includes/libs/rdbms/loadbalancer/LoadBalancer.php

   1 <?php
   2 /**
   3  * Database load balancing manager
   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  */
  22 namespace Wikimedia\Rdbms;
  23
  24 use Psr\Log\LoggerInterface;
  25 use Psr\Log\NullLogger;
  26 use Wikimedia\ScopedCallback;
  27 use BagOStuff;
  28 use EmptyBagOStuff;
  29 use WANObjectCache;
  30 use ArrayUtils;
  31 use LogicException;
  32 use UnexpectedValueException;
  33 use InvalidArgumentException;
  34 use RuntimeException;
  35 use Exception;
  36
  37 /**
  38  * Database connection, tracking, load balancing, and transaction manager for a cluster
  39  *
  40  * @ingroup Database
  41  */
  42 class LoadBalancer implements ILoadBalancer {
  43         /** @var ILoadMonitor */
  44         private $loadMonitor;
  45         /** @var callable|null Callback to run before the first connection attempt */
  46         private $chronologyCallback;
  47         /** @var BagOStuff */
  48         private $srvCache;
  49         /** @var WANObjectCache */
  50         private $wanCache;
  51         /** @var mixed Class name or object With profileIn/profileOut methods */
  52         private $profiler;
  53         /** @var TransactionProfiler */
  54         private $trxProfiler;
  55         /** @var LoggerInterface */
  56         private $replLogger;
  57         /** @var LoggerInterface */
  58         private $connLogger;
  59         /** @var LoggerInterface */
  60         private $queryLogger;
  61         /** @var LoggerInterface */
  62         private $perfLogger;
  63         /** @var callable Exception logger */
  64         private $errorLogger;
  65         /** @var callable Deprecation logger */
  66         private $deprecationLogger;
  67
  68         /** @var DatabaseDomain Local DB domain ID and default for selectDB() calls */
  69         private $localDomain;
  70
  71         /** @var Database[][][] Map of (connection category => server index => IDatabase[]) */
  72         private $conns;
  73
  74         /** @var array[] Map of (server index => server config array) */
  75         private $servers;
  76         /** @var array[] Map of (group => server index => weight) */
  77         private $groupLoads;
  78         /** @var bool Whether to disregard replica DB lag as a factor in replica DB selection */
  79         private $allowLagged;
  80         /** @var int Seconds to spend waiting on replica DB lag to resolve */
  81         private $waitTimeout;
  82         /** @var array The LoadMonitor configuration */
  83         private $loadMonitorConfig;
  84         /** @var string Alternate local DB domain instead of DatabaseDomain::getId() */
  85         private $localDomainIdAlias;
  86         /** @var int Amount of replication lag, in seconds, that is considered "high" */
  87         private $maxLag;
  88         /** @var string|null Default query group to use with getConnection() */
  89         private $defaultGroup;
  90
  91         /** @var string Current server name */
  92         private $hostname;
  93         /** @var bool Whether this PHP instance is for a CLI script */
  94         private $cliMode;
  95         /** @var string Agent name for query profiling */
  96         private $agent;
  97
  98         /** @var array[] $aliases Map of (table => (dbname, schema, prefix) map) */
  99         private $tableAliases = [];
 100         /** @var string[] Map of (index alias => index) */
 101         private $indexAliases = [];
 102         /** @var array[] Map of (name => callable) */
 103         private $trxRecurringCallbacks = [];
 104
 105         /** @var Database Connection handle that caused a problem */
 106         private $errorConnection;
 107         /** @var int[] The group replica server indexes keyed by group */
 108         private $readIndexByGroup = [];
 109         /** @var bool|DBMasterPos Replication sync position or false if not set */
 110         private $waitForPos;
 111         /** @var bool Whether the generic reader fell back to a lagged replica DB */
 112         private $laggedReplicaMode = false;
 113         /** @var string The last DB selection or connection error */
 114         private $lastError = 'Unknown error';
 115         /** @var string|bool Reason this instance is read-only or false if not */
 116         private $readOnlyReason = false;
 117         /** @var int Total number of new connections ever made with this instance */
 118         private $connectionCounter = 0;
 119         /** @var bool */
 120         private $disabled = false;
 121         /** @var bool Whether any connection has been attempted yet */
 122         private $connectionAttempted = false;
 123
 124         /** var int An identifier for this class instance */
 125         private $id;
 126         /** @var int|null Integer ID of the managing LBFactory instance or null if none */
 127         private $ownerId;
 128         /** @var string|bool Explicit DBO_TRX transaction round active or false if none */
 129         private $trxRoundId = false;
 130         /** @var string Stage of the current transaction round in the transaction round life-cycle */
 131         private $trxRoundStage = self::ROUND_CURSORY;
 132
 133         /** @var int Warn when this many connection are held */
 134         const CONN_HELD_WARN_THRESHOLD = 10;
 135
 136         /** @var int Default 'maxLag' when unspecified */
 137         const MAX_LAG_DEFAULT = 6;
 138         /** @var int Default 'waitTimeout' when unspecified */
 139         const MAX_WAIT_DEFAULT = 10;
 140         /** @var int Seconds to cache master DB server read-only status */
 141         const TTL_CACHE_READONLY = 5;
 142
 143         const KEY_LOCAL = 'local';
 144         const KEY_FOREIGN_FREE = 'foreignFree';
 145         const KEY_FOREIGN_INUSE = 'foreignInUse';
 146
 147         const KEY_LOCAL_NOROUND = 'localAutoCommit';
 148         const KEY_FOREIGN_FREE_NOROUND = 'foreignFreeAutoCommit';
 149         const KEY_FOREIGN_INUSE_NOROUND = 'foreignInUseAutoCommit';
 150
 151         /** @var string Transaction round, explicit or implicit, has not finished writing */
 152         const ROUND_CURSORY = 'cursory';
 153         /** @var string Transaction round writes are complete and ready for pre-commit checks */
 154         const ROUND_FINALIZED = 'finalized';
 155         /** @var string Transaction round passed final pre-commit checks */
 156         const ROUND_APPROVED = 'approved';
 157         /** @var string Transaction round was committed and post-commit callbacks must be run */
 158         const ROUND_COMMIT_CALLBACKS = 'commit-callbacks';
 159         /** @var string Transaction round was rolled back and post-rollback callbacks must be run */
 160         const ROUND_ROLLBACK_CALLBACKS = 'rollback-callbacks';
 161         /** @var string Transaction round encountered an error */
 162         const ROUND_ERROR = 'error';
 163
 164         public function __construct( array $params ) {
 165                 if ( !isset( $params['servers'] ) || !count( $params['servers'] ) ) {
 166                         throw new InvalidArgumentException( 'Missing or empty "servers" parameter' );
 167                 }
 168
 169                 $listKey = -1;
 170                 $this->servers = [];
 171                 $this->groupLoads = [ self::GROUP_GENERIC => [] ];
 172                 foreach ( $params['servers'] as $i => $server ) {
 173                         if ( ++$listKey !== $i ) {
 174                                 throw new UnexpectedValueException( 'List expected for "servers" parameter' );
 175                         }
 176                         $this->servers[$i] = $server;
 177                         foreach ( ( $server['groupLoads'] ?? [] ) as $group => $ratio ) {
 178                                 $this->groupLoads[$group][$i] = $ratio;
 179                         }
 180                         $this->groupLoads[self::GROUP_GENERIC][$i] = $server['load'];
 181                 }
 182
 183                 $localDomain = isset( $params['localDomain'] )
 184                         ? DatabaseDomain::newFromId( $params['localDomain'] )
 185                         : DatabaseDomain::newUnspecified();
 186                 $this->setLocalDomain( $localDomain );
 187
 188                 $this->waitTimeout = $params['waitTimeout'] ?? self::MAX_WAIT_DEFAULT;
 189
 190                 $this->conns = self::newTrackedConnectionsArray();
 191                 $this->waitForPos = false;
 192                 $this->allowLagged = false;
 193
 194                 if ( isset( $params['readOnlyReason'] ) && is_string( $params['readOnlyReason'] ) ) {
 195                         $this->readOnlyReason = $params['readOnlyReason'];
 196                 }
 197
 198                 $this->maxLag = $params['maxLag'] ?? self::MAX_LAG_DEFAULT;
 199
 200                 $this->loadMonitorConfig = $params['loadMonitor'] ?? [ 'class' => 'LoadMonitorNull' ];
 201                 $this->loadMonitorConfig += [ 'lagWarnThreshold' => $this->maxLag ];
 202
 203                 $this->srvCache = $params['srvCache'] ?? new EmptyBagOStuff();
 204                 $this->wanCache = $params['wanCache'] ?? WANObjectCache::newEmpty();
 205                 $this->profiler = $params['profiler'] ?? null;
 206                 $this->trxProfiler = $params['trxProfiler'] ?? new TransactionProfiler();
 207
 208                 $this->errorLogger = $params['errorLogger'] ?? function ( Exception $e ) {
 209                         trigger_error( get_class( $e ) . ': ' . $e->getMessage(), E_USER_WARNING );
 210                 };
 211                 $this->deprecationLogger = $params['deprecationLogger'] ?? function ( $msg ) {
 212                         trigger_error( $msg, E_USER_DEPRECATED );
 213                 };
 214
 215                 foreach ( [ 'replLogger', 'connLogger', 'queryLogger', 'perfLogger' ] as $key ) {
 216                         $this->$key = $params[$key] ?? new NullLogger();
 217                 }
 218
 219                 $this->hostname = $params['hostname'] ?? ( gethostname() ?: 'unknown' );
 220                 $this->cliMode = $params['cliMode'] ?? ( PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg' );
 221                 $this->agent = $params['agent'] ?? '';
 222
 223                 if ( isset( $params['chronologyCallback'] ) ) {
 224                         $this->chronologyCallback = $params['chronologyCallback'];
 225                 }
 226
 227                 if ( isset( $params['roundStage'] ) ) {
 228                         if ( $params['roundStage'] === self::STAGE_POSTCOMMIT_CALLBACKS ) {
 229                                 $this->trxRoundStage = self::ROUND_COMMIT_CALLBACKS;
 230                         } elseif ( $params['roundStage'] === self::STAGE_POSTROLLBACK_CALLBACKS ) {
 231                                 $this->trxRoundStage = self::ROUND_ROLLBACK_CALLBACKS;
 232                         }
 233                 }
 234
 235                 $group = $params['defaultGroup'] ?? self::GROUP_GENERIC;
 236                 $this->defaultGroup = isset( $this->groupLoads[$group] ) ? $group : self::GROUP_GENERIC;
 237
 238                 static $nextId;
 239                 $this->id = $nextId = ( is_int( $nextId ) ? $nextId++ : mt_rand() );
 240                 $this->ownerId = $params['ownerId'] ?? null;
 241         }
 242
 243         private static function newTrackedConnectionsArray() {
 244                 return [
 245                         // Connection were transaction rounds may be applied
 246                         self::KEY_LOCAL => [],
 247                         self::KEY_FOREIGN_INUSE => [],
 248                         self::KEY_FOREIGN_FREE => [],
 249                         // Auto-committing counterpart connections that ignore transaction rounds
 250                         self::KEY_LOCAL_NOROUND => [],
 251                         self::KEY_FOREIGN_INUSE_NOROUND => [],
 252                         self::KEY_FOREIGN_FREE_NOROUND => []
 253                 ];
 254         }
 255
 256         public function getLocalDomainID() {
 257                 return $this->localDomain->getId();
 258         }
 259
 260         public function resolveDomainID( $domain ) {
 261                 if ( $domain === $this->localDomainIdAlias || $domain === false ) {
 262                         // Local connection requested via some backwards-compatibility domain alias
 263                         return $this->getLocalDomainID();
 264                 }
 265
 266                 return (string)$domain;
 267         }
 268
 269         /**
 270          * Resolve $groups into a list of query groups defining as having database servers
 271          *
 272          * @param string[]|string|bool $groups Query group(s) in preference order, [], or false
 273          * @param int $i Specific server index or DB_MASTER/DB_REPLICA
 274          * @return string[] Non-empty group list in preference order with the default group appended
 275          */
 276         private function resolveGroups( $groups, $i ) {
 277                 // If a specific replica server was specified, then $groups makes no sense
 278                 if ( $i > 0 && $groups !== [] && $groups !== false ) {
 279                         $list = implode( ', ', (array)$groups );
 280                         throw new LogicException( "Query group(s) ($list) given with server index (#$i)" );
 281                 }
 282
 283                 if ( $groups === [] || $groups === false || $groups === $this->defaultGroup ) {
 284                         $resolvedGroups = [ $this->defaultGroup ]; // common case
 285                 } elseif ( is_string( $groups ) && isset( $this->groupLoads[$groups] ) ) {
 286                         $resolvedGroups = [ $groups, $this->defaultGroup ];
 287                 } elseif ( is_array( $groups ) ) {
 288                         $resolvedGroups = array_keys( array_flip( $groups ) + [ self::GROUP_GENERIC => 1 ] );
 289                 } else {
 290                         $resolvedGroups = [ $this->defaultGroup ];
 291                 }
 292
 293                 return $resolvedGroups;
 294         }
 295
 296         /**
 297          * @param int $flags Bitfield of class CONN_* constants
 298          * @param int $i Specific server index or DB_MASTER/DB_REPLICA
 299          * @return int Sanitized bitfield
 300          */
 301         private function sanitizeConnectionFlags( $flags, $i ) {
 302                 // Whether an outside caller is explicitly requesting the master database server
 303                 if ( $i === self::DB_MASTER || $i === $this->getWriterIndex() ) {
 304                         $flags |= self::CONN_INTENT_WRITABLE;
 305                 }
 306
 307                 if ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT ) {
 308                         // Callers use CONN_TRX_AUTOCOMMIT to bypass REPEATABLE-READ staleness without
 309                         // resorting to row locks (e.g. FOR UPDATE) or to make small out-of-band commits
 310                         // during larger transactions. This is useful for avoiding lock contention.
