Merge "Type hint against LinkTarget in WatchedItemStore"

[lhc/web/wiklou.git] / includes / libs / rdbms / loadbalancer / LoadBalancer.php

<?php
/**
 * Database load balancing manager
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 */
namespace Wikimedia\Rdbms;

use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use Wikimedia\ScopedCallback;
use BagOStuff;
use EmptyBagOStuff;
use WANObjectCache;
use ArrayUtils;
use LogicException;
use UnexpectedValueException;
use InvalidArgumentException;
use RuntimeException;
use Exception;

/**
 * Database connection, tracking, load balancing, and transaction manager for a cluster
 *
 * @ingroup Database
 */
class LoadBalancer implements ILoadBalancer {
        /** @var ILoadMonitor */
        private $loadMonitor;
        /** @var callable|null Callback to run before the first connection attempt */
        private $chronologyCallback;
        /** @var BagOStuff */
        private $srvCache;
        /** @var WANObjectCache */
        private $wanCache;
        /** @var mixed Class name or object With profileIn/profileOut methods */
        private $profiler;
        /** @var TransactionProfiler */
        private $trxProfiler;
        /** @var LoggerInterface */
        private $replLogger;
        /** @var LoggerInterface */
        private $connLogger;
        /** @var LoggerInterface */
        private $queryLogger;
        /** @var LoggerInterface */
        private $perfLogger;
        /** @var callable Exception logger */
        private $errorLogger;
        /** @var callable Deprecation logger */
        private $deprecationLogger;

        /** @var DatabaseDomain Local DB domain ID and default for selectDB() calls */
        private $localDomain;

        /** @var Database[][][] Map of (connection category => server index => IDatabase[]) */
        private $conns;

        /** @var array[] Map of (server index => server config array) */
        private $servers;
        /** @var array[] Map of (group => server index => weight) */
        private $groupLoads;
        /** @var bool Whether to disregard replica DB lag as a factor in replica DB selection */
        private $allowLagged;
        /** @var int Seconds to spend waiting on replica DB lag to resolve */
        private $waitTimeout;
        /** @var array The LoadMonitor configuration */
        private $loadMonitorConfig;
        /** @var string Alternate local DB domain instead of DatabaseDomain::getId() */
        private $localDomainIdAlias;
        /** @var int Amount of replication lag, in seconds, that is considered "high" */
        private $maxLag;
        /** @var string|null Default query group to use with getConnection() */
        private $defaultGroup;

        /** @var string Current server name */
        private $hostname;
        /** @var bool Whether this PHP instance is for a CLI script */
        private $cliMode;
        /** @var string Agent name for query profiling */
        private $agent;

        /** @var array[] $aliases Map of (table => (dbname, schema, prefix) map) */
        private $tableAliases = [];
        /** @var string[] Map of (index alias => index) */
        private $indexAliases = [];
        /** @var array[] Map of (name => callable) */
        private $trxRecurringCallbacks = [];

        /** @var Database Connection handle that caused a problem */
        private $errorConnection;
        /** @var int[] The group replica server indexes keyed by group */
        private $readIndexByGroup = [];
        /** @var bool|DBMasterPos Replication sync position or false if not set */
        private $waitForPos;
        /** @var bool Whether the generic reader fell back to a lagged replica DB */
        private $laggedReplicaMode = false;
        /** @var string The last DB selection or connection error */
        private $lastError = 'Unknown error';
        /** @var string|bool Reason this instance is read-only or false if not */
        private $readOnlyReason = false;
        /** @var int Total number of new connections ever made with this instance */
        private $connectionCounter = 0;
        /** @var bool */
        private $disabled = false;
        /** @var bool Whether any connection has been attempted yet */
        private $connectionAttempted = false;

        /** @var int|null Integer ID of the managing LBFactory instance or null if none */
        private $ownerId;
        /** @var string|bool Explicit DBO_TRX transaction round active or false if none */
        private $trxRoundId = false;
        /** @var string Stage of the current transaction round in the transaction round life-cycle */
        private $trxRoundStage = self::ROUND_CURSORY;

        /** @var int Warn when this many connection are held */
        const CONN_HELD_WARN_THRESHOLD = 10;

        /** @var int Default 'maxLag' when unspecified */
        const MAX_LAG_DEFAULT = 6;
        /** @var int Default 'waitTimeout' when unspecified */
        const MAX_WAIT_DEFAULT = 10;
        /** @var int Seconds to cache master DB server read-only status */
        const TTL_CACHE_READONLY = 5;

        const KEY_LOCAL = 'local';
        const KEY_FOREIGN_FREE = 'foreignFree';
        const KEY_FOREIGN_INUSE = 'foreignInUse';

        const KEY_LOCAL_NOROUND = 'localAutoCommit';
        const KEY_FOREIGN_FREE_NOROUND = 'foreignFreeAutoCommit';
        const KEY_FOREIGN_INUSE_NOROUND = 'foreignInUseAutoCommit';

        /** @var string Transaction round, explicit or implicit, has not finished writing */
        const ROUND_CURSORY = 'cursory';
        /** @var string Transaction round writes are complete and ready for pre-commit checks */
        const ROUND_FINALIZED = 'finalized';
        /** @var string Transaction round passed final pre-commit checks */
        const ROUND_APPROVED = 'approved';
        /** @var string Transaction round was committed and post-commit callbacks must be run */
        const ROUND_COMMIT_CALLBACKS = 'commit-callbacks';
        /** @var string Transaction round was rolled back and post-rollback callbacks must be run */
        const ROUND_ROLLBACK_CALLBACKS = 'rollback-callbacks';
        /** @var string Transaction round encountered an error */
        const ROUND_ERROR = 'error';

        public function __construct( array $params ) {
                if ( !isset( $params['servers'] ) || !count( $params['servers'] ) ) {
                        throw new InvalidArgumentException( 'Missing or empty "servers" parameter' );
                }

                $listKey = -1;
                $this->servers = [];
                $this->groupLoads = [ self::GROUP_GENERIC => [] ];
                foreach ( $params['servers'] as $i => $server ) {
                        if ( ++$listKey !== $i ) {
                                throw new UnexpectedValueException( 'List expected for "servers" parameter' );
                        }
                        if ( $i == 0 ) {
                                $server['master'] = true;
                        } else {
                                $server['replica'] = true;
                        }
                        $this->servers[$i] = $server;
                        $serverGroupLoads = [ self::GROUP_GENERIC => $server['load'] ];
                        $serverGroupLoads += ( $server['groupLoads'] ?? [] );
                        foreach ( $serverGroupLoads as $group => $ratio ) {
                                $this->groupLoads[$group][$i] = $ratio;
                        }
                }

                $localDomain = isset( $params['localDomain'] )
                        ? DatabaseDomain::newFromId( $params['localDomain'] )
                        : DatabaseDomain::newUnspecified();
                $this->setLocalDomain( $localDomain );

                $this->waitTimeout = $params['waitTimeout'] ?? self::MAX_WAIT_DEFAULT;

                $this->conns = self::newTrackedConnectionsArray();
                $this->waitForPos = false;
                $this->allowLagged = false;

                if ( isset( $params['readOnlyReason'] ) && is_string( $params['readOnlyReason'] ) ) {
                        $this->readOnlyReason = $params['readOnlyReason'];
                }

                $this->maxLag = $params['maxLag'] ?? self::MAX_LAG_DEFAULT;

                $this->loadMonitorConfig = $params['loadMonitor'] ?? [ 'class' => 'LoadMonitorNull' ];
                $this->loadMonitorConfig += [ 'lagWarnThreshold' => $this->maxLag ];

                $this->srvCache = $params['srvCache'] ?? new EmptyBagOStuff();
                $this->wanCache = $params['wanCache'] ?? WANObjectCache::newEmpty();
                $this->profiler = $params['profiler'] ?? null;
                $this->trxProfiler = $params['trxProfiler'] ?? new TransactionProfiler();

                $this->errorLogger = $params['errorLogger'] ?? function ( Exception $e ) {
                        trigger_error( get_class( $e ) . ': ' . $e->getMessage(), E_USER_WARNING );
                };
                $this->deprecationLogger = $params['deprecationLogger'] ?? function ( $msg ) {
                        trigger_error( $msg, E_USER_DEPRECATED );
                };

                foreach ( [ 'replLogger', 'connLogger', 'queryLogger', 'perfLogger' ] as $key ) {
                        $this->$key = $params[$key] ?? new NullLogger();
                }

                $this->hostname = $params['hostname'] ?? ( gethostname() ?: 'unknown' );
                $this->cliMode = $params['cliMode'] ?? ( PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg' );
                $this->agent = $params['agent'] ?? '';

                if ( isset( $params['chronologyCallback'] ) ) {
                        $this->chronologyCallback = $params['chronologyCallback'];
                }

                if ( isset( $params['roundStage'] ) ) {
                        if ( $params['roundStage'] === self::STAGE_POSTCOMMIT_CALLBACKS ) {
                                $this->trxRoundStage = self::ROUND_COMMIT_CALLBACKS;
                        } elseif ( $params['roundStage'] === self::STAGE_POSTROLLBACK_CALLBACKS ) {
                                $this->trxRoundStage = self::ROUND_ROLLBACK_CALLBACKS;
                        }
                }

                $group = $params['defaultGroup'] ?? self::GROUP_GENERIC;
                $this->defaultGroup = isset( $this->groupLoads[$group] ) ? $group : self::GROUP_GENERIC;

