1.32.5/php/MessageCache_8php_source.html

<?php

use MediaWiki\MediaWikiServices;

use Wikimedia\ScopedCallback;

use MediaWiki\Logger\LoggerFactory;

use Wikimedia\Rdbms\Database;


define( 'MSG_CACHE_VERSION', 2 );


class MessageCache {

    const FOR_UPDATE = 1; // force message reload


    const WAIT_SEC = 15;

    const LOCK_TTL = 30;


    protected $cache;


    protected $overridable;


    protected $cacheVolatile = [];


    protected $mDisable;


    protected $mExpiry;


    protected $mParserOptions;

    protected $mParser;


    protected $mInParser = false;


    protected $wanCache;

    protected $clusterCache;

    protected $srvCache;

    protected $contLang;


    private $loadedLanguages = [];


    private static $instance;


    public static function singleton() {

        if ( self::$instance === null ) {

            global $wgUseDatabaseMessages, $wgMsgCacheExpiry, $wgUseLocalMessageCache;

            $services = MediaWikiServices::getInstance();

            self::$instance = new self(

                $services->getMainWANObjectCache(),

                wfGetMessageCacheStorage(),

                $wgUseLocalMessageCache

                    ? $services->getLocalServerObjectCache()

                    : new EmptyBagOStuff(),

                $wgUseDatabaseMessages,

                $wgMsgCacheExpiry,

                $services->getContentLanguage()

            );

        }


        return self::$instance;

    }


    public static function destroyInstance() {

        self::$instance = null;

    }


    public static function normalizeKey( $key ) {

        $lckey = strtr( $key, ' ', '_' );

        if ( ord( $lckey ) < 128 ) {

            $lckey[0] = strtolower( $lckey[0] );

        } else {

            $lckey = MediaWikiServices::getInstance()->getContentLanguage()->lcfirst( $lckey );

        }


        return $lckey;

    }


    public function __construct(

        WANObjectCache $wanCache,

        BagOStuff $clusterCache,

        BagOStuff $serverCache,

        $useDB,

        $expiry,

        Language $contLang = null

    ) {

        $this->wanCache = $wanCache;

        $this->clusterCache = $clusterCache;

        $this->srvCache = $serverCache;


        $this->cache = new MapCacheLRU( 5 ); // limit size for sanity


        $this->mDisable = !$useDB;

        $this->mExpiry = $expiry;

        $this->contLang = $contLang ?? MediaWikiServices::getInstance()->getContentLanguage();

    }


    function getParserOptions() {

        global $wgUser;


        if ( !$this->mParserOptions ) {

            if ( !$wgUser->isSafeToLoad() ) {

                // $wgUser isn't unstubbable 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->mParserOptions = new ParserOptions;

            // Messages may take parameters that could come

            // from malicious sources. As a precaution, disable

            // the <html> parser tag when parsing messages.

            $this->mParserOptions->setAllowUnsafeRawHtml( false );

        }


        return $this->mParserOptions;

    }


    protected function getLocalCache( $code ) {

        $cacheKey = $this->srvCache->makeKey( __CLASS__, $code );


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

    }


    protected function saveToLocalCache( $code, $cache ) {

        $cacheKey = $this->srvCache->makeKey( __CLASS__, $code );

        $this->srvCache->set( $cacheKey, $cache );

    }


    protected function load( $code, $mode = null ) {

        if ( !is_string( $code ) ) {

            throw new InvalidArgumentException( "Missing language code" );

        }


        # Don't do double loading...

        if ( isset( $this->loadedLanguages[$code] ) && $mode != self::FOR_UPDATE ) {

            return true;

        }


        $this->overridable = array_flip( Language::getMessageKeysFor( $code ) );


        # 8 lines of code just to say (once) that message cache is disabled

        if ( $this->mDisable ) {

            static $shownDisabled = false;

            if ( !$shownDisabled ) {

                wfDebug( __METHOD__ . ": disabled\n" );

                $shownDisabled = true;

            }


            return true;

        }


        # Loading code starts

        $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


        # Hash of the contents is stored in memcache, to detect if data-center cache

        # or local cache goes out of date (e.g. due to replace() on some other server)

        list( $hash, $hashVolatile ) = $this->getValidationHash( $code );

