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