maintenance/storage/recompressTracked.php

   1 <?php
   2
   3 $optionsWithArgs = RecompressTracked::getOptionsWithArgs();
   4 require( dirname( __FILE__ ) . '/../commandLine.inc' );
   5
   6 if ( count( $args ) < 1 ) {
   7         echo "Usage: php recompressTracked.php [options] <cluster> [... <cluster>...]
   8 Moves blobs indexed by trackBlobs.php to a specified list of destination clusters, and recompresses them in the process. Restartable.
   9
  10 Options:
  11     --procs <procs>         Set the number of child processes (default 1)
  12         --copy-only             Copy only, do not update the text table. Restart without this option to complete.
  13         --debug-log <file>      Log debugging data to the specified file
  14         --info-log <file>       Log progress messages to the specified file
  15         --critical-log <file>   Log error messages to the specified file
  16 ";
  17         exit( 1 );
  18 }
  19
  20 $job = RecompressTracked::newFromCommandLine( $args, $options );
  21 $job->execute();
  22
  23 class RecompressTracked {
  24         var $destClusters;
  25         var $batchSize = 1000;
  26         var $orphanBatchSize = 1000;
  27         var $reportingInterval = 10;
  28         var $numProcs = 1;
  29         var $useDiff, $pageBlobClass, $orphanBlobClass;
  30         var $slavePipes, $slaveProcs, $prevSlaveId;
  31         var $copyOnly = false;
  32         var $isChild = false;
  33         var $slaveId = false;
  34         var $noCount = false;
  35         var $debugLog, $infoLog, $criticalLog;
  36         var $store;
  37
  38         static $optionsWithArgs = array( 'procs', 'slave-id', 'debug-log', 'info-log', 'critical-log' );
  39         static $cmdLineOptionMap = array(
  40                 'no-count' => 'noCount',
  41                 'procs' => 'numProcs',
  42                 'copy-only' => 'copyOnly',
  43                 'child' => 'isChild',
  44                 'slave-id' => 'slaveId',
  45                 'debug-log' => 'debugLog',
  46                 'info-log' => 'infoLog',
  47                 'critical-log' => 'criticalLog',
  48         );
  49
  50         static function getOptionsWithArgs() {
  51                 return self::$optionsWithArgs;
  52         }
  53
  54         static function newFromCommandLine( $args, $options ) {
  55                 $jobOptions = array( 'destClusters' => $args );
  56                 foreach ( self::$cmdLineOptionMap as $cmdOption => $classOption ) {
  57                         if ( isset( $options[$cmdOption] ) ) {
  58                                 $jobOptions[$classOption] = $options[$cmdOption];
  59                         }
  60                 }
  61                 return new self( $jobOptions );
  62         }
  63
  64         function __construct( $options ) {
  65                 foreach ( $options as $name => $value ) {
  66                         $this->$name = $value;
  67                 }
  68                 $this->store = new ExternalStoreDB;
  69                 if ( !$this->isChild ) {
  70                         $GLOBALS['wgDebugLogPrefix'] = "RCT M: ";
  71                 } elseif ( $this->slaveId !== false ) {
  72                         $GLOBALS['wgDebugLogPrefix'] = "RCT {$this->slaveId}: ";
  73                 }
  74                 $this->useDiff = function_exists( 'xdiff_string_bdiff' );
  75                 $this->pageBlobClass = $this->useDiff ? 'DiffHistoryBlob' : 'ConcatenatedGzipHistoryBlob';
  76                 $this->orphanBlobClass = 'ConcatenatedGzipHistoryBlob';
  77         }
  78
  79         function debug( $msg ) {
  80                 wfDebug( "$msg\n" );
  81                 if ( $this->debugLog ) {
  82                         $this->logToFile( $msg, $this->debugLog );
  83                 }
  84
  85         }
  86
  87         function info( $msg ) {
  88                 echo "$msg\n";
  89                 if ( $this->infoLog ) {
  90                         $this->logToFile( $msg, $this->infoLog );
  91                 }
  92         }
  93
  94         function critical( $msg ) {
  95                 echo "$msg\n";
  96                 if ( $this->criticalLog ) {
  97                         $this->logToFile( $msg, $this->criticalLog );
  98                 }
  99         }
 100
 101         function logToFile( $msg, $file ) {
 102                 $header = '[' . date( 'd\TH:i:s' ) . '] ' . wfHostname() . ' ' . posix_getpid();
 103                 if ( $this->slaveId !== false ) {
 104                         $header .= "({$this->slaveId})";
 105                 }
 106                 $header .= ' ' . wfWikiID();
 107                 wfErrorLog( sprintf( "%-50s %s\n", $header, $msg ), $file );
 108         }
 109
 110         /**
 111          * Wait until the selected slave has caught up to the master.
