wgLang;
        }

        $validCodes = array_keys( $this->languageNameUtils->getLanguageNames() );
        if ( in_array( $langcode, $validCodes ) ) {
            # $langcode corresponds to a valid language.
            return $this->langFactory->getLanguage( $langcode );
        }

        # $langcode is a string, but not a valid language code; use content language.
        $this->logger->debug( 'Invalid language code passed to' . __METHOD__ . ', falling back to content language.' );
        return $this->contLang;
    }

    /**
     * Given a language, try and fetch messages from that language.
     *
     * Will also consider fallbacks of that language, the site language, and fallbacks for
     * the site language.
     *
     * @see MessageCache::get
     * @param Language|StubObject $lang Preferred language
     * @param string $lckey Lowercase key for the message (as for localisation cache)
     * @param bool $useDB Whether to include messages from the wiki database
     * @return string|false The message, or false if not found
     */
    private function getMessageFromFallbackChain( $lang, $lckey, $useDB ) {
        $alreadyTried = [];

        // First try the requested language.
        $message = $this->getMessageForLang( $lang, $lckey, $useDB, $alreadyTried );
        if ( $message !== false ) {
            return $message;
        }

        // Now try checking the site language.
        $message = $this->getMessageForLang( $this->contLang, $lckey, $useDB, $alreadyTried );
        return $message;
    }

    /**
     * Given a language, try and fetch messages from that language and its fallbacks.
     *
     * @see MessageCache::get
     * @param Language|StubObject $lang Preferred language
     * @param string $lckey Lowercase key for the message (as for localisation cache)
     * @param bool $useDB Whether to include messages from the wiki database
     * @param bool[] &$alreadyTried Contains true for each language that has been tried already
     * @return string|false The message, or false if not found
     */
    private function getMessageForLang( $lang, $lckey, $useDB, &$alreadyTried ) {
        $langcode = $lang->getCode();

        // Try checking the database for the requested language
        if ( $useDB ) {
            $uckey = $this->contLang->ucfirst( $lckey );

            if ( !isset( $alreadyTried[$langcode] ) ) {
                $message = $this->getMsgFromNamespace(
                    $this->getMessagePageName( $langcode, $uckey ),
                    $langcode
                );
                if ( $message !== false ) {
                    return $message;
                }
                $alreadyTried[$langcode] = true;
            }
        } else {
            $uckey = null;
        }

        // Return a special value handled in Message::format() to display the message key
        // (and fallback keys) and the parameters passed to the message.
        // TODO: Move to a better place.
        if ( $langcode === 'qqx' ) {
            return '($*)';
        } elseif (
            $langcode === 'x-xss' &&
            $this->useXssLanguage &&
            !in_array( $lckey, $this->rawHtmlMessages, true )
        ) {
            $xssViaInnerHtml = "<script>alert('$lckey')</script>";
            $xssViaAttribute = '">' . $xssViaInnerHtml . '<x y="';
            return $xssViaInnerHtml . $xssViaAttribute . '($*)';
        }

        // Check the localisation cache
        [ $defaultMessage, $messageSource ] =
            $this->localisationCache->getSubitemWithSource( $langcode, 'messages', $lckey );
        if ( $messageSource === $langcode ) {
            return $defaultMessage;
        }

        // Try checking the database for all of the fallback languages
        if ( $useDB ) {
            $fallbackChain = $this->languageFallback->getAll( $langcode );

            foreach ( $fallbackChain as $code ) {
                if ( isset( $alreadyTried[$code] ) ) {
                    continue;
                }

                $message = $this->getMsgFromNamespace(
                    // @phan-suppress-next-line PhanTypeMismatchArgumentNullable uckey is set when used
                    $this->getMessagePageName( $code, $uckey ), $code );

                if ( $message !== false ) {
                    return $message;
                }
                $alreadyTried[$code] = true;

                // Reached the source language of the default message. Don't look for DB overrides
                // further back in the fallback chain. (T229992)
                if ( $code === $messageSource ) {
                    return $defaultMessage;
                }
            }
        }

        return $defaultMessage ?? false;
    }

    /**
     * Get the message page name for a given language
     *
     * @param string $langcode
     * @param string $uckey Uppercase key for the message
     * @return string The page name
     */
    private function getMessagePageName( $langcode, $uckey ) {
        if ( $langcode === $this->contLangCode ) {
            // Messages created in the content language will not have the /lang extension
            return $uckey;
        } else {
            return "$uckey/$langcode";
        }
    }

