Merge "Removed unused and poorly supported time argument to BagOStuff::delete"

[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
 */

/**
 * MediaWiki message cache structure version.
 * Bump this whenever the message cache format has changed.
 */
define( 'MSG_CACHE_VERSION', 1 );

/**
 * Memcached timeout when loading a key.
 * See MessageCache::load()
 */
define( 'MSG_LOAD_TIMEOUT', 60 );

/**
 * Memcached timeout when locking a key for a writing operation.
 * See MessageCache::lock()
 */
define( 'MSG_LOCK_TIMEOUT', 30 );
/**
 * Number of times we will try to acquire a lock from Memcached.
 * This comes in addition to MSG_LOCK_TIMEOUT.
 */
define( 'MSG_WAIT_TIMEOUT', 30 );

/**
 * Message cache
 * Performs various MediaWiki namespace-related functions
 * @ingroup Cache
 */
class MessageCache {
        /**
         * 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.
         */
        protected $mCache;

        /**
         * 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.
         */
        protected $mParserOptions, $mParser;

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

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

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

        /**
         * Get the signleton instance of this class
         *
         * @since 1.18
         * @return MessageCache
         */
        public static function singleton() {
                if ( is_null( self::$instance ) ) {
                        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;
        }

        /**
         * @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( $memCached, $useDB, $expiry ) {
                if ( !$memCached ) {
                        $memCached = wfGetCache( CACHE_NONE );
                }

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

        /**
         * ParserOptions is lazy initialised.
         *
         * @return ParserOptions
         */
        function getParserOptions() {
                if ( !$this->mParserOptions ) {
                        $this->mParserOptions = new ParserOptions;
                        $this->mParserOptions->setEditSection( false );
                }

                return $this->mParserOptions;
        }

        /**
         * Try to load the cache from a local file.
         *
         * @param string $hash The hash of contents, to check validity.
         * @param string $code Optional language code, see documenation of load().
         * @return array The cache array
         */
        function getLocalCache( $hash, $code ) {
                global $wgCacheDirectory;

                $filename = "$wgCacheDirectory/messages-" . wfWikiID() . "-$code";

                # Check file existence
                wfSuppressWarnings();
                $file = fopen( $filename, 'r' );
                wfRestoreWarnings();
                if ( !$file ) {
                        return false; // No cache file
                }

                // Check to see if the file has the hash specified
                $localHash = fread( $file, 32 );
                if ( $hash === $localHash ) {
                        // All good, get the rest of it
                        $serialized = '';
                        while ( !feof( $file ) ) {
                                $serialized .= fread( $file, 100000 );
                        }
                        fclose( $file );

                        return unserialize( $serialized );
                } else {
                        fclose( $file );

                        return false; // Wrong hash
                }
        }

        /**
         * Save the cache to a local file.
         * @param string $serialized
         * @param string $hash
         * @param string $code
         */
        function saveToLocal( $serialized, $hash, $code ) {
                global $wgCacheDirectory;

                $filename = "$wgCacheDirectory/messages-" . wfWikiID() . "-$code";
                wfMkdirParents( $wgCacheDirectory, null, __METHOD__ ); // might fail

                wfSuppressWarnings();
                $file = fopen( $filename, 'w' );
                wfRestoreWarnings();

                if ( !$file ) {
                        wfDebug( "Unable to open local cache file for writing\n" );

                        return;
                }

                fwrite( $file, $hash . $serialized );
                fclose( $file );
                wfSuppressWarnings();
                chmod( $filename, 0666 );
                wfRestoreWarnings();
        }

        /**
         * 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 bool|string $code Language to which load messages
         * @throws MWException
         * @return bool
         */
        function load( $code = false ) {
                global $wgUseLocalMessageCache;

                if ( !is_string( $code ) ) {
                        # This isn't really nice, so at least make a note about it and try to
                        # fall back
                        wfDebug( __METHOD__ . " called without providing a language code\n" );
                        $code = 'en';
                }

                # Don't do double loading...
                if ( isset( $this->mLoadedLanguages[$code] ) ) {
                        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 = array(); # Debug info, delayed to avoid spamming debug log too much
                $cacheKey = wfMemcKey( 'messages', $code ); # Key in memc for messages

                # Local cache
                # Hash of the contents is stored in memcache, to detect if local cache goes
                # out of date (e.g. due to replace() on some other server)
                if ( $wgUseLocalMessageCache ) {

