includes/cache/MessageCache.php

   1 <?php
   2 /**
   3  * Localisation messages cache.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  * @ingroup Cache
  22  */
  23 use MediaWiki\MediaWikiServices;
  24 use Wikimedia\ScopedCallback;
  25 use MediaWiki\Logger\LoggerFactory;
  26
  27 /**
  28  * MediaWiki message cache structure version.
  29  * Bump this whenever the message cache format has changed.
  30  */
  31 define( 'MSG_CACHE_VERSION', 2 );
  32
  33 /**
  34  * Message cache
  35  * Performs various MediaWiki namespace-related functions
  36  * @ingroup Cache
  37  */
  38 class MessageCache {
  39         const FOR_UPDATE = 1; // force message reload
  40
  41         /** How long to wait for memcached locks */
  42         const WAIT_SEC = 15;
  43         /** How long memcached locks last */
  44         const LOCK_TTL = 30;
  45
  46         /**
  47          * Process local cache of loaded messages that are defined in
  48          * MediaWiki namespace. First array level is a language code,
  49          * second level is message key and the values are either message
  50          * content prefixed with space, or !NONEXISTENT for negative
  51          * caching.
  52          * @var array $mCache
  53          */
  54         protected $mCache;
  55
  56         /**
  57          * @var bool[] Map of (language code => boolean)
  58          */
  59         protected $mCacheVolatile = [];
  60
  61         /**
  62          * Should mean that database cannot be used, but check
  63          * @var bool $mDisable
  64          */
  65         protected $mDisable;
  66
  67         /**
  68          * Lifetime for cache, used by object caching.
  69          * Set on construction, see __construct().
  70          */
  71         protected $mExpiry;
  72
  73         /**
  74          * Message cache has its own parser which it uses to transform messages
  75          * @var ParserOptions
  76          */
  77         protected $mParserOptions;
  78         /** @var Parser */
  79         protected $mParser;
  80
  81         /**
  82          * Variable for tracking which variables are already loaded
  83          * @var array $mLoadedLanguages
  84          */
  85         protected $mLoadedLanguages = [];
  86
  87         /**
  88          * @var bool $mInParser
  89          */
  90         protected $mInParser = false;
  91
  92         /** @var WANObjectCache */
  93         protected $wanCache;
  94         /** @var BagOStuff */
  95         protected $clusterCache;
  96         /** @var BagOStuff */
  97         protected $srvCache;
  98
  99         /**
 100          * Singleton instance
 101          *
 102          * @var MessageCache $instance
 103          */
 104         private static $instance;
 105
 106         /**
 107          * Get the signleton instance of this class
 108          *
 109          * @since 1.18
 110          * @return MessageCache
 111          */
 112         public static function singleton() {
 113                 if ( self::$instance === null ) {
 114                         global $wgUseDatabaseMessages, $wgMsgCacheExpiry, $wgUseLocalMessageCache;
 115                         self::$instance = new self(
 116                                 MediaWikiServices::getInstance()->getMainWANObjectCache(),
 117                                 wfGetMessageCacheStorage(),
 118                                 $wgUseLocalMessageCache
 119                                         ? MediaWikiServices::getInstance()->getLocalServerObjectCache()
 120                                         : new EmptyBagOStuff(),
 121                                 $wgUseDatabaseMessages,
 122                                 $wgMsgCacheExpiry
 123                         );
 124                 }
 125
 126                 return self::$instance;
 127         }
 128
 129         /**
 130          * Destroy the singleton instance
 131          *
 132          * @since 1.18
 133          */
 134         public static function destroyInstance() {
 135                 self::$instance = null;
 136         }
 137
 138         /**
 139          * Normalize message key input
 140          *
 141          * @param string $key Input message key to be normalized
 142          * @return string Normalized message key
 143          */
 144         public static function normalizeKey( $key ) {
 145                 global $wgContLang;
 146
 147                 $lckey = strtr( $key, ' ', '_' );
 148                 if ( ord( $lckey ) < 128 ) {
 149                         $lckey[0] = strtolower( $lckey[0] );
 150                 } else {
 151                         $lckey = $wgContLang->lcfirst( $lckey );
 152                 }
 153
 154                 return $lckey;
 155         }
 156
 157         /**
 158          * @param WANObjectCache $wanCache WAN cache instance
 159          * @param BagOStuff $clusterCache Cluster cache instance
 160          * @param BagOStuff $srvCache Server cache instance
 161          * @param bool $useDB Whether to look for message overrides (e.g. MediaWiki: pages)
 162          * @param int $expiry Lifetime for cache. @see $mExpiry.
 163          */
 164         public function __construct(
 165                 WANObjectCache $wanCache,
 166                 BagOStuff $clusterCache,
 167                 BagOStuff $srvCache,
 168                 $useDB,
 169                 $expiry
 170         ) {
 171                 $this->wanCache = $wanCache;
 172                 $this->clusterCache = $clusterCache;
 173                 $this->srvCache = $srvCache;
 174
 175                 $this->mDisable = !$useDB;
 176                 $this->mExpiry = $expiry;
 177         }
 178
 179         /**
 180          * ParserOptions is lazy initialised.
 181          *
 182          * @return ParserOptions
 183          */
 184         function getParserOptions() {
 185                 global $wgUser;
 186
 187                 if ( !$this->mParserOptions ) {
 188                         if ( !$wgUser->isSafeToLoad() ) {
 189                                 // $wgUser isn't unstubbable yet, so don't try to get a
 190                                 // ParserOptions for it. And don't cache this ParserOptions
 191                                 // either.
 192                                 $po = ParserOptions::newFromAnon();
 193                                 $po->setEditSection( false );
 194                                 return $po;
 195                         }
 196
 197                         $this->mParserOptions = new ParserOptions;
 198                         $this->mParserOptions->setEditSection( false );
 199                 }
 200
 201                 return $this->mParserOptions;
 202         }
 203
 204         /**
 205          * Try to load the cache from APC.
 206          *
 207          * @param string $code Optional language code, see documenation of load().
 208          * @return array|bool The cache array, or false if not in cache.
 209          */
 210         protected function getLocalCache( $code ) {
 211                 $cacheKey = wfMemcKey( __CLASS__, $code );
 212
 213                 return $this->srvCache->get( $cacheKey );
 214         }
 215
 216         /**
 217          * Save the cache to APC.