        $this->cacheVolatile[$code] = $hashVolatile;


        # 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 ) {

            $where[] = 'local cache validation key is expired/volatile';

            $staleCache = $cache;

        } else {

            $where[] = 'got from local cache';

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

            $success = true;

        }


        if ( !$success ) {

            $cacheKey = $this->clusterCache->makeKey( 'messages', $code );

            # Try the global cache. If it is empty, try to acquire a lock. If

            # the lock can't be acquired, wait for the other thread to finish

            # and then try the global cache a second time.

            for ( $failedAttempts = 0; $failedAttempts <= 1; $failedAttempts++ ) {

                if ( $hashVolatile && $staleCache ) {

                    # Do not bother fetching the whole cache blob to avoid I/O.

                    # Instead, just try to get the non-blocking $statusKey lock

                    # below, and use the local stale value if it was not acquired.

                    $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 ) {

                        # DB results are replica DB lag prone until the holdoff TTL passes.

                        # By then, updates should be reflected in loadFromDBWithLock().

                        # One thread regenerates the cache while others use old values.

                        $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;

                    }

                }


                if ( $success ) {

                    # Done, no need to retry

                    break;

                }


                # We need to call loadFromDB. Limit the concurrency to one process.

                # This prevents the site from going down when the cache expires.

                # Note that the DB slam protection lock here is non-blocking.

                $loadStatus = $this->loadFromDBWithLock( $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 ) {

                    # 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.

                    $where[] = "could not acquire status key.";

                    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[] = 'waited for other thread to complete';

                    $this->getReentrantScopedLock( $cacheKey );

                } else {

                    # Disable cache; $loadStatus is 'disabled'

                    break;

                }

            }

        }


        if ( !$success ) {

            $where[] = 'loading FAILED - cache is disabled';

            $this->mDisable = true;

            $this->cache->set( $code, [] );

            wfDebugLog( 'MessageCacheError', __METHOD__ . ": Failed to load $code\n" );

            # 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

        } else {

            # All good, just record the success

            $this->loadedLanguages[$code] = true;

        }


        if ( !$this->cache->has( $code ) ) { // sanity

            throw new LogicException( "Process cache for '$code' should be set by now." );

        }


        $info = implode( ', ', $where );

        wfDebugLog( 'MessageCache', __METHOD__ . ": Loading $code... $info\n" );


        return $success;

    }


    protected function loadFromDBWithLock( $code, array &$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 database';


        # 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 need to wait for any current thread.

        $cacheKey = $this->clusterCache->makeKey( 'messages', $code );

        $scopedLock = $this->getReentrantScopedLock( $cacheKey, 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 ) {

            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;

    }


    protected function loadFromDB( $code, $mode = null ) {

        global $wgMaxMsgCacheEntrySize, $wgLanguageCode, $wgAdaptiveMessageCache;


        // (T164666) The query here performs really poorly on WMF's

        // contributions replicas. We don't have a way to say "any group except

        // contributions", so for the moment let's specify 'api'.

        // @todo: Get rid of this hack.

        $dbr = wfGetDB( ( $mode == self::FOR_UPDATE ) ? DB_MASTER : DB_REPLICA, 'api' );


        $cache = [];


        $mostused = []; // list of "<cased message key>/<code>"

        if ( $wgAdaptiveMessageCache && $code !== $wgLanguageCode ) {

            if ( !$this->cache->has( $wgLanguageCode ) ) {

                $this->load( $wgLanguageCode );

            }

            $mostused = array_keys( $this->cache->get( $wgLanguageCode ) );

            foreach ( $mostused as $key => $value ) {

                $mostused[$key] = "$value/$code";

            }

        }


        // Get the list of software-defined messages in core/extensions

        $overridable = array_flip( Language::getMessageKeysFor( $wgLanguageCode ) );


        // Common conditions

        $conds = [

            'page_is_redirect' => 0,

            'page_namespace' => NS_MEDIAWIKI,

        ];

        if ( count( $mostused ) ) {

            $conds['page_title'] = $mostused;

        } elseif ( $code !== $wgLanguageCode ) {

            $conds[] = 'page_title' . $dbr->buildLike( $dbr->anyString(), '/', $code );

        } else {

            # Effectively disallows use of '/' character in NS_MEDIAWIKI for uses

            # other than language code.

            $conds[] = 'page_title NOT' .