 112          * This allows us to use the slave for things that were committed in a
 113          * previous part of this batch process.
 114          */
 115         function syncDBs() {
 116                 $dbw = wfGetDB( DB_MASTER );
 117                 $dbr = wfGetDB( DB_SLAVE );
 118                 $pos = $dbw->getMasterPos();
 119                 $dbr->masterPosWait( $pos, 100000 );
 120         }
 121
 122         /**
 123          * Execute parent or child depending on the isChild option
 124          */
 125         function execute() {
 126                 if ( $this->isChild ) {
 127                         $this->executeChild();
 128                 } else {
 129                         $this->executeParent();
 130                 }
 131         }
 132
 133         /**
 134          * Execute the parent process
 135          */
 136         function executeParent() {
 137                 if ( !$this->checkTrackingTable() ) {
 138                         return;
 139                 }
 140
 141                 $this->syncDBs();
 142                 $this->startSlaveProcs();
 143                 $this->doAllPages();
 144                 $this->doAllOrphans();
 145                 $this->killSlaveProcs();
 146         }
 147
 148         /**
 149          * Make sure the tracking table exists and isn't empty
 150          */
 151         function checkTrackingTable() {
 152                 $dbr = wfGetDB( DB_SLAVE );
 153                 if ( !$dbr->tableExists( 'blob_tracking' ) ) {
 154                         $this->critical( "Error: blob_tracking table does not exist" );
 155                         return false;
 156                 }
 157                 $row = $dbr->selectRow( 'blob_tracking', '*', false, __METHOD__ );
 158                 if ( !$row ) {
 159                         $this->info( "Warning: blob_tracking table contains no rows, skipping this wiki." );
 160                         return false;
 161                 }
 162                 return true;
 163         }
 164
 165         /**
 166          * Start the worker processes.
 167          * These processes will listen on stdin for commands.
 168          * This necessary because text recompression is slow: loading, compressing and
 169          * writing are all slow.
 170          */
 171         function startSlaveProcs() {
 172                 $cmd = 'php ' . wfEscapeShellArg( __FILE__ );
 173                 foreach ( self::$cmdLineOptionMap as $cmdOption => $classOption ) {
 174                         if ( $cmdOption == 'slave-id' ) {
 175                                 continue;
 176                         } elseif ( in_array( $cmdOption, self::$optionsWithArgs ) && isset( $this->$classOption ) ) {
 177                                 $cmd .= " --$cmdOption " . wfEscapeShellArg( $this->$classOption );
 178                         } elseif ( $this->$classOption ) {
 179                                 $cmd .= " --$cmdOption";
 180                         }
 181                 }
 182                 $cmd .= ' --child' .
 183                         ' --wiki ' . wfEscapeShellArg( wfWikiID() ) .
 184                         ' ' . call_user_func_array( 'wfEscapeShellArg', $this->destClusters );
 185
 186                 $this->slavePipes = $this->slaveProcs = array();
 187                 for ( $i = 0; $i < $this->numProcs; $i++ ) {
 188                         $pipes = false;
 189                         $spec = array(
 190                                 array( 'pipe', 'r' ),
 191                                 array( 'file', 'php://stdout', 'w' ),
 192                                 array( 'file', 'php://stderr', 'w' )
 193                         );
 194                         wfSuppressWarnings();
 195                         $proc = proc_open( "$cmd --slave-id $i", $spec, $pipes );
 196                         wfRestoreWarnings();
 197                         if ( !$proc ) {
 198                                 $this->critical( "Error opening slave process: $cmd" );
 199                                 exit( 1 );
 200                         }
 201                         $this->slaveProcs[$i] = $proc;
 202                         $this->slavePipes[$i] = $pipes[0];
 203                 }
 204                 $this->prevSlaveId = -1;
 205         }
 206
 207         /**
 208          * Gracefully terminate the child processes
 209          */
 210         function killSlaveProcs() {
 211                 $this->info( "Waiting for slave processes to finish..." );
 212                 for ( $i = 0; $i < $this->numProcs; $i++ ) {
 213                         $this->dispatchToSlave( $i, 'quit' );
 214                 }
 215                 for ( $i = 0; $i < $this->numProcs; $i++ ) {
 216                         $status = proc_close( $this->slaveProcs[$i] );
 217                         if ( $status ) {
 218                                 $this->critical( "Warning: child #$i exited with status $status" );
 219                         }
 220                 }
 221                 $this->info( "Done." );
 222         }
 223
 224         /**
 225          * Dispatch a command to the next available slave.