 311
 312                         // Master DB server attributes (should match those of the replica DB servers)
 313                         $attributes = $this->getServerAttributes( $this->getWriterIndex() );
 314                         if ( $attributes[Database::ATTR_DB_LEVEL_LOCKING] ) {
 315                                 // The RDBMS does not support concurrent writes (e.g. SQLite), so attempts
 316                                 // to use separate connections would just cause self-deadlocks. Note that
 317                                 // REPEATABLE-READ staleness is not an issue since DB-level locking means
 318                                 // that transactions are Strict Serializable anyway.
 319                                 $flags &= ~self::CONN_TRX_AUTOCOMMIT;
 320                                 $type = $this->getServerType( $this->getWriterIndex() );
 321                                 $this->connLogger->info( __METHOD__ . ": CONN_TRX_AUTOCOMMIT disallowed ($type)" );
 322                         }
 323                 }
 324
 325                 return $flags;
 326         }
 327
 328         /**
 329          * @param IDatabase $conn
 330          * @param int $flags
 331          * @throws DBUnexpectedError
 332          */
 333         private function enforceConnectionFlags( IDatabase $conn, $flags ) {
 334                 if ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT ) {
 335                         if ( $conn->trxLevel() ) { // sanity
 336                                 throw new DBUnexpectedError(
 337                                         $conn,
 338                                         'Handle requested with CONN_TRX_AUTOCOMMIT yet it has a transaction'
 339                                 );
 340                         }
 341
 342                         $conn->clearFlag( $conn::DBO_TRX ); // auto-commit mode
 343                 }
 344         }
 345
 346         /**
 347          * Get a LoadMonitor instance
 348          *
 349          * @return ILoadMonitor
 350          */
 351         private function getLoadMonitor() {
 352                 if ( !isset( $this->loadMonitor ) ) {
 353                         $compat = [
 354                                 'LoadMonitor' => LoadMonitor::class,
 355                                 'LoadMonitorNull' => LoadMonitorNull::class,
 356                                 'LoadMonitorMySQL' => LoadMonitorMySQL::class,
 357                         ];
 358
 359                         $class = $this->loadMonitorConfig['class'];
 360                         if ( isset( $compat[$class] ) ) {
 361                                 $class = $compat[$class];
 362                         }
 363
 364                         $this->loadMonitor = new $class(
 365                                 $this, $this->srvCache, $this->wanCache, $this->loadMonitorConfig );
 366                         $this->loadMonitor->setLogger( $this->replLogger );
 367                 }
 368
 369                 return $this->loadMonitor;
 370         }
 371
 372         /**
 373          * @param array $loads
 374          * @param bool|string $domain Domain to get non-lagged for
 375          * @param int $maxLag Restrict the maximum allowed lag to this many seconds
 376          * @return bool|int|string
 377          */
 378         private function getRandomNonLagged( array $loads, $domain = false, $maxLag = INF ) {
 379                 $lags = $this->getLagTimes( $domain );
 380
 381                 # Unset excessively lagged servers
 382                 foreach ( $lags as $i => $lag ) {
 383                         if ( $i !== $this->getWriterIndex() ) {
 384                                 # How much lag this server nominally is allowed to have
 385                                 $maxServerLag = $this->servers[$i]['max lag'] ?? $this->maxLag; // default
 386                                 # Constrain that futher by $maxLag argument
 387                                 $maxServerLag = min( $maxServerLag, $maxLag );
 388
 389                                 $host = $this->getServerName( $i );
 390                                 if ( $lag === false && !is_infinite( $maxServerLag ) ) {
 391                                         $this->replLogger->debug(
 392                                                 __METHOD__ .
 393                                                 ": server {host} is not replicating?", [ 'host' => $host ] );
 394                                         unset( $loads[$i] );
 395                                 } elseif ( $lag > $maxServerLag ) {
 396                                         $this->replLogger->debug(
 397                                                 __METHOD__ .
 398                                                 ": server {host} has {lag} seconds of lag (>= {maxlag})",
 399                                                 [ 'host' => $host, 'lag' => $lag, 'maxlag' => $maxServerLag ]
 400                                         );
 401                                         unset( $loads[$i] );
 402                                 }
 403                         }
 404                 }
 405
 406                 # Find out if all the replica DBs with non-zero load are lagged
 407                 $sum = 0;
 408                 foreach ( $loads as $load ) {
 409                         $sum += $load;
 410                 }
 411                 if ( $sum == 0 ) {
 412                         # No appropriate DB servers except maybe the master and some replica DBs with zero load
 413                         # Do NOT use the master
 414                         # Instead, this function will return false, triggering read-only mode,
 415                         # and a lagged replica DB will be used instead.
 416                         return false;
 417                 }
 418
 419                 if ( count( $loads ) == 0 ) {
 420                         return false;
 421                 }
 422
 423                 # Return a random representative of the remainder
 424                 return ArrayUtils::pickRandom( $loads );
 425         }
 426
 427         /**
 428          * Get the server index to use for a specified server index and query group list
 429          *
 430          * @param int $i Specific server index or DB_MASTER/DB_REPLICA
 431          * @param string[] $groups Non-empty query group list in preference order
 432          * @param string|bool $domain
 433          * @return int A specific server index (replica DBs are checked for connectivity)
 434          */
 435         private function getConnectionIndex( $i, array $groups, $domain ) {
 436                 if ( $i === self::DB_MASTER ) {
 437                         $i = $this->getWriterIndex();
 438                 } elseif ( $i === self::DB_REPLICA ) {
 439                         foreach ( $groups as $group ) {
 440                                 $groupIndex = $this->getReaderIndex( $group, $domain );
 441                                 if ( $groupIndex !== false ) {
 442                                         $i = $groupIndex; // group connection succeeded
 443                                         break;
 444                                 }
 445                         }
 446                 } elseif ( !isset( $this->servers[$i] ) ) {
 447                         throw new UnexpectedValueException( "Invalid server index index #$i" );
 448                 }
 449
 450                 if ( $i === self::DB_REPLICA ) {
 451                         $this->lastError = 'Unknown error'; // set here in case of worse failure
 452                         $this->lastError = 'No working replica DB server: ' . $this->lastError;
 453                         $this->reportConnectionError();
 454                         return null; // unreachable due to exception
 455                 }
 456
 457                 return $i;
 458         }
 459
 460         public function getReaderIndex( $group = false, $domain = false ) {
 461                 if ( $this->getServerCount() == 1 ) {
 462                         // Skip the load balancing if there's only one server
 463                         return $this->getWriterIndex();
 464                 }
 465
 466                 $group = is_string( $group ) ? $group : self::GROUP_GENERIC;
 467
 468                 $index = $this->getExistingReaderIndex( $group );
 469                 if ( $index >= 0 ) {
 470                         // A reader index was already selected and "waitForPos" was handled
 471                         return $index;
 472                 }
 473
 474                 // Use the server weight array for this load group
 475                 if ( isset( $this->groupLoads[$group] ) ) {
 476                         $loads = $this->groupLoads[$group];
 477                 } else {
 478                         $this->connLogger->info( __METHOD__ . ": no loads for group $group" );
 479
 480                         return false;
 481                 }
 482
 483                 // Scale the configured load ratios according to each server's load and state
 484                 $this->getLoadMonitor()->scaleLoads( $loads, $domain );
 485
 486                 // Pick a server to use, accounting for weights, load, lag, and "waitForPos"
 487                 $this->lazyLoadReplicationPositions(); // optimizes server candidate selection
 488                 list( $i, $laggedReplicaMode ) = $this->pickReaderIndex( $loads, $domain );
 489                 if ( $i === false ) {
 490                         // DB connection unsuccessful
 491                         return false;
 492                 }
 493
 494                 // If data seen by queries is expected to reflect the transactions committed as of
 495                 // or after a given replication position then wait for the DB to apply those changes
 496                 if ( $this->waitForPos && $i !== $this->getWriterIndex() && !$this->doWait( $i ) ) {
 497                         // Data will be outdated compared to what was expected
 498                         $laggedReplicaMode = true;
 499                 }
 500
 501                 // Cache the reader index for future DB_REPLICA handles
 502                 $this->setExistingReaderIndex( $group, $i );
 503                 // Record whether the generic reader index is in "lagged replica DB" mode
 504                 if ( $group === self::GROUP_GENERIC && $laggedReplicaMode ) {
 505                         $this->laggedReplicaMode = true;
 506                 }
 507
 508                 $serverName = $this->getServerName( $i );
 509                 $this->connLogger->debug( __METHOD__ . ": using server $serverName for group '$group'" );
 510
 511                 return $i;
 512         }
 513
 514         /**
 515          * Get the server index chosen by the load balancer for use with the given query group
 516          *
 517          * @param string $group Query group; use false for the generic group
 518          * @return int Server index or -1 if none was chosen
 519          */
 520         protected function getExistingReaderIndex( $group ) {
 521                 return $this->readIndexByGroup[$group] ?? -1;
 522         }
 523
 524         /**
 525          * Set the server index chosen by the load balancer for use with the given query group
 526          *
 527          * @param string $group Query group; use false for the generic group
 528          * @param int $index The index of a specific server
 529          */
 530         private function setExistingReaderIndex( $group, $index ) {
 531                 if ( $index < 0 ) {
 532                         throw new UnexpectedValueException( "Cannot set a negative read server index" );
 533                 }
 534                 $this->readIndexByGroup[$group] = $index;
 535         }
 536
 537         /**
 538          * @param array $loads List of server weights
 539          * @param string|bool $domain
 540          * @return array (reader index, lagged replica mode) or (false, false) on failure
 541          */
 542         private function pickReaderIndex( array $loads, $domain = false ) {
 543                 if ( $loads === [] ) {
 544                         throw new InvalidArgumentException( "Server configuration array is empty" );
 545                 }
 546
 547                 /** @var int|bool $i Index of selected server */
 548                 $i = false;
 549                 /** @var bool $laggedReplicaMode Whether server is considered lagged */
 550                 $laggedReplicaMode = false;
 551
 552                 // Quickly look through the available servers for a server that meets criteria...