                $this->ownerId = $params['ownerId'] ?? null;
        }

        private static function newTrackedConnectionsArray() {
                return [
                        // Connection were transaction rounds may be applied
                        self::KEY_LOCAL => [],
                        self::KEY_FOREIGN_INUSE => [],
                        self::KEY_FOREIGN_FREE => [],
                        // Auto-committing counterpart connections that ignore transaction rounds
                        self::KEY_LOCAL_NOROUND => [],
                        self::KEY_FOREIGN_INUSE_NOROUND => [],
                        self::KEY_FOREIGN_FREE_NOROUND => []
                ];
        }

        public function getLocalDomainID() {
                return $this->localDomain->getId();
        }

        public function resolveDomainID( $domain ) {
                if ( $domain === $this->localDomainIdAlias || $domain === false ) {
                        // Local connection requested via some backwards-compatibility domain alias
                        return $this->getLocalDomainID();
                }

                return (string)$domain;
        }

        /**
         * Resolve $groups into a list of query groups defining as having database servers
         *
         * @param string[]|string|bool $groups Query group(s) in preference order, [], or false
         * @param int $i Specific server index or DB_MASTER/DB_REPLICA
         * @return string[] Non-empty group list in preference order with the default group appended
         */
        private function resolveGroups( $groups, $i ) {
                // If a specific replica server was specified, then $groups makes no sense
                if ( $i > 0 && $groups !== [] && $groups !== false ) {
                        $list = implode( ', ', (array)$groups );
                        throw new LogicException( "Query group(s) ($list) given with server index (#$i)" );
                }

                if ( $groups === [] || $groups === false || $groups === $this->defaultGroup ) {
                        $resolvedGroups = [ $this->defaultGroup ]; // common case
                } elseif ( is_string( $groups ) && isset( $this->groupLoads[$groups] ) ) {
                        $resolvedGroups = [ $groups, $this->defaultGroup ];
                } elseif ( is_array( $groups ) ) {
                        $resolvedGroups = array_keys( array_flip( $groups ) + [ self::GROUP_GENERIC => 1 ] );
                } else {
                        $resolvedGroups = [ $this->defaultGroup ];
                }

                return $resolvedGroups;
        }

        /**
         * @param int $flags Bitfield of class CONN_* constants
         * @param int $i Specific server index or DB_MASTER/DB_REPLICA
         * @return int Sanitized bitfield
         */
        private function sanitizeConnectionFlags( $flags, $i ) {
                // Whether an outside caller is explicitly requesting the master database server
                if ( $i === self::DB_MASTER || $i === $this->getWriterIndex() ) {
                        $flags |= self::CONN_INTENT_WRITABLE;
                }

                if ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT ) {
                        // Callers use CONN_TRX_AUTOCOMMIT to bypass REPEATABLE-READ staleness without
                        // resorting to row locks (e.g. FOR UPDATE) or to make small out-of-band commits
                        // during larger transactions. This is useful for avoiding lock contention.

                        // Master DB server attributes (should match those of the replica DB servers)
                        $attributes = $this->getServerAttributes( $this->getWriterIndex() );
                        if ( $attributes[Database::ATTR_DB_LEVEL_LOCKING] ) {
                                // The RDBMS does not support concurrent writes (e.g. SQLite), so attempts
                                // to use separate connections would just cause self-deadlocks. Note that
                                // REPEATABLE-READ staleness is not an issue since DB-level locking means
                                // that transactions are Strict Serializable anyway.
                                $flags &= ~self::CONN_TRX_AUTOCOMMIT;
                                $type = $this->getServerType( $this->getWriterIndex() );
                                $this->connLogger->info( __METHOD__ . ": CONN_TRX_AUTOCOMMIT disallowed ($type)" );
                        }
                }

                return $flags;
        }

        /**
         * @param IDatabase $conn
         * @param int $flags
         * @throws DBUnexpectedError
         */
        private function enforceConnectionFlags( IDatabase $conn, $flags ) {
                if ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT ) {
                        if ( $conn->trxLevel() ) { // sanity
                                throw new DBUnexpectedError(
                                        $conn,
                                        'Handle requested with CONN_TRX_AUTOCOMMIT yet it has a transaction'
                                );
                        }

                        $conn->clearFlag( $conn::DBO_TRX ); // auto-commit mode
                }
        }

        /**
         * Get a LoadMonitor instance
         *
         * @return ILoadMonitor
         */
        private function getLoadMonitor() {
                if ( !isset( $this->loadMonitor ) ) {
                        $compat = [
                                'LoadMonitor' => LoadMonitor::class,
                                'LoadMonitorNull' => LoadMonitorNull::class,
                                'LoadMonitorMySQL' => LoadMonitorMySQL::class,
                        ];

                        $class = $this->loadMonitorConfig['class'];
                        if ( isset( $compat[$class] ) ) {
                                $class = $compat[$class];
                        }

                        $this->loadMonitor = new $class(
                                $this, $this->srvCache, $this->wanCache, $this->loadMonitorConfig );
                        $this->loadMonitor->setLogger( $this->replLogger );
                }

                return $this->loadMonitor;
        }

        /**
         * @param array $loads
         * @param bool|string $domain Domain to get non-lagged for
         * @param int $maxLag Restrict the maximum allowed lag to this many seconds
         * @return bool|int|string
         */
        private function getRandomNonLagged( array $loads, $domain = false, $maxLag = INF ) {
                $lags = $this->getLagTimes( $domain );

                # Unset excessively lagged servers
                foreach ( $lags as $i => $lag ) {
                        if ( $i !== $this->getWriterIndex() ) {
                                # How much lag this server nominally is allowed to have
                                $maxServerLag = $this->servers[$i]['max lag'] ?? $this->maxLag; // default
                                # Constrain that futher by $maxLag argument
                                $maxServerLag = min( $maxServerLag, $maxLag );

                                $host = $this->getServerName( $i );
                                if ( $lag === false && !is_infinite( $maxServerLag ) ) {
                                        $this->replLogger->debug(
                                                __METHOD__ .
                                                ": server {host} is not replicating?", [ 'host' => $host ] );
                                        unset( $loads[$i] );
                                } elseif ( $lag > $maxServerLag ) {
                                        $this->replLogger->debug(
                                                __METHOD__ .
                                                ": server {host} has {lag} seconds of lag (>= {maxlag})",
                                                [ 'host' => $host, 'lag' => $lag, 'maxlag' => $maxServerLag ]
                                        );
                                        unset( $loads[$i] );
                                }
                        }
                }

                # Find out if all the replica DBs with non-zero load are lagged
                $sum = 0;
                foreach ( $loads as $load ) {
                        $sum += $load;
                }
                if ( $sum == 0 ) {
                        # No appropriate DB servers except maybe the master and some replica DBs with zero load
                        # Do NOT use the master
                        # Instead, this function will return false, triggering read-only mode,
                        # and a lagged replica DB will be used instead.
                        return false;
                }

                if ( count( $loads ) == 0 ) {
                        return false;
                }

                # Return a random representative of the remainder
                return ArrayUtils::pickRandom( $loads );
        }

        /**
         * Get the server index to use for a specified server index and query group list
         *
         * @param int $i Specific server index or DB_MASTER/DB_REPLICA
         * @param string[] $groups Non-empty query group list in preference order
         * @param string|bool $domain
         * @return int A specific server index (replica DBs are checked for connectivity)
         */
        private function getConnectionIndex( $i, array $groups, $domain ) {
                if ( $i === self::DB_MASTER ) {
                        $i = $this->getWriterIndex();
                } elseif ( $i === self::DB_REPLICA ) {
                        foreach ( $groups as $group ) {
                                $groupIndex = $this->getReaderIndex( $group, $domain );
                                if ( $groupIndex !== false ) {
                                        $i = $groupIndex; // group connection succeeded
                                        break;
                                }
                        }
                } elseif ( !isset( $this->servers[$i] ) ) {
                        throw new UnexpectedValueException( "Invalid server index index #$i" );
                }

                if ( $i === self::DB_REPLICA ) {
                        $this->lastError = 'Unknown error'; // set here in case of worse failure
                        $this->lastError = 'No working replica DB server: ' . $this->lastError;
                        $this->reportConnectionError();
                        return null; // unreachable due to exception
                }

                return $i;
        }

        public function getReaderIndex( $group = false, $domain = false ) {
                if ( $this->getServerCount() == 1 ) {
                        // Skip the load balancing if there's only one server
                        return $this->getWriterIndex();
                }

                $group = is_string( $group ) ? $group : self::GROUP_GENERIC;

                $index = $this->getExistingReaderIndex( $group );
                if ( $index >= 0 ) {
                        // A reader index was already selected and "waitForPos" was handled
                        return $index;
                }

                // Use the server weight array for this load group
                if ( isset( $this->groupLoads[$group] ) ) {
                        $loads = $this->groupLoads[$group];
                } else {
                        $this->connLogger->info( __METHOD__ . ": no loads for group $group" );

                        return false;
                }

                // Scale the configured load ratios according to each server's load and state
                $this->getLoadMonitor()->scaleLoads( $loads, $domain );

                // Pick a server to use, accounting for weights, load, lag, and "waitForPos"
                $this->lazyLoadReplicationPositions(); // optimizes server candidate selection
                list( $i, $laggedReplicaMode ) = $this->pickReaderIndex( $loads, $domain );
                if ( $i === false ) {
                        // DB connection unsuccessful
                        return false;
                }

                // If data seen by queries is expected to reflect the transactions committed as of
                // or after a given replication position then wait for the DB to apply those changes
                if ( $this->waitForPos && $i !== $this->getWriterIndex() && !$this->doWait( $i ) ) {
                        // Data will be outdated compared to what was expected
                        $laggedReplicaMode = true;
                }

                // Cache the reader index for future DB_REPLICA handles
                $this->setExistingReaderIndex( $group, $i );
                // Record whether the generic reader index is in "lagged replica DB" mode
                if ( $group === self::GROUP_GENERIC && $laggedReplicaMode ) {
                        $this->laggedReplicaMode = true;
                }

                $serverName = $this->getServerName( $i );
                $this->connLogger->debug( __METHOD__ . ": using server $serverName for group '$group'" );

                return $i;
        }

        /**
         * Get the server index chosen by the load balancer for use with the given query group
         *
         * @param string $group Query group; use false for the generic group
         * @return int Server index or -1 if none was chosen
         */
        protected function getExistingReaderIndex( $group ) {
                return $this->readIndexByGroup[$group] ?? -1;
        }

        /**
         * Set the server index chosen by the load balancer for use with the given query group
         *
         * @param string $group Query group; use false for the generic group
         * @param int $index The index of a specific server
         */
        private function setExistingReaderIndex( $group, $index ) {
                if ( $index < 0 ) {
                        throw new UnexpectedValueException( "Cannot set a negative read server index" );
                }
                $this->readIndexByGroup[$group] = $index;
        }

        /**
         * @param array $loads List of server weights
         * @param string|bool $domain
         * @return array (reader index, lagged replica mode) or (false, false) on failure
         */
        private function pickReaderIndex( array $loads, $domain = false ) {
                if ( $loads === [] ) {
                        throw new InvalidArgumentException( "Server configuration array is empty" );
                }

