includes/db/loadbalancer/LoadBalancer.php

   1 <?php
   2 /**
   3  * Database load balancing.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  * @ingroup Database
  22  */
  23
  24 /**
  25  * Database load balancing object
  26  *
  27  * @todo document
  28  * @ingroup Database
  29  */
  30 class LoadBalancer {
  31         /** @var array[] Map of (server index => server config array) */
  32         private $mServers;
  33         /** @var array[] Map of (local/foreignUsed/foreignFree => server index => DatabaseBase array) */
  34         private $mConns;
  35         /** @var array Map of (server index => weight) */
  36         private $mLoads;
  37         /** @var array[] Map of (group => server index => weight) */
  38         private $mGroupLoads;
  39         /** @var bool Whether to disregard slave lag as a factor in slave selection */
  40         private $mAllowLagged;
  41         /** @var integer Seconds to spend waiting on slave lag to resolve */
  42         private $mWaitTimeout;
  43         /** @var array LBFactory information */
  44         private $mParentInfo;
  45
  46         /** @var string The LoadMonitor subclass name */
  47         private $mLoadMonitorClass;
  48         /** @var LoadMonitor */
  49         private $mLoadMonitor;
  50         /** @var BagOStuff */
  51         private $srvCache;
  52
  53         /** @var bool|DatabaseBase Database connection that caused a problem */
  54         private $mErrorConnection;
  55         /** @var integer The generic (not query grouped) slave index (of $mServers) */
  56         private $mReadIndex;
  57         /** @var bool|DBMasterPos False if not set */
  58         private $mWaitForPos;
  59         /** @var bool Whether the generic reader fell back to a lagged slave */
  60         private $laggedSlaveMode = false;
  61         /** @var bool Whether the generic reader fell back to a lagged slave */
  62         private $slavesDownMode = false;
  63         /** @var string The last DB selection or connection error */
  64         private $mLastError = 'Unknown error';
  65         /** @var string|bool Reason the LB is read-only or false if not */
  66         private $readOnlyReason = false;
  67         /** @var integer Total connections opened */
  68         private $connsOpened = 0;
  69
  70         /** @var TransactionProfiler */
  71         protected $trxProfiler;
  72
  73         /** @var integer Warn when this many connection are held */
  74         const CONN_HELD_WARN_THRESHOLD = 10;
  75         /** @var integer Default 'max lag' when unspecified */
  76         const MAX_LAG = 10;
  77         /** @var integer Max time to wait for a slave to catch up (e.g. ChronologyProtector) */
  78         const POS_WAIT_TIMEOUT = 10;
  79
  80         /**
  81          * @param array $params Array with keys:
  82          *  - servers : Required. Array of server info structures.
  83          *  - loadMonitor : Name of a class used to fetch server lag and load.
  84          *  - readOnlyReason : Reason the master DB is read-only if so [optional]
  85          * @throws MWException
  86          */
  87         public function __construct( array $params ) {
  88                 if ( !isset( $params['servers'] ) ) {
  89                         throw new MWException( __CLASS__ . ': missing servers parameter' );
  90                 }
  91                 $this->mServers = $params['servers'];
  92                 $this->mWaitTimeout = self::POS_WAIT_TIMEOUT;
  93
  94                 $this->mReadIndex = -1;
  95                 $this->mWriteIndex = -1;
  96                 $this->mConns = [
  97                         'local' => [],
  98                         'foreignUsed' => [],
  99                         'foreignFree' => [] ];
 100                 $this->mLoads = [];
 101                 $this->mWaitForPos = false;
 102                 $this->mErrorConnection = false;
 103                 $this->mAllowLagged = false;
 104
 105                 if ( isset( $params['readOnlyReason'] ) && is_string( $params['readOnlyReason'] ) ) {
 106                         $this->readOnlyReason = $params['readOnlyReason'];
 107                 }
 108
 109                 if ( isset( $params['loadMonitor'] ) ) {
 110                         $this->mLoadMonitorClass = $params['loadMonitor'];
 111                 } else {
 112                         $master = reset( $params['servers'] );
 113                         if ( isset( $master['type'] ) && $master['type'] === 'mysql' ) {
 114                                 $this->mLoadMonitorClass = 'LoadMonitorMySQL';
 115                         } else {
 116                                 $this->mLoadMonitorClass = 'LoadMonitorNull';
 117                         }
 118                 }
 119
 120                 foreach ( $params['servers'] as $i => $server ) {
 121                         $this->mLoads[$i] = $server['load'];
 122                         if ( isset( $server['groupLoads'] ) ) {
 123                                 foreach ( $server['groupLoads'] as $group => $ratio ) {
 124                                         if ( !isset( $this->mGroupLoads[$group] ) ) {
 125                                                 $this->mGroupLoads[$group] = [];
 126                                         }
 127                                         $this->mGroupLoads[$group][$i] = $ratio;
 128                                 }
 129                         }
 130                 }
 131
 132                 $this->srvCache = ObjectCache::getLocalServerInstance();
 133
 134                 if ( isset( $params['trxProfiler'] ) ) {
 135                         $this->trxProfiler = $params['trxProfiler'];
 136                 } else {
 137                         $this->trxProfiler = new TransactionProfiler();
 138                 }
 139         }
 140
 141         /**
 142          * Get a LoadMonitor instance
 143          *
 144          * @return LoadMonitor
 145          */
 146         private function getLoadMonitor() {
 147                 if ( !isset( $this->mLoadMonitor ) ) {
 148                         $class = $this->mLoadMonitorClass;
 149                         $this->mLoadMonitor = new $class( $this );
 150                 }
 151
 152                 return $this->mLoadMonitor;
 153         }
 154
 155         /**
 156          * Get or set arbitrary data used by the parent object, usually an LBFactory
 157          * @param mixed $x
 158          * @return mixed
 159          */
 160         public function parentInfo( $x = null ) {
 161                 return wfSetVar( $this->mParentInfo, $x );
 162         }
 163
 164         /**
 165          * @param array $loads
 166          * @param bool|string $wiki Wiki to get non-lagged for
 167          * @param int $maxLag Restrict the maximum allowed lag to this many seconds
 168          * @return bool|int|string
 169          */
 170         private function getRandomNonLagged( array $loads, $wiki = false, $maxLag = self::MAX_LAG ) {
 171                 $lags = $this->getLagTimes( $wiki );
 172
 173                 # Unset excessively lagged servers
 174                 foreach ( $lags as $i => $lag ) {
 175                         if ( $i != 0 ) {
 176                                 $maxServerLag = $maxLag;
 177                                 if ( isset( $this->mServers[$i]['max lag'] ) ) {
 178                                         $maxServerLag = min( $maxServerLag, $this->mServers[$i]['max lag'] );
 179                                 }
 180
 181                                 $host = $this->getServerName( $i );
 182                                 if ( $lag === false ) {
 183                                         wfDebugLog( 'replication', "Server $host (#$i) is not replicating?" );
 184                                         unset( $loads[$i] );
 185                                 } elseif ( $lag > $maxServerLag ) {
 186                                         wfDebugLog( 'replication', "Server $host (#$i) has >= $lag seconds of lag" );
 187                                         unset( $loads[$i] );
 188                                 }
 189                         }
 190                 }
 191
 192                 # Find out if all the slaves with non-zero load are lagged
 193                 $sum = 0;
 194                 foreach ( $loads as $load ) {
 195                         $sum += $load;
 196                 }
 197                 if ( $sum == 0 ) {
 198                         # No appropriate DB servers except maybe the master and some slaves with zero load
 199                         # Do NOT use the master
 200                         # Instead, this function will return false, triggering read-only mode,
 201                         # and a lagged slave will be used instead.
