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[mediawiki.git] / includes / cache / MessageCache.php

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

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

/**
 * Message cache
 * Performs various MediaWiki namespace-related functions
 * @ingroup Cache
 */
class MessageCache {
        const FOR_UPDATE = 1; // force message reload

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

        /**
         * Process local cache of loaded messages that are defined in
         * MediaWiki namespace. First array level is a language code,
         * second level is message key and the values are either message
         * content prefixed with space, or !NONEXISTENT for negative
         * caching.
         * @var array $mCache
         */
        protected $mCache;

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

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

        /**
         * Lifetime for cache, used by object caching.
         * Set on construction, see __construct().
         */
        protected $mExpiry;

        /**
         * Message cache has its own parser which it uses to transform messages
         * @var ParserOptions
         */
        protected $mParserOptions;
        /** @var Parser */
        protected $mParser;

        /**
         * Variable for tracking which variables are already loaded
         * @var array $mLoadedLanguages
         */
        protected $mLoadedLanguages = [];

        /**
         * @var bool $mInParser
         */
        protected $mInParser = false;

        /** @var BagOStuff */
        protected $mMemc;
        /** @var WANObjectCache */
        protected $wanCache;

        /**
         * Singleton instance
         *
         * @var MessageCache $instance
         */
        private static $instance;

        /**
         * Get the signleton instance of this class
         *
         * @since 1.18
         * @return MessageCache
         */
        public static function singleton() {
                if ( self::$instance === null ) {
                        global $wgUseDatabaseMessages, $wgMsgCacheExpiry;
                        self::$instance = new self(
                                wfGetMessageCacheStorage(),
                                $wgUseDatabaseMessages,
                                $wgMsgCacheExpiry
                        );
                }

                return self::$instance;
        }

        /**
         * Destroy the singleton instance
         *
         * @since 1.18
         */
        public static function destroyInstance() {
                self::$instance = null;
        }

        /**
         * Normalize message key input
         *
         * @param string $key Input message key to be normalized
         * @return string Normalized message key
         */
        public static function normalizeKey( $key ) {
                global $wgContLang;

                $lckey = strtr( $key, ' ', '_' );
                if ( ord( $lckey ) < 128 ) {
                        $lckey[0] = strtolower( $lckey[0] );
                } else {
                        $lckey = $wgContLang->lcfirst( $lckey );
                }

                return $lckey;
        }

        /**
         * @param BagOStuff $memCached A cache instance. If none, fall back to CACHE_NONE.
         * @param bool $useDB
         * @param int $expiry Lifetime for cache. @see $mExpiry.
         */
        function __construct( BagOStuff $memCached, $useDB, $expiry ) {
                global $wgUseLocalMessageCache;

                $this->mMemc = $memCached;
                $this->mDisable = !$useDB;
                $this->mExpiry = $expiry;

                if ( $wgUseLocalMessageCache ) {
                        $this->localCache = MediaWikiServices::getInstance()->getLocalServerObjectCache();
                } else {
                        $this->localCache = new EmptyBagOStuff();
                }

                $this->wanCache = ObjectCache::getMainWANInstance();
        }

        /**
         * ParserOptions is lazy initialised.
         *
         * @return ParserOptions
         */
        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->setEditSection( false );
                                return $po;
                        }

                        $this->mParserOptions = new ParserOptions;
                        $this->mParserOptions->setEditSection( false );
                }

                return $this->mParserOptions;
        }

        /**
         * Try to load the cache from APC.
         *
         * @param string $code Optional language code, see documenation of load().
         * @return array|bool The cache array, or false if not in cache.
         */
        protected function getLocalCache( $code ) {
                $cacheKey = wfMemcKey( __CLASS__, $code );