 553                 $currentLoads = $loads;
 554                 while ( count( $currentLoads ) ) {
 555                         if ( $this->allowLagged || $laggedReplicaMode ) {
 556                                 $i = ArrayUtils::pickRandom( $currentLoads );
 557                         } else {
 558                                 $i = false;
 559                                 if ( $this->waitForPos && $this->waitForPos->asOfTime() ) {
 560                                         $this->replLogger->debug( __METHOD__ . ": replication positions detected" );
 561                                         // "chronologyCallback" sets "waitForPos" for session consistency.
 562                                         // This triggers doWait() after connect, so it's especially good to
 563                                         // avoid lagged servers so as to avoid excessive delay in that method.
 564                                         $ago = microtime( true ) - $this->waitForPos->asOfTime();
 565                                         // Aim for <= 1 second of waiting (being too picky can backfire)
 566                                         $i = $this->getRandomNonLagged( $currentLoads, $domain, $ago + 1 );
 567                                 }
 568                                 if ( $i === false ) {
 569                                         // Any server with less lag than it's 'max lag' param is preferable
 570                                         $i = $this->getRandomNonLagged( $currentLoads, $domain );
 571                                 }
 572                                 if ( $i === false && count( $currentLoads ) ) {
 573                                         // All replica DBs lagged. Switch to read-only mode
 574                                         $this->replLogger->error(
 575                                                 __METHOD__ . ": all replica DBs lagged. Switch to read-only mode" );
 576                                         $i = ArrayUtils::pickRandom( $currentLoads );
 577                                         $laggedReplicaMode = true;
 578                                 }
 579                         }
 580
 581                         if ( $i === false ) {
 582                                 // pickRandom() returned false.
 583                                 // This is permanent and means the configuration or the load monitor
 584                                 // wants us to return false.
 585                                 $this->connLogger->debug( __METHOD__ . ": pickRandom() returned false" );
 586
 587                                 return [ false, false ];
 588                         }
 589
 590                         $serverName = $this->getServerName( $i );
 591                         $this->connLogger->debug( __METHOD__ . ": Using reader #$i: $serverName..." );
 592
 593                         // Get a connection to this server without triggering other server connections
 594                         $conn = $this->getServerConnection( $i, $domain, self::CONN_SILENCE_ERRORS );
 595                         if ( !$conn ) {
 596                                 $this->connLogger->warning( __METHOD__ . ": Failed connecting to $i/$domain" );
 597                                 unset( $currentLoads[$i] ); // avoid this server next iteration
 598                                 $i = false;
 599                                 continue;
 600                         }
 601
 602                         // Decrement reference counter, we are finished with this connection.
 603                         // It will be incremented for the caller later.
 604                         if ( $domain !== false ) {
 605                                 $this->reuseConnection( $conn );
 606                         }
 607
 608                         // Return this server
 609                         break;
 610                 }
 611
 612                 // If all servers were down, quit now
 613                 if ( $currentLoads === [] ) {
 614                         $this->connLogger->error( __METHOD__ . ": all servers down" );
 615                 }
 616
 617                 return [ $i, $laggedReplicaMode ];
 618         }
 619
 620         public function waitFor( $pos ) {
 621                 $oldPos = $this->waitForPos;
 622                 try {
 623                         $this->waitForPos = $pos;
 624                         // If a generic reader connection was already established, then wait now
 625                         $i = $this->getExistingReaderIndex( self::GROUP_GENERIC );
 626                         if ( $i > 0 && !$this->doWait( $i ) ) {
 627                                 $this->laggedReplicaMode = true;
 628                         }
 629                         // Otherwise, wait until a connection is established in getReaderIndex()
 630                 } finally {
 631                         // Restore the older position if it was higher since this is used for lag-protection
 632                         $this->setWaitForPositionIfHigher( $oldPos );
 633                 }
 634         }
 635
 636         public function waitForOne( $pos, $timeout = null ) {
 637                 $oldPos = $this->waitForPos;
 638                 try {
 639                         $this->waitForPos = $pos;
 640
 641                         $i = $this->getExistingReaderIndex( self::GROUP_GENERIC );
 642                         if ( $i <= 0 ) {
 643                                 // Pick a generic replica DB if there isn't one yet
 644                                 $readLoads = $this->groupLoads[self::GROUP_GENERIC];
 645                                 unset( $readLoads[$this->getWriterIndex()] ); // replica DBs only
 646                                 $readLoads = array_filter( $readLoads ); // with non-zero load
 647                                 $i = ArrayUtils::pickRandom( $readLoads );
 648                         }
 649
 650                         if ( $i > 0 ) {
 651                                 $ok = $this->doWait( $i, true, $timeout );
 652                         } else {
 653                                 $ok = true; // no applicable loads
 654                         }
 655                 } finally {
 656                         // Restore the old position; this is used for throttling, not lag-protection
 657                         $this->waitForPos = $oldPos;
 658                 }
 659
 660                 return $ok;
 661         }
 662
 663         public function waitForAll( $pos, $timeout = null ) {
 664                 $timeout = $timeout ?: $this->waitTimeout;
 665
 666                 $oldPos = $this->waitForPos;
 667                 try {
 668                         $this->waitForPos = $pos;
 669                         $serverCount = $this->getServerCount();
 670
 671                         $ok = true;
 672                         for ( $i = 1; $i < $serverCount; $i++ ) {
 673                                 if ( $this->serverHasLoadInAnyGroup( $i ) ) {
 674                                         $start = microtime( true );
 675                                         $ok = $this->doWait( $i, true, $timeout ) && $ok;
 676                                         $timeout -= intval( microtime( true ) - $start );
 677                                         if ( $timeout <= 0 ) {
 678                                                 break; // timeout reached
 679                                         }
 680                                 }
 681                         }
 682                 } finally {
 683                         // Restore the old position; this is used for throttling, not lag-protection
 684                         $this->waitForPos = $oldPos;
 685                 }
 686
 687                 return $ok;
 688         }
 689
 690         /**
 691          * @param int $i Specific server index
 692          * @return bool
 693          */
 694         private function serverHasLoadInAnyGroup( $i ) {
 695                 foreach ( $this->groupLoads as $loadsByIndex ) {
 696                         if ( ( $loadsByIndex[$i] ?? 0 ) > 0 ) {
 697                                 return true;
 698                         }
 699                 }
 700
 701                 return false;
 702         }
 703
 704         /**
 705          * @param DBMasterPos|bool $pos
 706          */
 707         private function setWaitForPositionIfHigher( $pos ) {
 708                 if ( !$pos ) {
 709                         return;
 710                 }
 711
 712                 if ( !$this->waitForPos || $pos->hasReached( $this->waitForPos ) ) {
 713                         $this->waitForPos = $pos;
 714                 }
 715         }
 716
 717         public function getAnyOpenConnection( $i, $flags = 0 ) {
 718                 $i = ( $i === self::DB_MASTER ) ? $this->getWriterIndex() : $i;
 719                 // Connection handles required to be in auto-commit mode use a separate connection
 720                 // pool since the main pool is effected by implicit and explicit transaction rounds
 721                 $autocommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );
 722
 723                 $conn = false;
 724                 foreach ( $this->conns as $connsByServer ) {
 725                         // Get the connection array server indexes to inspect
 726                         if ( $i === self::DB_REPLICA ) {
 727                                 $indexes = array_keys( $connsByServer );
 728                         } else {
 729                                 $indexes = isset( $connsByServer[$i] ) ? [ $i ] : [];
 730                         }
 731
 732                         foreach ( $indexes as $index ) {
 733                                 $conn = $this->pickAnyOpenConnection( $connsByServer[$index], $autocommit );
 734                                 if ( $conn ) {
 735                                         break;
 736                                 }
 737                         }
 738                 }
 739
 740                 if ( $conn ) {
 741                         $this->enforceConnectionFlags( $conn, $flags );
 742                 }
 743
 744                 return $conn;
 745         }
 746
 747         /**
 748          * @param IDatabase[] $candidateConns
 749          * @param bool $autocommit Whether to only look for auto-commit connections
 750          * @return IDatabase|false An appropriate open connection or false if none found
 751          */
 752         private function pickAnyOpenConnection( $candidateConns, $autocommit ) {
 753                 $conn = false;
 754
 755                 foreach ( $candidateConns as $candidateConn ) {
 756                         if ( !$candidateConn->isOpen() ) {
 757                                 continue; // some sort of error occured?
 758                         } elseif (
 759                                 $autocommit &&
 760                                 (
 761                                         // Connection is transaction round aware
 762                                         !$candidateConn->getLBInfo( 'autoCommitOnly' ) ||
 763                                         // Some sort of error left a transaction open?
 764                                         $candidateConn->trxLevel()
 765                                 )
 766                         ) {
 767                                 continue; // some sort of error left a transaction open?
 768                         }
 769
 770                         $conn = $candidateConn;
 771                 }
 772
 773                 return $conn;
 774         }
 775
 776         /**
 777          * Wait for a given replica DB to catch up to the master pos stored in "waitForPos"
 778          * @param int $index Specific server index
 779          * @param bool $open Check the server even if a new connection has to be made
 780          * @param int|null $timeout Max seconds to wait; default is "waitTimeout"
 781          * @return bool
 782          */
 783         protected function doWait( $index, $open = false, $timeout = null ) {
 784                 $timeout = max( 1, intval( $timeout ?: $this->waitTimeout ) );
 785
 786                 // Check if we already know that the DB has reached this point
 787                 $server = $this->getServerName( $index );
 788                 $key = $this->srvCache->makeGlobalKey( __CLASS__, 'last-known-pos', $server, 'v1' );
 789                 /** @var DBMasterPos $knownReachedPos */
 790                 $knownReachedPos = $this->srvCache->get( $key );
 791                 if (
 792                         $knownReachedPos instanceof DBMasterPos &&
 793                         $knownReachedPos->hasReached( $this->waitForPos )
 794                 ) {
 795                         $this->replLogger->debug(
 796                                 __METHOD__ .
 797                                 ': replica DB {dbserver} known to be caught up (pos >= $knownReachedPos).',
 798                                 [ 'dbserver' => $server ]
 799                         );
 800                         return true;
 801                 }
 802
 803                 // Find a connection to wait on, creating one if needed and allowed
 804                 $close = false; // close the connection afterwards
 805                 $flags = self::CONN_SILENCE_ERRORS;
 806                 $conn = $this->getAnyOpenConnection( $index, $flags );
 807                 if ( !$conn ) {
 808                         if ( !$open ) {
 809                                 $this->replLogger->debug(
 810                                         __METHOD__ . ': no connection open for {dbserver}',
 811                                         [ 'dbserver' => $server ]
 812                                 );
 813
 814                                 return false;
 815                         }
 816                         // Get a connection to this server without triggering other server connections
 817                         $conn = $this->getServerConnection( $index, self::DOMAIN_ANY, $flags );
 818                         if ( !$conn ) {
 819                                 $this->replLogger->warning(
 820                                         __METHOD__ . ': failed to connect to {dbserver}',
 821                                         [ 'dbserver' => $server ]
 822                                 );
 823
 824                                 return false;
 825                         }
 826                         // Avoid connection spam in waitForAll() when connections
 827                         // are made just for the sake of doing this lag check.
 828                         $close = true;
 829                 }
 830
 831                 $this->replLogger->info(
 832                         __METHOD__ .