 218          *
 219          * @param string $code
 220          * @param array $cache The cache array
 221          */
 222         protected function saveToLocalCache( $code, $cache ) {
 223                 $cacheKey = wfMemcKey( __CLASS__, $code );
 224                 $this->srvCache->set( $cacheKey, $cache );
 225         }
 226
 227         /**
 228          * Loads messages from caches or from database in this order:
 229          * (1) local message cache (if $wgUseLocalMessageCache is enabled)
 230          * (2) memcached
 231          * (3) from the database.
 232          *
 233          * When succesfully loading from (2) or (3), all higher level caches are
 234          * updated for the newest version.
 235          *
 236          * Nothing is loaded if member variable mDisable is true, either manually
 237          * set by calling code or if message loading fails (is this possible?).
 238          *
 239          * Returns true if cache is already populated or it was succesfully populated,
 240          * or false if populating empty cache fails. Also returns true if MessageCache
 241          * is disabled.
 242          *
 243          * @param string $code Language to which load messages
 244          * @param integer $mode Use MessageCache::FOR_UPDATE to skip process cache [optional]
 245          * @throws MWException
 246          * @return bool
 247          */
 248         protected function load( $code, $mode = null ) {
 249                 if ( !is_string( $code ) ) {
 250                         throw new InvalidArgumentException( "Missing language code" );
 251                 }
 252
 253                 # Don't do double loading...
 254                 if ( isset( $this->mLoadedLanguages[$code] ) && $mode != self::FOR_UPDATE ) {
 255                         return true;
 256                 }
 257
 258                 # 8 lines of code just to say (once) that message cache is disabled
 259                 if ( $this->mDisable ) {
 260                         static $shownDisabled = false;
 261                         if ( !$shownDisabled ) {
 262                                 wfDebug( __METHOD__ . ": disabled\n" );
 263                                 $shownDisabled = true;
 264                         }
 265
 266                         return true;
 267                 }
 268
 269                 # Loading code starts
 270                 $success = false; # Keep track of success
 271                 $staleCache = false; # a cache array with expired data, or false if none has been loaded
 272                 $where = []; # Debug info, delayed to avoid spamming debug log too much
 273
 274                 # Hash of the contents is stored in memcache, to detect if data-center cache
 275                 # or local cache goes out of date (e.g. due to replace() on some other server)
 276                 list( $hash, $hashVolatile ) = $this->getValidationHash( $code );
 277                 $this->mCacheVolatile[$code] = $hashVolatile;
 278
 279                 # Try the local cache and check against the cluster hash key...
 280                 $cache = $this->getLocalCache( $code );
 281                 if ( !$cache ) {
 282                         $where[] = 'local cache is empty';
 283                 } elseif ( !isset( $cache['HASH'] ) || $cache['HASH'] !== $hash ) {
 284                         $where[] = 'local cache has the wrong hash';
 285                         $staleCache = $cache;
 286                 } elseif ( $this->isCacheExpired( $cache ) ) {
 287                         $where[] = 'local cache is expired';
 288                         $staleCache = $cache;
 289                 } elseif ( $hashVolatile ) {
 290                         $where[] = 'local cache validation key is expired/volatile';
 291                         $staleCache = $cache;
 292                 } else {
 293                         $where[] = 'got from local cache';
 294                         $success = true;
 295                         $this->mCache[$code] = $cache;
 296                 }
 297
 298                 if ( !$success ) {
 299                         $cacheKey = wfMemcKey( 'messages', $code ); # Key in memc for messages
 300                         # Try the global cache. If it is empty, try to acquire a lock. If
 301                         # the lock can't be acquired, wait for the other thread to finish
 302                         # and then try the global cache a second time.
 303                         for ( $failedAttempts = 0; $failedAttempts <= 1; $failedAttempts++ ) {
 304                                 if ( $hashVolatile && $staleCache ) {
 305                                         # Do not bother fetching the whole cache blob to avoid I/O.
 306                                         # Instead, just try to get the non-blocking $statusKey lock
 307                                         # below, and use the local stale value if it was not acquired.
 308                                         $where[] = 'global cache is presumed expired';
 309                                 } else {
 310                                         $cache = $this->clusterCache->get( $cacheKey );
 311                                         if ( !$cache ) {
 312                                                 $where[] = 'global cache is empty';
 313                                         } elseif ( $this->isCacheExpired( $cache ) ) {
 314                                                 $where[] = 'global cache is expired';
 315                                                 $staleCache = $cache;
 316                                         } elseif ( $hashVolatile ) {
 317                                                 # DB results are replica DB lag prone until the holdoff TTL passes.
 318                                                 # By then, updates should be reflected in loadFromDBWithLock().
 319                                                 # One thread renerates the cache while others use old values.
 320                                                 $where[] = 'global cache is expired/volatile';
 321                                                 $staleCache = $cache;
 322                                         } else {
 323                                                 $where[] = 'got from global cache';
 324                                                 $this->mCache[$code] = $cache;
 325                                                 $this->saveToCaches( $cache, 'local-only', $code );
 326                                                 $success = true;
 327                                         }
 328                                 }
 329
 330                                 if ( $success ) {
 331                                         # Done, no need to retry
 332                                         break;
 333                                 }
 334
 335                                 # We need to call loadFromDB. Limit the concurrency to one process.
 336                                 # This prevents the site from going down when the cache expires.
 337                                 # Note that the DB slam protection lock here is non-blocking.
 338                                 $loadStatus = $this->loadFromDBWithLock( $code, $where, $mode );
 339                                 if ( $loadStatus === true ) {
 340                                         $success = true;
 341                                         break;
 342                                 } elseif ( $staleCache ) {
 343                                         # Use the stale cache while some other thread constructs the new one
 344                                         $where[] = 'using stale cache';
 345                                         $this->mCache[$code] = $staleCache;
 346                                         $success = true;
 347                                         break;
 348                                 } elseif ( $failedAttempts > 0 ) {
 349                                         # Already blocked once, so avoid another lock/unlock cycle.
 350                                         # This case will typically be hit if memcached is down, or if
 351                                         # loadFromDB() takes longer than LOCK_WAIT.
 352                                         $where[] = "could not acquire status key.";
 353                                         break;
 354                                 } elseif ( $loadStatus === 'cantacquire' ) {
 355                                         # Wait for the other thread to finish, then retry. Normally,
 356                                         # the memcached get() will then yeild the other thread's result.