    /**
     * Get a message from the MediaWiki namespace, with caching. The key must
     * first be converted to two-part lang/msg form if necessary.
     *
     * Unlike self::get(), this function doesn't resolve fallback chains, and
     * some callers require this behavior. LanguageConverter::parseCachedTable()
     * and self::get() are some examples in core.
     *
     * @param string $title Message cache key with the initial uppercase letter
     * @param string $code Code denoting the language to try
     * @return string|false The message, or false if it does not exist or on error
     */
    public function getMsgFromNamespace( $title, $code ) {
        // Load all MediaWiki page definitions into cache. Note that individual keys
        // already loaded into the cache during this request remain in the cache, which
        // includes the value of hook-defined messages.
        $this->load( $code );

        $entry = $this->cache->getField( $code, $title );

        if ( $entry !== null ) {
            // Message page exists as an override of a software messages
            if ( substr( $entry, 0, 1 ) === ' ' ) {
                // The message exists and is not '!TOO BIG' or '!ERROR'
                return (string)substr( $entry, 1 );
            } elseif ( $entry === '!NONEXISTENT' ) {
                // The text might be '-' or missing due to some data loss
                return false;
            }
            // Load the message page, utilizing the individual message cache.
            // If the page does not exist, there will be no hook handler fallbacks.
            $entry = $this->loadCachedMessagePageEntry(
                $title,
                $code,
                $this->cache->getField( $code, 'HASH' )
            );
        } else {
            // Message page either does not exist or does not override a software message
            if ( !$this->isMainCacheable( $title, $code ) ) {
                // Message page does not override any software-defined message. A custom
                // message might be defined to have content or settings specific to the wiki.
                // Load the message page, utilizing the individual message cache as needed.
                $entry = $this->loadCachedMessagePageEntry(
                    $title,
                    $code,
                    $this->cache->getField( $code, 'HASH' )
                );
            }
            if ( $entry === null || substr( $entry, 0, 1 ) !== ' ' ) {
                // Message does not have a MediaWiki page definition; try hook handlers
                $message = false;
                // @phan-suppress-next-line PhanTypeMismatchArgument Type mismatch on pass-by-ref args
                $this->hookRunner->onMessagesPreLoad( $title, $message, $code );
                if ( $message !== false ) {
                    $this->cache->setField( $code, $title, ' ' . $message );
                } else {
                    $this->cache->setField( $code, $title, '!NONEXISTENT' );
                }

                return $message;
            }
        }

        if ( $entry !== false && substr( $entry, 0, 1 ) === ' ' ) {
            if ( $this->cacheVolatile[$code] ) {
                // Make sure that individual keys respect the WAN cache holdoff period too
                $this->logger->debug(
                    __METHOD__ . ': loading volatile key \'{titleKey}\'',
                    [ 'titleKey' => $title, 'code' => $code ] );
            } else {
                $this->cache->setField( $code, $title, $entry );
            }
            // The message exists, so make sure a string is returned
            return (string)substr( $entry, 1 );
        }

        $this->cache->setField( $code, $title, '!NONEXISTENT' );