 833                         ': waiting for replica DB {dbserver} to catch up...',
 834                         [ 'dbserver' => $server ]
 835                 );
 836
 837                 $result = $conn->masterPosWait( $this->waitForPos, $timeout );
 838
 839                 if ( $result === null ) {
 840                         $this->replLogger->warning(
 841                                 __METHOD__ . ': Errored out waiting on {host} pos {pos}',
 842                                 [
 843                                         'host' => $server,
 844                                         'pos' => $this->waitForPos,
 845                                         'trace' => ( new RuntimeException() )->getTraceAsString()
 846                                 ]
 847                         );
 848                         $ok = false;
 849                 } elseif ( $result == -1 ) {
 850                         $this->replLogger->warning(
 851                                 __METHOD__ . ': Timed out waiting on {host} pos {pos}',
 852                                 [
 853                                         'host' => $server,
 854                                         'pos' => $this->waitForPos,
 855                                         'trace' => ( new RuntimeException() )->getTraceAsString()
 856                                 ]
 857                         );
 858                         $ok = false;
 859                 } else {
 860                         $this->replLogger->debug( __METHOD__ . ": done waiting" );
 861                         $ok = true;
 862                         // Remember that the DB reached this point
 863                         $this->srvCache->set( $key, $this->waitForPos, BagOStuff::TTL_DAY );
 864                 }
 865
 866                 if ( $close ) {
 867                         $this->closeConnection( $conn );
 868                 }
 869
 870                 return $ok;
 871         }
 872
 873         public function getConnection( $i, $groups = [], $domain = false, $flags = 0 ) {
 874                 $domain = $this->resolveDomainID( $domain );
 875                 $groups = $this->resolveGroups( $groups, $i );
 876                 $flags = $this->sanitizeConnectionFlags( $flags, $i );
 877                 // If given DB_MASTER/DB_REPLICA, resolve it to a specific server index. Resolving
 878                 // DB_REPLICA might trigger getServerConnection() calls due to the getReaderIndex()
 879                 // connectivity checks or LoadMonitor::scaleLoads() server state cache regeneration.
 880                 // The use of getServerConnection() instead of getConnection() avoids infinite loops.
 881                 $serverIndex = $this->getConnectionIndex( $i, $groups, $domain );
 882                 // Get an open connection to that server (might trigger a new connection)
 883                 $conn = $this->getServerConnection( $serverIndex, $domain, $flags );
 884                 // Set master DB handles as read-only if there is high replication lag
 885                 if ( $serverIndex === $this->getWriterIndex() && $this->getLaggedReplicaMode( $domain ) ) {
 886                         $reason = ( $this->getExistingReaderIndex( self::GROUP_GENERIC ) >= 0 )
 887                                 ? 'The database is read-only until replication lag decreases.'
 888                                 : 'The database is read-only until replica database servers becomes reachable.';
 889                         $conn->setLBInfo( 'readOnlyReason', $reason );
 890                 }
 891
 892                 return $conn;
 893         }
 894
 895         /**
 896          * @param int $i Specific server index
 897          * @param string $domain Resolved DB domain
 898          * @param int $flags Bitfield of class CONN_* constants
 899          * @return IDatabase|bool
 900          * @throws InvalidArgumentException When the server index is invalid
 901          */
 902         public function getServerConnection( $i, $domain, $flags = 0 ) {
 903                 // Number of connections made before getting the server index and handle
 904                 $priorConnectionsMade = $this->connectionCounter;
 905                 // Get an open connection to this server (might trigger a new connection)
 906                 $conn = $this->localDomain->equals( $domain )
 907                         ? $this->getLocalConnection( $i, $flags )
 908                         : $this->getForeignConnection( $i, $domain, $flags );
 909                 // Throw an error or otherwise bail out if the connection attempt failed
 910                 if ( !( $conn instanceof IDatabase ) ) {
 911                         if ( ( $flags & self::CONN_SILENCE_ERRORS ) != self::CONN_SILENCE_ERRORS ) {
 912                                 $this->reportConnectionError();
 913                         }
 914
 915                         return false;
 916                 }
 917
 918                 // Profile any new connections caused by this method
 919                 if ( $this->connectionCounter > $priorConnectionsMade ) {
 920                         $this->trxProfiler->recordConnection(
 921                                 $conn->getServer(),
 922                                 $conn->getDBname(),
 923                                 ( ( $flags & self::CONN_INTENT_WRITABLE ) == self::CONN_INTENT_WRITABLE )
 924                         );
 925                 }
 926
 927                 if ( !$conn->isOpen() ) {
 928                         $this->errorConnection = $conn;
 929                         // Connection was made but later unrecoverably lost for some reason.
 930                         // Do not return a handle that will just throw exceptions on use, but
 931                         // let the calling code, e.g. getReaderIndex(), try another server.
 932                         return false;
 933                 }
 934
 935                 // Make sure that flags like CONN_TRX_AUTOCOMMIT are respected by this handle
 936                 $this->enforceConnectionFlags( $conn, $flags );
 937                 // Set master DB handles as read-only if the load balancer is configured as read-only
 938                 // or the master database server is running in server-side read-only mode. Note that
 939                 // replica DB handles are always read-only via Database::assertIsWritableMaster().
 940                 // Read-only mode due to replication lag is *avoided* here to avoid recursion.
 941                 if ( $conn->getLBInfo( 'serverIndex' ) === $this->getWriterIndex() ) {
 942                         if ( $this->readOnlyReason !== false ) {
 943                                 $conn->setLBInfo( 'readOnlyReason', $this->readOnlyReason );
 944                         } elseif ( $this->masterRunningReadOnly( $domain, $conn ) ) {
 945                                 $conn->setLBInfo(
 946                                         'readOnlyReason',
 947                                         'The master database server is running in read-only mode.'
 948                                 );
 949                         }
 950                 }
 951
 952                 return $conn;
 953         }
 954
 955         public function reuseConnection( IDatabase $conn ) {
 956                 $serverIndex = $conn->getLBInfo( 'serverIndex' );
 957                 $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 958                 if ( $serverIndex === null || $refCount === null ) {
 959                         return; // non-foreign connection; no domain-use tracking to update
 960                 } elseif ( $conn instanceof DBConnRef ) {
 961                         // DBConnRef already handles calling reuseConnection() and only passes the live
 962                         // Database instance to this method. Any caller passing in a DBConnRef is broken.
 963                         $this->connLogger->error(
 964                                 __METHOD__ . ": got DBConnRef instance.\n" .
 965                                 ( new LogicException() )->getTraceAsString() );
 966
 967                         return;
 968                 }
 969
 970                 if ( $this->disabled ) {
 971                         return; // DBConnRef handle probably survived longer than the LoadBalancer
 972                 }
 973
 974                 if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
 975                         $connFreeKey = self::KEY_FOREIGN_FREE_NOROUND;
 976                         $connInUseKey = self::KEY_FOREIGN_INUSE_NOROUND;
 977                 } else {
 978                         $connFreeKey = self::KEY_FOREIGN_FREE;
 979                         $connInUseKey = self::KEY_FOREIGN_INUSE;
 980                 }
 981
 982                 $domain = $conn->getDomainID();
 983                 if ( !isset( $this->conns[$connInUseKey][$serverIndex][$domain] ) ) {
 984                         throw new InvalidArgumentException(
 985                                 "Connection $serverIndex/$domain not found; it may have already been freed" );
 986                 } elseif ( $this->conns[$connInUseKey][$serverIndex][$domain] !== $conn ) {
 987                         throw new InvalidArgumentException(
 988                                 "Connection $serverIndex/$domain mismatched; it may have already been freed" );
 989                 }
 990
 991                 $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
 992                 if ( $refCount <= 0 ) {
 993                         $this->conns[$connFreeKey][$serverIndex][$domain] = $conn;
 994                         unset( $this->conns[$connInUseKey][$serverIndex][$domain] );
 995                         if ( !$this->conns[$connInUseKey][$serverIndex] ) {
 996                                 unset( $this->conns[$connInUseKey][$serverIndex] ); // clean up
 997                         }
 998                         $this->connLogger->debug( __METHOD__ . ": freed connection $serverIndex/$domain" );
 999                 } else {
1000                         $this->connLogger->debug( __METHOD__ .
1001                                 ": reference count for $serverIndex/$domain reduced to $refCount" );
1002                 }
1003         }
1004
1005         public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
1006                 $domain = $this->resolveDomainID( $domain );
1007                 $role = $this->getRoleFromIndex( $i );
1008
1009                 return new DBConnRef( $this, $this->getConnection( $i, $groups, $domain, $flags ), $role );
1010         }
1011
1012         public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
1013                 $domain = $this->resolveDomainID( $domain );
1014                 $role = $this->getRoleFromIndex( $i );
1015
1016                 return new DBConnRef( $this, [ $i, $groups, $domain, $flags ], $role );
1017         }
1018
1019         public function getMaintenanceConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
1020                 $domain = $this->resolveDomainID( $domain );
1021                 $role = $this->getRoleFromIndex( $i );
1022
1023                 return new MaintainableDBConnRef(
1024                         $this, $this->getConnection( $i, $groups, $domain, $flags ), $role );
1025         }
1026
1027         /**
1028          * @param int $i Server index or DB_MASTER/DB_REPLICA
1029          * @return int One of DB_MASTER/DB_REPLICA
1030          */
1031         private function getRoleFromIndex( $i ) {
1032                 return ( $i === self::DB_MASTER || $i === $this->getWriterIndex() )
1033                         ? self::DB_MASTER
1034                         : self::DB_REPLICA;
1035         }
1036
1037         /**
1038          * @param int $i
1039          * @param string|bool $domain
1040          * @param int $flags
1041          * @return Database|bool Live database handle or false on failure
1042          * @deprecated Since 1.34 Use getConnection() instead
1043          */
1044         public function openConnection( $i, $domain = false, $flags = 0 ) {
1045                 return $this->getConnection( $i, [], $domain, $flags | self::CONN_SILENCE_ERRORS );
1046         }
1047
1048         /**
1049          * Open a connection to a local DB, or return one if it is already open.
1050          *
1051          * On error, returns false, and the connection which caused the
1052          * error will be available via $this->errorConnection.
1053          *
1054          * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
1055          *
1056          * @param int $i Specific server index
1057          * @param int $flags Class CONN_* constant bitfield
1058          * @return Database
1059          * @throws InvalidArgumentException When the server index is invalid
1060          * @throws UnexpectedValueException When the DB domain of the connection is corrupted
1061          */
1062         private function getLocalConnection( $i, $flags = 0 ) {
1063                 $autoCommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );
1064                 // Connection handles required to be in auto-commit mode use a separate connection
1065                 // pool since the main pool is effected by implicit and explicit transaction rounds
1066                 $connKey = $autoCommit ? self::KEY_LOCAL_NOROUND : self::KEY_LOCAL;
1067
1068                 if ( isset( $this->conns[$connKey][$i][0] ) ) {
1069                         $conn = $this->conns[$connKey][$i][0];
1070                 } else {
1071                         $conn = $this->reallyOpenConnection(
1072                                 $i,
1073                                 $this->localDomain,
1074                                 [ 'autoCommitOnly' => $autoCommit ]
1075                         );
1076                         if ( $conn->isOpen() ) {
1077                                 $this->connLogger->debug( __METHOD__ . ": opened new connection for $i" );
1078                                 $this->conns[$connKey][$i][0] = $conn;
1079                         } else {
1080                                 $this->connLogger->warning( __METHOD__ . ": connection error for $i" );
1081                                 $this->errorConnection = $conn;
1082                                 $conn = false;
1083                         }
1084                 }
1085
1086                 // Sanity check to make sure that the right domain is selected
1087                 if (
1088                         $conn instanceof IDatabase &&
1089                         !$this->localDomain->isCompatible( $conn->getDomainID() )
1090                 ) {
1091                         throw new UnexpectedValueException(
1092                                 "Got connection to '{$conn->getDomainID()}', " .
1093                                 "but expected local domain ('{$this->localDomain}')"
1094                         );
1095                 }
1096
1097                 return $conn;
1098         }
1099
1100         /**
1101          * Open a connection to a foreign DB, or return one if it is already open.
1102          *
1103          * Increments a reference count on the returned connection which locks the
1104          * connection to the requested domain. This reference count can be
1105          * decremented by calling reuseConnection().
1106          *
1107          * If a connection is open to the appropriate server already, but with the wrong
1108          * database, it will be switched to the right database and returned, as long as
1109          * it has been freed first with reuseConnection().