 202                         return false;
 203                 }
 204
 205                 if ( count( $loads ) == 0 ) {
 206                         return false;
 207                 }
 208
 209                 # Return a random representative of the remainder
 210                 return ArrayUtils::pickRandom( $loads );
 211         }
 212
 213         /**
 214          * Get the index of the reader connection, which may be a slave
 215          * This takes into account load ratios and lag times. It should
 216          * always return a consistent index during a given invocation
 217          *
 218          * Side effect: opens connections to databases
 219          * @param string|bool $group Query group, or false for the generic reader
 220          * @param string|bool $wiki Wiki ID, or false for the current wiki
 221          * @throws MWException
 222          * @return bool|int|string
 223          */
 224         public function getReaderIndex( $group = false, $wiki = false ) {
 225                 global $wgDBtype;
 226
 227                 # @todo FIXME: For now, only go through all this for mysql databases
 228                 if ( $wgDBtype != 'mysql' ) {
 229                         return $this->getWriterIndex();
 230                 }
 231
 232                 if ( count( $this->mServers ) == 1 ) {
 233                         # Skip the load balancing if there's only one server
 234                         return 0;
 235                 } elseif ( $group === false && $this->mReadIndex >= 0 ) {
 236                         # Shortcut if generic reader exists already
 237                         return $this->mReadIndex;
 238                 }
 239
 240                 # Find the relevant load array
 241                 if ( $group !== false ) {
 242                         if ( isset( $this->mGroupLoads[$group] ) ) {
 243                                 $nonErrorLoads = $this->mGroupLoads[$group];
 244                         } else {
 245                                 # No loads for this group, return false and the caller can use some other group
 246                                 wfDebugLog( 'connect', __METHOD__ . ": no loads for group $group\n" );
 247
 248                                 return false;
 249                         }
 250                 } else {
 251                         $nonErrorLoads = $this->mLoads;
 252                 }
 253
 254                 if ( !count( $nonErrorLoads ) ) {
 255                         throw new MWException( "Empty server array given to LoadBalancer" );
 256                 }
 257
 258                 # Scale the configured load ratios according to the dynamic load (if the load monitor supports it)
 259                 $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $group, $wiki );
 260
 261                 $laggedSlaveMode = false;
 262
 263                 # No server found yet
 264                 $i = false;
 265                 $conn = false;
 266                 # First try quickly looking through the available servers for a server that
 267                 # meets our criteria
 268                 $currentLoads = $nonErrorLoads;
 269                 while ( count( $currentLoads ) ) {
 270                         if ( $this->mAllowLagged || $laggedSlaveMode ) {
 271                                 $i = ArrayUtils::pickRandom( $currentLoads );
 272                         } else {
 273                                 $i = false;
 274                                 if ( $this->mWaitForPos && $this->mWaitForPos->asOfTime() ) {
 275                                         # ChronologyProtecter causes mWaitForPos to be set via sessions.
 276                                         # This triggers doWait() after connect, so it's especially good to
 277                                         # avoid lagged servers so as to avoid just blocking in that method.
 278                                         $ago = microtime( true ) - $this->mWaitForPos->asOfTime();
 279                                         # Aim for <= 1 second of waiting (being too picky can backfire)
 280                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki, $ago + 1 );
 281                                 }
 282                                 if ( $i === false ) {
 283                                         # Any server with less lag than it's 'max lag' param is preferable
 284                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki );
 285                                 }
 286                                 if ( $i === false && count( $currentLoads ) != 0 ) {
 287                                         # All slaves lagged. Switch to read-only mode
 288                                         wfDebugLog( 'replication', "All slaves lagged. Switch to read-only mode" );
 289                                         $i = ArrayUtils::pickRandom( $currentLoads );
 290                                         $laggedSlaveMode = true;
 291                                 }
 292                         }
 293
 294                         if ( $i === false ) {
 295                                 # pickRandom() returned false
 296                                 # This is permanent and means the configuration or the load monitor
 297                                 # wants us to return false.
 298                                 wfDebugLog( 'connect', __METHOD__ . ": pickRandom() returned false" );
 299
 300                                 return false;
 301                         }
 302
 303                         $serverName = $this->getServerName( $i );
 304                         wfDebugLog( 'connect', __METHOD__ . ": Using reader #$i: $serverName..." );
 305
 306                         $conn = $this->openConnection( $i, $wiki );
 307                         if ( !$conn ) {
 308                                 wfDebugLog( 'connect', __METHOD__ . ": Failed connecting to $i/$wiki" );
 309                                 unset( $nonErrorLoads[$i] );
 310                                 unset( $currentLoads[$i] );
 311                                 $i = false;
 312                                 continue;
 313                         }
 314
 315                         // Decrement reference counter, we are finished with this connection.
 316                         // It will be incremented for the caller later.
 317                         if ( $wiki !== false ) {
 318                                 $this->reuseConnection( $conn );
 319                         }
 320
 321                         # Return this server
 322                         break;
 323                 }
 324
 325                 # If all servers were down, quit now
 326                 if ( !count( $nonErrorLoads ) ) {
 327                         wfDebugLog( 'connect', "All servers down" );
 328                 }
 329
 330                 if ( $i !== false ) {
 331                         # Slave connection successful
 332                         # Wait for the session master pos for a short time
 333                         if ( $this->mWaitForPos && $i > 0 ) {
 334                                 if ( !$this->doWait( $i ) ) {
 335                                         $this->mServers[$i]['slave pos'] = $conn->getSlavePos();
 336                                 }
 337                         }
 338                         if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group === false ) {
 339                                 $this->mReadIndex = $i;
 340                                 # Record if the generic reader index is in "lagged slave" mode
 341                                 if ( $laggedSlaveMode ) {
 342                                         $this->laggedSlaveMode = true;
 343                                 }
 344                         }
 345                         $serverName = $this->getServerName( $i );
 346                         wfDebugLog( 'connect', __METHOD__ .