                $dbr->buildLike( $dbr->anyString(), '/', $dbr->anyString() );

        }


        // Set the stubs for oversized software-defined messages in the main cache map

        $res = $dbr->select(

            'page',

            [ 'page_title', 'page_latest' ],

            array_merge( $conds, [ 'page_len > ' . intval( $wgMaxMsgCacheEntrySize ) ] ),

            __METHOD__ . "($code)-big"

        );

        foreach ( $res as $row ) {

            $name = $this->contLang->lcfirst( $row->page_title );

            // Include entries/stubs for all keys in $mostused in adaptive mode

            if ( $wgAdaptiveMessageCache || $this->isMainCacheable( $name, $overridable ) ) {

                $cache[$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;

        }


        // Set the text for small software-defined messages in the main cache map

        $res = $dbr->select(

            [ 'page', 'revision', 'text' ],

            [ 'page_title', 'page_latest', 'old_id', 'old_text', 'old_flags' ],

            array_merge( $conds, [ 'page_len <= ' . intval( $wgMaxMsgCacheEntrySize ) ] ),

            __METHOD__ . "($code)-small",

            [],

            [

                'revision' => [ 'JOIN', 'page_latest=rev_id' ],

                'text' => [ 'JOIN', 'rev_text_id=old_id' ],

            ]

        );

        foreach ( $res as $row ) {

            $name = $this->contLang->lcfirst( $row->page_title );

            // Include entries/stubs for all keys in $mostused in adaptive mode

            if ( $wgAdaptiveMessageCache || $this->isMainCacheable( $name, $overridable ) ) {

                $text = Revision::getRevisionText( $row );

                if ( $text === false ) {

                    // Failed to fetch data; possible ES errors?

                    // Store a marker to fetch on-demand as a workaround...

                    // TODO Use a differnt marker

                    $entry = '!TOO BIG';

                    wfDebugLog(

                        'MessageCache',

                        __METHOD__

                        . ": failed to load message page text for {$row->page_title} ($code)"

                    );

                } else {

                    $entry = ' ' . $text;

                }

                $cache[$row->page_title] = $entry;

            } else {

                // T193271: cache object gets too big and slow to generate.

                // At least include revision ID so 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() + $this->mExpiry );

        unset( $cache['EXCESSIVE'] ); // only needed for hash


        return $cache;

    }


    private function isMainCacheable( $name, array $overridable ) {

        // Include common conversion table pages. This also avoids problems with

        // Installer::parse() bailing out due to disallowed DB queries (T207979).

        return ( isset( $overridable[$name] ) || strpos( $name, 'conversiontable/' ) === 0 );

    }


    public function replace( $title, $text ) {

        global $wgLanguageCode;


        if ( $this->mDisable ) {

            return;

        }


        list( $msg, $code ) = $this->figureMessage( $title );

        if ( strpos( $title, '/' ) !== false && $code === $wgLanguageCode ) {

            // 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

        );

    }


    public function refreshAndReplaceInternal( $code, array $replacements ) {

        global $wgMaxMsgCacheEntrySize;


        // Allow one caller at a time to avoid race conditions

        $scopedLock = $this->getReentrantScopedLock(

            $this->clusterCache->makeKey( 'messages', $code )

        );

        if ( !$scopedLock ) {

            foreach ( $replacements as list( $title ) ) {

                LoggerFactory::getInstance( 'MessageCache' )->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()

        foreach ( $replacements as list( $title ) ) {

            $page = WikiPage::factory( Title::makeTitle( NS_MEDIAWIKI, $title ) );

            $page->loadPageData( $page::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 ) > $wgMaxMsgCacheEntrySize ) {

                $cache[$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],

                $this->mExpiry

            );

        }

        // 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

        // blacklist 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

        $resourceloader = RequestContext::getMain()->getOutput()->getResourceLoader();

        $blobStore = $resourceloader->getMessageBlobStore();

        foreach ( $replacements as list( $title, $msg ) ) {

            $blobStore->updateMessage( $this->contLang->lcfirst( $msg ) );

            Hooks::run( 'MessageCacheReplace', [ $title, $newTextByTitle[$title] ] );

        }

    }


    protected function isCacheExpired( $cache ) {

        if ( !isset( $cache['VERSION'] ) || !isset( $cache['EXPIRY'] ) ) {

            return true;

        }

        if ( $cache['VERSION'] != MSG_CACHE_VERSION ) {

            return true;

        }

        if ( wfTimestampNow() >= $cache['EXPIRY'] ) {

            return true;

        }


        return false;

    }


    protected 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;

    }


    protected 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.