1110          *
1111          * On error, returns false, and the connection which caused the
1112          * error will be available via $this->errorConnection.
1113          *
1114          * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
1115          *
1116          * @param int $i Specific server index
1117          * @param string $domain Domain ID to open
1118          * @param int $flags Class CONN_* constant bitfield
1119          * @return Database|bool Returns false on connection error
1120          * @throws DBError When database selection fails
1121          * @throws InvalidArgumentException When the server index is invalid
1122          * @throws UnexpectedValueException When the DB domain of the connection is corrupted
1123          */
1124         private function getForeignConnection( $i, $domain, $flags = 0 ) {
1125                 $domainInstance = DatabaseDomain::newFromId( $domain );
1126                 $autoCommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );
1127                 // Connection handles required to be in auto-commit mode use a separate connection
1128                 // pool since the main pool is effected by implicit and explicit transaction rounds
1129                 if ( $autoCommit ) {
1130                         $connFreeKey = self::KEY_FOREIGN_FREE_NOROUND;
1131                         $connInUseKey = self::KEY_FOREIGN_INUSE_NOROUND;
1132                 } else {
1133                         $connFreeKey = self::KEY_FOREIGN_FREE;
1134                         $connInUseKey = self::KEY_FOREIGN_INUSE;
1135                 }
1136
1137                 /** @var Database $conn */
1138                 $conn = null;
1139
1140                 if ( isset( $this->conns[$connInUseKey][$i][$domain] ) ) {
1141                         // Reuse an in-use connection for the same domain
1142                         $conn = $this->conns[$connInUseKey][$i][$domain];
1143                         $this->connLogger->debug( __METHOD__ . ": reusing connection $i/$domain" );
1144                 } elseif ( isset( $this->conns[$connFreeKey][$i][$domain] ) ) {
1145                         // Reuse a free connection for the same domain
1146                         $conn = $this->conns[$connFreeKey][$i][$domain];
1147                         unset( $this->conns[$connFreeKey][$i][$domain] );
1148                         $this->conns[$connInUseKey][$i][$domain] = $conn;
1149                         $this->connLogger->debug( __METHOD__ . ": reusing free connection $i/$domain" );
1150                 } elseif ( !empty( $this->conns[$connFreeKey][$i] ) ) {
1151                         // Reuse a free connection from another domain if possible
1152                         foreach ( $this->conns[$connFreeKey][$i] as $oldDomain => $conn ) {
1153                                 if ( $domainInstance->getDatabase() !== null ) {
1154                                         // Check if changing the database will require a new connection.
1155                                         // In that case, leave the connection handle alone and keep looking.
1156                                         // This prevents connections from being closed mid-transaction and can
1157                                         // also avoid overhead if the same database will later be requested.
1158                                         if (
1159                                                 $conn->databasesAreIndependent() &&
1160                                                 $conn->getDBname() !== $domainInstance->getDatabase()
1161                                         ) {
1162                                                 continue;
1163                                         }
1164                                         // Select the new database, schema, and prefix
1165                                         $conn->selectDomain( $domainInstance );
1166                                 } else {
1167                                         // Stay on the current database, but update the schema/prefix
1168                                         $conn->dbSchema( $domainInstance->getSchema() );
1169                                         $conn->tablePrefix( $domainInstance->getTablePrefix() );
1170                                 }
1171                                 unset( $this->conns[$connFreeKey][$i][$oldDomain] );
1172                                 // Note that if $domain is an empty string, getDomainID() might not match it
1173                                 $this->conns[$connInUseKey][$i][$conn->getDomainID()] = $conn;
1174                                 $this->connLogger->debug( __METHOD__ .
1175                                         ": reusing free connection from $oldDomain for $domain" );
1176                                 break;
1177                         }
1178                 }
1179
1180                 if ( !$conn ) {
1181                         $conn = $this->reallyOpenConnection(
1182                                 $i,
1183                                 $domainInstance,
1184                                 [
1185                                         'autoCommitOnly' => $autoCommit,
1186                                         'foreign' => true,
1187                                         'foreignPoolRefCount' => 0
1188                                 ]
1189                         );
1190                         if ( $conn->isOpen() ) {
1191                                 // Note that if $domain is an empty string, getDomainID() might not match it
1192                                 $this->conns[$connInUseKey][$i][$conn->getDomainID()] = $conn;
1193                                 $this->connLogger->debug( __METHOD__ . ": opened new connection for $i/$domain" );
1194                         } else {
1195                                 $this->connLogger->warning( __METHOD__ . ": connection error for $i/$domain" );
1196                                 $this->errorConnection = $conn;
1197                                 $conn = false;
1198                         }
1199                 }
1200
1201                 if ( $conn instanceof IDatabase ) {
1202                         // Sanity check to make sure that the right domain is selected
1203                         if ( !$domainInstance->isCompatible( $conn->getDomainID() ) ) {
1204                                 throw new UnexpectedValueException(
1205                                         "Got connection to '{$conn->getDomainID()}', but expected '$domain'" );
1206                         }
1207                         // Increment reference count
1208                         $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
1209                         $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
1210                 }
1211
1212                 return $conn;
1213         }
1214
1215         public function getServerAttributes( $i ) {
1216                 return Database::attributesFromType(
1217                         $this->getServerType( $i ),
1218                         $this->servers[$i]['driver'] ?? null
1219                 );
1220         }
1221
1222         /**
1223          * Test if the specified index represents an open connection
1224          *
1225          * @param int $index Server index
1226          * @return bool
1227          */
1228         private function isOpen( $index ) {
1229                 return (bool)$this->getAnyOpenConnection( $index );
1230         }
1231
1232         /**
1233          * Open a new network connection to a server (uncached)
1234          *
1235          * Returns a Database object whether or not the connection was successful.
1236          *
1237          * @param int $i Specific server index
1238          * @param DatabaseDomain $domain Domain the connection is for, possibly unspecified
1239          * @param array $lbInfo Additional information for setLBInfo()
1240          * @return Database
1241          * @throws DBAccessError
1242          * @throws InvalidArgumentException
1243          */
1244         protected function reallyOpenConnection( $i, DatabaseDomain $domain, array $lbInfo ) {
1245                 if ( $this->disabled ) {
1246                         throw new DBAccessError();
1247                 }
1248
1249                 $server = $this->getServerInfoStrict( $i );
1250                 $server = array_merge( $server, [
1251                         // Use the database specified in $domain (null means "none or entrypoint DB");
1252                         // fallback to the $server default if the RDBMs is an embedded library using a file
1253                         // on disk since there would be nothing to access to without a DB/file name.
1254                         'dbname' => $this->getServerAttributes( $i )[Database::ATTR_DB_IS_FILE]
1255                                 ? ( $domain->getDatabase() ?? $server['dbname'] ?? null )
1256                                 : $domain->getDatabase(),
1257                         // Override the $server default schema with that of $domain if specified
1258                         'schema' => $domain->getSchema() ?? $server['schema'] ?? null,
1259                         // Use the table prefix specified in $domain
1260                         'tablePrefix' => $domain->getTablePrefix(),
1261                         // Participate in transaction rounds if $server does not specify otherwise
1262                         'flags' => $server['flags'] ?? IDatabase::DBO_DEFAULT,
1263                         // Inject the PHP execution mode and the agent string
1264                         'cliMode' => $this->cliMode,
1265                         'agent' => $this->agent,
1266                         // Inject object and callback dependencies
1267                         'srvCache' => $this->srvCache,
1268                         'connLogger' => $this->connLogger,
1269                         'queryLogger' => $this->queryLogger,
1270                         'errorLogger' => $this->errorLogger,
1271                         'deprecationLogger' => $this->deprecationLogger,
1272                         'profiler' => $this->profiler,
1273                         'trxProfiler' => $this->trxProfiler
1274                 ] );
1275
1276                 $lbInfo = array_merge( $lbInfo, [
1277                         'ownerId' => $this->id,
1278                         'serverIndex' => $i,
1279                         'master' => ( $i === $this->getWriterIndex() ),
1280                         'replica' => ( $i !== $this->getWriterIndex() ),
1281                         // Name of the master server of the relevant DB cluster (e.g. "db1052")
1282                         'clusterMasterHost' => $this->getServerName( $this->getWriterIndex() )
1283                 ] );
1284
1285                 $conn = Database::factory( $server['type'], $server, Database::NEW_UNCONNECTED );
1286                 try {
1287                         $conn->initConnection();
1288                         ++$this->connectionCounter;
1289                 } catch ( DBConnectionError $e ) {
1290                         // ignore; let the DB handle the logging
1291                 }
1292
1293                 $conn->setLBInfo( $lbInfo );
1294                 if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
1295                         if ( $this->cliMode ) {
1296                                 $conn->clearFlag( $conn::DBO_TRX );
1297                         } else {
1298                                 $conn->setFlag( $conn::DBO_TRX );
1299                         }
1300                 }
1301                 if ( $i === $this->getWriterIndex() ) {
1302                         if ( $this->trxRoundId !== false ) {
1303                                 $this->applyTransactionRoundFlags( $conn );
1304                         }
1305                         foreach ( $this->trxRecurringCallbacks as $name => $callback ) {
1306                                 $conn->setTransactionListener( $name, $callback );
1307                         }
1308                 }
1309                 $conn->setTableAliases( $this->tableAliases );
1310                 $conn->setIndexAliases( $this->indexAliases );
1311
1312                 $conn->setLazyMasterHandle(
1313                         $this->getLazyConnectionRef( self::DB_MASTER, [], $conn->getDomainID() )
1314                 );
1315
1316                 $this->lazyLoadReplicationPositions(); // session consistency
1317
1318                 // Log when many connection are made on requests
1319                 $count = $this->getCurrentConnectionCount();
1320                 if ( $count >= self::CONN_HELD_WARN_THRESHOLD ) {
1321                         $this->perfLogger->warning(
1322                                 __METHOD__ . ": {connections}+ connections made (master={masterdb})",
1323                                 [
1324                                         'connections' => $count,
1325                                         'dbserver' => $conn->getServer(),
1326                                         'masterdb' => $conn->getLBInfo( 'clusterMasterHost' )
1327                                 ]
1328                         );
1329                 }
1330
1331                 return $conn;
1332         }
1333
1334         /**
1335          * Make sure that any "waitForPos" positions are loaded and available to doWait()
1336          */
1337         private function lazyLoadReplicationPositions() {
1338                 if ( !$this->connectionAttempted && $this->chronologyCallback ) {
1339                         $this->connectionAttempted = true;
1340                         ( $this->chronologyCallback )( $this ); // generally calls waitFor()
1341                         $this->connLogger->debug( __METHOD__ . ': executed chronology callback.' );
1342                 }
1343         }
1344
1345         /**
1346          * @throws DBConnectionError
1347          */
1348         private function reportConnectionError() {
1349                 $conn = $this->errorConnection; // the connection which caused the error
1350                 $context = [
1351                         'method' => __METHOD__,
1352                         'last_error' => $this->lastError,
1353                 ];
1354
1355                 if ( $conn instanceof IDatabase ) {
1356                         $context['db_server'] = $conn->getServer();
1357                         $this->connLogger->warning(
1358                                 __METHOD__ . ": connection error: {last_error} ({db_server})",
1359                                 $context
1360                         );
1361
1362                         throw new DBConnectionError( $conn, "{$this->lastError} ({$context['db_server']})" );
1363                 } else {
1364                         // No last connection, probably due to all servers being too busy
1365                         $this->connLogger->error(
1366                                 __METHOD__ .