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

        /**
         * Save the cache to APC.
         *
         * @param string $code
         * @param array $cache The cache array
         */
        protected function saveToLocalCache( $code, $cache ) {
                $cacheKey = wfMemcKey( __CLASS__, $code );
                $this->localCache->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 succesfully 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 succesfully populated,
         * or false if populating empty cache fails. Also returns true if MessageCache
         * is disabled.
         *
         * @param string $code Language to which load messages
         * @param integer $mode Use MessageCache::FOR_UPDATE to skip process cache [optional]
         * @throws MWException
         * @return bool
         */
        protected function load( $code, $mode = null ) {
                if ( !is_string( $code ) ) {
                        throw new InvalidArgumentException( "Missing language code" );
                }

                # Don't do double loading...
                if ( isset( $this->mLoadedLanguages[$code] ) && $mode != self::FOR_UPDATE ) {
                        return true;
                }

                # 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 );
                $this->mCacheVolatile[$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';
                        $success = true;
                        $this->mCache[$code] = $cache;
                }

                if ( !$success ) {
                        $cacheKey = wfMemcKey( 'messages', $code ); # Key in memc for messages
                        # 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->mMemc->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 renerates the cache while others use old values.
                                                $where[] = 'global cache is expired/volatile';
                                                $staleCache = $cache;
                                        } else {
                                                $where[] = 'got from global cache';
                                                $this->mCache[$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';
                                        $this->mCache[$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 yeild 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->mCache = false;
                        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
                        $this->mLoadedLanguages[$code] = true;
                }

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

                return $success;
        }

        /**
         * @param string $code
         * @param array $where List of wfDebug() comments
         * @param integer $mode Use MessageCache::FOR_UPDATE to use DB_MASTER
         * @return bool|string True on success or one of ("cantacquire", "disabled")
         */
        protected function loadFromDBWithLock( $code, array &$where, $mode = null ) {
                global $wgUseLocalMessageCache;

                # If cache updates on all levels fail, give up on message overrides.
                # This is to avoid easy site outages; see $saveSuccess comments below.
                $statusKey = wfMemcKey( 'messages', $code, 'status' );
                $status = $this->mMemc->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 = wfMemcKey( 'messages', $code );
                $scopedLock = $this->getReentrantScopedLock( $cacheKey, 0 );
                if ( !$scopedLock ) {
                        $where[] = 'could not acquire main lock';
                        return 'cantacquire';
                }

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

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

                return true;
        }

        /**
         * 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 integer $mode Use MessageCache::FOR_UPDATE to skip process cache
         * @return array Loaded messages for storing in caches
         */
        function loadFromDB( $code, $mode = null ) {
                global $wgMaxMsgCacheEntrySize, $wgLanguageCode, $wgAdaptiveMessageCache;

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

                $cache = [];

                # Common conditions
                $conds = [
                        'page_is_redirect' => 0,
                        'page_namespace' => NS_MEDIAWIKI,
                ];

                $mostused = [];
                if ( $wgAdaptiveMessageCache && $code !== $wgLanguageCode ) {
                        if ( !isset( $this->mCache[$wgLanguageCode] ) ) {
                                $this->load( $wgLanguageCode );
                        }
                        $mostused = array_keys( $this->mCache[$wgLanguageCode] );
                        foreach ( $mostused as $key => $value ) {
                                $mostused[$key] = "$value/$code";
                        }
                }

                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() );
                }

                # Conditions to fetch oversized pages to ignore them
                $bigConds = $conds;
                $bigConds[] = 'page_len > ' . intval( $wgMaxMsgCacheEntrySize );

                # Load titles for all oversized pages in the MediaWiki namespace
                $res = $dbr->select(
                        'page',
                        [ 'page_title', 'page_latest' ],
                        $bigConds,
                        __METHOD__ . "($code)-big"
                );
                foreach ( $res as $row ) {
                        $cache[$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;
                }

                # Conditions to load the remaining pages with their contents
                $smallConds = $conds;
                $smallConds[] = 'page_latest=rev_id';
                $smallConds[] = 'rev_text_id=old_id';
                $smallConds[] = 'page_len <= ' . intval( $wgMaxMsgCacheEntrySize );

                $res = $dbr->select(
                        [ 'page', 'revision', 'text' ],
                        [ 'page_title', 'old_text', 'old_flags' ],
                        $smallConds,
                        __METHOD__ . "($code)-small"
                );

                foreach ( $res as $row ) {
                        $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;
                }