 357                                         $where[] = 'waited for other thread to complete';
 358                                         $this->getReentrantScopedLock( $cacheKey );
 359                                 } else {
 360                                         # Disable cache; $loadStatus is 'disabled'
 361                                         break;
 362                                 }
 363                         }
 364                 }
 365
 366                 if ( !$success ) {
 367                         $where[] = 'loading FAILED - cache is disabled';
 368                         $this->mDisable = true;
 369                         $this->mCache = false;
 370                         wfDebugLog( 'MessageCacheError', __METHOD__ . ": Failed to load $code\n" );
 371                         # This used to throw an exception, but that led to nasty side effects like
 372                         # the whole wiki being instantly down if the memcached server died
 373                 } else {
 374                         # All good, just record the success
 375                         $this->mLoadedLanguages[$code] = true;
 376                 }
 377
 378                 $info = implode( ', ', $where );
 379                 wfDebugLog( 'MessageCache', __METHOD__ . ": Loading $code... $info\n" );
 380
 381                 return $success;
 382         }
 383
 384         /**
 385          * @param string $code
 386          * @param array $where List of wfDebug() comments
 387          * @param integer $mode Use MessageCache::FOR_UPDATE to use DB_MASTER
 388          * @return bool|string True on success or one of ("cantacquire", "disabled")
 389          */
 390         protected function loadFromDBWithLock( $code, array &$where, $mode = null ) {
 391                 # If cache updates on all levels fail, give up on message overrides.
 392                 # This is to avoid easy site outages; see $saveSuccess comments below.
 393                 $statusKey = wfMemcKey( 'messages', $code, 'status' );
 394                 $status = $this->clusterCache->get( $statusKey );
 395                 if ( $status === 'error' ) {
 396                         $where[] = "could not load; method is still globally disabled";
 397                         return 'disabled';
 398                 }
 399
 400                 # Now let's regenerate
 401                 $where[] = 'loading from database';
 402
 403                 # Lock the cache to prevent conflicting writes.
 404                 # This lock is non-blocking so stale cache can quickly be used.
 405                 # Note that load() will call a blocking getReentrantScopedLock()
 406                 # after this if it really need to wait for any current thread.
 407                 $cacheKey = wfMemcKey( 'messages', $code );
 408                 $scopedLock = $this->getReentrantScopedLock( $cacheKey, 0 );
 409                 if ( !$scopedLock ) {
 410                         $where[] = 'could not acquire main lock';
 411                         return 'cantacquire';
 412                 }
 413
 414                 $cache = $this->loadFromDB( $code, $mode );
 415                 $this->mCache[$code] = $cache;
 416                 $saveSuccess = $this->saveToCaches( $cache, 'all', $code );
 417
 418                 if ( !$saveSuccess ) {
 419                         /**
 420                          * Cache save has failed.
 421                          *
 422                          * There are two main scenarios where this could be a problem:
 423                          * - The cache is more than the maximum size (typically 1MB compressed).
 424                          * - Memcached has no space remaining in the relevant slab class. This is
 425                          *   unlikely with recent versions of memcached.
 426                          *
 427                          * Either way, if there is a local cache, nothing bad will happen. If there
 428                          * is no local cache, disabling the message cache for all requests avoids
 429                          * incurring a loadFromDB() overhead on every request, and thus saves the
 430                          * wiki from complete downtime under moderate traffic conditions.
 431                          */
 432                         if ( $this->srvCache instanceof EmptyBagOStuff ) {
 433                                 $this->clusterCache->set( $statusKey, 'error', 60 * 5 );
 434                                 $where[] = 'could not save cache, disabled globally for 5 minutes';
 435                         } else {
 436                                 $where[] = "could not save global cache";
 437                         }
 438                 }
 439
 440                 return true;
 441         }
 442
 443         /**
 444          * Loads cacheable messages from the database. Messages bigger than
 445          * $wgMaxMsgCacheEntrySize are assigned a special value, and are loaded
 446          * on-demand from the database later.
 447          *
 448          * @param string $code Language code
 449          * @param integer $mode Use MessageCache::FOR_UPDATE to skip process cache
 450          * @return array Loaded messages for storing in caches
 451          */
 452         protected function loadFromDB( $code, $mode = null ) {
 453                 global $wgMaxMsgCacheEntrySize, $wgLanguageCode, $wgAdaptiveMessageCache;
 454
 455                 $dbr = wfGetDB( ( $mode == self::FOR_UPDATE ) ? DB_MASTER : DB_REPLICA );
 456
 457                 $cache = [];
 458
 459                 # Common conditions
 460                 $conds = [
 461                         'page_is_redirect' => 0,
 462                         'page_namespace' => NS_MEDIAWIKI,
 463                 ];
 464
 465                 $mostused = [];
 466                 if ( $wgAdaptiveMessageCache && $code !== $wgLanguageCode ) {
 467                         if ( !isset( $this->mCache[$wgLanguageCode] ) ) {
 468                                 $this->load( $wgLanguageCode );
 469                         }
 470                         $mostused = array_keys( $this->mCache[$wgLanguageCode] );
 471                         foreach ( $mostused as $key => $value ) {
 472                                 $mostused[$key] = "$value/$code";
 473                         }
 474                 }
 475
 476                 if ( count( $mostused ) ) {
 477                         $conds['page_title'] = $mostused;
 478                 } elseif ( $code !== $wgLanguageCode ) {
 479                         $conds[] = 'page_title' . $dbr->buildLike( $dbr->anyString(), '/', $code );
 480                 } else {
 481                         # Effectively disallows use of '/' character in NS_MEDIAWIKI for uses
 482                         # other than language code.
 483                         $conds[] = 'page_title NOT' . $dbr->buildLike( $dbr->anyString(), '/', $dbr->anyString() );
 484                 }
 485
 486                 # Conditions to fetch oversized pages to ignore them
 487                 $bigConds = $conds;
 488                 $bigConds[] = 'page_len > ' . intval( $wgMaxMsgCacheEntrySize );
 489
 490                 # Load titles for all oversized pages in the MediaWiki namespace
 491                 $res = $dbr->select(
 492                         'page',
 493                         [ 'page_title', 'page_latest' ],
 494                         $bigConds,
 495                         __METHOD__ . "($code)-big"
 496                 );
 497                 foreach ( $res as $row ) {
 498                         $cache[$row->page_title] = '!TOO BIG';
 499                         // At least include revision ID so page changes are reflected in the hash
 500                         $cache['EXCESSIVE'][$row->page_title] = $row->page_latest;
 501                 }
 502
 503                 # Conditions to load the remaining pages with their contents
 504                 $smallConds = $conds;
 505                 $smallConds[] = 'page_latest=rev_id';
 506                 $smallConds[] = 'rev_text_id=old_id';
 507                 $smallConds[] = 'page_len <= ' . intval( $wgMaxMsgCacheEntrySize );
 508
 509                 $res = $dbr->select(
 510                         [ 'page', 'revision', 'text' ],
 511                         [ 'page_title', 'old_text', 'old_flags' ],
 512                         $smallConds,
 513                         __METHOD__ . "($code)-small"
 514                 );
 515
 516                 foreach ( $res as $row ) {
 517                         $text = Revision::getRevisionText( $row );
 518                         if ( $text === false ) {
 519                                 // Failed to fetch data; possible ES errors?