1367                                 ": LB failure with no last connection. Connection error: {last_error}",
1368                                 $context
1369                         );
1370
1371                         // If all servers were busy, "lastError" will contain something sensible
1372                         throw new DBConnectionError( null, $this->lastError );
1373                 }
1374         }
1375
1376         public function getWriterIndex() {
1377                 return 0;
1378         }
1379
1380         /**
1381          * Returns true if the specified index is a valid server index
1382          *
1383          * @param int $i
1384          * @return bool
1385          * @deprecated Since 1.34
1386          */
1387         public function haveIndex( $i ) {
1388                 return array_key_exists( $i, $this->servers );
1389         }
1390
1391         /**
1392          * Returns true if the specified index is valid and has non-zero load
1393          *
1394          * @param int $i
1395          * @return bool
1396          * @deprecated Since 1.34
1397          */
1398         public function isNonZeroLoad( $i ) {
1399                 return ( isset( $this->servers[$i] ) && $this->groupLoads[self::GROUP_GENERIC][$i] > 0 );
1400         }
1401
1402         public function getServerCount() {
1403                 return count( $this->servers );
1404         }
1405
1406         public function hasReplicaServers() {
1407                 return ( $this->getServerCount() > 1 );
1408         }
1409
1410         public function hasStreamingReplicaServers() {
1411                 foreach ( $this->servers as $i => $server ) {
1412                         if ( $i !== $this->getWriterIndex() && empty( $server['is static'] ) ) {
1413                                 return true;
1414                         }
1415                 }
1416
1417                 return false;
1418         }
1419
1420         public function getServerName( $i ) {
1421                 $name = $this->servers[$i]['hostName'] ?? ( $this->servers[$i]['host'] ?? '' );
1422
1423                 return ( $name != '' ) ? $name : 'localhost';
1424         }
1425
1426         public function getServerInfo( $i ) {
1427                 return $this->servers[$i] ?? false;
1428         }
1429
1430         public function getServerType( $i ) {
1431                 return $this->servers[$i]['type'] ?? 'unknown';
1432         }
1433
1434         public function getMasterPos() {
1435                 $index = $this->getWriterIndex();
1436
1437                 $conn = $this->getAnyOpenConnection( $index );
1438                 if ( $conn ) {
1439                         return $conn->getMasterPos();
1440                 }
1441
1442                 $conn = $this->getConnection( $index, self::CONN_SILENCE_ERRORS );
1443                 if ( !$conn ) {
1444                         $this->reportConnectionError();
1445                         return null; // unreachable due to exception
1446                 }
1447
1448                 try {
1449                         $pos = $conn->getMasterPos();
1450                 } finally {
1451                         $this->closeConnection( $conn );
1452                 }
1453
1454                 return $pos;
1455         }
1456
1457         public function getReplicaResumePos() {
1458                 // Get the position of any existing master server connection
1459                 $masterConn = $this->getAnyOpenConnection( $this->getWriterIndex() );
1460                 if ( $masterConn ) {
1461                         return $masterConn->getMasterPos();
1462                 }
1463
1464                 // Get the highest position of any existing replica server connection
1465                 $highestPos = false;
1466                 $serverCount = $this->getServerCount();
1467                 for ( $i = 1; $i < $serverCount; $i++ ) {
1468                         if ( !empty( $this->servers[$i]['is static'] ) ) {
1469                                 continue; // server does not use replication
1470                         }
1471
1472                         $conn = $this->getAnyOpenConnection( $i );
1473                         $pos = $conn ? $conn->getReplicaPos() : false;
1474                         if ( !$pos ) {
1475                                 continue; // no open connection or could not get position
1476                         }
1477
1478                         $highestPos = $highestPos ?: $pos;
1479                         if ( $pos->hasReached( $highestPos ) ) {
1480                                 $highestPos = $pos;
1481                         }
1482                 }
1483
1484                 return $highestPos;
1485         }
1486
1487         public function disable() {
1488                 $this->closeAll();
1489                 $this->disabled = true;
1490         }
1491
1492         public function closeAll() {
1493                 $fname = __METHOD__;
1494                 $this->forEachOpenConnection( function ( IDatabase $conn ) use ( $fname ) {
1495                         $host = $conn->getServer();
1496                         $this->connLogger->debug(
1497                                 $fname . ": closing connection to database '$host'." );
1498                         $conn->close();
1499                 } );
1500
1501                 $this->conns = self::newTrackedConnectionsArray();
1502         }
1503
1504         public function closeConnection( IDatabase $conn ) {
1505                 if ( $conn instanceof DBConnRef ) {
1506                         // Avoid calling close() but still leaving the handle in the pool
1507                         throw new RuntimeException( 'Cannot close DBConnRef instance; it must be shareable' );
1508                 }
1509
1510                 $serverIndex = $conn->getLBInfo( 'serverIndex' );
1511                 foreach ( $this->conns as $type => $connsByServer ) {
1512                         if ( !isset( $connsByServer[$serverIndex] ) ) {
1513                                 continue;
1514                         }
1515
1516                         foreach ( $connsByServer[$serverIndex] as $i => $trackedConn ) {
1517                                 if ( $conn === $trackedConn ) {
1518                                         $host = $this->getServerName( $i );
1519                                         $this->connLogger->debug(
1520                                                 __METHOD__ . ": closing connection to database $i at '$host'." );
1521                                         unset( $this->conns[$type][$serverIndex][$i] );
1522                                         break 2;
1523                                 }
1524                         }
1525                 }
1526
1527                 $conn->close();
1528         }
1529
1530         public function commitAll( $fname = __METHOD__, $owner = null ) {
1531                 $this->commitMasterChanges( $fname, $owner );
1532                 $this->flushMasterSnapshots( $fname );
1533                 $this->flushReplicaSnapshots( $fname );
1534         }
1535
1536         public function finalizeMasterChanges( $fname = __METHOD__, $owner = null ) {
1537                 $this->assertOwnership( $fname, $owner );
1538                 $this->assertTransactionRoundStage( [ self::ROUND_CURSORY, self::ROUND_FINALIZED ] );
1539
1540                 $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
1541                 // Loop until callbacks stop adding callbacks on other connections
1542                 $total = 0;
1543                 do {
1544                         $count = 0; // callbacks execution attempts
1545                         $this->forEachOpenMasterConnection( function ( Database $conn ) use ( &$count ) {
1546                                 // Run any pre-commit callbacks while leaving the post-commit ones suppressed.
1547                                 // Any error should cause all (peer) transactions to be rolled back together.
1548                                 $count += $conn->runOnTransactionPreCommitCallbacks();
1549                         } );
1550                         $total += $count;
1551                 } while ( $count > 0 );
1552                 // Defer post-commit callbacks until after COMMIT/ROLLBACK happens on all handles
1553                 $this->forEachOpenMasterConnection( function ( Database $conn ) {
1554                         $conn->setTrxEndCallbackSuppression( true );
1555                 } );
1556                 $this->trxRoundStage = self::ROUND_FINALIZED;
1557
1558                 return $total;
1559         }
1560
1561         public function approveMasterChanges( array $options, $fname = __METHOD__, $owner = null ) {
1562                 $this->assertOwnership( $fname, $owner );
1563                 $this->assertTransactionRoundStage( self::ROUND_FINALIZED );
1564
1565                 $limit = $options['maxWriteDuration'] ?? 0;
1566
1567                 $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
1568                 $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $limit ) {
1569                         // If atomic sections or explicit transactions are still open, some caller must have
1570                         // caught an exception but failed to properly rollback any changes. Detect that and
1571                         // throw and error (causing rollback).
1572                         $conn->assertNoOpenTransactions();
1573                         // Assert that the time to replicate the transaction will be sane.
1574                         // If this fails, then all DB transactions will be rollback back together.
1575                         $time = $conn->pendingWriteQueryDuration( $conn::ESTIMATE_DB_APPLY );
1576                         if ( $limit > 0 && $time > $limit ) {
1577                                 throw new DBTransactionSizeError(
1578                                         $conn,
1579                                         "Transaction spent $time second(s) in writes, exceeding the limit of $limit",
1580                                         [ $time, $limit ]
1581                                 );
1582                         }
1583                         // If a connection sits idle while slow queries execute on another, that connection
1584                         // may end up dropped before the commit round is reached. Ping servers to detect this.
1585                         if ( $conn->writesOrCallbacksPending() && !$conn->ping() ) {
1586                                 throw new DBTransactionError(
1587                                         $conn,
1588                                         "A connection to the {$conn->getDBname()} database was lost before commit"
1589                                 );
1590                         }
1591                 } );
1592                 $this->trxRoundStage = self::ROUND_APPROVED;
1593         }
1594
1595         public function beginMasterChanges( $fname = __METHOD__, $owner = null ) {
1596                 $this->assertOwnership( $fname, $owner );
1597                 if ( $this->trxRoundId !== false ) {
1598                         throw new DBTransactionError(
1599                                 null,
1600                                 "$fname: Transaction round '{$this->trxRoundId}' already started"
1601                         );
1602                 }
1603                 $this->assertTransactionRoundStage( self::ROUND_CURSORY );
1604
1605                 // Clear any empty transactions (no writes/callbacks) from the implicit round
1606                 $this->flushMasterSnapshots( $fname );
1607
1608                 $this->trxRoundId = $fname;
1609                 $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
1610                 // Mark applicable handles as participating in this explicit transaction round.
1611                 // For each of these handles, any writes and callbacks will be tied to a single
1612                 // transaction. The (peer) handles will reject begin()/commit() calls unless they
1613                 // are part of an en masse commit or an en masse rollback.
1614                 $this->forEachOpenMasterConnection( function ( Database $conn ) {
1615                         $this->applyTransactionRoundFlags( $conn );
1616                 } );
1617                 $this->trxRoundStage = self::ROUND_CURSORY;
1618         }
1619
1620         public function commitMasterChanges( $fname = __METHOD__, $owner = null ) {
1621                 $this->assertOwnership( $fname, $owner );
1622                 $this->assertTransactionRoundStage( self::ROUND_APPROVED );
1623
1624                 $failures = [];
1625
1626                 /** @noinspection PhpUnusedLocalVariableInspection */
1627                 $scope = ScopedCallback::newScopedIgnoreUserAbort(); // try to ignore client aborts
1628
1629                 $restore = ( $this->trxRoundId !== false );
1630                 $this->trxRoundId = false;
1631                 $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
1632                 // Commit any writes and clear any snapshots as well (callbacks require AUTOCOMMIT).
1633                 // Note that callbacks should already be suppressed due to finalizeMasterChanges().
1634                 $this->forEachOpenMasterConnection(
1635                         function ( IDatabase $conn ) use ( $fname, &$failures ) {
1636                                 try {
1637                                         $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
1638                                 } catch ( DBError $e ) {
1639                                         ( $this->errorLogger )( $e );
1640                                         $failures[] = "{$conn->getServer()}: {$e->getMessage()}";
1641                                 }
1642                         }
1643                 );
1644                 if ( $failures ) {
1645                         throw new DBTransactionError(
1646                                 null,
1647                                 "$fname: Commit failed on server(s) " . implode( "\n", array_unique( $failures ) )
1648                         );
1649                 }
1650                 if ( $restore ) {
1651                         // Unmark handles as participating in this explicit transaction round
1652                         $this->forEachOpenMasterConnection( function ( Database $conn ) {
1653                                 $this->undoTransactionRoundFlags( $conn );
1654                         } );
1655                 }
1656                 $this->trxRoundStage = self::ROUND_COMMIT_CALLBACKS;
1657         }
1658
1659         public function runMasterTransactionIdleCallbacks( $fname = __METHOD__, $owner = null ) {
1660                 $this->assertOwnership( $fname, $owner );
1661                 if ( $this->trxRoundStage === self::ROUND_COMMIT_CALLBACKS ) {
1662                         $type = IDatabase::TRIGGER_COMMIT;
1663                 } elseif ( $this->trxRoundStage === self::ROUND_ROLLBACK_CALLBACKS ) {
1664                         $type = IDatabase::TRIGGER_ROLLBACK;
1665                 } else {
1666                         throw new DBTransactionError(
1667                                 null,
1668                                 "Transaction should be in the callback stage (not '{$this->trxRoundStage}')"
1669                         );
1670                 }
1671
1672                 $oldStage = $this->trxRoundStage;
1673                 $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
1674
1675                 // Now that the COMMIT/ROLLBACK step is over, enable post-commit callback runs
1676                 $this->forEachOpenMasterConnection( function ( Database $conn ) {
1677                         $conn->setTrxEndCallbackSuppression( false );
1678                 } );
1679
1680                 $e = null; // first exception
1681                 $fname = __METHOD__;
1682                 // Loop until callbacks stop adding callbacks on other connections
1683                 do {
1684                         // Run any pending callbacks for each connection...