 226          * This may block until a slave finishes its work and becomes available.
 227          */
 228         function dispatch( /*...*/ ) {
 229                 $args = func_get_args();
 230                 $pipes = $this->slavePipes;
 231                 $numPipes = stream_select( $x = array(), $pipes, $y = array(), 3600 );
 232                 if ( !$numPipes ) {
 233                         $this->critical( "Error waiting to write to slaves. Aborting" );
 234                         exit( 1 );
 235                 }
 236                 for ( $i = 0; $i < $this->numProcs; $i++ ) {
 237                         $slaveId = ( $i + $this->prevSlaveId + 1 ) % $this->numProcs;
 238                         if ( isset( $pipes[$slaveId] ) ) {
 239                                 $this->prevSlaveId = $slaveId;
 240                                 $this->dispatchToSlave( $slaveId, $args );
 241                                 return;
 242                         }
 243                 }
 244                 $this->critical( "Unreachable" );
 245                 exit( 1 );
 246         }
 247
 248         /**
 249          * Dispatch a command to a specified slave
 250          */
 251         function dispatchToSlave( $slaveId, $args ) {
 252                 $args = (array)$args;
 253                 $cmd = implode( ' ',  $args );
 254                 fwrite( $this->slavePipes[$slaveId], "$cmd\n" );
 255         }
 256
 257         /**
 258          * Move all tracked pages to the new clusters
 259          */
 260         function doAllPages() {
 261                 $dbr = wfGetDB( DB_SLAVE );
 262                 $i = 0;
 263                 $startId = 0;
 264                 if ( $this->noCount ) {
 265                         $numPages = '[unknown]';
 266                 } else {
 267                         $numPages = $dbr->selectField( 'blob_tracking',
 268                                 'COUNT(DISTINCT bt_page)',
 269                                 # A condition is required so that this query uses the index
 270                                 array( 'bt_moved' => 0 ),
 271                                 __METHOD__
 272                         );
 273                 }
 274                 if ( $this->copyOnly ) {
 275                         $this->info( "Copying pages..." );
 276                 } else {
 277                         $this->info( "Moving pages..." );
 278                 }
 279                 while ( true ) {
 280                         $res = $dbr->select( 'blob_tracking',
 281                                 array( 'bt_page' ),
 282                                 array(
 283                                         'bt_moved' => 0,
 284                                         'bt_page > ' . $dbr->addQuotes( $startId )
 285                                 ),
 286                                 __METHOD__,
 287                                 array(
 288                                         'DISTINCT',
 289                                         'ORDER BY' => 'bt_page',
 290                                         'LIMIT' => $this->batchSize,
 291                                 )
 292                         );
 293                         if ( !$res->numRows() ) {
 294                                 break;
 295                         }
 296                         foreach ( $res as $row ) {
 297                                 $this->dispatch( 'doPage', $row->bt_page );
 298                                 $i++;
 299                         }
 300                         $startId = $row->bt_page;
 301                         $this->report( 'pages', $i, $numPages );
 302                 }
 303                 $this->report( 'pages', $i, $numPages );
 304                 if ( $this->copyOnly ) {
 305                         $this->info( "All page copies queued." );
 306                 } else {
 307                         $this->info( "All page moves queued." );
 308                 }
 309         }
 310
 311         /**
 312          * Display a progress report
 313          */
 314         function report( $label, $current, $end ) {
 315                 $this->numBatches++;
 316                 if ( $current == $end || $this->numBatches >= $this->reportingInterval ) {
 317                         $this->numBatches = 0;
 318                         $this->info( "$label: $current / $end" );
 319                         $this->waitForSlaves();
 320                 }
 321         }
 322
 323         /**
 324          * Move all orphan text to the new clusters
 325          */
 326         function doAllOrphans() {
 327                 $dbr = wfGetDB( DB_SLAVE );
 328                 $startId = 0;
 329                 $i = 0;
 330                 if ( $this->noCount ) {
 331                         $numOrphans = '[unknown]';
 332                 } else {
 333                         $numOrphans = $dbr->selectField( 'blob_tracking',