                /** @var int|bool $i Index of selected server */
                $i = false;
                /** @var bool $laggedReplicaMode Whether server is considered lagged */
                $laggedReplicaMode = false;

                // Quickly look through the available servers for a server that meets criteria...
                $currentLoads = $loads;
                while ( count( $currentLoads ) ) {
                        if ( $this->allowLagged || $laggedReplicaMode ) {
                                $i = ArrayUtils::pickRandom( $currentLoads );
                        } else {
                                $i = false;
                                if ( $this->waitForPos && $this->waitForPos->asOfTime() ) {
                                        $this->replLogger->debug( __METHOD__ . ": replication positions detected" );
                                        // "chronologyCallback" sets "waitForPos" for session consistency.
                                        // This triggers doWait() after connect, so it's especially good to
                                        // avoid lagged servers so as to avoid excessive delay in that method.
                                        $ago = microtime( true ) - $this->waitForPos->asOfTime();
                                        // Aim for <= 1 second of waiting (being too picky can backfire)
                                        $i = $this->getRandomNonLagged( $currentLoads, $domain, $ago + 1 );
                                }
                                if ( $i === false ) {
                                        // Any server with less lag than it's 'max lag' param is preferable
                                        $i = $this->getRandomNonLagged( $currentLoads, $domain );
                                }
                                if ( $i === false && count( $currentLoads ) ) {
                                        // All replica DBs lagged. Switch to read-only mode
                                        $this->replLogger->error(
                                                __METHOD__ . ": all replica DBs lagged. Switch to read-only mode" );
                                        $i = ArrayUtils::pickRandom( $currentLoads );
                                        $laggedReplicaMode = true;
                                }
                        }

                        if ( $i === false ) {
                                // pickRandom() returned false.
                                // This is permanent and means the configuration or the load monitor
                                // wants us to return false.
                                $this->connLogger->debug( __METHOD__ . ": pickRandom() returned false" );

                                return [ false, false ];
                        }

                        $serverName = $this->getServerName( $i );
                        $this->connLogger->debug( __METHOD__ . ": Using reader #$i: $serverName..." );

                        // Get a connection to this server without triggering other server connections
                        $flags = self::CONN_SILENCE_ERRORS;
                        $conn = $this->getServerConnection( $i, $domain, $flags );
                        if ( !$conn ) {
                                $this->connLogger->warning( __METHOD__ . ": Failed connecting to $i/$domain" );
                                unset( $currentLoads[$i] ); // avoid this server next iteration
                                $i = false;
                                continue;
                        }

                        // Decrement reference counter, we are finished with this connection.
                        // It will be incremented for the caller later.
                        if ( $domain !== false ) {
                                $this->reuseConnection( $conn );
                        }

                        // Return this server
                        break;
                }

                // If all servers were down, quit now
                if ( $currentLoads === [] ) {
                        $this->connLogger->error( __METHOD__ . ": all servers down" );
                }

                return [ $i, $laggedReplicaMode ];
        }

        public function waitFor( $pos ) {
                $oldPos = $this->waitForPos;
                try {
                        $this->waitForPos = $pos;
                        // If a generic reader connection was already established, then wait now
                        $i = $this->getExistingReaderIndex( self::GROUP_GENERIC );
                        if ( $i > 0 && !$this->doWait( $i ) ) {
                                $this->laggedReplicaMode = true;
                        }
                        // Otherwise, wait until a connection is established in getReaderIndex()
                } finally {
                        // Restore the older position if it was higher since this is used for lag-protection
                        $this->setWaitForPositionIfHigher( $oldPos );
                }
        }

        public function waitForOne( $pos, $timeout = null ) {
                $oldPos = $this->waitForPos;
                try {
                        $this->waitForPos = $pos;

                        $i = $this->getExistingReaderIndex( self::GROUP_GENERIC );
                        if ( $i <= 0 ) {
                                // Pick a generic replica DB if there isn't one yet
                                $readLoads = $this->groupLoads[self::GROUP_GENERIC];
                                unset( $readLoads[$this->getWriterIndex()] ); // replica DBs only
                                $readLoads = array_filter( $readLoads ); // with non-zero load
                                $i = ArrayUtils::pickRandom( $readLoads );
                        }

                        if ( $i > 0 ) {
                                $ok = $this->doWait( $i, true, $timeout );
                        } else {
                                $ok = true; // no applicable loads
                        }
                } finally {
                        # Restore the old position, as this is not used for lag-protection but for throttling
                        $this->waitForPos = $oldPos;
                }

                return $ok;
        }

        public function waitForAll( $pos, $timeout = null ) {
                $timeout = $timeout ?: $this->waitTimeout;

                $oldPos = $this->waitForPos;
                try {
                        $this->waitForPos = $pos;
                        $serverCount = $this->getServerCount();

                        $ok = true;
                        for ( $i = 1; $i < $serverCount; $i++ ) {
                                if ( $this->groupLoads[self::GROUP_GENERIC][$i] > 0 ) {
                                        $start = microtime( true );
                                        $ok = $this->doWait( $i, true, $timeout ) && $ok;
                                        $timeout -= intval( microtime( true ) - $start );
                                        if ( $timeout <= 0 ) {
                                                break; // timeout reached
                                        }
                                }
                        }
                } finally {
                        # Restore the old position, as this is not used for lag-protection but for throttling
                        $this->waitForPos = $oldPos;
                }

                return $ok;
        }

        /**
         * @param DBMasterPos|bool $pos
         */
        private function setWaitForPositionIfHigher( $pos ) {
                if ( !$pos ) {
                        return;
                }

                if ( !$this->waitForPos || $pos->hasReached( $this->waitForPos ) ) {
                        $this->waitForPos = $pos;
                }
        }

        public function getAnyOpenConnection( $i, $flags = 0 ) {
                $i = ( $i === self::DB_MASTER ) ? $this->getWriterIndex() : $i;
                // Connection handles required to be in auto-commit mode use a separate connection
                // pool since the main pool is effected by implicit and explicit transaction rounds
                $autocommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );

                $conn = false;
                foreach ( $this->conns as $connsByServer ) {
                        // Get the connection array server indexes to inspect
                        if ( $i === self::DB_REPLICA ) {
                                $indexes = array_keys( $connsByServer );
                        } else {
                                $indexes = isset( $connsByServer[$i] ) ? [ $i ] : [];
                        }

                        foreach ( $indexes as $index ) {
                                $conn = $this->pickAnyOpenConnection( $connsByServer[$index], $autocommit );
                                if ( $conn ) {
                                        break;
                                }
                        }
                }

                if ( $conn ) {
                        $this->enforceConnectionFlags( $conn, $flags );
                }

                return $conn;
        }

        /**
         * @param IDatabase[] $candidateConns
         * @param bool $autocommit Whether to only look for auto-commit connections
         * @return IDatabase|false An appropriate open connection or false if none found
         */
        private function pickAnyOpenConnection( $candidateConns, $autocommit ) {
                $conn = false;

                foreach ( $candidateConns as $candidateConn ) {
                        if ( !$candidateConn->isOpen() ) {
                                continue; // some sort of error occured?
                        } elseif (
                                $autocommit &&
                                (
                                        // Connection is transaction round aware
                                        !$candidateConn->getLBInfo( 'autoCommitOnly' ) ||
                                        // Some sort of error left a transaction open?
                                        $candidateConn->trxLevel()
                                )
                        ) {
                                continue; // some sort of error left a transaction open?
                        }

                        $conn = $candidateConn;
                }

                return $conn;
        }

        /**
         * Wait for a given replica DB to catch up to the master pos stored in "waitForPos"
         * @param int $index Specific server index
         * @param bool $open Check the server even if a new connection has to be made
         * @param int|null $timeout Max seconds to wait; default is "waitTimeout"
         * @return bool
         */
        protected function doWait( $index, $open = false, $timeout = null ) {
                $timeout = max( 1, intval( $timeout ?: $this->waitTimeout ) );

                // Check if we already know that the DB has reached this point
                $server = $this->getServerName( $index );
                $key = $this->srvCache->makeGlobalKey( __CLASS__, 'last-known-pos', $server, 'v1' );
                /** @var DBMasterPos $knownReachedPos */
                $knownReachedPos = $this->srvCache->get( $key );
                if (
                        $knownReachedPos instanceof DBMasterPos &&
                        $knownReachedPos->hasReached( $this->waitForPos )
                ) {
                        $this->replLogger->debug(
                                __METHOD__ .
                                ': replica DB {dbserver} known to be caught up (pos >= $knownReachedPos).',
                                [ 'dbserver' => $server ]
                        );
                        return true;
                }

                // Find a connection to wait on, creating one if needed and allowed
                $close = false; // close the connection afterwards
                $flags = self::CONN_SILENCE_ERRORS;
                $conn = $this->getAnyOpenConnection( $index, $flags );
                if ( !$conn ) {
                        if ( !$open ) {
                                $this->replLogger->debug(
                                        __METHOD__ . ': no connection open for {dbserver}',
                                        [ 'dbserver' => $server ]
                                );

                                return false;
                        }
                        // Get a connection to this server without triggering other server connections
                        $conn = $this->getServerConnection( $index, self::DOMAIN_ANY, $flags );
                        if ( !$conn ) {
                                $this->replLogger->warning(
                                        __METHOD__ . ': failed to connect to {dbserver}',
                                        [ 'dbserver' => $server ]
                                );

                                return false;
                        }
                        // Avoid connection spam in waitForAll() when connections
                        // are made just for the sake of doing this lag check.
                        $close = true;
                }

                $this->replLogger->info(
                        __METHOD__ .
                        ': waiting for replica DB {dbserver} to catch up...',
                        [ 'dbserver' => $server ]
                );