                $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 );

                return $cache;
        }

        /**
         * Updates cache as necessary when message page is changed
         *
         * @param string|bool $title Name of the page changed (false if deleted)
         * @param string|bool $text New contents of the page (false if deleted)
         */
        public function replace( $title, $text ) {
                global $wgMaxMsgCacheEntrySize, $wgContLang, $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;
                }

                // Note that if the cache is volatile, load() may trigger a DB fetch.
                // In that case we reenter/reuse the existing cache key lock to avoid
                // a self-deadlock. This is safe as no reads happen *directly* in this
                // method between getReentrantScopedLock() and load() below. There is
                // no risk of data "changing under our feet" for replace().
                $scopedLock = $this->getReentrantScopedLock( wfMemcKey( 'messages', $code ) );
                // Load the messages from the master DB to avoid race conditions
                $this->load( $code, self::FOR_UPDATE );

                // Load the new value into the process cache...
                if ( $text === false ) {
                        $this->mCache[$code][$title] = '!NONEXISTENT';
                } elseif ( strlen( $text ) > $wgMaxMsgCacheEntrySize ) {
                        $this->mCache[$code][$title] = '!TOO BIG';
                        // Pre-fill the individual key cache with the known latest message text
                        $key = $this->wanCache->makeKey( 'messages-big', $this->mCache[$code]['HASH'], $title );
                        $this->wanCache->set( $key, " $text", $this->mExpiry );
                } else {
                        $this->mCache[$code][$title] = ' ' . $text;
                }
                // Mark this cache as definitely being "latest" (non-volatile) so
                // load() calls do not try to refresh the cache with replica DB data
                $this->mCache[$code]['LATEST'] = time();

                // Update caches if the lock was acquired
                if ( $scopedLock ) {
                        $this->saveToCaches( $this->mCache[$code], 'all', $code );
                } else {
                        LoggerFactory::getInstance( 'MessageCache' )->error(
                                __METHOD__ . ': could not acquire lock to update {title} ({code})',
                                [ 'title' => $title, 'code' => $code ] );
                }

                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( wfMemcKey( 'messages', $code ) );

                // Also delete cached sidebar... just in case it is affected
                $codes = [ $code ];
                if ( $code === 'en' ) {
                        // Delete all sidebars, like for example on action=purge on the
                        // sidebar messages
                        $codes = array_keys( Language::fetchLanguageNames() );
                }

                foreach ( $codes as $code ) {
                        $sidebarKey = wfMemcKey( 'sidebar', $code );
                        $this->wanCache->delete( $sidebarKey );
                }

                // Update the message in the message blob store
                $resourceloader = RequestContext::getMain()->getOutput()->getResourceLoader();
                $blobStore = $resourceloader->getMessageBlobStore();
                $blobStore->updateMessage( $wgContLang->lcfirst( $msg ) );

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

        /**
         * Is the given cache array expired due to time passing or a version change?
         *
         * @param array $cache
         * @return bool
         */
        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;
        }

        /**
         * 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|bool $code Language code (default: false)
         * @return bool
         */
        protected function saveToCaches( array $cache, $dest, $code = false ) {
                if ( $dest === 'all' ) {
                        $cacheKey = wfMemcKey( 'messages', $code );
                        $success = $this->mMemc->set( $cacheKey, $cache );
                        $this->setValidationHash( $code, $cache );
                } else {
                        $success = true;
                }

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

                return $success;
        }

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

        /**
         * Set the md5 used to validate the local disk 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
         */
        protected function setValidationHash( $code, array $cache ) {
                $this->wanCache->set(
                        $this->wanCache->makeKey( 'messages', $code, 'hash', 'v1' ),
                        [
                                'hash' => $cache['HASH'],
                                'latest' => isset( $cache['LATEST'] ) ? $cache['LATEST'] : 0
                        ],
                        WANObjectCache::TTL_INDEFINITE
                );
        }