 347                                 ": using server $serverName for group '$group'\n" );
 348                 }
 349
 350                 return $i;
 351         }
 352
 353         /**
 354          * Set the master wait position
 355          * If a DB_SLAVE connection has been opened already, waits
 356          * Otherwise sets a variable telling it to wait if such a connection is opened
 357          * @param DBMasterPos $pos
 358          */
 359         public function waitFor( $pos ) {
 360                 $this->mWaitForPos = $pos;
 361                 $i = $this->mReadIndex;
 362
 363                 if ( $i > 0 ) {
 364                         if ( !$this->doWait( $i ) ) {
 365                                 $this->mServers[$i]['slave pos'] = $this->getAnyOpenConnection( $i )->getSlavePos();
 366                                 $this->laggedSlaveMode = true;
 367                         }
 368                 }
 369         }
 370
 371         /**
 372          * Set the master wait position and wait for a "generic" slave to catch up to it
 373          *
 374          * This can be used a faster proxy for waitForAll()
 375          *
 376          * @param DBMasterPos $pos
 377          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 378          * @return bool Success (able to connect and no timeouts reached)
 379          * @since 1.26
 380          */
 381         public function waitForOne( $pos, $timeout = null ) {
 382                 $this->mWaitForPos = $pos;
 383
 384                 $i = $this->mReadIndex;
 385                 if ( $i <= 0 ) {
 386                         // Pick a generic slave if there isn't one yet
 387                         $readLoads = $this->mLoads;
 388                         unset( $readLoads[$this->getWriterIndex()] ); // slaves only
 389                         $readLoads = array_filter( $readLoads ); // with non-zero load
 390                         $i = ArrayUtils::pickRandom( $readLoads );
 391                 }
 392
 393                 if ( $i > 0 ) {
 394                         $ok = $this->doWait( $i, true, $timeout );
 395                 } else {
 396                         $ok = true; // no applicable loads
 397                 }
 398
 399                 return $ok;
 400         }
 401
 402         /**
 403          * Set the master wait position and wait for ALL slaves to catch up to it
 404          * @param DBMasterPos $pos
 405          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 406          * @return bool Success (able to connect and no timeouts reached)
 407          */
 408         public function waitForAll( $pos, $timeout = null ) {
 409                 $this->mWaitForPos = $pos;
 410                 $serverCount = count( $this->mServers );
 411
 412                 $ok = true;
 413                 for ( $i = 1; $i < $serverCount; $i++ ) {
 414                         if ( $this->mLoads[$i] > 0 ) {
 415                                 $ok = $this->doWait( $i, true, $timeout ) && $ok;
 416                         }
 417                 }
 418
 419                 return $ok;
 420         }
 421
 422         /**
 423          * Get any open connection to a given server index, local or foreign
 424          * Returns false if there is no connection open
 425          *
 426          * @param int $i
 427          * @return DatabaseBase|bool False on failure
 428          */
 429         public function getAnyOpenConnection( $i ) {
 430                 foreach ( $this->mConns as $conns ) {
 431                         if ( !empty( $conns[$i] ) ) {
 432                                 return reset( $conns[$i] );
 433                         }
 434                 }
 435
 436                 return false;
 437         }
 438
 439         /**
 440          * Wait for a given slave to catch up to the master pos stored in $this
 441          * @param int $index Server index
 442          * @param bool $open Check the server even if a new connection has to be made
 443          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 444          * @return bool
 445          */
 446         protected function doWait( $index, $open = false, $timeout = null ) {
 447                 $close = false; // close the connection afterwards
 448
 449                 // Check if we already know that the DB has reached this point
 450                 $server = $this->getServerName( $index );
 451                 $key = $this->srvCache->makeGlobalKey( __CLASS__, 'last-known-pos', $server );
 452                 /** @var DBMasterPos $knownReachedPos */
 453                 $knownReachedPos = $this->srvCache->get( $key );
 454                 if ( $knownReachedPos && $knownReachedPos->hasReached( $this->mWaitForPos ) ) {
 455                         wfDebugLog( 'replication', __METHOD__ .
 456                                 ": slave $server known to be caught up (pos >= $knownReachedPos).\n" );
 457                         return true;
 458                 }
 459
 460                 // Find a connection to wait on, creating one if needed and allowed
 461                 $conn = $this->getAnyOpenConnection( $index );
 462                 if ( !$conn ) {
 463                         if ( !$open ) {
 464                                 wfDebugLog( 'replication', __METHOD__ . ": no connection open for $server\n" );
 465
 466                                 return false;
 467                         } else {
 468                                 $conn = $this->openConnection( $index, '' );
 469                                 if ( !$conn ) {
 470                                         wfDebugLog( 'replication', __METHOD__ . ": failed to connect to $server\n" );
 471
 472                                         return false;
 473                                 }
 474                                 // Avoid connection spam in waitForAll() when connections
 475                                 // are made just for the sake of doing this lag check.
 476                                 $close = true;
 477                         }
 478                 }
 479
 480                 wfDebugLog( 'replication', __METHOD__ . ": Waiting for slave $server to catch up...\n" );
 481                 $timeout = $timeout ?: $this->mWaitTimeout;
 482                 $result = $conn->masterPosWait( $this->mWaitForPos, $timeout );
 483
 484                 if ( $result == -1 || is_null( $result ) ) {
 485                         // Timed out waiting for slave, use master instead
 486                         $msg = __METHOD__ . ": Timed out waiting on $server pos {$this->mWaitForPos}";
 487                         wfDebugLog( 'replication', "$msg\n" );
 488                         wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );
 489                         $ok = false;
 490                 } else {
 491                         wfDebugLog( 'replication', __METHOD__ . ": Done\n" );
 492                         $ok = true;
 493                         // Remember that the DB reached this point
 494                         $this->srvCache->set( $key, $this->mWaitForPos, BagOStuff::TTL_DAY );
 495                 }
 496
 497                 if ( $close ) {
 498                         $this->closeConnection( $conn );
 499                 }
 500
 501                 return $ok;
 502         }
 503
 504         /**
 505          * Get a connection by index
 506          * This is the main entry point for this class.