 334                                 'COUNT(DISTINCT bt_text_id)',
 335                                 array( 'bt_moved' => 0, 'bt_page' => 0 ),
 336                                 __METHOD__ );
 337                         if ( !$numOrphans ) {
 338                                 return;
 339                         }
 340                 }
 341                 if ( $this->copyOnly ) {
 342                         $this->info( "Copying orphans..." );
 343                 } else {
 344                         $this->info( "Moving orphans..." );
 345                 }
 346
 347                 while ( true ) {
 348                         $res = $dbr->select( 'blob_tracking',
 349                                 array( 'bt_text_id' ),
 350                                 array(
 351                                         'bt_moved' => 0,
 352                                         'bt_page' => 0,
 353                                         'bt_text_id > ' . $dbr->addQuotes( $startId )
 354                                 ),
 355                                 __METHOD__,
 356                                 array(
 357                                         'DISTINCT',
 358                                         'ORDER BY' => 'bt_text_id',
 359                                         'LIMIT' => $this->batchSize
 360                                 )
 361                         );
 362                         if ( !$res->numRows() ) {
 363                                 break;
 364                         }
 365                         $ids = array();
 366                         foreach ( $res as $row ) {
 367                                 $ids[] = $row->bt_text_id;
 368                                 $i++;
 369                         }
 370                         // Need to send enough orphan IDs to the child at a time to fill a blob,
 371                         // so orphanBatchSize needs to be at least ~100.
 372                         // batchSize can be smaller or larger.
 373                         while ( count( $ids ) > $this->orphanBatchSize ) {
 374                                 $args = array_slice( $ids, 0, $this->orphanBatchSize );
 375                                 $ids = array_slice( $ids, $this->orphanBatchSize );
 376                                 array_unshift( $args, 'doOrphanList' );
 377                                 call_user_func_array( array( $this, 'dispatch' ), $args );
 378                         }
 379                         if ( count( $ids ) ) {
 380                                 $args = $ids;
 381                                 array_unshift( $args, 'doOrphanList' );
 382                                 call_user_func_array( array( $this, 'dispatch' ), $args );
 383                         }
 384
 385                         $startId = $row->bt_text_id;
 386                         $this->report( 'orphans', $i, $numOrphans );
 387                 }
 388                 $this->report( 'orphans', $i, $numOrphans );
 389                 $this->info( "All orphans queued." );
 390         }
 391
 392         /**
 393          * Main entry point for worker processes
 394          */
 395         function executeChild() {
 396                 $this->debug( 'starting' );
 397                 $this->syncDBs();
 398
 399                 while ( !feof( STDIN ) ) {
 400                         $line = rtrim( fgets( STDIN ) );
 401                         if ( $line == '' ) {
 402                                 continue;
 403                         }
 404                         $this->debug( $line );
 405                         $args = explode( ' ', $line );
 406                         $cmd = array_shift( $args );
 407                         switch ( $cmd ) {
 408                         case 'doPage':
 409                                 $this->doPage( intval( $args[0] ) );
 410                                 break;
 411                         case 'doOrphanList':
 412                                 $this->doOrphanList( array_map( 'intval', $args ) );
 413                                 break;
 414                         case 'quit':
 415                                 return;
 416                         }
 417                         $this->waitForSlaves();
 418                 }
 419         }
 420
 421         /**
 422          * Move tracked text in a given page
 423          */
 424         function doPage( $pageId ) {
 425                 $title = Title::newFromId( $pageId );
 426                 if ( $title ) {
 427                         $titleText = $title->getPrefixedText();
 428                 } else {
 429                         $titleText = '[deleted]';
 430                 }
 431                 $dbr = wfGetDB( DB_SLAVE );
 432
 433                 // Finish any incomplete transactions
 434                 if ( !$this->copyOnly ) {