        return false;
    }

    /**
     * @param string $dbKey
     * @param string $code
     * @param string $hash
     * @return string Either " <MESSAGE>" or "!NONEXISTANT"
     */
    private function loadCachedMessagePageEntry( $dbKey, $code, $hash ) {
        $fname = __METHOD__;
        return $this->srvCache->getWithSetCallback(
            $this->srvCache->makeKey( 'messages-big', $hash, $dbKey ),
            BagOStuff::TTL_HOUR,
            function () use ( $code, $dbKey, $hash, $fname ) {
                return $this->wanCache->getWithSetCallback(
                    $this->bigMessageCacheKey( $hash, $dbKey ),
                    self::WAN_TTL,
                    function ( $oldValue, &$ttl, &$setOpts ) use ( $dbKey, $code, $fname ) {
                        // Try loading the message from the database
                        $setOpts += Database::getCacheSetOptions(
                            MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase()
                        );
                        // Use newKnownCurrent() to avoid querying revision/user tables
                        $title = Title::makeTitle( NS_MEDIAWIKI, $dbKey );
                        // Injecting RevisionStore breaks installer since it
                        // instantiates MessageCache before DB.
                        $revision = MediaWikiServices::getInstance()
                            ->getRevisionLookup()
                            ->getKnownCurrentRevision( $title );
                        if ( !$revision ) {
                            // The wiki doesn't have a local override page. Cache absence with normal TTL.
                            // When overrides are created, self::replace() takes care of the cache.
                            return '!NONEXISTENT';
                        }
                        $content = $revision->getContent( SlotRecord::MAIN );
                        if ( $content ) {
                            $message = $this->getMessageTextFromContent( $content );
                        } else {
                            $this->logger->warning(
                                $fname . ': failed to load page text for \'{titleKey}\'',
                                [ 'titleKey' => $dbKey, 'code' => $code ]
                            );
                            $message = null;
                        }

                        if ( !is_string( $message ) ) {
                            // Revision failed to load Content, or Content is incompatible with wikitext.
                            // Possibly a temporary loading failure.
                            $ttl = 5;

                            return '!NONEXISTENT';
                        }

                        return ' ' . $message;
                    }
                );
            }
        );
    }

    /**
     * @param string $message
     * @param bool $interface
     * @param Language|null $language
     * @param PageReference|null $page
     * @return string
     */
    public function transform( $message, $interface = false, $language = null, PageReference $page = null ) {
        // Avoid creating parser if nothing to transform
        if ( $this->inParser || !str_contains( $message, '{{' ) ) {
            return $message;
        }

        $parser = $this->getParser();
        $popts = $this->getParserOptions();
        $popts->setInterfaceMessage( $interface );
        $popts->setTargetLanguage( $language );

        $userlang = $popts->setUserLang( $language );
        $this->inParser = true;
        $message = $parser->transformMsg( $message, $popts, $page );
        $this->inParser = false;
        $popts->setUserLang( $userlang );

        return $message;
    }

    /**
     * @return Parser
     */
    public function getParser() {
        if ( !$this->parser ) {
            $this->parser = $this->parserFactory->create();
        }

        return $this->parser;
    }

    /**
     * @param string $text
     * @param PageReference|null $page
     * @param bool $linestart Whether this is at the start of a line
     * @param bool $interface Whether this is an interface message
     * @param Language|StubUserLang|string|null $language Language code
     * @return ParserOutput|string
     */
    public function parse( $text, PageReference $page = null, $linestart = true,
        $interface = false, $language = null
    ) {
        global $wgTitle;

        if ( $this->inParser ) {
            return htmlspecialchars( $text );
        }

        $parser = $this->getParser();
        $popts = $this->getParserOptions();
        $popts->setInterfaceMessage( $interface );

        if ( is_string( $language ) ) {
            $language = $this->langFactory->getLanguage( $language );
        }
        $popts->setTargetLanguage( $language );

        if ( !$page ) {
            $logger = LoggerFactory::getInstance( 'GlobalTitleFail' );
            $logger->info(
                __METHOD__ . ' called with no title set.',
                [ 'exception' => new RuntimeException ]
            );
            $page = $wgTitle;
        }
        // Sometimes $wgTitle isn't set either...
        if ( !$page ) {
            // It's not uncommon having a null $wgTitle in scripts. See r80898
            // Create a ghost title in such case
            $page = PageReferenceValue::localReference(
                NS_SPECIAL,
                'Badtitle/title not set in ' . __METHOD__
            );
        }

        $this->inParser = true;
        $res = $parser->parse( $text, $page, $popts, $linestart );
        $this->inParser = false;

        return $res;
    }

    public function disable() {
        $this->disable = true;
    }

    public function enable() {
        $this->disable = false;
    }

    /**
     * Whether DB/cache usage is disabled for determining messages
     *
     * If so, this typically indicates either:
     *   - a) load() failed to find a cached copy nor query the DB
     *   - b) we are in a special context or error mode that cannot use the DB
     * If the DB is ignored, any derived HTML output or cached objects may be wrong.
     * To avoid long-term cache pollution, TTLs can be adjusted accordingly.
     *
     * @return bool
     * @since 1.27
     */
    public function isDisabled() {
        return $this->disable;
    }