 520                                 // Store a marker to fetch on-demand as a workaround...
 521                                 // TODO Use a differnt marker
 522                                 $entry = '!TOO BIG';
 523                                 wfDebugLog(
 524                                         'MessageCache',
 525                                         __METHOD__
 526                                         . ": failed to load message page text for {$row->page_title} ($code)"
 527                                 );
 528                         } else {
 529                                 $entry = ' ' . $text;
 530                         }
 531                         $cache[$row->page_title] = $entry;
 532                 }
 533
 534                 $cache['VERSION'] = MSG_CACHE_VERSION;
 535                 ksort( $cache );
 536
 537                 # Hash for validating local cache (APC). No need to take into account
 538                 # messages larger than $wgMaxMsgCacheEntrySize, since those are only
 539                 # stored and fetched from memcache.
 540                 $cache['HASH'] = md5( serialize( $cache ) );
 541                 $cache['EXPIRY'] = wfTimestamp( TS_MW, time() + $this->mExpiry );
 542
 543                 return $cache;
 544         }
 545
 546         /**
 547          * Updates cache as necessary when message page is changed
 548          *
 549          * @param string $title Message cache key with initial uppercase letter.
 550          * @param string|bool $text New contents of the page (false if deleted)
 551          */
 552         public function replace( $title, $text ) {
 553                 global $wgLanguageCode;
 554
 555                 if ( $this->mDisable ) {
 556                         return;
 557                 }
 558
 559                 list( $msg, $code ) = $this->figureMessage( $title );
 560                 if ( strpos( $title, '/' ) !== false && $code === $wgLanguageCode ) {
 561                         // Content language overrides do not use the /<code> suffix
 562                         return;
 563                 }
 564
 565                 // (a) Update the process cache with the new message text
 566                 if ( $text === false ) {
 567                         // Page deleted
 568                         $this->mCache[$code][$title] = '!NONEXISTENT';
 569                 } else {
 570                         // Ignore $wgMaxMsgCacheEntrySize so the process cache is up to date
 571                         $this->mCache[$code][$title] = ' ' . $text;
 572                 }
 573
 574                 // (b) Update the shared caches in a deferred update with a fresh DB snapshot
 575                 DeferredUpdates::addCallableUpdate(
 576                         function () use ( $title, $msg, $code ) {
 577                                 global $wgContLang, $wgMaxMsgCacheEntrySize;
 578                                 // Allow one caller at a time to avoid race conditions
 579                                 $scopedLock = $this->getReentrantScopedLock( wfMemcKey( 'messages', $code ) );
 580                                 if ( !$scopedLock ) {
 581                                         LoggerFactory::getInstance( 'MessageCache' )->error(
 582                                                 __METHOD__ . ': could not acquire lock to update {title} ({code})',
 583                                                 [ 'title' => $title, 'code' => $code ] );
 584                                         return;
 585                                 }
 586                                 // Load the messages from the master DB to avoid race conditions
 587                                 $this->loadFromDB( $code, self::FOR_UPDATE );
 588                                 // Load the process cache values and set the per-title cache keys
 589                                 $page = WikiPage::factory( Title::makeTitle( NS_MEDIAWIKI, $title ) );
 590                                 $page->loadPageData( $page::READ_LATEST );
 591                                 $text = $this->getMessageTextFromContent( $page->getContent() );
 592                                 // Check if an individual cache key should exist and update cache accordingly
 593                                 $titleKey = $this->wanCache->makeKey(
 594                                         'messages-big', $this->mCache[$code]['HASH'], $title );
 595                                 if ( is_string( $text ) && strlen( $text ) > $wgMaxMsgCacheEntrySize ) {
 596                                         $this->wanCache->set( $titleKey, ' ' . $text, $this->mExpiry );
 597                                 }
 598                                 // Mark this cache as definitely being "latest" (non-volatile) so
 599                                 // load() calls do try to refresh the cache with replica DB data
 600                                 $this->mCache[$code]['LATEST'] = time();
 601                                 // Pre-emptively update the local datacenter cache so things like edit filter and
 602                                 // blacklist changes are reflect immediately, as these often use MediaWiki: pages.
 603                                 // The datacenter handling replace() calls should be the same one handling edits
 604                                 // as they require HTTP POST.
 605                                 $this->saveToCaches( $this->mCache[$code], 'all', $code );
 606                                 // Release the lock now that the cache is saved
 607                                 ScopedCallback::consume( $scopedLock );
 608
 609                                 // Relay the purge. Touching this check key expires cache contents
 610                                 // and local cache (APC) validation hash across all datacenters.
 611                                 $this->wanCache->touchCheckKey( wfMemcKey( 'messages', $code ) );
 612                                 // Also delete cached sidebar... just in case it is affected
 613                                 // @TODO: shouldn't this be $code === $wgLanguageCode?
 614                                 if ( $code === 'en' ) {
 615                                         // Purge all language sidebars, e.g. on ?action=purge to the sidebar messages
 616                                         $codes = array_keys( Language::fetchLanguageNames() );
 617                                 } else {
 618                                         // Purge only the sidebar for this language
 619                                         $codes = [ $code ];
 620                                 }
 621                                 foreach ( $codes as $code ) {
 622                                         $this->wanCache->delete( wfMemcKey( 'sidebar', $code ) );
 623                                 }
 624
 625                                 // Purge the message in the message blob store
 626                                 $resourceloader = RequestContext::getMain()->getOutput()->getResourceLoader();
 627                                 $blobStore = $resourceloader->getMessageBlobStore();
 628                                 $blobStore->updateMessage( $wgContLang->lcfirst( $msg ) );
 629
 630                                 Hooks::run( 'MessageCacheReplace', [ $title, $text ] );
 631                         },
 632                         DeferredUpdates::PRESEND
 633                 );
 634         }
 635
 636         /**
 637          * Is the given cache array expired due to time passing or a version change?