                        $hash = $this->mMemc->get( wfMemcKey( 'messages', $code, 'hash' ) );
                        if ( $hash ) {
                                $cache = $this->getLocalCache( $hash, $code );
                                if ( !$cache ) {
                                        $where[] = 'local cache is empty or has the wrong hash';
                                } elseif ( $this->isCacheExpired( $cache ) ) {
                                        $where[] = 'local cache is expired';
                                        $staleCache = $cache;
                                } else {
                                        $where[] = 'got from local cache';
                                        $success = true;
                                        $this->mCache[$code] = $cache;
                                }
                        }
                }

                if ( !$success ) {
                        # 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 < 2; $failedAttempts++ ) {
                                $cache = $this->mMemc->get( $cacheKey );
                                if ( !$cache ) {
                                        $where[] = 'global cache is empty';
                                } elseif ( $this->isCacheExpired( $cache ) ) {
                                        $where[] = 'global cache is expired';
                                        $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 a single
                                # process. This prevents the site from going down when the cache
                                # expires.
                                $statusKey = wfMemcKey( 'messages', $code, 'status' );
                                $acquired = $this->mMemc->add( $statusKey, 'loading', MSG_LOAD_TIMEOUT );
                                if ( $acquired ) {
                                        # Unlock the status key if there is an exception
                                        $that = $this;
                                        $statusUnlocker = new ScopedCallback( function () use ( $that, $statusKey ) {
                                                $that->mMemc->delete( $statusKey );
                                        } );

                                        # Now let's regenerate
                                        $where[] = 'loading from database';

                                        # Lock the cache to prevent conflicting writes
                                        # If this lock fails, it doesn't really matter, it just means the
                                        # write is potentially non-atomic, e.g. the results of a replace()
                                        # may be discarded.
                                        if ( $this->lock( $cacheKey ) ) {
                                                $mainUnlocker = new ScopedCallback( function () use ( $that, $cacheKey ) {
                                                        $that->unlock( $cacheKey );
                                                } );
                                        } else {
                                                $mainUnlocker = null;
                                                $where[] = 'could not acquire main lock';
                                        }

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

                                        # Unlock
                                        ScopedCallback::consume( $mainUnlocker );
                                        ScopedCallback::consume( $statusUnlocker );

                                        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";
                                                }
                                        }

                                        # Load from DB complete, no need to retry
                                        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 retried once, still failed, so don't do another lock/unlock cycle
                                        # This case will typically be hit if memcached is down, or if
                                        # loadFromDB() takes longer than MSG_WAIT_TIMEOUT
                                        $where[] = "could not acquire status key.";
                                        break;
                                } else {
                                        $status = $this->mMemc->get( $statusKey );
                                        if ( $status === 'error' ) {
                                                # Disable cache
                                                break;
                                        } else {
                                                # Wait for the other thread to finish, then retry
                                                $where[] = 'waited for other thread to complete';
                                                $this->lock( $cacheKey );
                                                $this->unlock( $cacheKey );
                                        }
                                }
                        }
                }

                if ( !$success ) {
                        $where[] = 'loading FAILED - cache is disabled';
                        $this->mDisable = true;
                        $this->mCache = false;
                        # 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;
        }

        /**
         * 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.
         * @return array Loaded messages for storing in caches.
         */
        function loadFromDB( $code ) {
                global $wgMaxMsgCacheEntrySize, $wgLanguageCode, $wgAdaptiveMessageCache;
                $dbr = wfGetDB( DB_SLAVE );
                $cache = array();

                # Common conditions
                $conds = array(
                        'page_is_redirect' => 0,
                        'page_namespace' => NS_MEDIAWIKI,
                );

                $mostused = array();
                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', $bigConds, __METHOD__ . "($code)-big" );
                foreach ( $res as $row ) {
                        $cache[$row->page_title] = '!TOO BIG';
                }

                # 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(
                        array( 'page', 'revision', 'text' ),
                        array( '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...
                                $entry = '!TOO BIG';
                                wfDebugLog(
                                        'MessageCache',
                                        __METHOD__
                                                . ": failed to load message page text for {$row->page_title} ($code)"
                                );
                        } else {
                                $entry = ' ' . $text;
                        }
                        $cache[$row->page_title] = $entry;
                }

                $cache['VERSION'] = MSG_CACHE_VERSION;
                $cache['EXPIRY'] = wfTimestamp( TS_MW, time() + $this->mExpiry );

                return $cache;
        }

        /**
         * Updates cache as necessary when message page is changed
         *
         * @param string $title Name of the page changed.
         * @param mixed $text New contents of the page.
         */
        public function replace( $title, $text ) {
                global $wgMaxMsgCacheEntrySize;

                if ( $this->mDisable ) {

                        return;
                }

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

                $cacheKey = wfMemcKey( 'messages', $code );
                $this->load( $code );
                $this->lock( $cacheKey );