                $result = $conn->masterPosWait( $this->waitForPos, $timeout );

                if ( $result === null ) {
                        $this->replLogger->warning(
                                __METHOD__ . ': Errored out waiting on {host} pos {pos}',
                                [
                                        'host' => $server,
                                        'pos' => $this->waitForPos,
                                        'trace' => ( new RuntimeException() )->getTraceAsString()
                                ]
                        );
                        $ok = false;
                } elseif ( $result == -1 ) {
                        $this->replLogger->warning(
                                __METHOD__ . ': Timed out waiting on {host} pos {pos}',
                                [
                                        'host' => $server,
                                        'pos' => $this->waitForPos,
                                        'trace' => ( new RuntimeException() )->getTraceAsString()
                                ]
                        );
                        $ok = false;
                } else {
                        $this->replLogger->debug( __METHOD__ . ": done waiting" );
                        $ok = true;
                        // Remember that the DB reached this point
                        $this->srvCache->set( $key, $this->waitForPos, BagOStuff::TTL_DAY );
                }

                if ( $close ) {
                        $this->closeConnection( $conn );
                }

                return $ok;
        }

        public function getConnection( $i, $groups = [], $domain = false, $flags = 0 ) {
                $domain = $this->resolveDomainID( $domain );
                $groups = $this->resolveGroups( $groups, $i );
                $flags = $this->sanitizeConnectionFlags( $flags, $i );
                // If given DB_MASTER/DB_REPLICA, resolve it to a specific server index. Resolving
                // DB_REPLICA might trigger getServerConnection() calls due to the getReaderIndex()
                // connectivity checks or LoadMonitor::scaleLoads() server state cache regeneration.
                // The use of getServerConnection() instead of getConnection() avoids infinite loops.
                $serverIndex = $this->getConnectionIndex( $i, $groups, $domain );
                // Get an open connection to that server (might trigger a new connection)
                $conn = $this->getServerConnection( $serverIndex, $domain, $flags );
                // Set master DB handles as read-only if there is high replication lag
                if ( $serverIndex === $this->getWriterIndex() && $this->getLaggedReplicaMode( $domain ) ) {
                        $reason = ( $this->getExistingReaderIndex( self::GROUP_GENERIC ) >= 0 )
                                ? 'The database is read-only until replication lag decreases.'
                                : 'The database is read-only until replica database servers becomes reachable.';
                        $conn->setLBInfo( 'readOnlyReason', $reason );
                }

                return $conn;
        }

        /**
         * @param int $i Specific server index
         * @param string $domain Resolved DB domain
         * @param int $flags Bitfield of class CONN_* constants
         * @return IDatabase|bool
         * @throws InvalidArgumentException When the server index is invalid
         */
        public function getServerConnection( $i, $domain, $flags = 0 ) {
                // Number of connections made before getting the server index and handle
                $priorConnectionsMade = $this->connectionCounter;
                // Get an open connection to this server (might trigger a new connection)
                $conn = $this->localDomain->equals( $domain )
                        ? $this->getLocalConnection( $i, $flags )
                        : $this->getForeignConnection( $i, $domain, $flags );
                // Throw an error or otherwise bail out if the connection attempt failed
                if ( !( $conn instanceof IDatabase ) ) {
                        if ( ( $flags & self::CONN_SILENCE_ERRORS ) != self::CONN_SILENCE_ERRORS ) {
                                $this->reportConnectionError();
                        }

                        return false;
                }

                // Profile any new connections caused by this method
                if ( $this->connectionCounter > $priorConnectionsMade ) {
                        $this->trxProfiler->recordConnection(
                                $conn->getServer(),
                                $conn->getDBname(),
                                ( ( $flags & self::CONN_INTENT_WRITABLE ) == self::CONN_INTENT_WRITABLE )
                        );
                }

                if ( !$conn->isOpen() ) {
                        $this->errorConnection = $conn;
                        // Connection was made but later unrecoverably lost for some reason.
                        // Do not return a handle that will just throw exceptions on use, but
                        // let the calling code, e.g. getReaderIndex(), try another server.
                        return false;
                }

                // Make sure that flags like CONN_TRX_AUTOCOMMIT are respected by this handle
                $this->enforceConnectionFlags( $conn, $flags );
                // Set master DB handles as read-only if the load balancer is configured as read-only
                // or the master database server is running in server-side read-only mode. Note that
                // replica DB handles are always read-only via Database::assertIsWritableMaster().
                // Read-only mode due to replication lag is *avoided* here to avoid recursion.
                if ( $conn->getLBInfo( 'serverIndex' ) === $this->getWriterIndex() ) {
                        if ( $this->readOnlyReason !== false ) {
                                $conn->setLBInfo( 'readOnlyReason', $this->readOnlyReason );
                        } elseif ( $this->masterRunningReadOnly( $domain, $conn ) ) {
                                $conn->setLBInfo(
                                        'readOnlyReason',
                                        'The master database server is running in read-only mode.'
                                );
                        }
                }

                return $conn;
        }

        public function reuseConnection( IDatabase $conn ) {
                $serverIndex = $conn->getLBInfo( 'serverIndex' );
                $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
                if ( $serverIndex === null || $refCount === null ) {
                        /**
                         * This can happen in code like:
                         *   foreach ( $dbs as $db ) {
                         *     $conn = $lb->getConnection( $lb::DB_REPLICA, [], $db );
                         *     ...
                         *     $lb->reuseConnection( $conn );
                         *   }
                         * When a connection to the local DB is opened in this way, reuseConnection()
                         * should be ignored
                         */
                        return;
                } elseif ( $conn instanceof DBConnRef ) {
                        // DBConnRef already handles calling reuseConnection() and only passes the live
                        // Database instance to this method. Any caller passing in a DBConnRef is broken.
                        $this->connLogger->error(
                                __METHOD__ . ": got DBConnRef instance.\n" .
                                ( new LogicException() )->getTraceAsString() );

                        return;
                }

                if ( $this->disabled ) {
                        return; // DBConnRef handle probably survived longer than the LoadBalancer
                }

                if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
                        $connFreeKey = self::KEY_FOREIGN_FREE_NOROUND;
                        $connInUseKey = self::KEY_FOREIGN_INUSE_NOROUND;
                } else {
                        $connFreeKey = self::KEY_FOREIGN_FREE;
                        $connInUseKey = self::KEY_FOREIGN_INUSE;
                }

                $domain = $conn->getDomainID();
                if ( !isset( $this->conns[$connInUseKey][$serverIndex][$domain] ) ) {
                        throw new InvalidArgumentException(
                                "Connection $serverIndex/$domain not found; it may have already been freed" );
                } elseif ( $this->conns[$connInUseKey][$serverIndex][$domain] !== $conn ) {
                        throw new InvalidArgumentException(
                                "Connection $serverIndex/$domain mismatched; it may have already been freed" );
                }

                $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
                if ( $refCount <= 0 ) {
                        $this->conns[$connFreeKey][$serverIndex][$domain] = $conn;
                        unset( $this->conns[$connInUseKey][$serverIndex][$domain] );
                        if ( !$this->conns[$connInUseKey][$serverIndex] ) {
                                unset( $this->conns[$connInUseKey][$serverIndex] ); // clean up
                        }
                        $this->connLogger->debug( __METHOD__ . ": freed connection $serverIndex/$domain" );
                } else {
                        $this->connLogger->debug( __METHOD__ .
                                ": reference count for $serverIndex/$domain reduced to $refCount" );
                }
        }

        public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
                $domain = $this->resolveDomainID( $domain );
                $role = $this->getRoleFromIndex( $i );

                return new DBConnRef( $this, $this->getConnection( $i, $groups, $domain, $flags ), $role );
        }

        public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
                $domain = $this->resolveDomainID( $domain );
                $role = $this->getRoleFromIndex( $i );

                return new DBConnRef( $this, [ $i, $groups, $domain, $flags ], $role );
        }

        public function getMaintenanceConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
                $domain = $this->resolveDomainID( $domain );
                $role = $this->getRoleFromIndex( $i );

                return new MaintainableDBConnRef(
                        $this, $this->getConnection( $i, $groups, $domain, $flags ), $role );
        }

        /**
         * @param int $i Server index or DB_MASTER/DB_REPLICA
         * @return int One of DB_MASTER/DB_REPLICA
         */
        private function getRoleFromIndex( $i ) {
                return ( $i === self::DB_MASTER || $i === $this->getWriterIndex() )
                        ? self::DB_MASTER
                        : self::DB_REPLICA;
        }

        /**
         * @param int $i
         * @param string|bool $domain
         * @param int $flags
         * @return Database|bool Live database handle or false on failure
         * @deprecated Since 1.34 Use getConnection() instead
         */
        public function openConnection( $i, $domain = false, $flags = 0 ) {
                return $this->getConnection( $i, [], $domain, $flags | self::CONN_SILENCE_ERRORS );
        }

        /**
         * Open a connection to a local DB, or return one if it is already open.
         *
         * On error, returns false, and the connection which caused the
         * error will be available via $this->errorConnection.
         *
         * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
         *
         * @param int $i Server index
         * @param int $flags Class CONN_* constant bitfield
         * @return Database
         * @throws InvalidArgumentException When the server index is invalid
         * @throws UnexpectedValueException When the DB domain of the connection is corrupted
         */
        private function getLocalConnection( $i, $flags = 0 ) {
                // Connection handles required to be in auto-commit mode use a separate connection
                // pool since the main pool is effected by implicit and explicit transaction rounds
                $autoCommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );

                $connKey = $autoCommit ? self::KEY_LOCAL_NOROUND : self::KEY_LOCAL;
                if ( isset( $this->conns[$connKey][$i][0] ) ) {
                        $conn = $this->conns[$connKey][$i][0];
                } else {
                        // Open a new connection
                        $server = $this->getServerInfoStrict( $i );
                        $server['serverIndex'] = $i;
                        $server['autoCommitOnly'] = $autoCommit;
                        $conn = $this->reallyOpenConnection( $server, $this->localDomain );
                        $host = $this->getServerName( $i );
                        if ( $conn->isOpen() ) {
                                $this->connLogger->debug(
                                        __METHOD__ . ": connected to database $i at '$host'." );
                                $this->conns[$connKey][$i][0] = $conn;
                        } else {
                                $this->connLogger->warning(
                                        __METHOD__ . ": failed to connect to database $i at '$host'." );
                                $this->errorConnection = $conn;
                                $conn = false;
                        }
                }