 638          *
 639          * @param array $cache
 640          * @return bool
 641          */
 642         protected function isCacheExpired( $cache ) {
 643                 if ( !isset( $cache['VERSION'] ) || !isset( $cache['EXPIRY'] ) ) {
 644                         return true;
 645                 }
 646                 if ( $cache['VERSION'] != MSG_CACHE_VERSION ) {
 647                         return true;
 648                 }
 649                 if ( wfTimestampNow() >= $cache['EXPIRY'] ) {
 650                         return true;
 651                 }
 652
 653                 return false;
 654         }
 655
 656         /**
 657          * Shortcut to update caches.
 658          *
 659          * @param array $cache Cached messages with a version.
 660          * @param string $dest Either "local-only" to save to local caches only
 661          *   or "all" to save to all caches.
 662          * @param string|bool $code Language code (default: false)
 663          * @return bool
 664          */
 665         protected function saveToCaches( array $cache, $dest, $code = false ) {
 666                 if ( $dest === 'all' ) {
 667                         $cacheKey = wfMemcKey( 'messages', $code );
 668                         $success = $this->clusterCache->set( $cacheKey, $cache );
 669                         $this->setValidationHash( $code, $cache );
 670                 } else {
 671                         $success = true;
 672                 }
 673
 674                 $this->saveToLocalCache( $code, $cache );
 675
 676                 return $success;
 677         }
 678
 679         /**
 680          * Get the md5 used to validate the local APC cache
 681          *
 682          * @param string $code
 683          * @return array (hash or false, bool expiry/volatility status)
 684          */
 685         protected function getValidationHash( $code ) {
 686                 $curTTL = null;
 687                 $value = $this->wanCache->get(
 688                         $this->wanCache->makeKey( 'messages', $code, 'hash', 'v1' ),
 689                         $curTTL,
 690                         [ wfMemcKey( 'messages', $code ) ]
 691                 );
 692
 693                 if ( $value ) {
 694                         $hash = $value['hash'];
 695                         if ( ( time() - $value['latest'] ) < WANObjectCache::TTL_MINUTE ) {
 696                                 // Cache was recently updated via replace() and should be up-to-date.
 697                                 // That method is only called in the primary datacenter and uses FOR_UPDATE.
 698                                 // Also, it is unlikely that the current datacenter is *now* secondary one.
 699                                 $expired = false;
 700                         } else {
 701                                 // See if the "check" key was bumped after the hash was generated
 702                                 $expired = ( $curTTL < 0 );
 703                         }
 704                 } else {
 705                         // No hash found at all; cache must regenerate to be safe
 706                         $hash = false;
 707                         $expired = true;
 708                 }
 709
 710                 return [ $hash, $expired ];
 711         }
 712
 713         /**
 714          * Set the md5 used to validate the local disk cache
 715          *
 716          * If $cache has a 'LATEST' UNIX timestamp key, then the hash will not
 717          * be treated as "volatile" by getValidationHash() for the next few seconds.
 718          * This is triggered when $cache is generated using FOR_UPDATE mode.
 719          *
 720          * @param string $code
 721          * @param array $cache Cached messages with a version
 722          */
 723         protected function setValidationHash( $code, array $cache ) {
 724                 $this->wanCache->set(
 725                         $this->wanCache->makeKey( 'messages', $code, 'hash', 'v1' ),
 726                         [
 727                                 'hash' => $cache['HASH'],
 728                                 'latest' => isset( $cache['LATEST'] ) ? $cache['LATEST'] : 0
 729                         ],
 730                         WANObjectCache::TTL_INDEFINITE
 731                 );
 732         }
 733
 734         /**
 735          * @param string $key A language message cache key that stores blobs
 736          * @param integer $timeout Wait timeout in seconds
 737          * @return null|ScopedCallback
 738          */
 739         protected function getReentrantScopedLock( $key, $timeout = self::WAIT_SEC ) {
 740                 return $this->clusterCache->getScopedLock( $key, $timeout, self::LOCK_TTL, __METHOD__ );
 741         }
 742
 743         /**
 744          * Get a message from either the content language or the user language.
 745          *
 746          * First, assemble a list of languages to attempt getting the message from. This
 747          * chain begins with the requested language and its fallbacks and then continues with
 748          * the content language and its fallbacks. For each language in the chain, the following
 749          * process will occur (in this order):
 750          *  1. If a language-specific override, i.e., [[MW:msg/lang]], is available, use that.
 751          *     Note: for the content language, there is no /lang subpage.
 752          *  2. Fetch from the static CDB cache.
 753          *  3. If available, check the database for fallback language overrides.
 754          *
 755          * This process provides a number of guarantees. When changing this code, make sure all
 756          * of these guarantees are preserved.
 757          *  * If the requested language is *not* the content language, then the CDB cache for that
 758          *    specific language will take precedence over the root database page ([[MW:msg]]).
 759          *  * Fallbacks will be just that: fallbacks. A fallback language will never be reached if
 760          *    the message is available *anywhere* in the language for which it is a fallback.
 761          *
 762          * @param string $key The message key
 763          * @param bool $useDB If true, look for the message in the DB, false
 764          *   to use only the compiled l10n cache.
 765          * @param bool|string|object $langcode Code of the language to get the message for.
 766          *   - If string and a valid code, will create a standard language object
 767          *   - If string but not a valid code, will create a basic language object
 768          *   - If boolean and false, create object from the current users language
 769          *   - If boolean and true, create object from the wikis content language
 770          *   - If language object, use it as given
 771          * @param bool $isFullKey Specifies whether $key is a two part key "msg/lang".