1685                         $count = 0; // callback execution attempts
1686                         $this->forEachOpenMasterConnection(
1687                                 function ( Database $conn ) use ( $type, &$e, &$count ) {
1688                                         if ( $conn->trxLevel() ) {
1689                                                 return; // retry in the next iteration, after commit() is called
1690                                         }
1691                                         try {
1692                                                 $count += $conn->runOnTransactionIdleCallbacks( $type );
1693                                         } catch ( Exception $ex ) {
1694                                                 $e = $e ?: $ex;
1695                                         }
1696                                 }
1697                         );
1698                         // Clear out any active transactions left over from callbacks...
1699                         $this->forEachOpenMasterConnection( function ( Database $conn ) use ( &$e, $fname ) {
1700                                 if ( $conn->writesPending() ) {
1701                                         // A callback from another handle wrote to this one and DBO_TRX is set
1702                                         $this->queryLogger->warning( $fname . ": found writes pending." );
1703                                         $fnames = implode( ', ', $conn->pendingWriteAndCallbackCallers() );
1704                                         $this->queryLogger->warning(
1705                                                 $fname . ": found writes pending ($fnames).",
1706                                                 [
1707                                                         'db_server' => $conn->getServer(),
1708                                                         'db_name' => $conn->getDBname()
1709                                                 ]
1710                                         );
1711                                 } elseif ( $conn->trxLevel() ) {
1712                                         // A callback from another handle read from this one and DBO_TRX is set,
1713                                         // which can easily happen if there is only one DB (no replicas)
1714                                         $this->queryLogger->debug( $fname . ": found empty transaction." );
1715                                 }
1716                                 try {
1717                                         $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
1718                                 } catch ( Exception $ex ) {
1719                                         $e = $e ?: $ex;
1720                                 }
1721                         } );
1722                 } while ( $count > 0 );
1723
1724                 $this->trxRoundStage = $oldStage;
1725
1726                 return $e;
1727         }
1728
1729         public function runMasterTransactionListenerCallbacks( $fname = __METHOD__, $owner = null ) {
1730                 $this->assertOwnership( $fname, $owner );
1731                 if ( $this->trxRoundStage === self::ROUND_COMMIT_CALLBACKS ) {
1732                         $type = IDatabase::TRIGGER_COMMIT;
1733                 } elseif ( $this->trxRoundStage === self::ROUND_ROLLBACK_CALLBACKS ) {
1734                         $type = IDatabase::TRIGGER_ROLLBACK;
1735                 } else {
1736                         throw new DBTransactionError(
1737                                 null,
1738                                 "Transaction should be in the callback stage (not '{$this->trxRoundStage}')"
1739                         );
1740                 }
1741
1742                 $e = null;
1743
1744                 $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
1745                 $this->forEachOpenMasterConnection( function ( Database $conn ) use ( $type, &$e ) {
1746                         try {
1747                                 $conn->runTransactionListenerCallbacks( $type );
1748                         } catch ( Exception $ex ) {
1749                                 $e = $e ?: $ex;
1750                         }
1751                 } );
1752                 $this->trxRoundStage = self::ROUND_CURSORY;
1753
1754                 return $e;
1755         }
1756
1757         public function rollbackMasterChanges( $fname = __METHOD__, $owner = null ) {
1758                 $this->assertOwnership( $fname, $owner );
1759
1760                 $restore = ( $this->trxRoundId !== false );
1761                 $this->trxRoundId = false;
1762                 $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
1763                 $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $fname ) {
1764                         $conn->rollback( $fname, $conn::FLUSHING_ALL_PEERS );
1765                 } );
1766                 if ( $restore ) {
1767                         // Unmark handles as participating in this explicit transaction round
1768                         $this->forEachOpenMasterConnection( function ( Database $conn ) {
1769                                 $this->undoTransactionRoundFlags( $conn );
1770                         } );
1771                 }
1772                 $this->trxRoundStage = self::ROUND_ROLLBACK_CALLBACKS;
1773         }
1774
1775         /**
1776          * @param string|string[] $stage
1777          * @throws DBTransactionError
1778          */
1779         private function assertTransactionRoundStage( $stage ) {
1780                 $stages = (array)$stage;
1781
1782                 if ( !in_array( $this->trxRoundStage, $stages, true ) ) {
1783                         $stageList = implode(
1784                                 '/',
1785                                 array_map( function ( $v ) {
1786                                         return "'$v'";
1787                                 }, $stages )
1788                         );
1789                         throw new DBTransactionError(
1790                                 null,
1791                                 "Transaction round stage must be $stageList (not '{$this->trxRoundStage}')"
1792                         );
1793                 }
1794         }
1795
1796         /**
1797          * @param string $fname
1798          * @param int|null $owner Owner ID of the caller
1799          * @throws DBTransactionError
1800          */
1801         private function assertOwnership( $fname, $owner ) {
1802                 if ( $this->ownerId !== null && $owner !== $this->ownerId ) {
1803                         throw new DBTransactionError(
1804                                 null,
1805                                 "$fname: LoadBalancer is owned by LBFactory #{$this->ownerId} (got '$owner')."
1806                         );
1807                 }
1808         }
1809
1810         /**
1811          * Make all DB servers with DBO_DEFAULT/DBO_TRX set join the transaction round
1812          *
1813          * Some servers may have neither flag enabled, meaning that they opt out of such
1814          * transaction rounds and remain in auto-commit mode. Such behavior might be desired
1815          * when a DB server is used for something like simple key/value storage.
1816          *
1817          * @param Database $conn
1818          */
1819         private function applyTransactionRoundFlags( Database $conn ) {
1820                 if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
1821                         return; // transaction rounds do not apply to these connections
1822                 }
1823
1824                 if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
1825                         // DBO_TRX is controlled entirely by CLI mode presence with DBO_DEFAULT.
1826                         // Force DBO_TRX even in CLI mode since a commit round is expected soon.
1827                         $conn->setFlag( $conn::DBO_TRX, $conn::REMEMBER_PRIOR );
1828                 }
1829
1830                 if ( $conn->getFlag( $conn::DBO_TRX ) ) {
1831                         $conn->setLBInfo( 'trxRoundId', $this->trxRoundId );
1832                 }
1833         }
1834
1835         /**
1836          * @param Database $conn
1837          */
1838         private function undoTransactionRoundFlags( Database $conn ) {
1839                 if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
1840                         return; // transaction rounds do not apply to these connections
1841                 }
1842
1843                 if ( $conn->getFlag( $conn::DBO_TRX ) ) {
1844                         $conn->setLBInfo( 'trxRoundId', null ); // remove the round ID
1845                 }
1846
1847                 if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
1848                         $conn->restoreFlags( $conn::RESTORE_PRIOR );
1849                 }
1850         }
1851
1852         public function flushReplicaSnapshots( $fname = __METHOD__ ) {
1853                 $this->forEachOpenReplicaConnection( function ( IDatabase $conn ) use ( $fname ) {
1854                         $conn->flushSnapshot( $fname );
1855                 } );
1856         }
1857
1858         public function flushMasterSnapshots( $fname = __METHOD__ ) {
1859                 $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $fname ) {
1860                         $conn->flushSnapshot( $fname );
1861                 } );
1862         }
1863
1864         /**
1865          * @return string
1866          * @since 1.32
1867          */
1868         public function getTransactionRoundStage() {
1869                 return $this->trxRoundStage;
1870         }
1871
1872         public function hasMasterConnection() {
1873                 return $this->isOpen( $this->getWriterIndex() );
1874         }
1875
1876         public function hasMasterChanges() {
1877                 $pending = 0;
1878                 $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$pending ) {
1879                         $pending |= $conn->writesOrCallbacksPending();
1880                 } );
1881
1882                 return (bool)$pending;
1883         }
1884
1885         public function lastMasterChangeTimestamp() {
1886                 $lastTime = false;
1887                 $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$lastTime ) {
1888                         $lastTime = max( $lastTime, $conn->lastDoneWrites() );
1889                 } );
1890
1891                 return $lastTime;
1892         }
1893
1894         public function hasOrMadeRecentMasterChanges( $age = null ) {
1895                 $age = ( $age === null ) ? $this->waitTimeout : $age;
1896
1897                 return ( $this->hasMasterChanges()
1898                         || $this->lastMasterChangeTimestamp() > microtime( true ) - $age );
1899         }
1900
1901         public function pendingMasterChangeCallers() {
1902                 $fnames = [];
1903                 $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$fnames ) {
1904                         $fnames = array_merge( $fnames, $conn->pendingWriteCallers() );
1905                 } );
1906
1907                 return $fnames;
1908         }
1909
1910         public function getLaggedReplicaMode( $domain = false ) {
1911                 if ( $this->laggedReplicaMode ) {
1912                         return true; // stay in lagged replica mode
1913                 }
1914
1915                 if ( $this->hasStreamingReplicaServers() ) {
1916                         // This will set "laggedReplicaMode" as needed
1917                         $this->getReaderIndex( self::GROUP_GENERIC, $domain );
1918                 }
1919
1920                 return $this->laggedReplicaMode;
1921         }
1922
1923         public function laggedReplicaUsed() {
1924                 return $this->laggedReplicaMode;
1925         }
1926
1927         public function getReadOnlyReason( $domain = false, IDatabase $conn = null ) {
1928                 if ( $this->readOnlyReason !== false ) {
1929                         return $this->readOnlyReason;
1930                 } elseif ( $this->masterRunningReadOnly( $domain, $conn ) ) {
1931                         return 'The master database server is running in read-only mode.';
1932                 } elseif ( $this->getLaggedReplicaMode( $domain ) ) {
1933                         return ( $this->getExistingReaderIndex( self::GROUP_GENERIC ) >= 0 )
1934                                 ? 'The database is read-only until replication lag decreases.'