                // Final sanity check to make sure the right domain is selected
                if (
                        $conn instanceof IDatabase &&
                        !$this->localDomain->isCompatible( $conn->getDomainID() )
                ) {
                        throw new UnexpectedValueException(
                                "Got connection to '{$conn->getDomainID()}', " .
                                "but expected local domain ('{$this->localDomain}')" );
                }

                return $conn;
        }

        /**
         * Open a connection to a foreign DB, or return one if it is already open.
         *
         * Increments a reference count on the returned connection which locks the
         * connection to the requested domain. This reference count can be
         * decremented by calling reuseConnection().
         *
         * If a connection is open to the appropriate server already, but with the wrong
         * database, it will be switched to the right database and returned, as long as
         * it has been freed first with reuseConnection().
         *
         * On error, returns false, and the connection which caused the
         * error will be available via $this->errorConnection.
         *
         * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
         *
         * @param int $i Server index
         * @param string $domain Domain ID to open
         * @param int $flags Class CONN_* constant bitfield
         * @return Database|bool Returns false on connection error
         * @throws DBError When database selection fails
         * @throws InvalidArgumentException When the server index is invalid
         * @throws UnexpectedValueException When the DB domain of the connection is corrupted
         */
        private function getForeignConnection( $i, $domain, $flags = 0 ) {
                $domainInstance = DatabaseDomain::newFromId( $domain );
                // Connection handles required to be in auto-commit mode use a separate connection
                // pool since the main pool is effected by implicit and explicit transaction rounds
                $autoCommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );

                if ( $autoCommit ) {
                        $connFreeKey = self::KEY_FOREIGN_FREE_NOROUND;
                        $connInUseKey = self::KEY_FOREIGN_INUSE_NOROUND;
                } else {
                        $connFreeKey = self::KEY_FOREIGN_FREE;
                        $connInUseKey = self::KEY_FOREIGN_INUSE;
                }

                /** @var Database $conn */
                $conn = null;

                if ( isset( $this->conns[$connInUseKey][$i][$domain] ) ) {
                        // Reuse an in-use connection for the same domain
                        $conn = $this->conns[$connInUseKey][$i][$domain];
                        $this->connLogger->debug( __METHOD__ . ": reusing connection $i/$domain" );
                } elseif ( isset( $this->conns[$connFreeKey][$i][$domain] ) ) {
                        // Reuse a free connection for the same domain
                        $conn = $this->conns[$connFreeKey][$i][$domain];
                        unset( $this->conns[$connFreeKey][$i][$domain] );
                        $this->conns[$connInUseKey][$i][$domain] = $conn;
                        $this->connLogger->debug( __METHOD__ . ": reusing free connection $i/$domain" );
                } elseif ( !empty( $this->conns[$connFreeKey][$i] ) ) {
                        // Reuse a free connection from another domain if possible
                        foreach ( $this->conns[$connFreeKey][$i] as $oldDomain => $conn ) {
                                if ( $domainInstance->getDatabase() !== null ) {
                                        // Check if changing the database will require a new connection.
                                        // In that case, leave the connection handle alone and keep looking.
                                        // This prevents connections from being closed mid-transaction and can
                                        // also avoid overhead if the same database will later be requested.
                                        if (
                                                $conn->databasesAreIndependent() &&
                                                $conn->getDBname() !== $domainInstance->getDatabase()
                                        ) {
                                                continue;
                                        }
                                        // Select the new database, schema, and prefix
                                        $conn->selectDomain( $domainInstance );
                                } else {
                                        // Stay on the current database, but update the schema/prefix
                                        $conn->dbSchema( $domainInstance->getSchema() );
                                        $conn->tablePrefix( $domainInstance->getTablePrefix() );
                                }
                                unset( $this->conns[$connFreeKey][$i][$oldDomain] );
                                // Note that if $domain is an empty string, getDomainID() might not match it
                                $this->conns[$connInUseKey][$i][$conn->getDomainID()] = $conn;
                                $this->connLogger->debug( __METHOD__ .
                                        ": reusing free connection from $oldDomain for $domain" );
                                break;
                        }
                }

                if ( !$conn ) {
                        // Open a new connection
                        $server = $this->getServerInfoStrict( $i );
                        $server['serverIndex'] = $i;
                        $server['foreignPoolRefCount'] = 0;
                        $server['foreign'] = true;
                        $server['autoCommitOnly'] = $autoCommit;
                        $conn = $this->reallyOpenConnection( $server, $domainInstance );
                        if ( !$conn->isOpen() ) {
                                $this->connLogger->warning( __METHOD__ . ": connection error for $i/$domain" );
                                $this->errorConnection = $conn;
                                $conn = false;
                        } else {
                                // Note that if $domain is an empty string, getDomainID() might not match it
                                $this->conns[$connInUseKey][$i][$conn->getDomainID()] = $conn;
                                $this->connLogger->debug( __METHOD__ . ": opened new connection for $i/$domain" );
                        }
                }

                if ( $conn instanceof IDatabase ) {
                        // Final sanity check to make sure the right domain is selected
                        if ( !$domainInstance->isCompatible( $conn->getDomainID() ) ) {
                                throw new UnexpectedValueException(
                                        "Got connection to '{$conn->getDomainID()}', but expected '$domain'" );
                        }
                        // Increment reference count
                        $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
                        $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
                }

                return $conn;
        }

        public function getServerAttributes( $i ) {
                return Database::attributesFromType(
                        $this->getServerType( $i ),
                        $this->servers[$i]['driver'] ?? null
                );
        }

        /**
         * Test if the specified index represents an open connection
         *
         * @param int $index Server index
         * @return bool
         */
        private function isOpen( $index ) {
                return (bool)$this->getAnyOpenConnection( $index );
        }

        /**
         * Open a new network connection to a server (uncached)
         *
         * Returns a Database object whether or not the connection was successful.
         *
         * @param array $server
         * @param DatabaseDomain $domain Domain the connection is for, possibly unspecified
         * @return Database
         * @throws DBAccessError
         * @throws InvalidArgumentException
         */
        protected function reallyOpenConnection( array $server, DatabaseDomain $domain ) {
                if ( $this->disabled ) {
                        throw new DBAccessError();
                }

                if ( $domain->getDatabase() === null ) {
                        // The database domain does not specify a DB name and some database systems require a
                        // valid DB specified on connection. The $server configuration array contains a default
                        // DB name to use for connections in such cases.
                        if ( $server['type'] === 'mysql' ) {
                                // For MySQL, DATABASE and SCHEMA are synonyms, connections need not specify a DB,
                                // and the DB name in $server might not exist due to legacy reasons (the default
                                // domain used to ignore the local LB domain, even when mismatched).
                                $server['dbname'] = null;
                        }
                } else {
                        $server['dbname'] = $domain->getDatabase();
                }

                if ( $domain->getSchema() !== null ) {
                        $server['schema'] = $domain->getSchema();
                }

                // It is always possible to connect with any prefix, even the empty string
                $server['tablePrefix'] = $domain->getTablePrefix();

                // Let the handle know what the cluster master is (e.g. "db1052")
                $masterName = $this->getServerName( $this->getWriterIndex() );
                $server['clusterMasterHost'] = $masterName;

                $server['srvCache'] = $this->srvCache;
                // Set loggers and profilers
                $server['connLogger'] = $this->connLogger;
                $server['queryLogger'] = $this->queryLogger;
                $server['errorLogger'] = $this->errorLogger;
                $server['deprecationLogger'] = $this->deprecationLogger;
                $server['profiler'] = $this->profiler;
                $server['trxProfiler'] = $this->trxProfiler;
                // Use the same agent and PHP mode for all DB handles
                $server['cliMode'] = $this->cliMode;
                $server['agent'] = $this->agent;
                // Use DBO_DEFAULT flags by default for LoadBalancer managed databases. Assume that the
                // application calls LoadBalancer::commitMasterChanges() before the PHP script completes.
                $server['flags'] = $server['flags'] ?? IDatabase::DBO_DEFAULT;

                // Create a live connection object
                try {
                        $db = Database::factory( $server['type'], $server );
                        // Log when many connection are made on requests
                        ++$this->connectionCounter;
                        $currentConnCount = $this->getCurrentConnectionCount();
                        if ( $currentConnCount >= self::CONN_HELD_WARN_THRESHOLD ) {
                                $this->perfLogger->warning(
                                        __METHOD__ . ": {connections}+ connections made (master={masterdb})",
                                        [ 'connections' => $currentConnCount, 'masterdb' => $masterName ]
                                );
                        }
                } catch ( DBConnectionError $e ) {
                        // FIXME: This is probably the ugliest thing I have ever done to
                        // PHP. I'm half-expecting it to segfault, just out of disgust. -- TS
                        $db = $e->db;
                }

                $db->setLBInfo( $server );
                $db->setLazyMasterHandle(
                        $this->getLazyConnectionRef( self::DB_MASTER, [], $db->getDomainID() )
                );
                $db->setTableAliases( $this->tableAliases );
                $db->setIndexAliases( $this->indexAliases );

                if ( $server['serverIndex'] === $this->getWriterIndex() ) {
                        if ( $this->trxRoundId !== false ) {
                                $this->applyTransactionRoundFlags( $db );
                        }
                        foreach ( $this->trxRecurringCallbacks as $name => $callback ) {
                                $db->setTransactionListener( $name, $callback );
                        }
                }

                $this->lazyLoadReplicationPositions(); // session consistency

                return $db;
        }

        /**
         * Make sure that any "waitForPos" positions are loaded and available to doWait()
         */
        private function lazyLoadReplicationPositions() {
                if ( !$this->connectionAttempted && $this->chronologyCallback ) {
                        $this->connectionAttempted = true;
                        ( $this->chronologyCallback )( $this ); // generally calls waitFor()
                        $this->connLogger->debug( __METHOD__ . ': executed chronology callback.' );
                }
        }

        /**
         * @throws DBConnectionError
         */
        private function reportConnectionError() {
                $conn = $this->errorConnection; // the connection which caused the error
                $context = [
                        'method' => __METHOD__,
                        'last_error' => $this->lastError,
                ];

                if ( $conn instanceof IDatabase ) {
                        $context['db_server'] = $conn->getServer();
                        $this->connLogger->warning(
                                __METHOD__ . ": connection error: {last_error} ({db_server})",
                                $context
                        );

                        throw new DBConnectionError( $conn, "{$this->lastError} ({$context['db_server']})" );
                } else {
                        // No last connection, probably due to all servers being too busy
                        $this->connLogger->error(
                                __METHOD__ .
                                ": LB failure with no last connection. Connection error: {last_error}",
                                $context
                        );