 772          *
 773          * @throws MWException When given an invalid key
 774          * @return string|bool False if the message doesn't exist, otherwise the
 775          *   message (which can be empty)
 776          */
 777         function get( $key, $useDB = true, $langcode = true, $isFullKey = false ) {
 778                 if ( is_int( $key ) ) {
 779                         // Fix numerical strings that somehow become ints
 780                         // on their way here
 781                         $key = (string)$key;
 782                 } elseif ( !is_string( $key ) ) {
 783                         throw new MWException( 'Non-string key given' );
 784                 } elseif ( $key === '' ) {
 785                         // Shortcut: the empty key is always missing
 786                         return false;
 787                 }
 788
 789                 // For full keys, get the language code from the key
 790                 $pos = strrpos( $key, '/' );
 791                 if ( $isFullKey && $pos !== false ) {
 792                         $langcode = substr( $key, $pos + 1 );
 793                         $key = substr( $key, 0, $pos );
 794                 }
 795
 796                 // Normalise title-case input (with some inlining)
 797                 $lckey = MessageCache::normalizeKey( $key );
 798
 799                 Hooks::run( 'MessageCache::get', [ &$lckey ] );
 800
 801                 // Loop through each language in the fallback list until we find something useful
 802                 $lang = wfGetLangObj( $langcode );
 803                 $message = $this->getMessageFromFallbackChain(
 804                         $lang,
 805                         $lckey,
 806                         !$this->mDisable && $useDB
 807                 );
 808
 809                 // If we still have no message, maybe the key was in fact a full key so try that
 810                 if ( $message === false ) {
 811                         $parts = explode( '/', $lckey );
 812                         // We may get calls for things that are http-urls from sidebar
 813                         // Let's not load nonexistent languages for those
 814                         // They usually have more than one slash.
 815                         if ( count( $parts ) == 2 && $parts[1] !== '' ) {
 816                                 $message = Language::getMessageFor( $parts[0], $parts[1] );
 817                                 if ( $message === null ) {
 818                                         $message = false;
 819                                 }
 820                         }
 821                 }
 822
 823                 // Post-processing if the message exists
 824                 if ( $message !== false ) {
 825                         // Fix whitespace
 826                         $message = str_replace(
 827                                 [
 828                                         # Fix for trailing whitespace, removed by textarea
 829                                         '&#32;',
 830                                         # Fix for NBSP, converted to space by firefox
 831                                         '&nbsp;',
 832                                         '&#160;',
 833                                         '&shy;'
 834                                 ],
 835                                 [
 836                                         ' ',
 837                                         "\xc2\xa0",
 838                                         "\xc2\xa0",
 839                                         "\xc2\xad"
 840                                 ],
 841                                 $message
 842                         );
 843                 }
 844
 845                 return $message;
 846         }
 847
 848         /**
 849          * Given a language, try and fetch messages from that language.
 850          *
 851          * Will also consider fallbacks of that language, the site language, and fallbacks for
 852          * the site language.
 853          *
 854          * @see MessageCache::get
 855          * @param Language|StubObject $lang Preferred language
 856          * @param string $lckey Lowercase key for the message (as for localisation cache)
 857          * @param bool $useDB Whether to include messages from the wiki database
 858          * @return string|bool The message, or false if not found
 859          */
 860         protected function getMessageFromFallbackChain( $lang, $lckey, $useDB ) {
 861                 global $wgContLang;
 862
 863                 $alreadyTried = [];
 864
 865                 // First try the requested language.
 866                 $message = $this->getMessageForLang( $lang, $lckey, $useDB, $alreadyTried );
 867                 if ( $message !== false ) {
 868                         return $message;
 869                 }
 870
 871                 // Now try checking the site language.
 872                 $message = $this->getMessageForLang( $wgContLang, $lckey, $useDB, $alreadyTried );
 873                 return $message;
 874         }
 875
 876         /**
 877          * Given a language, try and fetch messages from that language and its fallbacks.
 878          *
 879          * @see MessageCache::get
 880          * @param Language|StubObject $lang Preferred language
 881          * @param string $lckey Lowercase key for the message (as for localisation cache)
 882          * @param bool $useDB Whether to include messages from the wiki database
 883          * @param bool[] $alreadyTried Contains true for each language that has been tried already
 884          * @return string|bool The message, or false if not found
 885          */
 886         private function getMessageForLang( $lang, $lckey, $useDB, &$alreadyTried ) {
 887                 global $wgContLang;
 888
 889                 $langcode = $lang->getCode();
 890
 891                 // Try checking the database for the requested language
 892                 if ( $useDB ) {
 893                         $uckey = $wgContLang->ucfirst( $lckey );
 894
 895                         if ( !isset( $alreadyTried[ $langcode ] ) ) {
 896                                 $message = $this->getMsgFromNamespace(
 897                                         $this->getMessagePageName( $langcode, $uckey ),
 898                                         $langcode
 899                                 );
 900
 901                                 if ( $message !== false ) {
 902                                         return $message;
 903                                 }
 904                                 $alreadyTried[ $langcode ] = true;
 905                         }
 906                 } else {
 907                         $uckey = null;
 908                 }
 909
 910                 // Check the CDB cache
 911                 $message = $lang->getMessage( $lckey );
 912                 if ( $message !== null ) {
 913                         return $message;
 914                 }
 915
 916                 // Try checking the database for all of the fallback languages
 917                 if ( $useDB ) {
 918                         $fallbackChain = Language::getFallbacksFor( $langcode );
 919
 920                         foreach ( $fallbackChain as $code ) {
 921                                 if ( isset( $alreadyTried[ $code ] ) ) {
 922                                         continue;
 923                                 }
 924
 925                                 $message = $this->getMsgFromNamespace(
 926                                         $this->getMessagePageName( $code, $uckey ), $code );
 927
 928                                 if ( $message !== false ) {
 929                                         return $message;
 930                                 }
 931                                 $alreadyTried[ $code ] = true;
 932                         }
 933                 }
 934
 935                 return false;
 936         }
 937
 938         /**
 939          * Get the message page name for a given language
 940          *
 941          * @param string $langcode
 942          * @param string $uckey Uppercase key for the message
 943          * @return string The page name
 944          */
 945         private function getMessagePageName( $langcode, $uckey ) {
 946                 global $wgLanguageCode;
 947
 948                 if ( $langcode === $wgLanguageCode ) {
 949                         // Messages created in the content language will not have the /lang extension
 950                         return $uckey;
 951                 } else {
 952                         return "$uckey/$langcode";
 953                 }
 954         }
 955
 956         /**
 957          * Get a message from the MediaWiki namespace, with caching. The key must
 958          * first be converted to two-part lang/msg form if necessary.
 959          *
 960          * Unlike self::get(), this function doesn't resolve fallback chains, and
 961          * some callers require this behavior. LanguageConverter::parseCachedTable()
 962          * and self::get() are some examples in core.
 963          *
 964          * @param string $title Message cache key with initial uppercase letter.