1935                                 : 'The database is read-only until a replica database server becomes reachable.';
1936                 }
1937
1938                 return false;
1939         }
1940
1941         /**
1942          * @param string $domain Domain ID, or false for the current domain
1943          * @param IDatabase|null $conn DB master connectionl used to avoid loops [optional]
1944          * @return bool
1945          */
1946         private function masterRunningReadOnly( $domain, IDatabase $conn = null ) {
1947                 $cache = $this->wanCache;
1948                 $masterServer = $this->getServerName( $this->getWriterIndex() );
1949
1950                 return (bool)$cache->getWithSetCallback(
1951                         $cache->makeGlobalKey( __CLASS__, 'server-read-only', $masterServer ),
1952                         self::TTL_CACHE_READONLY,
1953                         function () use ( $domain, $conn ) {
1954                                 $old = $this->trxProfiler->setSilenced( true );
1955                                 try {
1956                                         $index = $this->getWriterIndex();
1957                                         $dbw = $conn ?: $this->getServerConnection( $index, $domain );
1958                                         $readOnly = (int)$dbw->serverIsReadOnly();
1959                                         if ( !$conn ) {
1960                                                 $this->reuseConnection( $dbw );
1961                                         }
1962                                 } catch ( DBError $e ) {
1963                                         $readOnly = 0;
1964                                 }
1965                                 $this->trxProfiler->setSilenced( $old );
1966
1967                                 return $readOnly;
1968                         },
1969                         [ 'pcTTL' => $cache::TTL_PROC_LONG, 'busyValue' => 0 ]
1970                 );
1971         }
1972
1973         public function allowLagged( $mode = null ) {
1974                 if ( $mode === null ) {
1975                         return $this->allowLagged;
1976                 }
1977                 $this->allowLagged = $mode;
1978
1979                 return $this->allowLagged;
1980         }
1981
1982         public function pingAll() {
1983                 $success = true;
1984                 $this->forEachOpenConnection( function ( IDatabase $conn ) use ( &$success ) {
1985                         if ( !$conn->ping() ) {
1986                                 $success = false;
1987                         }
1988                 } );
1989
1990                 return $success;
1991         }
1992
1993         public function forEachOpenConnection( $callback, array $params = [] ) {
1994                 foreach ( $this->conns as $connsByServer ) {
1995                         foreach ( $connsByServer as $serverConns ) {
1996                                 foreach ( $serverConns as $conn ) {
1997                                         $callback( $conn, ...$params );
1998                                 }
1999                         }
2000                 }
2001         }
2002
2003         public function forEachOpenMasterConnection( $callback, array $params = [] ) {
2004                 $masterIndex = $this->getWriterIndex();
2005                 foreach ( $this->conns as $connsByServer ) {
2006                         if ( isset( $connsByServer[$masterIndex] ) ) {
2007                                 /** @var IDatabase $conn */
2008                                 foreach ( $connsByServer[$masterIndex] as $conn ) {
2009                                         $callback( $conn, ...$params );
2010                                 }
2011                         }
2012                 }
2013         }
2014
2015         public function forEachOpenReplicaConnection( $callback, array $params = [] ) {
2016                 foreach ( $this->conns as $connsByServer ) {
2017                         foreach ( $connsByServer as $i => $serverConns ) {
2018                                 if ( $i === $this->getWriterIndex() ) {
2019                                         continue; // skip master
2020                                 }
2021                                 foreach ( $serverConns as $conn ) {
2022                                         $callback( $conn, ...$params );
2023                                 }
2024                         }
2025                 }
2026         }
2027
2028         /**
2029          * @return int
2030          */
2031         private function getCurrentConnectionCount() {
2032                 $count = 0;
2033                 foreach ( $this->conns as $connsByServer ) {
2034                         foreach ( $connsByServer as $serverConns ) {
2035                                 $count += count( $serverConns );
2036                         }
2037                 }
2038
2039                 return $count;
2040         }
2041
2042         public function getMaxLag( $domain = false ) {
2043                 $host = '';
2044                 $maxLag = -1;
2045                 $maxIndex = 0;
2046
2047                 if ( $this->hasReplicaServers() ) {
2048                         $lagTimes = $this->getLagTimes( $domain );
2049                         foreach ( $lagTimes as $i => $lag ) {
2050                                 if ( $this->groupLoads[self::GROUP_GENERIC][$i] > 0 && $lag > $maxLag ) {
2051                                         $maxLag = $lag;
2052                                         $host = $this->getServerInfoStrict( $i, 'host' );
2053                                         $maxIndex = $i;
2054                                 }
2055                         }
2056                 }
2057
2058                 return [ $host, $maxLag, $maxIndex ];
2059         }
2060
2061         public function getLagTimes( $domain = false ) {
2062                 if ( !$this->hasReplicaServers() ) {
2063                         return [ $this->getWriterIndex() => 0 ]; // no replication = no lag
2064                 }
2065
2066                 $knownLagTimes = []; // map of (server index => 0 seconds)
2067                 $indexesWithLag = [];
2068                 foreach ( $this->servers as $i => $server ) {
2069                         if ( empty( $server['is static'] ) ) {
2070                                 $indexesWithLag[] = $i; // DB server might have replication lag
2071                         } else {
2072                                 $knownLagTimes[$i] = 0; // DB server is a non-replicating and read-only archive
2073                         }
2074                 }
2075
2076                 return $this->getLoadMonitor()->getLagTimes( $indexesWithLag, $domain ) + $knownLagTimes;
2077         }
2078
2079         /**
2080          * Get the lag in seconds for a given connection, or zero if this load
2081          * balancer does not have replication enabled.
2082          *
2083          * This should be used in preference to Database::getLag() in cases where
2084          * replication may not be in use, since there is no way to determine if
2085          * replication is in use at the connection level without running
2086          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
2087          * function instead of Database::getLag() avoids a fatal error in this
2088          * case on many installations.
2089          *
2090          * @param IDatabase $conn
2091          * @return int|bool Returns false on error
2092          * @deprecated Since 1.34 Use IDatabase::getLag() instead
2093          */
2094         public function safeGetLag( IDatabase $conn ) {
2095                 if ( $conn->getLBInfo( 'is static' ) ) {
2096                         return 0; // static dataset
2097                 } elseif ( $conn->getLBInfo( 'serverIndex' ) == $this->getWriterIndex() ) {
2098                         return 0; // this is the master
2099                 }
2100
2101                 return $conn->getLag();
2102         }
2103
2104         public function waitForMasterPos( IDatabase $conn, $pos = false, $timeout = null ) {
2105                 $timeout = max( 1, $timeout ?: $this->waitTimeout );
2106
2107                 if ( $conn->getLBInfo( 'serverIndex' ) === $this->getWriterIndex() ) {
2108                         return true; // not a replica DB server
2109                 }
2110
2111                 if ( !$pos ) {
2112                         // Get the current master position, opening a connection if needed
2113                         $index = $this->getWriterIndex();
2114                         $flags = self::CONN_SILENCE_ERRORS;
2115                         $masterConn = $this->getAnyOpenConnection( $index, $flags );
2116                         if ( $masterConn ) {
2117                                 $pos = $masterConn->getMasterPos();
2118                         } else {
2119                                 $masterConn = $this->getServerConnection( $index, self::DOMAIN_ANY, $flags );
2120                                 if ( !$masterConn ) {
2121                                         throw new DBReplicationWaitError(
2122                                                 null,
2123                                                 "Could not obtain a master database connection to get the position"
2124                                         );
2125                                 }
2126                                 $pos = $masterConn->getMasterPos();
2127                                 $this->closeConnection( $masterConn );
2128                         }
2129                 }
2130
2131                 if ( $pos instanceof DBMasterPos ) {
2132                         $start = microtime( true );
2133                         $result = $conn->masterPosWait( $pos, $timeout );
2134                         $seconds = max( microtime( true ) - $start, 0 );
2135                         if ( $result == -1 || is_null( $result ) ) {
2136                                 $msg = __METHOD__ . ': timed out waiting on {host} pos {pos} [{seconds}s]';
2137                                 $this->replLogger->warning( $msg, [
2138                                         'host' => $conn->getServer(),
2139                                         'pos' => $pos,
2140                                         'seconds' => round( $seconds, 6 ),
2141                                         'trace' => ( new RuntimeException() )->getTraceAsString()
2142                                 ] );
2143                                 $ok = false;
2144                         } else {
2145                                 $this->replLogger->debug( __METHOD__ . ': done waiting' );
2146                                 $ok = true;
2147                         }
2148                 } else {
2149                         $ok = false; // something is misconfigured
2150                         $this->replLogger->error(
2151                                 __METHOD__ . ': could not get master pos for {host}',
2152                                 [
2153                                         'host' => $conn->getServer(),
2154                                         'trace' => ( new RuntimeException() )->getTraceAsString()
2155                                 ]
2156                         );
2157                 }
2158
2159                 return $ok;
2160         }
2161
2162         /**
2163          * Wait for a replica DB to reach a specified master position
2164          *
2165          * This will connect to the master to get an accurate position if $pos is not given
2166          *
2167          * @param IDatabase $conn Replica DB
2168          * @param DBMasterPos|bool $pos Master position; default: current position
2169          * @param int $timeout Timeout in seconds [optional]
2170          * @return bool Success
2171          * @since 1.28
2172          * @deprecated Since 1.34 Use waitForMasterPos() instead
2173          */
2174         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = null ) {
2175                 return $this->waitForMasterPos( $conn, $pos, $timeout );
2176         }
2177
2178         public function setTransactionListener( $name, callable $callback = null ) {
2179                 if ( $callback ) {
2180                         $this->trxRecurringCallbacks[$name] = $callback;
2181                 } else {
2182                         unset( $this->trxRecurringCallbacks[$name] );
2183                 }
2184                 $this->forEachOpenMasterConnection(
2185                         function ( IDatabase $conn ) use ( $name, $callback ) {
2186                                 $conn->setTransactionListener( $name, $callback );
2187                         }
2188                 );
2189         }
2190
2191         public function setTableAliases( array $aliases ) {
2192                 $this->tableAliases = $aliases;
2193         }
2194
2195         public function setIndexAliases( array $aliases ) {
2196                 $this->indexAliases = $aliases;
2197         }
2198
2199         public function setLocalDomainPrefix( $prefix ) {
2200                 // Find connections to explicit foreign domains still marked as in-use...
2201                 $domainsInUse = [];
2202                 $this->forEachOpenConnection( function ( IDatabase $conn ) use ( &$domainsInUse ) {
2203                         // Once reuseConnection() is called on a handle, its reference count goes from 1 to 0.
2204                         // Until then, it is still in use by the caller (explicitly or via DBConnRef scope).
2205                         if ( $conn->getLBInfo( 'foreignPoolRefCount' ) > 0 ) {
2206                                 $domainsInUse[] = $conn->getDomainID();
2207                         }
2208                 } );
2209
2210                 // Do not switch connections to explicit foreign domains unless marked as safe
2211                 if ( $domainsInUse ) {
2212                         $domains = implode( ', ', $domainsInUse );
2213                         throw new DBUnexpectedError( null,
2214                                 "Foreign domain connections are still in use ($domains)" );
2215                 }
2216
2217                 $this->setLocalDomain( new DatabaseDomain(
2218                         $this->localDomain->getDatabase(),
2219                         $this->localDomain->getSchema(),
2220                         $prefix
2221                 ) );
2222
2223                 // Update the prefix for all local connections...
2224                 $this->forEachOpenConnection( function ( IDatabase $conn ) use ( $prefix ) {
2225                         if ( !$conn->getLBInfo( 'foreign' ) ) {
2226                                 $conn->tablePrefix( $prefix );
2227                         }
2228                 } );
2229         }
2230
2231         public function redefineLocalDomain( $domain ) {
2232                 $this->closeAll();
2233
2234                 $this->setLocalDomain( DatabaseDomain::newFromId( $domain ) );
2235         }
2236
2237         /**
2238          * @param DatabaseDomain $domain
2239          */
2240         private function setLocalDomain( DatabaseDomain $domain ) {
2241                 $this->localDomain = $domain;
2242                 // In case a caller assumes that the domain ID is simply <db>-<prefix>, which is almost
2243                 // always true, gracefully handle the case when they fail to account for escaping.
2244                 if ( $this->localDomain->getTablePrefix() != '' ) {
2245                         $this->localDomainIdAlias =
2246                                 $this->localDomain->getDatabase() . '-' . $this->localDomain->getTablePrefix();
2247                 } else {
2248                         $this->localDomainIdAlias = $this->localDomain->getDatabase();
2249                 }
2250         }
2251
2252         /**
2253          * @param int $i Server index
2254          * @param string|null $field Server index field [optional]
2255          * @return array|mixed
2256          * @throws InvalidArgumentException
2257          */
2258         private function getServerInfoStrict( $i, $field = null ) {
2259                 if ( !isset( $this->servers[$i] ) || !is_array( $this->servers[$i] ) ) {
2260                         throw new InvalidArgumentException( "No server with index '$i'" );
2261                 }
2262
2263                 if ( $field !== null ) {
2264                         if ( !array_key_exists( $field, $this->servers[$i] ) ) {
2265                                 throw new InvalidArgumentException( "No field '$field' in server index '$i'" );
2266                         }
2267
2268                         return $this->servers[$i][$field];
2269                 }
2270
2271                 return $this->servers[$i];
2272         }
2273
2274         function __destruct() {
2275                 // Avoid connection leaks for sanity
2276                 $this->disable();
2277         }
2278 }
2279
2280 /**
2281  * @deprecated since 1.29
2282  */
2283 class_alias( LoadBalancer::class, 'LoadBalancer' );