 507          *
 508          * @param int $i Server index
 509          * @param array|string|bool $groups Query group(s), or false for the generic reader
 510          * @param string|bool $wiki Wiki ID, or false for the current wiki
 511          *
 512          * @throws MWException
 513          * @return DatabaseBase
 514          */
 515         public function getConnection( $i, $groups = [], $wiki = false ) {
 516                 if ( $i === null || $i === false ) {
 517                         throw new MWException( 'Attempt to call ' . __METHOD__ .
 518                                 ' with invalid server index' );
 519                 }
 520
 521                 if ( $wiki === wfWikiID() ) {
 522                         $wiki = false;
 523                 }
 524
 525                 $groups = ( $groups === false || $groups === [] )
 526                         ? [ false ] // check one "group": the generic pool
 527                         : (array)$groups;
 528
 529                 $masterOnly = ( $i == DB_MASTER || $i == $this->getWriterIndex() );
 530                 $oldConnsOpened = $this->connsOpened; // connections open now
 531
 532                 if ( $i == DB_MASTER ) {
 533                         $i = $this->getWriterIndex();
 534                 } else {
 535                         # Try to find an available server in any the query groups (in order)
 536                         foreach ( $groups as $group ) {
 537                                 $groupIndex = $this->getReaderIndex( $group, $wiki );
 538                                 if ( $groupIndex !== false ) {
 539                                         $i = $groupIndex;
 540                                         break;
 541                                 }
 542                         }
 543                 }
 544
 545                 # Operation-based index
 546                 if ( $i == DB_SLAVE ) {
 547                         $this->mLastError = 'Unknown error'; // reset error string
 548                         # Try the general server pool if $groups are unavailable.
 549                         $i = in_array( false, $groups, true )
 550                                 ? false // don't bother with this if that is what was tried above
 551                                 : $this->getReaderIndex( false, $wiki );
 552                         # Couldn't find a working server in getReaderIndex()?
 553                         if ( $i === false ) {
 554                                 $this->mLastError = 'No working slave server: ' . $this->mLastError;
 555
 556                                 return $this->reportConnectionError();
 557                         }
 558                 }
 559
 560                 # Now we have an explicit index into the servers array
 561                 $conn = $this->openConnection( $i, $wiki );
 562                 if ( !$conn ) {
 563                         return $this->reportConnectionError();
 564                 }
 565
 566                 # Profile any new connections that happen
 567                 if ( $this->connsOpened > $oldConnsOpened ) {
 568                         $host = $conn->getServer();
 569                         $dbname = $conn->getDBname();
 570                         $trxProf = Profiler::instance()->getTransactionProfiler();
 571                         $trxProf->recordConnection( $host, $dbname, $masterOnly );
 572                 }
 573
 574                 if ( $masterOnly ) {
 575                         # Make master-requested DB handles inherit any read-only mode setting
 576                         $conn->setLBInfo( 'readOnlyReason', $this->getReadOnlyReason( $wiki ) );
 577                 }
 578
 579                 return $conn;
 580         }
 581
 582         /**
 583          * Mark a foreign connection as being available for reuse under a different
 584          * DB name or prefix. This mechanism is reference-counted, and must be called
 585          * the same number of times as getConnection() to work.
 586          *
 587          * @param DatabaseBase $conn
 588          * @throws MWException
 589          */
 590         public function reuseConnection( $conn ) {
 591                 $serverIndex = $conn->getLBInfo( 'serverIndex' );
 592                 $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 593                 if ( $serverIndex === null || $refCount === null ) {
 594                         wfDebug( __METHOD__ . ": this connection was not opened as a foreign connection\n" );
 595                         /**
 596                          * This can happen in code like:
 597                          *   foreach ( $dbs as $db ) {
 598                          *     $conn = $lb->getConnection( DB_SLAVE, array(), $db );
 599                          *     ...
 600                          *     $lb->reuseConnection( $conn );
 601                          *   }
 602                          * When a connection to the local DB is opened in this way, reuseConnection()
 603                          * should be ignored
 604                          */
 605                         return;
 606                 }
 607
 608                 $dbName = $conn->getDBname();
 609                 $prefix = $conn->tablePrefix();
 610                 if ( strval( $prefix ) !== '' ) {
 611                         $wiki = "$dbName-$prefix";
 612                 } else {
 613                         $wiki = $dbName;
 614                 }
 615                 if ( $this->mConns['foreignUsed'][$serverIndex][$wiki] !== $conn ) {
 616                         throw new MWException( __METHOD__ . ": connection not found, has " .
 617                                 "the connection been freed already?" );
 618                 }
 619                 $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
 620                 if ( $refCount <= 0 ) {
 621                         $this->mConns['foreignFree'][$serverIndex][$wiki] = $conn;
 622                         unset( $this->mConns['foreignUsed'][$serverIndex][$wiki] );
 623                         wfDebug( __METHOD__ . ": freed connection $serverIndex/$wiki\n" );
 624                 } else {
 625                         wfDebug( __METHOD__ . ": reference count for $serverIndex/$wiki reduced to $refCount\n" );
 626                 }
 627         }
 628
 629         /**
 630          * Get a database connection handle reference
 631          *
 632          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 633          *
 634          * @see LoadBalancer::getConnection() for parameter information
 635          *
 636          * @param int $db
 637          * @param array|string|bool $groups Query group(s), or false for the generic reader
 638          * @param string|bool $wiki Wiki ID, or false for the current wiki
 639          * @return DBConnRef
 640          */
 641         public function getConnectionRef( $db, $groups = [], $wiki = false ) {
 642                 return new DBConnRef( $this, $this->getConnection( $db, $groups, $wiki ) );
 643         }
 644
 645         /**
 646          * Get a database connection handle reference without connecting yet
 647          *
 648          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 649          *
 650          * @see LoadBalancer::getConnection() for parameter information
 651          *
 652          * @param int $db
 653          * @param array|string|bool $groups Query group(s), or false for the generic reader
 654          * @param string|bool $wiki Wiki ID, or false for the current wiki
 655          * @return DBConnRef
 656          */
 657         public function getLazyConnectionRef( $db, $groups = [], $wiki = false ) {
 658                 return new DBConnRef( $this, [ $db, $groups, $wiki ] );
 659         }
 660
 661         /**
 662          * Open a connection to the server given by the specified index
 663          * Index must be an actual index into the array.
 664          * If the server is already open, returns it.
 665          *
 666          * On error, returns false, and the connection which caused the
 667          * error will be available via $this->mErrorConnection.