 965          * @param string $code Code denoting the language to try.
 966          * @return string|bool The message, or false if it does not exist or on error
 967          */
 968         public function getMsgFromNamespace( $title, $code ) {
 969                 $this->load( $code );
 970
 971                 if ( isset( $this->mCache[$code][$title] ) ) {
 972                         $entry = $this->mCache[$code][$title];
 973                         if ( substr( $entry, 0, 1 ) === ' ' ) {
 974                                 // The message exists, so make sure a string is returned.
 975                                 return (string)substr( $entry, 1 );
 976                         } elseif ( $entry === '!NONEXISTENT' ) {
 977                                 return false;
 978                         } elseif ( $entry === '!TOO BIG' ) {
 979                                 // Fall through and try invididual message cache below
 980                         }
 981                 } else {
 982                         // XXX: This is not cached in process cache, should it?
 983                         $message = false;
 984                         Hooks::run( 'MessagesPreLoad', [ $title, &$message, $code ] );
 985                         if ( $message !== false ) {
 986                                 return $message;
 987                         }
 988
 989                         return false;
 990                 }
 991
 992                 // Try the individual message cache
 993                 $titleKey = $this->wanCache->makeKey( 'messages-big', $this->mCache[$code]['HASH'], $title );
 994
 995                 if ( $this->mCacheVolatile[$code] ) {
 996                         $entry = false;
 997                         // Make sure that individual keys respect the WAN cache holdoff period too
 998                         LoggerFactory::getInstance( 'MessageCache' )->debug(
 999                                 __METHOD__ . ': loading volatile key \'{titleKey}\'',
1000                                 [ 'titleKey' => $titleKey, 'code' => $code ] );
1001                 } else {
1002                         $entry = $this->wanCache->get( $titleKey );
1003                 }
1004
1005                 if ( $entry !== false ) {
1006                         if ( substr( $entry, 0, 1 ) === ' ' ) {
1007                                 $this->mCache[$code][$title] = $entry;
1008                                 // The message exists, so make sure a string is returned
1009                                 return (string)substr( $entry, 1 );
1010                         } elseif ( $entry === '!NONEXISTENT' ) {
1011                                 $this->mCache[$code][$title] = '!NONEXISTENT';
1012
1013                                 return false;
1014                         } else {
1015                                 // Corrupt/obsolete entry, delete it
1016                                 $this->wanCache->delete( $titleKey );
1017                         }
1018                 }
1019
1020                 // Try loading the message from the database
1021                 $dbr = wfGetDB( DB_REPLICA );
1022                 $cacheOpts = Database::getCacheSetOptions( $dbr );
1023                 // Use newKnownCurrent() to avoid querying revision/user tables
1024                 $titleObj = Title::makeTitle( NS_MEDIAWIKI, $title );
1025                 if ( $titleObj->getLatestRevID() ) {
1026                         $revision = Revision::newKnownCurrent(
1027                                 $dbr,
1028                                 $titleObj->getArticleID(),
1029                                 $titleObj->getLatestRevID()
1030                         );
1031                 } else {
1032                         $revision = false;
1033                 }
1034
1035                 if ( $revision ) {
1036                         $content = $revision->getContent();
1037                         if ( $content ) {
1038                                 $message = $this->getMessageTextFromContent( $content );
1039                                 if ( is_string( $message ) ) {
1040                                         $this->mCache[$code][$title] = ' ' . $message;
1041                                         $this->wanCache->set( $titleKey, ' ' . $message, $this->mExpiry, $cacheOpts );
1042                                 }
1043                         } else {
1044                                 // A possibly temporary loading failure
1045                                 LoggerFactory::getInstance( 'MessageCache' )->warning(
1046                                         __METHOD__ . ': failed to load message page text for \'{titleKey}\'',
1047                                         [ 'titleKey' => $titleKey, 'code' => $code ] );
1048                                 $message = null; // no negative caching
1049                         }
1050                 } else {
1051                         $message = false; // negative caching
1052                 }
1053
1054                 if ( $message === false ) { // negative caching
1055                         $this->mCache[$code][$title] = '!NONEXISTENT';
1056                         $this->wanCache->set( $titleKey, '!NONEXISTENT', $this->mExpiry, $cacheOpts );
1057                 }
1058
1059                 return $message;
1060         }
1061
1062         /**
1063          * @param string $message
1064          * @param bool $interface
1065          * @param string $language Language code
1066          * @param Title $title
1067          * @return string
1068          */
1069         function transform( $message, $interface = false, $language = null, $title = null ) {
1070                 // Avoid creating parser if nothing to transform
1071                 if ( strpos( $message, '{{' ) === false ) {
1072                         return $message;
1073                 }
1074
1075                 if ( $this->mInParser ) {
1076                         return $message;
1077                 }
1078
1079                 $parser = $this->getParser();
1080                 if ( $parser ) {
1081                         $popts = $this->getParserOptions();
1082                         $popts->setInterfaceMessage( $interface );
1083                         $popts->setTargetLanguage( $language );
1084
1085                         $userlang = $popts->setUserLang( $language );
1086                         $this->mInParser = true;
1087                         $message = $parser->transformMsg( $message, $popts, $title );
1088                         $this->mInParser = false;
1089                         $popts->setUserLang( $userlang );
1090                 }
1091
1092                 return $message;
1093         }
1094
1095         /**
1096          * @return Parser
1097          */
1098         function getParser() {
1099                 global $wgParser, $wgParserConf;
1100
1101                 if ( !$this->mParser && isset( $wgParser ) ) {
1102                         # Do some initialisation so that we don't have to do it twice
1103                         $wgParser->firstCallInit();
1104                         # Clone it and store it
1105                         $class = $wgParserConf['class'];
1106                         if ( $class == 'ParserDiffTest' ) {
1107                                 # Uncloneable
1108                                 $this->mParser = new $class( $wgParserConf );
1109                         } else {
1110                                 $this->mParser = clone $wgParser;
1111                         }
1112                 }
1113
1114                 return $this->mParser;
1115         }
1116
1117         /**
1118          * @param string $text
1119          * @param Title $title
1120          * @param bool $linestart Whether or not this is at the start of a line
1121          * @param bool $interface Whether this is an interface message
1122          * @param Language|string $language Language code
1123          * @return ParserOutput|string
1124          */
1125         public function parse( $text, $title = null, $linestart = true,
1126                 $interface = false, $language = null
1127         ) {
1128                 global $wgTitle;
1129
1130                 if ( $this->mInParser ) {
1131                         return htmlspecialchars( $text );
1132                 }
1133
1134                 $parser = $this->getParser();
1135                 $popts = $this->getParserOptions();
1136                 $popts->setInterfaceMessage( $interface );
1137
1138                 if ( is_string( $language ) ) {
1139                         $language = Language::factory( $language );
1140                 }
1141                 $popts->setTargetLanguage( $language );
1142
1143                 if ( !$title || !$title instanceof Title ) {
1144                         wfDebugLog( 'GlobalTitleFail', __METHOD__ . ' called by ' .
