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