                        // If all servers were busy, "lastError" will contain something sensible
                        throw new DBConnectionError( null, $this->lastError );
                }
        }

        public function getWriterIndex() {
                return 0;
        }

        /**
         * Returns true if the specified index is a valid server index
         *
         * @param int $i
         * @return bool
         * @deprecated Since 1.34
         */
        public function haveIndex( $i ) {
                return array_key_exists( $i, $this->servers );
        }

        /**
         * Returns true if the specified index is valid and has non-zero load
         *
         * @param int $i
         * @return bool
         * @deprecated Since 1.34
         */
        public function isNonZeroLoad( $i ) {
                return ( isset( $this->servers[$i] ) && $this->groupLoads[self::GROUP_GENERIC][$i] > 0 );
        }

        public function getServerCount() {
                return count( $this->servers );
        }

        public function hasReplicaServers() {
                return ( $this->getServerCount() > 1 );
        }

        public function hasStreamingReplicaServers() {
                foreach ( $this->servers as $i => $server ) {
                        if ( $i !== $this->getWriterIndex() && empty( $server['is static'] ) ) {
                                return true;
                        }
                }

                return false;
        }

        public function getServerName( $i ) {
                $name = $this->servers[$i]['hostName'] ?? ( $this->servers[$i]['host'] ?? '' );

                return ( $name != '' ) ? $name : 'localhost';
        }

        public function getServerInfo( $i ) {
                return $this->servers[$i] ?? false;
        }

        public function getServerType( $i ) {
                return $this->servers[$i]['type'] ?? 'unknown';
        }

        public function getMasterPos() {
                $index = $this->getWriterIndex();

                $conn = $this->getAnyOpenConnection( $index );
                if ( $conn ) {
                        return $conn->getMasterPos();
                }

                $conn = $this->getConnection( $index, self::CONN_SILENCE_ERRORS );
                if ( !$conn ) {
                        $this->reportConnectionError();
                        return null; // unreachable due to exception
                }

                try {
                        $pos = $conn->getMasterPos();
                } finally {
                        $this->closeConnection( $conn );
                }

                return $pos;
        }

        public function getReplicaResumePos() {
                // Get the position of any existing master server connection
                $masterConn = $this->getAnyOpenConnection( $this->getWriterIndex() );
                if ( $masterConn ) {
                        return $masterConn->getMasterPos();
                }

                // Get the highest position of any existing replica server connection
                $highestPos = false;
                $serverCount = $this->getServerCount();
                for ( $i = 1; $i < $serverCount; $i++ ) {
                        if ( !empty( $this->servers[$i]['is static'] ) ) {
                                continue; // server does not use replication
                        }

                        $conn = $this->getAnyOpenConnection( $i );
                        $pos = $conn ? $conn->getReplicaPos() : false;
                        if ( !$pos ) {
                                continue; // no open connection or could not get position
                        }

                        $highestPos = $highestPos ?: $pos;
                        if ( $pos->hasReached( $highestPos ) ) {
                                $highestPos = $pos;
                        }
                }

                return $highestPos;
        }

        public function disable() {
                $this->closeAll();
                $this->disabled = true;
        }

        public function closeAll() {
                $fname = __METHOD__;
                $this->forEachOpenConnection( function ( IDatabase $conn ) use ( $fname ) {
                        $host = $conn->getServer();
                        $this->connLogger->debug(
                                $fname . ": closing connection to database '$host'." );
                        $conn->close();
                } );

                $this->conns = self::newTrackedConnectionsArray();
        }

        public function closeConnection( IDatabase $conn ) {
                if ( $conn instanceof DBConnRef ) {
                        // Avoid calling close() but still leaving the handle in the pool
                        throw new RuntimeException( 'Cannot close DBConnRef instance; it must be shareable' );
                }

                $serverIndex = $conn->getLBInfo( 'serverIndex' );
                foreach ( $this->conns as $type => $connsByServer ) {
                        if ( !isset( $connsByServer[$serverIndex] ) ) {
                                continue;
                        }

                        foreach ( $connsByServer[$serverIndex] as $i => $trackedConn ) {
                                if ( $conn === $trackedConn ) {
                                        $host = $this->getServerName( $i );
                                        $this->connLogger->debug(
                                                __METHOD__ . ": closing connection to database $i at '$host'." );
                                        unset( $this->conns[$type][$serverIndex][$i] );
                                        break 2;
                                }
                        }
                }

                $conn->close();
        }

        public function commitAll( $fname = __METHOD__, $owner = null ) {
                $this->commitMasterChanges( $fname, $owner );
                $this->flushMasterSnapshots( $fname );
                $this->flushReplicaSnapshots( $fname );
        }

        public function finalizeMasterChanges( $fname = __METHOD__, $owner = null ) {
                $this->assertOwnership( $fname, $owner );
                $this->assertTransactionRoundStage( [ self::ROUND_CURSORY, self::ROUND_FINALIZED ] );

                $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
                // Loop until callbacks stop adding callbacks on other connections
                $total = 0;
                do {
                        $count = 0; // callbacks execution attempts
                        $this->forEachOpenMasterConnection( function ( Database $conn ) use ( &$count ) {
                                // Run any pre-commit callbacks while leaving the post-commit ones suppressed.
                                // Any error should cause all (peer) transactions to be rolled back together.
                                $count += $conn->runOnTransactionPreCommitCallbacks();
                        } );
                        $total += $count;
                } while ( $count > 0 );
                // Defer post-commit callbacks until after COMMIT/ROLLBACK happens on all handles
                $this->forEachOpenMasterConnection( function ( Database $conn ) {
                        $conn->setTrxEndCallbackSuppression( true );
                } );
                $this->trxRoundStage = self::ROUND_FINALIZED;

                return $total;
        }

        public function approveMasterChanges( array $options, $fname = __METHOD__, $owner = null ) {
                $this->assertOwnership( $fname, $owner );
                $this->assertTransactionRoundStage( self::ROUND_FINALIZED );

                $limit = $options['maxWriteDuration'] ?? 0;

                $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
                $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $limit ) {
                        // If atomic sections or explicit transactions are still open, some caller must have
                        // caught an exception but failed to properly rollback any changes. Detect that and
                        // throw and error (causing rollback).
                        $conn->assertNoOpenTransactions();
                        // Assert that the time to replicate the transaction will be sane.
                        // If this fails, then all DB transactions will be rollback back together.
                        $time = $conn->pendingWriteQueryDuration( $conn::ESTIMATE_DB_APPLY );
                        if ( $limit > 0 && $time > $limit ) {
                                throw new DBTransactionSizeError(
                                        $conn,
                                        "Transaction spent $time second(s) in writes, exceeding the limit of $limit",
                                        [ $time, $limit ]
                                );
                        }
                        // If a connection sits idle while slow queries execute on another, that connection
                        // may end up dropped before the commit round is reached. Ping servers to detect this.
                        if ( $conn->writesOrCallbacksPending() && !$conn->ping() ) {
                                throw new DBTransactionError(
                                        $conn,
                                        "A connection to the {$conn->getDBname()} database was lost before commit"
                                );
                        }
                } );
                $this->trxRoundStage = self::ROUND_APPROVED;
        }

        public function beginMasterChanges( $fname = __METHOD__, $owner = null ) {
                $this->assertOwnership( $fname, $owner );
                if ( $this->trxRoundId !== false ) {
                        throw new DBTransactionError(
                                null,
                                "$fname: Transaction round '{$this->trxRoundId}' already started"
                        );
                }
                $this->assertTransactionRoundStage( self::ROUND_CURSORY );

                // Clear any empty transactions (no writes/callbacks) from the implicit round
                $this->flushMasterSnapshots( $fname );

                $this->trxRoundId = $fname;
                $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
                // Mark applicable handles as participating in this explicit transaction round.
                // For each of these handles, any writes and callbacks will be tied to a single
                // transaction. The (peer) handles will reject begin()/commit() calls unless they
                // are part of an en masse commit or an en masse rollback.
                $this->forEachOpenMasterConnection( function ( Database $conn ) {
                        $this->applyTransactionRoundFlags( $conn );
                } );
                $this->trxRoundStage = self::ROUND_CURSORY;
        }

        public function commitMasterChanges( $fname = __METHOD__, $owner = null ) {
                $this->assertOwnership( $fname, $owner );
                $this->assertTransactionRoundStage( self::ROUND_APPROVED );

                $failures = [];

                /** @noinspection PhpUnusedLocalVariableInspection */
                $scope = ScopedCallback::newScopedIgnoreUserAbort(); // try to ignore client aborts

                $restore = ( $this->trxRoundId !== false );
                $this->trxRoundId = false;
                $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
                // Commit any writes and clear any snapshots as well (callbacks require AUTOCOMMIT).
                // Note that callbacks should already be suppressed due to finalizeMasterChanges().
                $this->forEachOpenMasterConnection(
                        function ( IDatabase $conn ) use ( $fname, &$failures ) {
                                try {
                                        $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
                                } catch ( DBError $e ) {
                                        ( $this->errorLogger )( $e );
                                        $failures[] = "{$conn->getServer()}: {$e->getMessage()}";
                                }
                        }
                );
                if ( $failures ) {
                        throw new DBTransactionError(
                                null,
                                "$fname: Commit failed on server(s) " . implode( "\n", array_unique( $failures ) )
                        );
                }
                if ( $restore ) {
                        // Unmark handles as participating in this explicit transaction round
                        $this->forEachOpenMasterConnection( function ( Database $conn ) {
                                $this->undoTransactionRoundFlags( $conn );
                        } );
                }
                $this->trxRoundStage = self::ROUND_COMMIT_CALLBACKS;
        }

        public function runMasterTransactionIdleCallbacks( $fname = __METHOD__, $owner = null ) {
                $this->assertOwnership( $fname, $owner );
                if ( $this->trxRoundStage === self::ROUND_COMMIT_CALLBACKS ) {
                        $type = IDatabase::TRIGGER_COMMIT;
                } elseif ( $this->trxRoundStage === self::ROUND_ROLLBACK_CALLBACKS ) {
                        $type = IDatabase::TRIGGER_ROLLBACK;
                } else {
                        throw new DBTransactionError(
                                null,
                                "Transaction should be in the callback stage (not '{$this->trxRoundStage}')"
                        );
                }

                $oldStage = $this->trxRoundStage;
                $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise

                // Now that the COMMIT/ROLLBACK step is over, enable post-commit callback runs
                $this->forEachOpenMasterConnection( function ( Database $conn ) {
                        $conn->setTrxEndCallbackSuppression( false );
                } );