        /**
         * @param string $key A language message cache key that stores blobs
         * @param integer $timeout Wait timeout in seconds
         * @return null|ScopedCallback
         */
        protected function getReentrantScopedLock( $key, $timeout = self::WAIT_SEC ) {
                return $this->mMemc->getScopedLock( $key, $timeout, self::LOCK_TTL, __METHOD__ );
        }

        /**
         * 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|object $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 bool $isFullKey Specifies whether $key is a two part key "msg/lang".
         *
         * @throws MWException When given an invalid key
         * @return string|bool False if the message doesn't exist, otherwise the
         *   message (which can be empty)
         */
        function get( $key, $useDB = true, $langcode = true, $isFullKey = false ) {
                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;
                }

                // For full keys, get the language code from the key
                $pos = strrpos( $key, '/' );
                if ( $isFullKey && $pos !== false ) {
                        $langcode = substr( $key, $pos + 1 );
                        $key = substr( $key, 0, $pos );
                }

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

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

                // Loop through each language in the fallback list until we find something useful
                $lang = wfGetLangObj( $langcode );
                $message = $this->getMessageFromFallbackChain(
                        $lang,
                        $lckey,
                        !$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;'
                                ],
                                [
                                        ' ',
                                        "\xc2\xa0",
                                        "\xc2\xa0",
                                        "\xc2\xad"
                                ],
                                $message
                        );
                }

                return $message;
        }

        /**
         * 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|bool The message, or false if not found
         */
        protected function getMessageFromFallbackChain( $lang, $lckey, $useDB ) {
                global $wgContLang;

                $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( $wgContLang, $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|bool The message, or false if not found
         */
        private function getMessageForLang( $lang, $lckey, $useDB, &$alreadyTried ) {
                global $wgContLang;

                $langcode = $lang->getCode();

                // Try checking the database for the requested language
                if ( $useDB ) {
                        $uckey = $wgContLang->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;
        }

        /**
         * 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 ) {
                global $wgLanguageCode;

                if ( $langcode === $wgLanguageCode ) {
                        // 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 initial uppercase letter.
         * @param string $code Code denoting the language to try.
         * @return string|bool The message, or false if it does not exist or on error
         */
        public function getMsgFromNamespace( $title, $code ) {
                $this->load( $code );
                if ( isset( $this->mCache[$code][$title] ) ) {
                        $entry = $this->mCache[$code][$title];
                        if ( substr( $entry, 0, 1 ) === ' ' ) {
                                // The message exists, so make sure a string is returned.
                                return (string)substr( $entry, 1 );
                        } elseif ( $entry === '!NONEXISTENT' ) {
                                return false;
                        } elseif ( $entry === '!TOO BIG' ) {
                                // Fall through and try invididual message cache below
                        }
                } else {
                        // XXX: This is not cached in process cache, should it?
                        $message = false;
                        Hooks::run( 'MessagesPreLoad', [ $title, &$message ] );
                        if ( $message !== false ) {
                                return $message;
                        }

                        return false;
                }

                // Try the individual message cache
                $titleKey = $this->wanCache->makeKey( 'messages-big', $this->mCache[$code]['HASH'], $title );

                if ( $this->mCacheVolatile[$code] ) {
                        $entry = false;
                        // Make sure that individual keys respect the WAN cache holdoff period too
                        LoggerFactory::getInstance( 'MessageCache' )->debug(
                                __METHOD__ . ': loading volatile key \'{titleKey}\'',
                                [ 'titleKey' => $titleKey, 'code' => $code ] );
                } else {
                        $entry = $this->wanCache->get( $titleKey );
                }

                if ( $entry !== false ) {
                        if ( substr( $entry, 0, 1 ) === ' ' ) {
                                $this->mCache[$code][$title] = $entry;
                                // The message exists, so make sure a string is returned
                                return (string)substr( $entry, 1 );
                        } elseif ( $entry === '!NONEXISTENT' ) {
                                $this->mCache[$code][$title] = '!NONEXISTENT';