                // Also, it is unlikely that the current datacenter is *now* secondary one.

                $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 ];

    }


    protected 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

        );

    }


    protected function getReentrantScopedLock( $key, $timeout = self::WAIT_SEC ) {

        return $this->clusterCache->getScopedLock( $key, $timeout, self::LOCK_TTL, __METHOD__ );

    }


    function get( $key, $useDB = true, $langcode = true ) {

        if ( is_int( $key ) ) {

            // Fix numerical strings that somehow become ints

            // on their way here

            $key = (string)$key;

        } elseif ( !is_string( $key ) ) {

            throw new MWException( 'Non-string key given' );

        } elseif ( $key === '' ) {

            // Shortcut: the empty key is always missing

            return false;

        }


        // Normalise title-case input (with some inlining)

        $lckey = self::normalizeKey( $key );


        Hooks::run( 'MessageCache::get', [ &$lckey ] );


        // Loop through each language in the fallback list until we find something useful

        $message = $this->getMessageFromFallbackChain(

            wfGetLangObj( $langcode ),

            $lckey,

            !$this->mDisable && $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 = Language::getMessageFor( $parts[0], $parts[1] );

                if ( $message === null ) {

                    $message = 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;

    }


    protected 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;

    }


    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;

        }


        // Check the CDB cache

        $message = $lang->getMessage( $lckey );

        if ( $message !== null ) {

            return $message;

        }


        // Try checking the database for all of the fallback languages

        if ( $useDB ) {

            $fallbackChain = Language::getFallbacksFor( $langcode );


            foreach ( $fallbackChain as $code ) {

                if ( isset( $alreadyTried[$code] ) ) {

                    continue;

                }


                $message = $this->getMsgFromNamespace(

                    $this->getMessagePageName( $code, $uckey ), $code );


                if ( $message !== false ) {

                    return $message;

                }

                $alreadyTried[$code] = true;

            }

        }


        return false;

    }


    private function getMessagePageName( $langcode, $uckey ) {

        global $wgLanguageCode;


        if ( $langcode === $wgLanguageCode ) {

            // Messages created in the content language will not have the /lang extension

            return $uckey;

        } else {

            return "$uckey/$langcode";

        }

    }


    public function getMsgFromNamespace( $title, $code ) {

        // Load all MediaWiki page definitions into cache. Note that individual keys

        // already loaded into 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'

                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

            $name = $this->contLang->lcfirst( $title );

            if ( !$this->isMainCacheable( $name, $this->overridable ) ) {

                // 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;

                Hooks::run( 'MessagesPreLoad', [ $title, &$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

                LoggerFactory::getInstance( 'MessageCache' )->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;

    }


    private function loadCachedMessagePageEntry( $dbKey, $code, $hash ) {

        $fname = __METHOD__;

        return $this->srvCache->getWithSetCallback(

            $this->srvCache->makeKey( 'messages-big', $hash, $dbKey ),

            IExpiringStore::TTL_MINUTE,

            function () use ( $code, $dbKey, $hash, $fname ) {

                return $this->wanCache->getWithSetCallback(

                    $this->bigMessageCacheKey( $hash, $dbKey ),

                    $this->mExpiry,

                    function ( $oldValue, &$ttl, &$setOpts ) use ( $dbKey, $code, $fname ) {

                        // Try loading the message from the database

                        $dbr = wfGetDB( DB_REPLICA );

                        $setOpts += Database::getCacheSetOptions( $dbr );

                        // Use newKnownCurrent() to avoid querying revision/user tables

                        $title = Title::makeTitle( NS_MEDIAWIKI, $dbKey );

                        $revision = Revision::newKnownCurrent( $dbr, $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();

                        if ( $content ) {

                            $message = $this->getMessageTextFromContent( $content );

                        } else {

                            LoggerFactory::getInstance( 'MessageCache' )->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;

                    }

                );