 668          *
 669          * @param int $i Server index
 670          * @param string|bool $wiki Wiki ID, or false for the current wiki
 671          * @return DatabaseBase|bool Returns false on errors
 672          */
 673         public function openConnection( $i, $wiki = false ) {
 674                 if ( $wiki !== false ) {
 675                         $conn = $this->openForeignConnection( $i, $wiki );
 676                 } elseif ( isset( $this->mConns['local'][$i][0] ) ) {
 677                         $conn = $this->mConns['local'][$i][0];
 678                 } else {
 679                         $server = $this->mServers[$i];
 680                         $server['serverIndex'] = $i;
 681                         $conn = $this->reallyOpenConnection( $server, false );
 682                         $serverName = $this->getServerName( $i );
 683                         if ( $conn->isOpen() ) {
 684                                 wfDebugLog( 'connect', "Connected to database $i at $serverName\n" );
 685                                 $this->mConns['local'][$i][0] = $conn;
 686                         } else {
 687                                 wfDebugLog( 'connect', "Failed to connect to database $i at $serverName\n" );
 688                                 $this->mErrorConnection = $conn;
 689                                 $conn = false;
 690                         }
 691                 }
 692
 693                 if ( $conn && !$conn->isOpen() ) {
 694                         // Connection was made but later unrecoverably lost for some reason.
 695                         // Do not return a handle that will just throw exceptions on use,
 696                         // but let the calling code (e.g. getReaderIndex) try another server.
 697                         // See DatabaseMyslBase::ping() for how this can happen.
 698                         $this->mErrorConnection = $conn;
 699                         $conn = false;
 700                 }
 701
 702                 return $conn;
 703         }
 704
 705         /**
 706          * Open a connection to a foreign DB, or return one if it is already open.
 707          *
 708          * Increments a reference count on the returned connection which locks the
 709          * connection to the requested wiki. This reference count can be
 710          * decremented by calling reuseConnection().
 711          *
 712          * If a connection is open to the appropriate server already, but with the wrong
 713          * database, it will be switched to the right database and returned, as long as
 714          * it has been freed first with reuseConnection().
 715          *
 716          * On error, returns false, and the connection which caused the
 717          * error will be available via $this->mErrorConnection.
 718          *
 719          * @param int $i Server index
 720          * @param string $wiki Wiki ID to open
 721          * @return DatabaseBase
 722          */
 723         private function openForeignConnection( $i, $wiki ) {
 724                 list( $dbName, $prefix ) = wfSplitWikiID( $wiki );
 725                 if ( isset( $this->mConns['foreignUsed'][$i][$wiki] ) ) {
 726                         // Reuse an already-used connection
 727                         $conn = $this->mConns['foreignUsed'][$i][$wiki];
 728                         wfDebug( __METHOD__ . ": reusing connection $i/$wiki\n" );
 729                 } elseif ( isset( $this->mConns['foreignFree'][$i][$wiki] ) ) {
 730                         // Reuse a free connection for the same wiki
 731                         $conn = $this->mConns['foreignFree'][$i][$wiki];
 732                         unset( $this->mConns['foreignFree'][$i][$wiki] );
 733                         $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 734                         wfDebug( __METHOD__ . ": reusing free connection $i/$wiki\n" );
 735                 } elseif ( !empty( $this->mConns['foreignFree'][$i] ) ) {
 736                         // Reuse a connection from another wiki
 737                         $conn = reset( $this->mConns['foreignFree'][$i] );
 738                         $oldWiki = key( $this->mConns['foreignFree'][$i] );
 739
 740                         // The empty string as a DB name means "don't care".
 741                         // DatabaseMysqlBase::open() already handle this on connection.
 742                         if ( $dbName !== '' && !$conn->selectDB( $dbName ) ) {
 743                                 $this->mLastError = "Error selecting database $dbName on server " .
 744                                         $conn->getServer() . " from client host " . wfHostname() . "\n";
 745                                 $this->mErrorConnection = $conn;
 746                                 $conn = false;
 747                         } else {
 748                                 $conn->tablePrefix( $prefix );
 749                                 unset( $this->mConns['foreignFree'][$i][$oldWiki] );
 750                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 751                                 wfDebug( __METHOD__ . ": reusing free connection from $oldWiki for $wiki\n" );
 752                         }
 753                 } else {
 754                         // Open a new connection
 755                         $server = $this->mServers[$i];
 756                         $server['serverIndex'] = $i;
 757                         $server['foreignPoolRefCount'] = 0;
 758                         $server['foreign'] = true;
 759                         $conn = $this->reallyOpenConnection( $server, $dbName );
 760                         if ( !$conn->isOpen() ) {
 761                                 wfDebug( __METHOD__ . ": error opening connection for $i/$wiki\n" );
 762                                 $this->mErrorConnection = $conn;
 763                                 $conn = false;
 764                         } else {
 765                                 $conn->tablePrefix( $prefix );
 766                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 767                                 wfDebug( __METHOD__ . ": opened new connection for $i/$wiki\n" );
 768                         }
 769                 }
 770
 771                 // Increment reference count
 772                 if ( $conn ) {
 773                         $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 774                         $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
 775                 }
 776
 777                 return $conn;
 778         }
 779
 780         /**
 781          * Test if the specified index represents an open connection
 782          *
 783          * @param int $index Server index
 784          * @access private
 785          * @return bool
 786          */
 787         private function isOpen( $index ) {
 788                 if ( !is_integer( $index ) ) {
 789                         return false;
 790                 }
 791
 792                 return (bool)$this->getAnyOpenConnection( $index );
 793         }
 794
 795         /**
 796          * Really opens a connection. Uncached.
 797          * Returns a Database object whether or not the connection was successful.
 798          * @access private
 799          *
 800          * @param array $server
 801          * @param bool $dbNameOverride
 802          * @throws MWException
 803          * @return DatabaseBase
 804          */
 805         protected function reallyOpenConnection( $server, $dbNameOverride = false ) {
 806                 if ( !is_array( $server ) ) {
 807                         throw new MWException( 'You must update your load-balancing configuration. ' .
 808                                 'See DefaultSettings.php entry for $wgDBservers.' );
 809                 }
 810
 811                 if ( $dbNameOverride !== false ) {
 812                         $server['dbname'] = $dbNameOverride;
 813                 }
 814
 815                 // Let the handle know what the cluster master is (e.g. "db1052")
 816                 $masterName = $this->getServerName( 0 );
 817                 $server['clusterMasterHost'] = $masterName;
 818
 819                 // Log when many connection are made on requests
 820                 if ( ++$this->connsOpened >= self::CONN_HELD_WARN_THRESHOLD ) {
 821                         wfDebugLog( 'DBPerformance', __METHOD__ . ": " .
 822                                 "{$this->connsOpened}+ connections made (master=$masterName)\n" .