    /**
     * Clear all stored messages in global and local cache
     *
     * Mainly used after a mass rebuild
     */
    public function clear() {
        $langs = $this->languageNameUtils->getLanguageNames();
        foreach ( $langs as $code => $_ ) {
            $this->wanCache->touchCheckKey( $this->getCheckKey( $code ) );
        }
        $this->cache->clear();
    }

    /**
     * @param string $key
     * @return array
     */
    public function figureMessage( $key ) {
        $pieces = explode( '/', $key );
        if ( count( $pieces ) < 2 ) {
            return [ $key, $this->contLangCode ];
        }

        $lang = array_pop( $pieces );
        if ( !$this->languageNameUtils->getLanguageName(
            $lang,
            LanguageNameUtils::AUTONYMS,
            LanguageNameUtils::DEFINED
        ) ) {
            return [ $key, $this->contLangCode ];
        }

        $message = implode( '/', $pieces );

        return [ $message, $lang ];
    }

    /**
     * Get all message keys stored in the message cache for a given language.
     * If $code is the content language code, this will return all message keys
     * for which MediaWiki:msgkey exists. If $code is another language code, this
     * will ONLY return message keys for which MediaWiki:msgkey/$code exists.
     *
     * @param string $code Language code
     * @return string[]|null Array of message keys
     */
    public function getAllMessageKeys( $code ) {
        $this->load( $code );
        if ( !$this->cache->has( $code ) ) {
            // Apparently load() failed
            return null;
        }
        // Remove administrative keys
        $cache = $this->cache->get( $code );
        unset( $cache['VERSION'] );
        unset( $cache['EXPIRY'] );
        unset( $cache['EXCESSIVE'] );
        // Remove any !NONEXISTENT keys
        $cache = array_diff( $cache, [ '!NONEXISTENT' ] );

        // Keys may appear with a capital first letter. lcfirst them.
        return array_map( [ $this->contLang, 'lcfirst' ], array_keys( $cache ) );
    }

    /**
     * Purge message caches when a MediaWiki: page is created, updated, or deleted
     *
     * @param LinkTarget $linkTarget Message page title
     * @param Content|null $content New content for edit/create, null on deletion
     * @since 1.29
     */
    public function updateMessageOverride( LinkTarget $linkTarget, Content $content = null ) {
        // treat null as not existing
        $msgText = $this->getMessageTextFromContent( $content ) ?? false;

        $this->replace( $linkTarget->getDBkey(), $msgText );

        if ( $this->contLangConverter->hasVariants() ) {
            $this->contLangConverter->updateConversionTable( $linkTarget );
        }
    }

    /**
     * @param string $code Language code
     * @return string WAN cache key usable as a "check key" against language page edits
     */
    public function getCheckKey( $code ) {
        return $this->wanCache->makeKey( 'messages', $code );
    }

    /**
     * @param Content|null $content Content or null if the message page does not exist
     * @return string|false|null Returns false if $content is null and null on error
     */
    private function getMessageTextFromContent( Content $content = null ) {
        // @TODO: could skip pseudo-messages like js/css here, based on content model
        if ( $content ) {
            // Message page exists...
            // XXX: Is this the right way to turn a Content object into a message?
            // NOTE: $content is typically either WikitextContent, JavaScriptContent or
            //       CssContent. MessageContent is *not* used for storing messages, it's
            //       only used for wrapping them when needed.
            $msgText = $content->getWikitextForTransclusion();
            if ( $msgText === false || $msgText === null ) {
                // This might be due to some kind of misconfiguration...
                $msgText = null;
                $this->logger->warning(
                    __METHOD__ . ": message content doesn't provide wikitext "
                    . "(content model: " . $content->getModel() . ")" );
            }
        } else {
            // Message page does not exist...
            $msgText = false;
        }

        return $msgText;
    }

    /**
     * @param string $hash Hash for this version of the entire key/value overrides map
     * @param string $title Message cache key with the initial uppercase letter
     * @return string
     */
    private function bigMessageCacheKey( $hash, $title ) {
        return $this->wanCache->makeKey( 'messages-big', $hash, $title );
    }
}