1145                                 wfGetAllCallers( 6 ) . ' with no title set.' );
1146                         $title = $wgTitle;
1147                 }
1148                 // Sometimes $wgTitle isn't set either...
1149                 if ( !$title ) {
1150                         # It's not uncommon having a null $wgTitle in scripts. See r80898
1151                         # Create a ghost title in such case
1152                         $title = Title::makeTitle( NS_SPECIAL, 'Badtitle/title not set in ' . __METHOD__ );
1153                 }
1154
1155                 $this->mInParser = true;
1156                 $res = $parser->parse( $text, $title, $popts, $linestart );
1157                 $this->mInParser = false;
1158
1159                 return $res;
1160         }
1161
1162         function disable() {
1163                 $this->mDisable = true;
1164         }
1165
1166         function enable() {
1167                 $this->mDisable = false;
1168         }
1169
1170         /**
1171          * Whether DB/cache usage is disabled for determining messages
1172          *
1173          * If so, this typically indicates either:
1174          *   - a) load() failed to find a cached copy nor query the DB
1175          *   - b) we are in a special context or error mode that cannot use the DB
1176          * If the DB is ignored, any derived HTML output or cached objects may be wrong.
1177          * To avoid long-term cache pollution, TTLs can be adjusted accordingly.
1178          *
1179          * @return bool
1180          * @since 1.27
1181          */
1182         public function isDisabled() {
1183                 return $this->mDisable;
1184         }
1185
1186         /**
1187          * Clear all stored messages. Mainly used after a mass rebuild.
1188          */
1189         function clear() {
1190                 $langs = Language::fetchLanguageNames( null, 'mw' );
1191                 foreach ( array_keys( $langs ) as $code ) {
1192                         # Global and local caches
1193                         $this->wanCache->touchCheckKey( wfMemcKey( 'messages', $code ) );
1194                 }
1195
1196                 $this->mLoadedLanguages = [];
1197         }
1198
1199         /**
1200          * @param string $key
1201          * @return array
1202          */
1203         public function figureMessage( $key ) {
1204                 global $wgLanguageCode;
1205
1206                 $pieces = explode( '/', $key );
1207                 if ( count( $pieces ) < 2 ) {
1208                         return [ $key, $wgLanguageCode ];
1209                 }
1210
1211                 $lang = array_pop( $pieces );
1212                 if ( !Language::fetchLanguageName( $lang, null, 'mw' ) ) {
1213                         return [ $key, $wgLanguageCode ];
1214                 }
1215
1216                 $message = implode( '/', $pieces );
1217
1218                 return [ $message, $lang ];
1219         }
1220
1221         /**
1222          * Get all message keys stored in the message cache for a given language.
1223          * If $code is the content language code, this will return all message keys
1224          * for which MediaWiki:msgkey exists. If $code is another language code, this
1225          * will ONLY return message keys for which MediaWiki:msgkey/$code exists.
1226          * @param string $code Language code
1227          * @return array Array of message keys (strings)
1228          */
1229         public function getAllMessageKeys( $code ) {
1230                 global $wgContLang;
1231
1232                 $this->load( $code );
1233                 if ( !isset( $this->mCache[$code] ) ) {
1234                         // Apparently load() failed
1235                         return null;
1236                 }
1237                 // Remove administrative keys
1238                 $cache = $this->mCache[$code];
1239                 unset( $cache['VERSION'] );
1240                 unset( $cache['EXPIRY'] );
1241                 unset( $cache['EXCESSIVE'] );
1242                 // Remove any !NONEXISTENT keys
1243                 $cache = array_diff( $cache, [ '!NONEXISTENT' ] );
1244
1245                 // Keys may appear with a capital first letter. lcfirst them.
1246                 return array_map( [ $wgContLang, 'lcfirst' ], array_keys( $cache ) );
1247         }
1248
1249         /**
1250          * Purge message caches when a MediaWiki: page is created, updated, or deleted
1251          *
1252          * @param Title $title Message page title
1253          * @param Content|null $content New content for edit/create, null on deletion
1254          * @since 1.29
1255          */
1256         public function updateMessageOverride( Title $title, Content $content = null ) {
1257                 global $wgContLang;
1258
1259                 $msgText = $this->getMessageTextFromContent( $content );
1260                 if ( $msgText === null ) {
1261                         $msgText = false; // treat as not existing
1262                 }
1263
1264                 $this->replace( $title->getDBkey(), $msgText );
1265
1266                 if ( $wgContLang->hasVariants() ) {
1267                         $wgContLang->updateConversionTable( $title );
1268                 }
1269         }
1270
1271         /**
1272          * @param Content|null $content Content or null if the message page does not exist
1273          * @return string|bool|null Returns false if $content is null and null on error
1274          */
1275         private function getMessageTextFromContent( Content $content = null ) {
1276                 // @TODO: could skip pseudo-messages like js/css here, based on content model
1277                 if ( $content ) {
1278                         // Message page exists...
1279                         // XXX: Is this the right way to turn a Content object into a message?
1280                         // NOTE: $content is typically either WikitextContent, JavaScriptContent or
1281                         //       CssContent. MessageContent is *not* used for storing messages, it's
1282                         //       only used for wrapping them when needed.
1283                         $msgText = $content->getWikitextForTransclusion();
1284                         if ( $msgText === false || $msgText === null ) {
1285                                 // This might be due to some kind of misconfiguration...
1286                                 $msgText = null;
1287                                 LoggerFactory::getInstance( 'MessageCache' )->warning(
1288                                         __METHOD__ . ": message content doesn't provide wikitext "
1289                                         . "(content model: " . $content->getModel() . ")" );
1290                         }
1291                 } else {
1292                         // Message page does not exist...
1293                         $msgText = false;
1294                 }
1295
1296                 return $msgText;
1297         }
1298 }