 435                         $this->finishIncompleteMoves( array( 'bt_page' => $pageId ) );
 436                         $this->syncDBs();
 437                 }
 438
 439                 $startId = 0;
 440                 $trx = new CgzCopyTransaction( $this, $this->pageBlobClass );
 441
 442                 while ( true ) {
 443                         $res = $dbr->select(
 444                                 array( 'blob_tracking', 'text' ),
 445                                 '*',
 446                                 array(
 447                                         'bt_page' => $pageId,
 448                                         'bt_text_id > ' . $dbr->addQuotes( $startId ),
 449                                         'bt_moved' => 0,
 450                                         'bt_new_url IS NULL',
 451                                         'bt_text_id=old_id',
 452                                 ),
 453                                 __METHOD__,
 454                                 array(
 455                                         'ORDER BY' => 'bt_text_id',
 456                                         'LIMIT' => $this->batchSize
 457                                 )
 458                         );
 459                         if ( !$res->numRows() ) {
 460                                 break;
 461                         }
 462
 463                         $lastTextId = 0;
 464                         foreach ( $res as $row ) {
 465                                 if ( $lastTextId == $row->bt_text_id ) {
 466                                         // Duplicate (null edit)
 467                                         continue;
 468                                 }
 469                                 $lastTextId = $row->bt_text_id;
 470                                 // Load the text
 471                                 $text = Revision::getRevisionText( $row );
 472                                 if ( $text === false ) {
 473                                         $this->critical( "Error loading {$row->bt_rev_id}/{$row->bt_text_id}" );
 474                                         continue;
 475                                 }
 476
 477                                 // Queue it
 478                                 if ( !$trx->addItem( $text, $row->bt_text_id ) ) {
 479                                         $this->debug( "$titleText: committing blob with " . $trx->getSize() . " items" );
 480                                         $trx->commit();
 481                                         $trx = new CgzCopyTransaction( $this, $this->pageBlobClass );
 482                                         $this->waitForSlaves();
 483                                 }
 484                         }
 485                         $startId = $row->bt_text_id;
 486                 }
 487
 488                 $this->debug( "$titleText: committing blob with " . $trx->getSize() . " items" );
 489                 $trx->commit();
 490         }
 491
 492         /**
 493          * Atomic move operation.
 494          *
 495          * Write the new URL to the text table and set the bt_moved flag.
 496          *
 497          * This is done in a single transaction to provide restartable behaviour
 498          * without data loss.
 499          *
 500          * The transaction is kept short to reduce locking.
 501          */
 502         function moveTextRow( $textId, $url ) {
 503                 if ( $this->copyOnly ) {
 504                         $this->critical( "Internal error: can't call moveTextRow() in --copy-only mode" );
 505                         exit( 1 );
 506                 }
 507                 $dbw = wfGetDB( DB_MASTER );
 508                 $dbw->begin();
 509                 $dbw->update( 'text',
 510                         array( // set
 511                                 'old_text' => $url,
 512                                 'old_flags' => 'external,utf-8',
 513                         ),
 514                         array( // where
 515                                 'old_id' => $textId
 516                         ),
 517                         __METHOD__
 518                 );
 519                 $dbw->update( 'blob_tracking',
 520                         array( 'bt_moved' => 1 ),
 521                         array( 'bt_text_id' => $textId ),
 522                         __METHOD__
 523                 );
 524                 $dbw->commit();
 525         }
 526
 527         /**
 528          * Moves are done in two phases: bt_new_url and then bt_moved.
 529          *  - bt_new_url indicates that the text has been copied to the new cluster.
 530          *  - bt_moved indicates that the text table has been updated.
 531          *
 532          * This function completes any moves that only have done bt_new_url. This
 533          * can happen when the script is interrupted, or when --copy-only is used.