            }

        );

    }


    public function transform( $message, $interface = false, $language = null, $title = null ) {

        // Avoid creating parser if nothing to transform

        if ( strpos( $message, '{{' ) === false ) {

            return $message;

        }


        if ( $this->mInParser ) {

            return $message;

        }


        $parser = $this->getParser();

        if ( $parser ) {

            $popts = $this->getParserOptions();

            $popts->setInterfaceMessage( $interface );

            $popts->setTargetLanguage( $language );


            $userlang = $popts->setUserLang( $language );

            $this->mInParser = true;

            $message = $parser->transformMsg( $message, $popts, $title );

            $this->mInParser = false;

            $popts->setUserLang( $userlang );

        }


        return $message;

    }


    public function getParser() {

        global $wgParser, $wgParserConf;


        if ( !$this->mParser && isset( $wgParser ) ) {

            # Do some initialisation so that we don't have to do it twice

            $wgParser->firstCallInit();

            # Clone it and store it

            $class = $wgParserConf['class'];

            if ( $class == ParserDiffTest::class ) {

                # Uncloneable

                $this->mParser = new $class( $wgParserConf );

            } else {

                $this->mParser = clone $wgParser;

            }

        }


        return $this->mParser;

    }


    public function parse( $text, $title = null, $linestart = true,

        $interface = false, $language = null

    ) {

        global $wgTitle;


        if ( $this->mInParser ) {

            return htmlspecialchars( $text );

        }


        $parser = $this->getParser();

        $popts = $this->getParserOptions();

        $popts->setInterfaceMessage( $interface );


        if ( is_string( $language ) ) {

            $language = Language::factory( $language );

        }

        $popts->setTargetLanguage( $language );


        if ( !$title || !$title instanceof Title ) {

            wfDebugLog( 'GlobalTitleFail', __METHOD__ . ' called by ' .

                wfGetAllCallers( 6 ) . ' with no title set.' );

            $title = $wgTitle;

        }

        // Sometimes $wgTitle isn't set either...

        if ( !$title ) {

            # It's not uncommon having a null $wgTitle in scripts. See r80898

            # Create a ghost title in such case

            $title = Title::makeTitle( NS_SPECIAL, 'Badtitle/title not set in ' . __METHOD__ );

        }


        $this->mInParser = true;

        $res = $parser->parse( $text, $title, $popts, $linestart );

        $this->mInParser = false;


        return $res;

    }


    public function disable() {

        $this->mDisable = true;

    }


    public function enable() {

        $this->mDisable = false;

    }


    public function isDisabled() {

        return $this->mDisable;

    }


    public function clear() {

        $langs = Language::fetchLanguageNames( null, 'mw' );

        foreach ( array_keys( $langs ) as $code ) {

            $this->wanCache->touchCheckKey( $this->getCheckKey( $code ) );

        }

        $this->cache->clear();

        $this->loadedLanguages = [];

    }


    public function figureMessage( $key ) {

        global $wgLanguageCode;


        $pieces = explode( '/', $key );

        if ( count( $pieces ) < 2 ) {

            return [ $key, $wgLanguageCode ];

        }


        $lang = array_pop( $pieces );

        if ( !Language::fetchLanguageName( $lang, null, 'mw' ) ) {

            return [ $key, $wgLanguageCode ];

        }


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


        return [ $message, $lang ];

    }


    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 ) );

    }


    public function updateMessageOverride( Title $title, Content $content = null ) {

        $msgText = $this->getMessageTextFromContent( $content );

        if ( $msgText === null ) {

            $msgText = false; // treat as not existing

        }


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


        if ( $this->contLang->hasVariants() ) {

            $this->contLang->updateConversionTable( $title );

        }

    }


    public function getCheckKey( $code ) {

        return $this->wanCache->makeKey( 'messages', $code );

    }


    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;

                LoggerFactory::getInstance( 'MessageCache' )->warning(

                    __METHOD__ . ": message content doesn't provide wikitext "

                    . "(content model: " . $content->getModel() . ")" );

            }

        } else {

            // Message page does not exist...

            $msgText = false;

        }


        return $msgText;

    }


    private function bigMessageCacheKey( $hash, $title ) {

        return $this->wanCache->makeKey( 'messages-big', $hash, $title );

    }

}