 823                                 wfBacktrace( true ) );
 824                 }
 825
 826                 # Create object
 827                 try {
 828                         $db = DatabaseBase::factory( $server['type'], $server );
 829                 } catch ( DBConnectionError $e ) {
 830                         // FIXME: This is probably the ugliest thing I have ever done to
 831                         // PHP. I'm half-expecting it to segfault, just out of disgust. -- TS
 832                         $db = $e->db;
 833                 }
 834
 835                 $db->setLBInfo( $server );
 836                 $db->setLazyMasterHandle(
 837                         $this->getLazyConnectionRef( DB_MASTER, [], $db->getWikiID() )
 838                 );
 839                 $db->setTransactionProfiler( $this->trxProfiler );
 840
 841                 return $db;
 842         }
 843
 844         /**
 845          * @throws DBConnectionError
 846          * @return bool
 847          */
 848         private function reportConnectionError() {
 849                 $conn = $this->mErrorConnection; // The connection which caused the error
 850                 $context = [
 851                         'method' => __METHOD__,
 852                         'last_error' => $this->mLastError,
 853                 ];
 854
 855                 if ( !is_object( $conn ) ) {
 856                         // No last connection, probably due to all servers being too busy
 857                         wfLogDBError(
 858                                 "LB failure with no last connection. Connection error: {last_error}",
 859                                 $context
 860                         );
 861
 862                         // If all servers were busy, mLastError will contain something sensible
 863                         throw new DBConnectionError( null, $this->mLastError );
 864                 } else {
 865                         $context['db_server'] = $conn->getProperty( 'mServer' );
 866                         wfLogDBError(
 867                                 "Connection error: {last_error} ({db_server})",
 868                                 $context
 869                         );
 870
 871                         // throws DBConnectionError
 872                         $conn->reportConnectionError( "{$this->mLastError} ({$context['db_server']})" );
 873                 }
 874
 875                 return false; /* not reached */
 876         }
 877
 878         /**
 879          * @return int
 880          * @since 1.26
 881          */
 882         public function getWriterIndex() {
 883                 return 0;
 884         }
 885
 886         /**
 887          * Returns true if the specified index is a valid server index
 888          *
 889          * @param string $i
 890          * @return bool
 891          */
 892         public function haveIndex( $i ) {
 893                 return array_key_exists( $i, $this->mServers );
 894         }
 895
 896         /**
 897          * Returns true if the specified index is valid and has non-zero load
 898          *
 899          * @param string $i
 900          * @return bool
 901          */
 902         public function isNonZeroLoad( $i ) {
 903                 return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
 904         }
 905
 906         /**
 907          * Get the number of defined servers (not the number of open connections)
 908          *
 909          * @return int
 910          */
 911         public function getServerCount() {
 912                 return count( $this->mServers );
 913         }
 914
 915         /**
 916          * Get the host name or IP address of the server with the specified index
 917          * Prefer a readable name if available.
 918          * @param string $i
 919          * @return string
 920          */
 921         public function getServerName( $i ) {
 922                 if ( isset( $this->mServers[$i]['hostName'] ) ) {
 923                         $name = $this->mServers[$i]['hostName'];
 924                 } elseif ( isset( $this->mServers[$i]['host'] ) ) {
 925                         $name = $this->mServers[$i]['host'];
 926                 } else {
 927                         $name = '';
 928                 }
 929
 930                 return ( $name != '' ) ? $name : 'localhost';
 931         }
 932
 933         /**
 934          * Return the server info structure for a given index, or false if the index is invalid.
 935          * @param int $i
 936          * @return array|bool
 937          */
 938         public function getServerInfo( $i ) {
 939                 if ( isset( $this->mServers[$i] ) ) {
 940                         return $this->mServers[$i];
 941                 } else {
 942                         return false;
 943                 }
 944         }
 945
 946         /**
 947          * Sets the server info structure for the given index. Entry at index $i
 948          * is created if it doesn't exist
 949          * @param int $i
 950          * @param array $serverInfo
 951          */
 952         public function setServerInfo( $i, array $serverInfo ) {
 953                 $this->mServers[$i] = $serverInfo;
 954         }
 955
 956         /**
 957          * Get the current master position for chronology control purposes
 958          * @return mixed
 959          */
 960         public function getMasterPos() {
 961                 # If this entire request was served from a slave without opening a connection to the
 962                 # master (however unlikely that may be), then we can fetch the position from the slave.
 963                 $masterConn = $this->getAnyOpenConnection( 0 );
 964                 if ( !$masterConn ) {
 965                         $serverCount = count( $this->mServers );
 966                         for ( $i = 1; $i < $serverCount; $i++ ) {
 967                                 $conn = $this->getAnyOpenConnection( $i );
 968                                 if ( $conn ) {
 969                                         return $conn->getSlavePos();
 970                                 }
 971                         }
 972                 } else {
 973                         return $masterConn->getMasterPos();
 974                 }
 975
 976                 return false;
 977         }
 978
 979         /**
 980          * Close all open connections
 981          */
 982         public function closeAll() {
 983                 foreach ( $this->mConns as $conns2 ) {
 984                         foreach ( $conns2 as $conns3 ) {
 985                                 /** @var DatabaseBase $conn */
 986                                 foreach ( $conns3 as $conn ) {
 987                                         $conn->close();
 988                                 }
 989                         }
 990                 }
 991                 $this->mConns = [
 992                         'local' => [],
 993                         'foreignFree' => [],
 994                         'foreignUsed' => [],
 995                 ];
 996                 $this->connsOpened = 0;
 997         }
 998
 999         /**
1000          * Close a connection
1001          * Using this function makes sure the LoadBalancer knows the connection is closed.
1002          * If you use $conn->close() directly, the load balancer won't update its state.