 534          */
 535         function finishIncompleteMoves( $conds ) {
 536                 $dbr = wfGetDB( DB_SLAVE );
 537
 538                 $startId = 0;
 539                 $conds = array_merge( $conds, array(
 540                         'bt_moved' => 0,
 541                         'bt_new_url IS NOT NULL'
 542                 ) );
 543                 while ( true ) {
 544                         $res = $dbr->select( 'blob_tracking',
 545                                 '*',
 546                                 array_merge( $conds, array( 'bt_text_id > ' . $dbr->addQuotes( $startId ) ) ),
 547                                 __METHOD__,
 548                                 array(
 549                                         'ORDER BY' => 'bt_text_id',
 550                                         'LIMIT' => $this->batchSize,
 551                                 )
 552                         );
 553                         if ( !$res->numRows() ) {
 554                                 break;
 555                         }
 556                         $this->debug( 'Incomplete: ' . $res->numRows() . ' rows' );
 557                         foreach ( $res as $row ) {
 558                                 $this->moveTextRow( $row->bt_text_id, $row->bt_new_url );
 559                                 if ( $row->bt_text_id % 10 == 0 ) {
 560                                         $this->waitForSlaves();
 561                                 }
 562                         }
 563                         $startId = $row->bt_text_id;
 564                 }
 565         }
 566
 567         /**
 568          * Returns the name of the next target cluster
 569          */
 570         function getTargetCluster() {
 571                 $cluster = next( $this->destClusters );
 572                 if ( $cluster === false ) {
 573                         $cluster = reset( $this->destClusters );
 574                 }
 575                 return $cluster;
 576         }
 577
 578         /**
 579          * Gets a DB master connection for the given external cluster name
 580          */
 581         function getExtDB( $cluster ) {
 582                 $lb = wfGetLBFactory()->getExternalLB( $cluster );
 583                 return $lb->getConnection( DB_MASTER );
 584         }
 585
 586         /**
 587          * Move an orphan text_id to the new cluster
 588          */
 589         function doOrphanList( $textIds ) {
 590                 // Finish incomplete moves
 591                 if ( !$this->copyOnly ) {
 592                         $this->finishIncompleteMoves( array( 'bt_text_id' => $textIds ) );
 593                         $this->syncDBs();
 594                 }
 595
 596                 $trx = new CgzCopyTransaction( $this, $this->orphanBlobClass );
 597
 598                 $res = wfGetDB( DB_SLAVE )->select(
 599                         array( 'text', 'blob_tracking' ),
 600                         array( 'old_id', 'old_text', 'old_flags' ),
 601                         array(
 602                                 'old_id' => $textIds,
 603                                 'bt_text_id=old_id',
 604                                 'bt_moved' => 0,
 605                         ),
 606                         __METHOD__,
 607                         array( 'DISTINCT' )
 608                 );
 609
 610                 foreach ( $res as $row ) {
 611                         $text = Revision::getRevisionText( $row );
 612                         if ( $text === false ) {
 613                                 $this->critical( "Error: cannot load revision text for old_id=$textId" );
 614                                 continue;
 615                         }
 616
 617                         if ( !$trx->addItem( $text, $row->old_id ) ) {
 618                                 $this->debug( "[orphan]: committing blob with " . $trx->getSize() . " rows" );
 619                                 $trx->commit();
 620                                 $trx = new CgzCopyTransaction( $this, $this->orphanBlobClass );
 621                                 $this->waitForSlaves();
 622                         }
 623                 }
 624                 $this->debug( "[orphan]: committing blob with " . $trx->getSize() . " rows" );
 625                 $trx->commit();
 626         }
 627
 628         /**
 629          * Wait for slaves (quietly)
 630          */
 631         function waitForSlaves() {
 632                 $lb = wfGetLB();
 633                 while ( true ) {
 634                         list( $host, $maxLag ) = $lb->getMaxLag();
 635                         if ( $maxLag < 2 ) {
 636                                 break;
 637                         }
 638                         sleep( 5 );
 639                 }
 640         }
 641 }
 642
 643 /**
 644  * Class to represent a recompression operation for a single CGZ blob
 645  */
 646 class CgzCopyTransaction {
 647         var $parent;
 648         var $blobClass;
 649         var $cgz;
 650         var $referrers;
 651
 652         /**
 653          * Create a transaction from a RecompressTracked object
 654          */
 655         function __construct( $parent, $blobClass ) {
 656                 $this->blobClass = $blobClass;
 657                 $this->cgz = false;
 658                 $this->texts = array();
 659                 $this->parent = $parent;
 660         }
 661
 662         /**
 663          * Add text.
 664          * Returns false if it's ready to commit.