                                return false;
                        } else {
                                // Corrupt/obsolete entry, delete it
                                $this->wanCache->delete( $titleKey );
                        }
                }

                // Try loading the message from the database
                $dbr = wfGetDB( DB_REPLICA );
                $cacheOpts = Database::getCacheSetOptions( $dbr );
                // Use newKnownCurrent() to avoid querying revision/user tables
                $titleObj = Title::makeTitle( NS_MEDIAWIKI, $title );
                if ( $titleObj->getLatestRevID() ) {
                        $revision = Revision::newKnownCurrent(
                                $dbr,
                                $titleObj->getArticleID(),
                                $titleObj->getLatestRevID()
                        );
                } else {
                        $revision = false;
                }

                if ( $revision ) {
                        $content = $revision->getContent();
                        if ( $content ) {
                                $message = $this->getMessageTextFromContent( $content );
                                if ( is_string( $message ) ) {
                                        $this->mCache[$code][$title] = ' ' . $message;
                                        $this->wanCache->set( $titleKey, ' ' . $message, $this->mExpiry, $cacheOpts );
                                }
                        } else {
                                // A possibly temporary loading failure
                                LoggerFactory::getInstance( 'MessageCache' )->warning(
                                        __METHOD__ . ': failed to load message page text for \'{titleKey}\'',
                                        [ 'titleKey' => $titleKey, 'code' => $code ] );
                                $message = null; // no negative caching
                        }
                } else {
                        $message = false; // negative caching
                }

                if ( $message === false ) { // negative caching
                        $this->mCache[$code][$title] = '!NONEXISTENT';
                        $this->wanCache->set( $titleKey, '!NONEXISTENT', $this->mExpiry, $cacheOpts );
                }

                return $message;
        }

        /**
         * @param string $message
         * @param bool $interface
         * @param string $language Language code
         * @param Title $title
         * @return string
         */
        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;
        }

        /**
         * @return Parser
         */
        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' ) {
                                # Uncloneable
                                $this->mParser = new $class( $wgParserConf );
                        } else {
                                $this->mParser = clone $wgParser;
                        }
                }

                return $this->mParser;
        }

        /**
         * @param string $text
         * @param Title $title
         * @param bool $linestart Whether or not this is at the start of a line
         * @param bool $interface Whether this is an interface message
         * @param Language|string $language Language code
         * @return ParserOutput|string
         */
        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;
        }

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

        function enable() {
                $this->mDisable = 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->mDisable;
        }

        /**
         * Clear all stored messages. Mainly used after a mass rebuild.
         */
        function clear() {
                $langs = Language::fetchLanguageNames( null, 'mw' );
                foreach ( array_keys( $langs ) as $code ) {
                        # Global and local caches
                        $this->wanCache->touchCheckKey( wfMemcKey( 'messages', $code ) );
                }

                $this->mLoadedLanguages = [];
        }

        /**
         * @param string $key
         * @return array
         */
        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 ];
        }

        /**
         * 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 array Array of message keys (strings)
         */
        public function getAllMessageKeys( $code ) {
                global $wgContLang;

                $this->load( $code );
                if ( !isset( $this->mCache[$code] ) ) {
                        // Apparently load() failed
                        return null;
                }
                // Remove administrative keys
                $cache = $this->mCache[$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( [ $wgContLang, 'lcfirst' ], array_keys( $cache ) );
        }

        /**
         * Purge message caches when a MediaWiki: page is created, updated, or deleted
         *
         * @param Title $title Message page title
         * @param Content|null $content New content for edit/create, null on deletion
         * @since 1.29
         */
        public function updateMessageOverride( Title $title, Content $content = null ) {
                global $wgContLang;

                $msgText = $this->getMessageTextFromContent( $content );
                if ( $msgText === null ) {
                        $msgText = false; // treat as not existing
                }

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

                if ( $wgContLang->hasVariants() ) {
                        $wgContLang->updateConversionTable( $title );
                }
        }

        /**
         * @param Content|null $content Content or null if the message page does not exist
         * @return string|bool|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;
                                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;
        }
}