                $titleKey = wfMemcKey( 'messages', 'individual', $title );

                if ( $text === false ) {
                        # Article was deleted
                        $this->mCache[$code][$title] = '!NONEXISTENT';
                        $this->mMemc->delete( $titleKey );
                } elseif ( strlen( $text ) > $wgMaxMsgCacheEntrySize ) {
                        # Check for size
                        $this->mCache[$code][$title] = '!TOO BIG';
                        $this->mMemc->set( $titleKey, ' ' . $text, $this->mExpiry );
                } else {
                        $this->mCache[$code][$title] = ' ' . $text;
                        $this->mMemc->delete( $titleKey );
                }

                # Update caches
                $this->saveToCaches( $this->mCache[$code], 'all', $code );
                $this->unlock( $cacheKey );

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

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

                // Update the message in the message blob store
                global $wgContLang;
                MessageBlobStore::getInstance()->updateMessage( $wgContLang->lcfirst( $msg ) );

                Hooks::run( 'MessageCacheReplace', array( $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( $cache, $dest, $code = false ) {
                global $wgUseLocalMessageCache;

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

                if ( $dest === 'all' ) {
                        $success = $this->mMemc->set( $cacheKey, $cache );
                } else {
                        $success = true;
                }

                # Save to local cache
                if ( $wgUseLocalMessageCache ) {
                        $serialized = serialize( $cache );
                        $hash = md5( $serialized );
                        $this->mMemc->set( wfMemcKey( 'messages', $code, 'hash' ), $hash );
                        $this->saveToLocal( $serialized, $hash, $code );
                }

                return $success;
        }

        /**
         * Represents a write lock on the messages key.
         *
         * Will retry MessageCache::MSG_WAIT_TIMEOUT times, each operations having
         * a timeout of MessageCache::MSG_LOCK_TIMEOUT.
         *
         * @param string $key
         * @return bool Success
         */
        function lock( $key ) {
                $lockKey = $key . ':lock';
                $acquired = false;
                $testDone = false;
                for ( $i = 0; $i < MSG_WAIT_TIMEOUT && !$acquired; $i++ ) {
                        $acquired = $this->mMemc->add( $lockKey, 1, MSG_LOCK_TIMEOUT );
                        if ( $acquired ) {
                                break;
                        }

                        # Fail fast if memcached is totally down
                        if ( !$testDone ) {
                                $testDone = true;
                                if ( !$this->mMemc->set( wfMemcKey( 'test' ), 'test', 1 ) ) {
                                        break;
                                }
                        }
                        sleep( 1 );
                }

                return $acquired;
        }

        function unlock( $key ) {
                $lockKey = $key . ':lock';
                $this->mMemc->delete( $lockKey );
        }

        /**
         * 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 ) {
                global $wgContLang;

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

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

                if ( ord( $lckey ) < 128 ) {
                        $uckey = ucfirst( $lckey );
                } else {
                        $uckey = $wgContLang->ucfirst( $lckey );
                }

                // Loop through each language in the fallback list until we find something useful
                $lang = wfGetLangObj( $langcode );
                $message = $this->getMessageFromFallbackChain(
                        $lang,
                        $lckey,
                        $uckey,
                        !$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(
                                array(
                                        # Fix for trailing whitespace, removed by textarea
                                        '&#32;',
                                        # Fix for NBSP, converted to space by firefox
                                        '&nbsp;',
                                        '&#160;',
                                ),
                                array(
                                        ' ',
                                        "\xc2\xa0",
                                        "\xc2\xa0"
                                ),
                                $message
                        );
                }

                return $message;
        }

        /**
         * Given a language, try and fetch a message from that language, then the
         * fallbacks of that language, then the site language, then the fallbacks for the
         * site language.
         *
         * @param Language $lang Requested language
         * @param string $lckey Lowercase key for the message
         * @param string $uckey Uppercase key for the message
         * @param bool $useDB Whether to use the database
         *
         * @see MessageCache::get
         * @return string|bool The message, or false if not found
         */
        protected function getMessageFromFallbackChain( $lang, $lckey, $uckey, $useDB ) {
                global $wgLanguageCode, $wgContLang;

                $langcode = $lang->getCode();
                $message = false;

                // First try the requested language.
                if ( $useDB ) {
                        if ( $langcode === $wgLanguageCode ) {
                                // Messages created in the content language will not have the /lang extension
                                $message = $this->getMsgFromNamespace( $uckey, $langcode );
                        } else {
                                $message = $this->getMsgFromNamespace( "$uckey/$langcode", $langcode );
                        }
                }

                if ( $message !== false ) {
                        return $message;
                }