 665          */
 666         function addItem( $text, $textId ) {
 667                 if ( !$this->cgz ) {
 668                         $class = $this->blobClass;
 669                         $this->cgz = new $class;
 670                 }
 671                 $hash = $this->cgz->addItem( $text );
 672                 $this->referrers[$textId] = $hash;
 673                 $this->texts[$textId] = $text;
 674                 return $this->cgz->isHappy();
 675         }
 676
 677         function getSize() {
 678                 return count( $this->texts );
 679         }
 680
 681         /**
 682          * Recompress text after some aberrant modification
 683          */
 684         function recompress() {
 685                 $class = $this->blobClass;
 686                 $this->cgz = new $class;
 687                 $this->referrers = array();
 688                 foreach ( $this->texts as $textId => $text ) {
 689                         $hash = $this->cgz->addItem( $text );
 690                         $this->referrers[$textId] = $hash;
 691                 }
 692         }
 693
 694         /**
 695          * Commit the blob.
 696          * Does nothing if no text items have been added.
 697          * May skip the move if --copy-only is set.
 698          */
 699         function commit() {
 700                 $originalCount = count( $this->texts );
 701                 if ( !$originalCount ) {
 702                         return;
 703                 }
 704
 705                 // Check to see if the target text_ids have been moved already.
 706                 //
 707                 // We originally read from the slave, so this can happen when a single
 708                 // text_id is shared between multiple pages. It's rare, but possible
 709                 // if a delete/move/undelete cycle splits up a null edit.
 710                 //
 711                 // We do a locking read to prevent closer-run race conditions.
 712                 $dbw = wfGetDB( DB_MASTER );
 713                 $dbw->begin();
 714                 $res = $dbw->select( 'blob_tracking',
 715                         array( 'bt_text_id', 'bt_moved' ),
 716                         array( 'bt_text_id' => array_keys( $this->referrers ) ),
 717                         __METHOD__, array( 'FOR UPDATE' ) );
 718                 $dirty = false;
 719                 foreach ( $res as $row ) {
 720                         if ( $row->bt_moved ) {
 721                                 # This row has already been moved, remove it
 722                                 $this->parent->debug( "TRX: conflict detected in old_id={$row->bt_text_id}" );
 723                                 unset( $this->texts[$row->bt_text_id] );
 724                                 $dirty = true;
 725                         }
 726                 }
 727
 728                 // Recompress the blob if necessary
 729                 if ( $dirty ) {
 730                         if ( !count( $this->texts ) ) {
 731                                 // All have been moved already
 732                                 if ( $originalCount > 1 ) {
 733                                         // This is suspcious, make noise
 734                                         $this->critical( "Warning: concurrent operation detected, are there two conflicting " .
 735                                                 "processes running, doing the same job?" );
 736                                 }
 737                                 return;
 738                         }
 739                         $this->recompress();
 740                 }
 741
 742                 // Insert the data into the destination cluster
 743                 $targetCluster = $this->parent->getTargetCluster();
 744                 $store = $this->parent->store;
 745                 $targetDB = $store->getMaster( $targetCluster );
 746                 $targetDB->clearFlag( DBO_TRX ); // we manage the transactions
 747                 $targetDB->begin();
 748                 $baseUrl = $this->parent->store->store( $targetCluster, serialize( $this->cgz ) );
 749
 750                 // Write the new URLs to the blob_tracking table
 751                 foreach ( $this->referrers as $textId => $hash ) {
 752                         $url = $baseUrl . '/' . $hash;
 753                         $dbw->update( 'blob_tracking',
 754                                 array( 'bt_new_url' => $url ),
 755                                 array(
 756                                         'bt_text_id' => $textId,
 757                                         'bt_moved' => 0, # Check for concurrent conflicting update
 758                                 ),
 759                                 __METHOD__
 760                         );
 761                 }
 762
 763                 $targetDB->commit();
 764                 // Critical section here: interruption at this point causes blob duplication
 765                 // Reversing the order of the commits would cause data loss instead
 766                 $dbw->commit();
 767
 768                 // Write the new URLs to the text table and set the moved flag
 769                 if ( !$this->parent->copyOnly ) {
 770                         foreach ( $this->referrers as $textId => $hash ) {
 771                                 $url = $baseUrl . '/' . $hash;
 772                                 $this->parent->moveTextRow( $textId, $url );
 773                         }
 774                 }
 775         }
 776 }
 777