1003          * @param DatabaseBase $conn
1004          */
1005         public function closeConnection( $conn ) {
1006                 $done = false;
1007                 foreach ( $this->mConns as $i1 => $conns2 ) {
1008                         foreach ( $conns2 as $i2 => $conns3 ) {
1009                                 foreach ( $conns3 as $i3 => $candidateConn ) {
1010                                         if ( $conn === $candidateConn ) {
1011                                                 $conn->close();
1012                                                 unset( $this->mConns[$i1][$i2][$i3] );
1013                                                 --$this->connsOpened;
1014                                                 $done = true;
1015                                                 break;
1016                                         }
1017                                 }
1018                         }
1019                 }
1020                 if ( !$done ) {
1021                         $conn->close();
1022                 }
1023         }
1024
1025         /**
1026          * Commit transactions on all open connections
1027          * @param string $fname Caller name
1028          */
1029         public function commitAll( $fname = __METHOD__ ) {
1030                 foreach ( $this->mConns as $conns2 ) {
1031                         foreach ( $conns2 as $conns3 ) {
1032                                 /** @var DatabaseBase[] $conns3 */
1033                                 foreach ( $conns3 as $conn ) {
1034                                         if ( $conn->trxLevel() ) {
1035                                                 $conn->commit( $fname, 'flush' );
1036                                         }
1037                                 }
1038                         }
1039                 }
1040         }
1041
1042         /**
1043          * Issue COMMIT only on master, only if queries were done on connection
1044          * @param string $fname Caller name
1045          */
1046         public function commitMasterChanges( $fname = __METHOD__ ) {
1047                 $masterIndex = $this->getWriterIndex();
1048                 foreach ( $this->mConns as $conns2 ) {
1049                         if ( empty( $conns2[$masterIndex] ) ) {
1050                                 continue;
1051                         }
1052                         /** @var DatabaseBase $conn */
1053                         foreach ( $conns2[$masterIndex] as $conn ) {
1054                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1055                                         $conn->commit( $fname, 'flush' );
1056                                 }
1057                         }
1058                 }
1059         }
1060
1061         /**
1062          * Issue ROLLBACK only on master, only if queries were done on connection
1063          * @param string $fname Caller name
1064          * @throws DBExpectedError
1065          * @since 1.23
1066          */
1067         public function rollbackMasterChanges( $fname = __METHOD__ ) {
1068                 $failedServers = [];
1069
1070                 $masterIndex = $this->getWriterIndex();
1071                 foreach ( $this->mConns as $conns2 ) {
1072                         if ( empty( $conns2[$masterIndex] ) ) {
1073                                 continue;
1074                         }
1075                         /** @var DatabaseBase $conn */
1076                         foreach ( $conns2[$masterIndex] as $conn ) {
1077                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1078                                         try {
1079                                                 $conn->rollback( $fname, 'flush' );
1080                                         } catch ( DBError $e ) {
1081                                                 MWExceptionHandler::logException( $e );
1082                                                 $failedServers[] = $conn->getServer();
1083                                         }
1084                                 }
1085                         }
1086                 }
1087
1088                 if ( $failedServers ) {
1089                         throw new DBExpectedError( null, "Rollback failed on server(s) " .
1090                                 implode( ', ', array_unique( $failedServers ) ) );
1091                 }
1092         }
1093
1094         /**
1095          * @return bool Whether a master connection is already open
1096          * @since 1.24
1097          */
1098         public function hasMasterConnection() {
1099                 return $this->isOpen( $this->getWriterIndex() );
1100         }
1101
1102         /**
1103          * Determine if there are pending changes in a transaction by this thread
1104          * @since 1.23
1105          * @return bool
1106          */
1107         public function hasMasterChanges() {
1108                 $masterIndex = $this->getWriterIndex();
1109                 foreach ( $this->mConns as $conns2 ) {
1110                         if ( empty( $conns2[$masterIndex] ) ) {
1111                                 continue;
1112                         }
1113                         /** @var DatabaseBase $conn */
1114                         foreach ( $conns2[$masterIndex] as $conn ) {
1115                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1116                                         return true;
1117                                 }
1118                         }
1119                 }
1120                 return false;
1121         }
1122
1123         /**
1124          * Get the timestamp of the latest write query done by this thread
1125          * @since 1.25
1126          * @return float|bool UNIX timestamp or false
1127          */
1128         public function lastMasterChangeTimestamp() {
1129                 $lastTime = false;
1130                 $masterIndex = $this->getWriterIndex();
1131                 foreach ( $this->mConns as $conns2 ) {
1132                         if ( empty( $conns2[$masterIndex] ) ) {
1133                                 continue;
1134                         }
1135                         /** @var DatabaseBase $conn */
1136                         foreach ( $conns2[$masterIndex] as $conn ) {
1137                                 $lastTime = max( $lastTime, $conn->lastDoneWrites() );
1138                         }
1139                 }
1140                 return $lastTime;
1141         }
1142
1143         /**
1144          * Check if this load balancer object had any recent or still
1145          * pending writes issued against it by this PHP thread
1146          *
1147          * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
1148          * @return bool
1149          * @since 1.25
1150          */
1151         public function hasOrMadeRecentMasterChanges( $age = null ) {
1152                 $age = ( $age === null ) ? $this->mWaitTimeout : $age;
1153
1154                 return ( $this->hasMasterChanges()
1155                         || $this->lastMasterChangeTimestamp() > microtime( true ) - $age );
1156         }
1157
1158         /**
1159          * Get the list of callers that have pending master changes
1160          *
1161          * @return array
1162          * @since 1.27
1163          */
1164         public function pendingMasterChangeCallers() {
1165                 $fnames = [];
1166
1167                 $masterIndex = $this->getWriterIndex();
1168                 foreach ( $this->mConns as $conns2 ) {
1169                         if ( empty( $conns2[$masterIndex] ) ) {
1170                                 continue;
1171                         }
1172                         /** @var DatabaseBase $conn */
1173                         foreach ( $conns2[$masterIndex] as $conn ) {
1174                                 $fnames = array_merge( $fnames, $conn->pendingWriteCallers() );
1175                         }
1176                 }
1177
1178                 return $fnames;
1179         }
1180
1181         /**
1182          * @param mixed $value
1183          * @return mixed
1184          */
1185         public function waitTimeout( $value = null ) {
1186                 return wfSetVar( $this->mWaitTimeout, $value );
1187         }
1188
1189         /**
1190          * @note This method will trigger a DB connection if not yet done
1191          *
1192          * @param string|bool $wiki Wiki ID, or false for the current wiki
1193          * @return bool Whether the generic connection for reads is highly "lagged"
1194          */
1195         public function getLaggedSlaveMode( $wiki = false ) {
1196                 // No-op if there is only one DB (also avoids recursion)
1197                 if ( !$this->laggedSlaveMode && $this->getServerCount() > 1 ) {
1198                         try {
1199                                 // See if laggedSlaveMode gets set
1200                                 $conn = $this->getConnection( DB_SLAVE, false, $wiki );
1201                                 $this->reuseConnection( $conn );
1202                         } catch ( DBConnectionError $e ) {
1203                                 // Avoid expensive re-connect attempts and failures
1204                                 $this->slavesDownMode = true;
1205                                 $this->laggedSlaveMode = true;
1206                         }
1207                 }
1208
1209                 return $this->laggedSlaveMode;
1210         }
1211
1212         /**
1213          * @note This method will never cause a new DB connection
1214          * @return bool Whether any generic connection used for reads was highly "lagged"
1215          * @since 1.27
1216          */
1217         public function laggedSlaveUsed() {
1218                 return $this->laggedSlaveMode;
1219         }
1220
1221         /**
1222          * @note This method may trigger a DB connection if not yet done
1223          * @param string|bool $wiki Wiki ID, or false for the current wiki
1224          * @return string|bool Reason the master is read-only or false if it is not
1225          * @since 1.27
1226          */
1227         public function getReadOnlyReason( $wiki = false ) {
1228                 if ( $this->readOnlyReason !== false ) {
1229                         return $this->readOnlyReason;
1230                 } elseif ( $this->getLaggedSlaveMode( $wiki ) ) {
1231                         if ( $this->slavesDownMode ) {
1232                                 return 'The database has been automatically locked ' .