                $e = null; // first exception
                $fname = __METHOD__;
                // Loop until callbacks stop adding callbacks on other connections
                do {
                        // Run any pending callbacks for each connection...
                        $count = 0; // callback execution attempts
                        $this->forEachOpenMasterConnection(
                                function ( Database $conn ) use ( $type, &$e, &$count ) {
                                        if ( $conn->trxLevel() ) {
                                                return; // retry in the next iteration, after commit() is called
                                        }
                                        try {
                                                $count += $conn->runOnTransactionIdleCallbacks( $type );
                                        } catch ( Exception $ex ) {
                                                $e = $e ?: $ex;
                                        }
                                }
                        );
                        // Clear out any active transactions left over from callbacks...
                        $this->forEachOpenMasterConnection( function ( Database $conn ) use ( &$e, $fname ) {
                                if ( $conn->writesPending() ) {
                                        // A callback from another handle wrote to this one and DBO_TRX is set
                                        $this->queryLogger->warning( $fname . ": found writes pending." );
                                        $fnames = implode( ', ', $conn->pendingWriteAndCallbackCallers() );
                                        $this->queryLogger->warning(
                                                $fname . ": found writes pending ($fnames).",
                                                [
                                                        'db_server' => $conn->getServer(),
                                                        'db_name' => $conn->getDBname()
                                                ]
                                        );
                                } elseif ( $conn->trxLevel() ) {
                                        // A callback from another handle read from this one and DBO_TRX is set,
                                        // which can easily happen if there is only one DB (no replicas)
                                        $this->queryLogger->debug( $fname . ": found empty transaction." );
                                }
                                try {
                                        $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
                                } catch ( Exception $ex ) {
                                        $e = $e ?: $ex;
                                }
                        } );
                } while ( $count > 0 );

                $this->trxRoundStage = $oldStage;

                return $e;
        }

        public function runMasterTransactionListenerCallbacks( $fname = __METHOD__, $owner = null ) {
                $this->assertOwnership( $fname, $owner );
                if ( $this->trxRoundStage === self::ROUND_COMMIT_CALLBACKS ) {
                        $type = IDatabase::TRIGGER_COMMIT;
                } elseif ( $this->trxRoundStage === self::ROUND_ROLLBACK_CALLBACKS ) {
                        $type = IDatabase::TRIGGER_ROLLBACK;
                } else {
                        throw new DBTransactionError(
                                null,
                                "Transaction should be in the callback stage (not '{$this->trxRoundStage}')"
                        );
                }

                $e = null;

                $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
                $this->forEachOpenMasterConnection( function ( Database $conn ) use ( $type, &$e ) {
                        try {
                                $conn->runTransactionListenerCallbacks( $type );
                        } catch ( Exception $ex ) {
                                $e = $e ?: $ex;
                        }
                } );
                $this->trxRoundStage = self::ROUND_CURSORY;

                return $e;
        }

        public function rollbackMasterChanges( $fname = __METHOD__, $owner = null ) {
                $this->assertOwnership( $fname, $owner );

                $restore = ( $this->trxRoundId !== false );
                $this->trxRoundId = false;
                $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
                $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $fname ) {
                        $conn->rollback( $fname, $conn::FLUSHING_ALL_PEERS );
                } );
                if ( $restore ) {
                        // Unmark handles as participating in this explicit transaction round
                        $this->forEachOpenMasterConnection( function ( Database $conn ) {
                                $this->undoTransactionRoundFlags( $conn );
                        } );
                }
                $this->trxRoundStage = self::ROUND_ROLLBACK_CALLBACKS;
        }

        /**
         * @param string|string[] $stage
         * @throws DBTransactionError
         */
        private function assertTransactionRoundStage( $stage ) {
                $stages = (array)$stage;

                if ( !in_array( $this->trxRoundStage, $stages, true ) ) {
                        $stageList = implode(
                                '/',
                                array_map( function ( $v ) {
                                        return "'$v'";
                                }, $stages )
                        );
                        throw new DBTransactionError(
                                null,
                                "Transaction round stage must be $stageList (not '{$this->trxRoundStage}')"
                        );
                }
        }

        /**
         * @param string $fname
         * @param int|null $owner Owner ID of the caller
         * @throws DBTransactionError
         */
        private function assertOwnership( $fname, $owner ) {
                if ( $this->ownerId !== null && $owner !== $this->ownerId ) {
                        throw new DBTransactionError(
                                null,
                                "$fname: LoadBalancer is owned by LBFactory #{$this->ownerId} (got '$owner')."
                        );
                }
        }

        /**
         * Make all DB servers with DBO_DEFAULT/DBO_TRX set join the transaction round
         *
         * Some servers may have neither flag enabled, meaning that they opt out of such
         * transaction rounds and remain in auto-commit mode. Such behavior might be desired
         * when a DB server is used for something like simple key/value storage.
         *
         * @param Database $conn
         */
        private function applyTransactionRoundFlags( Database $conn ) {
                if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
                        return; // transaction rounds do not apply to these connections
                }

                if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
                        // DBO_TRX is controlled entirely by CLI mode presence with DBO_DEFAULT.
                        // Force DBO_TRX even in CLI mode since a commit round is expected soon.
                        $conn->setFlag( $conn::DBO_TRX, $conn::REMEMBER_PRIOR );
                }

                if ( $conn->getFlag( $conn::DBO_TRX ) ) {
                        $conn->setLBInfo( 'trxRoundId', $this->trxRoundId );
                }
        }

        /**
         * @param Database $conn
         */
        private function undoTransactionRoundFlags( Database $conn ) {
                if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
                        return; // transaction rounds do not apply to these connections
                }

                if ( $conn->getFlag( $conn::DBO_TRX ) ) {
                        $conn->setLBInfo( 'trxRoundId', null ); // remove the round ID
                }

                if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
                        $conn->restoreFlags( $conn::RESTORE_PRIOR );
                }
        }

        public function flushReplicaSnapshots( $fname = __METHOD__ ) {
                $this->forEachOpenReplicaConnection( function ( IDatabase $conn ) use ( $fname ) {
                        $conn->flushSnapshot( $fname );
                } );
        }

        public function flushMasterSnapshots( $fname = __METHOD__ ) {
                $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $fname ) {
                        $conn->flushSnapshot( $fname );
                } );
        }

        /**
         * @return string
         * @since 1.32
         */
        public function getTransactionRoundStage() {
                return $this->trxRoundStage;
        }

        public function hasMasterConnection() {
                return $this->isOpen( $this->getWriterIndex() );
        }

        public function hasMasterChanges() {
                $pending = 0;
                $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$pending ) {
                        $pending |= $conn->writesOrCallbacksPending();
                } );

                return (bool)$pending;
        }

        public function lastMasterChangeTimestamp() {
                $lastTime = false;
                $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$lastTime ) {
                        $lastTime = max( $lastTime, $conn->lastDoneWrites() );
                } );

                return $lastTime;
        }

        public function hasOrMadeRecentMasterChanges( $age = null ) {
                $age = ( $age === null ) ? $this->waitTimeout : $age;

                return ( $this->hasMasterChanges()
                        || $this->lastMasterChangeTimestamp() > microtime( true ) - $age );
        }

        public function pendingMasterChangeCallers() {
                $fnames = [];
                $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$fnames ) {
                        $fnames = array_merge( $fnames, $conn->pendingWriteCallers() );
                } );

                return $fnames;
        }

        public function getLaggedReplicaMode( $domain = false ) {
                if ( $this->laggedReplicaMode ) {
                        return true; // stay in lagged replica mode
                }

                if ( $this->hasStreamingReplicaServers() ) {
                        try {
                                // Set "laggedReplicaMode"
                                $this->getReaderIndex( self::GROUP_GENERIC, $domain );
                        } catch ( DBConnectionError $e ) {
                                // Sanity: avoid expensive re-connect attempts and failures
                                $this->laggedReplicaMode = true;
                        }
                }

                return $this->laggedReplicaMode;
        }

        public function laggedReplicaUsed() {
                return $this->laggedReplicaMode;
        }

        /**
         * @return bool
         * @since 1.27
         * @deprecated Since 1.28; use laggedReplicaUsed()
         */
        public function laggedSlaveUsed() {
                return $this->laggedReplicaUsed();
        }

        public function getReadOnlyReason( $domain = false, IDatabase $conn = null ) {
                if ( $this->readOnlyReason !== false ) {
                        return $this->readOnlyReason;
                } elseif ( $this->masterRunningReadOnly( $domain, $conn ) ) {
                        return 'The master database server is running in read-only mode.';
                } elseif ( $this->getLaggedReplicaMode( $domain ) ) {
                        return ( $this->getExistingReaderIndex( self::GROUP_GENERIC ) >= 0 )
                                ? 'The database is read-only until replication lag decreases.'
                                : 'The database is read-only until a replica database server becomes reachable.';
                }

                return false;
        }

        /**
         * @param string $domain Domain ID, or false for the current domain
         * @param IDatabase|null $conn DB master connectionl used to avoid loops [optional]
         * @return bool
         */
        private function masterRunningReadOnly( $domain, IDatabase $conn = null ) {
                $cache = $this->wanCache;
                $masterServer = $this->getServerName( $this->getWriterIndex() );

                return (bool)$cache->getWithSetCallback(
                        $cache->makeGlobalKey( __CLASS__, 'server-read-only', $masterServer ),
                        self::TTL_CACHE_READONLY,
                        function () use ( $domain, $conn ) {
                                $old = $this->trxProfiler->setSilenced( true );
                                try {
                                        $index = $this->getWriterIndex();
                                        $dbw = $conn ?: $this->getServerConnection( $index, $domain );
                                        $readOnly = (int)$dbw->serverIsReadOnly();
                                        if ( !$conn ) {
                                                $this->reuseConnection( $dbw );
                                        }
                                } catch ( DBError $e ) {
                                        $readOnly = 0;
                                }
                                $this->trxProfiler->setSilenced( $old );