                // Check the CDB cache
                $message = $lang->getMessage( $lckey );
                if ( $message !== null ) {
                        return $message;
                }

                list( $fallbackChain, $siteFallbackChain ) =
                        Language::getFallbacksIncludingSiteLanguage( $langcode );

                // Next try checking the database for all of the fallback languages of the requested language.
                if ( $useDB ) {
                        foreach ( $fallbackChain as $code ) {
                                if ( $code === $wgLanguageCode ) {
                                        // Messages created in the content language will not have the /lang extension
                                        $message = $this->getMsgFromNamespace( $uckey, $code );
                                } else {
                                        $message = $this->getMsgFromNamespace( "$uckey/$code", $code );
                                }

                                if ( $message !== false ) {
                                        // Found the message.
                                        return $message;
                                }
                        }
                }

                // Now try checking the site language.
                if ( $useDB ) {
                        $message = $this->getMsgFromNamespace( $uckey, $wgLanguageCode );
                        if ( $message !== false ) {
                                return $message;
                        }
                }

                $message = $wgContLang->getMessage( $lckey );
                if ( $message !== null ) {
                        return $message;
                }

                // Finally try the DB for the site language's fallbacks.
                if ( $useDB ) {
                        foreach ( $siteFallbackChain as $code ) {
                                $message = $this->getMsgFromNamespace( "$uckey/$code", $code );
                                if ( $message === false && $code === $wgLanguageCode ) {
                                        // Messages created in the content language will not have the /lang extension
                                        $message = $this->getMsgFromNamespace( $uckey, $code );
                                }

                                if ( $message !== false ) {
                                        // Found the message.
                                        return $message;
                                }
                        }
                }

                return false;
        }

        /**
         * 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
         */
        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', array( $title, &$message ) );
                        if ( $message !== false ) {
                                return $message;
                        }

                        return false;
                }

                # Try the individual message cache
                $titleKey = wfMemcKey( 'messages', 'individual', $title );
                $entry = $this->mMemc->get( $titleKey );
                if ( $entry ) {
                        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->mMemc->delete( $titleKey );
                        }
                }

                # Try loading it from the database
                $revision = Revision::newFromTitle(
                        Title::makeTitle( NS_MEDIAWIKI, $title ), false, Revision::READ_LATEST
                );
                if ( $revision ) {
                        $content = $revision->getContent();
                        if ( !$content ) {
                                // A possibly temporary loading failure.
                                wfDebugLog(
                                        'MessageCache',
                                        __METHOD__ . ": failed to load message page text for {$title} ($code)"
                                );
                                $message = null; // no negative caching
                        } else {
                                // 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.
                                $message = $content->getWikitextForTransclusion();

                                if ( $message === false || $message === null ) {
                                        wfDebugLog(
                                                'MessageCache',
                                                __METHOD__ . ": message content doesn't provide wikitext "
                                                        . "(content model: " . $content->getContentHandler() . ")"
                                        );

                                        $message = false; // negative caching
                                } else {
                                        $this->mCache[$code][$title] = ' ' . $message;
                                        $this->mMemc->set( $titleKey, ' ' . $message, $this->mExpiry );
                                }
                        }
                } else {
                        $message = false; // negative caching
                }

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

                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 string $language Language code
         * @return ParserOutput|string
         */
        public function parse( $text, $title = null, $linestart = true,
                $interface = false, $language = null
        ) {
                if ( $this->mInParser ) {
                        return htmlspecialchars( $text );
                }

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

                if ( !$title || !$title instanceof Title ) {
                        global $wgTitle;
                        wfDebugLog( 'GlobalTitleFail', __METHOD__ . ' called by ' . wfGetAllCallers( 5 ) . ' 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::newFromText( 'Dwimmerlaik' );
                }

                $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;
        }

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

        /**
         * @param string $key
         * @return array
         */
        public function figureMessage( $key ) {
                global $wgLanguageCode;
                $pieces = explode( '/', $key );
                if ( count( $pieces ) < 2 ) {
                        return array( $key, $wgLanguageCode );
                }

                $lang = array_pop( $pieces );
                if ( !Language::fetchLanguageName( $lang, null, 'mw' ) ) {
                        return array( $key, $wgLanguageCode );
                }

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

                return array( $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'] );
                // Remove any !NONEXISTENT keys
                $cache = array_diff( $cache, array( '!NONEXISTENT' ) );

                // Keys may appear with a capital first letter. lcfirst them.
                return array_map( array( $wgContLang, 'lcfirst' ), array_keys( $cache ) );
        }
}