1233                                         'until the slave database servers become available';
1234                         } else {
1235                                 return 'The database has been automatically locked ' .
1236                                         'while the slave database servers catch up to the master.';
1237                         }
1238                 }
1239
1240                 return false;
1241         }
1242
1243         /**
1244          * Disables/enables lag checks
1245          * @param null|bool $mode
1246          * @return bool
1247          */
1248         public function allowLagged( $mode = null ) {
1249                 if ( $mode === null ) {
1250                         return $this->mAllowLagged;
1251                 }
1252                 $this->mAllowLagged = $mode;
1253
1254                 return $this->mAllowLagged;
1255         }
1256
1257         /**
1258          * @return bool
1259          */
1260         public function pingAll() {
1261                 $success = true;
1262                 foreach ( $this->mConns as $conns2 ) {
1263                         foreach ( $conns2 as $conns3 ) {
1264                                 /** @var DatabaseBase[] $conns3 */
1265                                 foreach ( $conns3 as $conn ) {
1266                                         if ( !$conn->ping() ) {
1267                                                 $success = false;
1268                                         }
1269                                 }
1270                         }
1271                 }
1272
1273                 return $success;
1274         }
1275
1276         /**
1277          * Call a function with each open connection object
1278          * @param callable $callback
1279          * @param array $params
1280          */
1281         public function forEachOpenConnection( $callback, array $params = [] ) {
1282                 foreach ( $this->mConns as $conns2 ) {
1283                         foreach ( $conns2 as $conns3 ) {
1284                                 foreach ( $conns3 as $conn ) {
1285                                         $mergedParams = array_merge( [ $conn ], $params );
1286                                         call_user_func_array( $callback, $mergedParams );
1287                                 }
1288                         }
1289                 }
1290         }
1291
1292         /**
1293          * Get the hostname and lag time of the most-lagged slave
1294          *
1295          * This is useful for maintenance scripts that need to throttle their updates.
1296          * May attempt to open connections to slaves on the default DB. If there is
1297          * no lag, the maximum lag will be reported as -1.
1298          *
1299          * @param bool|string $wiki Wiki ID, or false for the default database
1300          * @return array ( host, max lag, index of max lagged host )
1301          */
1302         public function getMaxLag( $wiki = false ) {
1303                 $maxLag = -1;
1304                 $host = '';
1305                 $maxIndex = 0;
1306
1307                 if ( $this->getServerCount() <= 1 ) {
1308                         return [ $host, $maxLag, $maxIndex ]; // no replication = no lag
1309                 }
1310
1311                 $lagTimes = $this->getLagTimes( $wiki );
1312                 foreach ( $lagTimes as $i => $lag ) {
1313                         if ( $lag > $maxLag ) {
1314                                 $maxLag = $lag;
1315                                 $host = $this->mServers[$i]['host'];
1316                                 $maxIndex = $i;
1317                         }
1318                 }
1319
1320                 return [ $host, $maxLag, $maxIndex ];
1321         }
1322
1323         /**
1324          * Get an estimate of replication lag (in seconds) for each server
1325          *
1326          * Results are cached for a short time in memcached/process cache
1327          *
1328          * Values may be "false" if replication is too broken to estimate
1329          *
1330          * @param string|bool $wiki
1331          * @return int[] Map of (server index => float|int|bool)
1332          */
1333         public function getLagTimes( $wiki = false ) {
1334                 if ( $this->getServerCount() <= 1 ) {
1335                         return [ 0 => 0 ]; // no replication = no lag
1336                 }
1337
1338                 # Send the request to the load monitor
1339                 return $this->getLoadMonitor()->getLagTimes( array_keys( $this->mServers ), $wiki );
1340         }
1341
1342         /**
1343          * Get the lag in seconds for a given connection, or zero if this load
1344          * balancer does not have replication enabled.
1345          *
1346          * This should be used in preference to Database::getLag() in cases where
1347          * replication may not be in use, since there is no way to determine if
1348          * replication is in use at the connection level without running
1349          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
1350          * function instead of Database::getLag() avoids a fatal error in this
1351          * case on many installations.
1352          *
1353          * @param IDatabase $conn
1354          * @return int|bool Returns false on error
1355          */
1356         public function safeGetLag( IDatabase $conn ) {
1357                 if ( $this->getServerCount() == 1 ) {
1358                         return 0;
1359                 } else {
1360                         return $conn->getLag();
1361                 }
1362         }
1363
1364         /**
1365          * Wait for a slave DB to reach a specified master position
1366          *
1367          * This will connect to the master to get an accurate position if $pos is not given
1368          *
1369          * @param IDatabase $conn Slave DB
1370          * @param DBMasterPos|bool $pos Master position; default: current position
1371          * @param integer $timeout Timeout in seconds
1372          * @return bool Success
1373          * @since 1.27
1374          */
1375         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 ) {
1376                 if ( $this->getServerCount() == 1 || !$conn->getLBInfo( 'slave' ) ) {
1377                         return true; // server is not a slave DB
1378                 }
1379
1380                 $pos = $pos ?: $this->getConnection( DB_MASTER )->getMasterPos();
1381                 if ( !$pos ) {
1382                         return false; // something is misconfigured
1383                 }
1384
1385                 $result = $conn->masterPosWait( $pos, $timeout );
1386                 if ( $result == -1 || is_null( $result ) ) {
1387                         $msg = __METHOD__ . ": Timed out waiting on {$conn->getServer()} pos {$pos}";
1388                         wfDebugLog( 'replication', "$msg\n" );
1389                         wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );
1390                         $ok = false;
1391                 } else {
1392                         wfDebugLog( 'replication', __METHOD__ . ": Done\n" );
1393                         $ok = true;
1394                 }
1395
1396                 return $ok;
1397         }
1398
1399         /**
1400          * Clear the cache for slag lag delay times
1401          *
1402          * This is only used for testing
1403          */
1404         public function clearLagTimeCache() {
1405                 $this->getLoadMonitor()->clearCaches();
1406         }
1407 }