Move DBConnRef/MaintainableDBConnRef to Rdbms namespace

[mediawiki.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
 * @ingroup Database
 */
namespace Wikimedia\Rdbms;

use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use Wikimedia\ScopedCallback;
use Database;
use BagOStuff;
use EmptyBagOStuff;
use WANObjectCache;
use ArrayUtils;
use DBError;
use DBAccessError;
use DBExpectedError;
use DBUnexpectedError;
use DBTransactionError;
use DBTransactionSizeError;
use DBConnectionError;
use InvalidArgumentException;
use RuntimeException;
use Exception;

/**
 * Database connection, tracking, load balancing, and transaction manager for a cluster
 *
 * @ingroup Database
 */
class LoadBalancer implements ILoadBalancer {
        /** @var array[] Map of (server index => server config array) */
        private $mServers;
        /** @var \Database[][][] Map of local/foreignUsed/foreignFree => server index => IDatabase array */
        private $mConns;
        /** @var float[] Map of (server index => weight) */
        private $mLoads;
        /** @var array[] Map of (group => server index => weight) */
        private $mGroupLoads;
        /** @var bool Whether to disregard replica DB lag as a factor in replica DB selection */
        private $mAllowLagged;
        /** @var integer Seconds to spend waiting on replica DB lag to resolve */
        private $mWaitTimeout;
        /** @var array The LoadMonitor configuration */
        private $loadMonitorConfig;
        /** @var array[] $aliases Map of (table => (dbname, schema, prefix) map) */
        private $tableAliases = [];

        /** @var ILoadMonitor */
        private $loadMonitor;
        /** @var ChronologyProtector|null */
        private $chronProt;
        /** @var BagOStuff */
        private $srvCache;
        /** @var BagOStuff */
        private $memCache;
        /** @var WANObjectCache */
        private $wanCache;
        /** @var object|string Class name or object With profileIn/profileOut methods */
        protected $profiler;
        /** @var TransactionProfiler */
        protected $trxProfiler;
        /** @var LoggerInterface */
        protected $replLogger;
        /** @var LoggerInterface */
        protected $connLogger;
        /** @var LoggerInterface */
        protected $queryLogger;
        /** @var LoggerInterface */
        protected $perfLogger;

        /** @var \Database Database connection that caused a problem */
        private $errorConnection;
        /** @var integer The generic (not query grouped) replica DB index (of $mServers) */
        private $mReadIndex;
        /** @var bool|DBMasterPos False if not set */
        private $mWaitForPos;
        /** @var bool Whether the generic reader fell back to a lagged replica DB */
        private $laggedReplicaMode = false;
        /** @var bool Whether the generic reader fell back to a lagged replica DB */
        private $allReplicasDownMode = false;
        /** @var string The last DB selection or connection error */
        private $mLastError = 'Unknown error';
        /** @var string|bool Reason the LB is read-only or false if not */
        private $readOnlyReason = false;
        /** @var integer Total connections opened */
        private $connsOpened = 0;
        /** @var string|bool String if a requested DBO_TRX transaction round is active */
        private $trxRoundId = false;
        /** @var array[] Map of (name => callable) */
        private $trxRecurringCallbacks = [];
        /** @var DatabaseDomain Local Domain ID and default for selectDB() calls */
        private $localDomain;
        /** @var string Alternate ID string for the domain instead of DatabaseDomain::getId() */
        private $localDomainIdAlias;
        /** @var string Current server name */
        private $host;
        /** @var bool Whether this PHP instance is for a CLI script */
        protected $cliMode;
        /** @var string Agent name for query profiling */
        protected $agent;

        /** @var callable Exception logger */
        private $errorLogger;

        /** @var boolean */
        private $disabled = false;
        /** @var boolean */
        private $chronProtInitialized = false;

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

        /** @var integer Default 'max lag' when unspecified */
        const MAX_LAG_DEFAULT = 10;
        /** @var integer Seconds to cache master server read-only status */
        const TTL_CACHE_READONLY = 5;

        public function __construct( array $params ) {
                if ( !isset( $params['servers'] ) ) {
                        throw new InvalidArgumentException( __CLASS__ . ': missing servers parameter' );
                }
                $this->mServers = $params['servers'];