                                return $readOnly;
                        },
                        [ 'pcTTL' => $cache::TTL_PROC_LONG, 'busyValue' => 0 ]
                );
        }

        public function allowLagged( $mode = null ) {
                if ( $mode === null ) {
                        return $this->allowLagged;
                }
                $this->allowLagged = $mode;

                return $this->allowLagged;
        }

        public function pingAll() {
                $success = true;
                $this->forEachOpenConnection( function ( IDatabase $conn ) use ( &$success ) {
                        if ( !$conn->ping() ) {
                                $success = false;
                        }
                } );

                return $success;
        }

        public function forEachOpenConnection( $callback, array $params = [] ) {
                foreach ( $this->conns as $connsByServer ) {
                        foreach ( $connsByServer as $serverConns ) {
                                foreach ( $serverConns as $conn ) {
                                        $callback( $conn, ...$params );
                                }
                        }
                }
        }

        public function forEachOpenMasterConnection( $callback, array $params = [] ) {
                $masterIndex = $this->getWriterIndex();
                foreach ( $this->conns as $connsByServer ) {
                        if ( isset( $connsByServer[$masterIndex] ) ) {
                                /** @var IDatabase $conn */
                                foreach ( $connsByServer[$masterIndex] as $conn ) {
                                        $callback( $conn, ...$params );
                                }
                        }
                }
        }

        public function forEachOpenReplicaConnection( $callback, array $params = [] ) {
                foreach ( $this->conns as $connsByServer ) {
                        foreach ( $connsByServer as $i => $serverConns ) {
                                if ( $i === $this->getWriterIndex() ) {
                                        continue; // skip master
                                }
                                foreach ( $serverConns as $conn ) {
                                        $callback( $conn, ...$params );
                                }
                        }
                }
        }

        /**
         * @return int
         */
        private function getCurrentConnectionCount() {
                $count = 0;
                foreach ( $this->conns as $connsByServer ) {
                        foreach ( $connsByServer as $serverConns ) {
                                $count += count( $serverConns );
                        }
                }

                return $count;
        }

        public function getMaxLag( $domain = false ) {
                $host = '';
                $maxLag = -1;
                $maxIndex = 0;

                if ( $this->hasReplicaServers() ) {
                        $lagTimes = $this->getLagTimes( $domain );
                        foreach ( $lagTimes as $i => $lag ) {
                                if ( $this->groupLoads[self::GROUP_GENERIC][$i] > 0 && $lag > $maxLag ) {
                                        $maxLag = $lag;
                                        $host = $this->getServerInfoStrict( $i, 'host' );
                                        $maxIndex = $i;
                                }
                        }
                }

                return [ $host, $maxLag, $maxIndex ];
        }

        public function getLagTimes( $domain = false ) {
                if ( !$this->hasReplicaServers() ) {
                        return [ $this->getWriterIndex() => 0 ]; // no replication = no lag
                }

                $knownLagTimes = []; // map of (server index => 0 seconds)
                $indexesWithLag = [];
                foreach ( $this->servers as $i => $server ) {
                        if ( empty( $server['is static'] ) ) {
                                $indexesWithLag[] = $i; // DB server might have replication lag
                        } else {
                                $knownLagTimes[$i] = 0; // DB server is a non-replicating and read-only archive
                        }
                }

                return $this->getLoadMonitor()->getLagTimes( $indexesWithLag, $domain ) + $knownLagTimes;
        }

        /**
         * Get the lag in seconds for a given connection, or zero if this load
         * balancer does not have replication enabled.
         *
         * This should be used in preference to Database::getLag() in cases where
         * replication may not be in use, since there is no way to determine if
         * replication is in use at the connection level without running
         * potentially restricted queries such as SHOW SLAVE STATUS. Using this
         * function instead of Database::getLag() avoids a fatal error in this
         * case on many installations.
         *
         * @param IDatabase $conn
         * @return int|bool Returns false on error
         * @deprecated Since 1.34 Use IDatabase::getLag() instead
         */
        public function safeGetLag( IDatabase $conn ) {
                if ( $conn->getLBInfo( 'is static' ) ) {
                        return 0; // static dataset
                } elseif ( $conn->getLBInfo( 'serverIndex' ) == $this->getWriterIndex() ) {
                        return 0; // this is the master
                }

                return $conn->getLag();
        }

        public function waitForMasterPos( IDatabase $conn, $pos = false, $timeout = null ) {
                $timeout = max( 1, $timeout ?: $this->waitTimeout );

                if ( $this->getServerCount() <= 1 || !$conn->getLBInfo( 'replica' ) ) {
                        return true; // server is not a replica DB
                }

                if ( !$pos ) {
                        // Get the current master position, opening a connection if needed
                        $index = $this->getWriterIndex();
                        $flags = self::CONN_SILENCE_ERRORS;
                        $masterConn = $this->getAnyOpenConnection( $index, $flags );
                        if ( $masterConn ) {
                                $pos = $masterConn->getMasterPos();
                        } else {
                                $masterConn = $this->getServerConnection( $index, self::DOMAIN_ANY, $flags );
                                if ( !$masterConn ) {
                                        throw new DBReplicationWaitError(
                                                null,
                                                "Could not obtain a master database connection to get the position"
                                        );
                                }
                                $pos = $masterConn->getMasterPos();
                                $this->closeConnection( $masterConn );
                        }
                }

                if ( $pos instanceof DBMasterPos ) {
                        $start = microtime( true );
                        $result = $conn->masterPosWait( $pos, $timeout );
                        $seconds = max( microtime( true ) - $start, 0 );
                        if ( $result == -1 || is_null( $result ) ) {
                                $msg = __METHOD__ . ': timed out waiting on {host} pos {pos} [{seconds}s]';
                                $this->replLogger->warning( $msg, [
                                        'host' => $conn->getServer(),
                                        'pos' => $pos,
                                        'seconds' => round( $seconds, 6 ),
                                        'trace' => ( new RuntimeException() )->getTraceAsString()
                                ] );
                                $ok = false;
                        } else {
                                $this->replLogger->debug( __METHOD__ . ': done waiting' );
                                $ok = true;
                        }
                } else {
                        $ok = false; // something is misconfigured
                        $this->replLogger->error(
                                __METHOD__ . ': could not get master pos for {host}',
                                [
                                        'host' => $conn->getServer(),
                                        'trace' => ( new RuntimeException() )->getTraceAsString()
                                ]
                        );
                }

                return $ok;
        }

        /**
         * Wait for a replica DB to reach a specified master position
         *
         * This will connect to the master to get an accurate position if $pos is not given
         *
         * @param IDatabase $conn Replica DB
         * @param DBMasterPos|bool $pos Master position; default: current position
         * @param int $timeout Timeout in seconds [optional]
         * @return bool Success
         * @since 1.28
         * @deprecated Since 1.34 Use waitForMasterPos() instead
         */
        public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = null ) {
                return $this->waitForMasterPos( $conn, $pos, $timeout );
        }

        public function setTransactionListener( $name, callable $callback = null ) {
                if ( $callback ) {
                        $this->trxRecurringCallbacks[$name] = $callback;
                } else {
                        unset( $this->trxRecurringCallbacks[$name] );
                }
                $this->forEachOpenMasterConnection(
                        function ( IDatabase $conn ) use ( $name, $callback ) {
                                $conn->setTransactionListener( $name, $callback );
                        }
                );
        }

        public function setTableAliases( array $aliases ) {
                $this->tableAliases = $aliases;
        }

        public function setIndexAliases( array $aliases ) {
                $this->indexAliases = $aliases;
        }

        /**
         * @param string $prefix
         * @deprecated Since 1.33
         */
        public function setDomainPrefix( $prefix ) {
                $this->setLocalDomainPrefix( $prefix );
        }

        public function setLocalDomainPrefix( $prefix ) {
                // Find connections to explicit foreign domains still marked as in-use...
                $domainsInUse = [];
                $this->forEachOpenConnection( function ( IDatabase $conn ) use ( &$domainsInUse ) {
                        // Once reuseConnection() is called on a handle, its reference count goes from 1 to 0.
                        // Until then, it is still in use by the caller (explicitly or via DBConnRef scope).
                        if ( $conn->getLBInfo( 'foreignPoolRefCount' ) > 0 ) {
                                $domainsInUse[] = $conn->getDomainID();
                        }
                } );

                // Do not switch connections to explicit foreign domains unless marked as safe
                if ( $domainsInUse ) {
                        $domains = implode( ', ', $domainsInUse );
                        throw new DBUnexpectedError( null,
                                "Foreign domain connections are still in use ($domains)" );
                }

                $this->setLocalDomain( new DatabaseDomain(
                        $this->localDomain->getDatabase(),
                        $this->localDomain->getSchema(),
                        $prefix
                ) );

                // Update the prefix for all local connections...
                $this->forEachOpenConnection( function ( IDatabase $db ) use ( $prefix ) {
                        if ( !$db->getLBInfo( 'foreign' ) ) {
                                $db->tablePrefix( $prefix );
                        }
                } );
        }

        public function redefineLocalDomain( $domain ) {
                $this->closeAll();

                $this->setLocalDomain( DatabaseDomain::newFromId( $domain ) );
        }

        /**
         * @param DatabaseDomain $domain
         */
        private function setLocalDomain( DatabaseDomain $domain ) {
                $this->localDomain = $domain;
                // In case a caller assumes that the domain ID is simply <db>-<prefix>, which is almost
                // always true, gracefully handle the case when they fail to account for escaping.
                if ( $this->localDomain->getTablePrefix() != '' ) {
                        $this->localDomainIdAlias =
                                $this->localDomain->getDatabase() . '-' . $this->localDomain->getTablePrefix();
                } else {
                        $this->localDomainIdAlias = $this->localDomain->getDatabase();
                }
        }

        /**
         * @param int $i Server index
         * @param string|null $field Server index field [optional]
         * @return array|mixed
         * @throws InvalidArgumentException
         */
        private function getServerInfoStrict( $i, $field = null ) {
                if ( !isset( $this->servers[$i] ) || !is_array( $this->servers[$i] ) ) {
                        throw new InvalidArgumentException( "No server with index '$i'" );
                }

                if ( $field !== null ) {
                        if ( !array_key_exists( $field, $this->servers[$i] ) ) {
                                throw new InvalidArgumentException( "No field '$field' in server index '$i'" );
                        }

                        return $this->servers[$i][$field];
                }

                return $this->servers[$i];
        }

        function __destruct() {
                // Avoid connection leaks for sanity
                $this->disable();
        }
}

/**
 * @deprecated since 1.29
 */
class_alias( LoadBalancer::class, 'LoadBalancer' );