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

                $this->mWaitTimeout = isset( $params['waitTimeout'] ) ? $params['waitTimeout'] : 10;

                $this->mReadIndex = -1;
                $this->mConns = [
                        'local'       => [],
                        'foreignUsed' => [],
                        'foreignFree' => []
                ];
                $this->mLoads = [];
                $this->mWaitForPos = false;
                $this->mAllowLagged = false;

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

                if ( isset( $params['loadMonitor'] ) ) {
                        $this->loadMonitorConfig = $params['loadMonitor'];
                } else {
                        $this->loadMonitorConfig = [ 'class' => 'LoadMonitorNull' ];
                }

                foreach ( $params['servers'] as $i => $server ) {
                        $this->mLoads[$i] = $server['load'];
                        if ( isset( $server['groupLoads'] ) ) {
                                foreach ( $server['groupLoads'] as $group => $ratio ) {
                                        if ( !isset( $this->mGroupLoads[$group] ) ) {
                                                $this->mGroupLoads[$group] = [];
                                        }
                                        $this->mGroupLoads[$group][$i] = $ratio;
                                }
                        }
                }

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

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

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

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

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

        /**
         * 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->memCache, $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 != 0 ) {
                                # How much lag this server nominally is allowed to have
                                $maxServerLag = isset( $this->mServers[$i]['max lag'] )
                                        ? $this->mServers[$i]['max lag']
                                        : self::MAX_LAG_DEFAULT; // default
                                # Constrain that futher by $maxLag argument
                                $maxServerLag = min( $maxServerLag, $maxLag );

                                $host = $this->getServerName( $i );
                                if ( $lag === false && !is_infinite( $maxServerLag ) ) {
                                        $this->replLogger->error(
                                                "Server {host} (#$i) is not replicating?", [ 'host' => $host ] );
                                        unset( $loads[$i] );
                                } elseif ( $lag > $maxServerLag ) {
                                        $this->replLogger->warning(
                                                "Server {host} (#$i) 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 );
        }

        public function getReaderIndex( $group = false, $domain = false ) {
                if ( count( $this->mServers ) == 1 ) {
                        # Skip the load balancing if there's only one server
                        return $this->getWriterIndex();
                } elseif ( $group === false && $this->mReadIndex >= 0 ) {
                        # Shortcut if generic reader exists already
                        return $this->mReadIndex;
                }

                # Find the relevant load array
                if ( $group !== false ) {
                        if ( isset( $this->mGroupLoads[$group] ) ) {
                                $nonErrorLoads = $this->mGroupLoads[$group];
                        } else {
                                # No loads for this group, return false and the caller can use some other group
                                $this->connLogger->info( __METHOD__ . ": no loads for group $group" );

                                return false;
                        }
                } else {
                        $nonErrorLoads = $this->mLoads;
                }

                if ( !count( $nonErrorLoads ) ) {
                        throw new InvalidArgumentException( "Empty server array given to LoadBalancer" );
                }

                # Scale the configured load ratios according to the dynamic load if supported
                $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $domain );

                $laggedReplicaMode = false;

                # No server found yet
                $i = false;
                # First try quickly looking through the available servers for a server that
                # meets our criteria
                $currentLoads = $nonErrorLoads;
                while ( count( $currentLoads ) ) {
                        if ( $this->mAllowLagged || $laggedReplicaMode ) {
                                $i = ArrayUtils::pickRandom( $currentLoads );
                        } else {
                                $i = false;
                                if ( $this->mWaitForPos && $this->mWaitForPos->asOfTime() ) {
                                        # ChronologyProtecter causes mWaitForPos to be set via sessions.
                                        # This triggers doWait() after connect, so it's especially good to
                                        # avoid lagged servers so as to avoid just blocking in that method.
                                        $ago = microtime( true ) - $this->mWaitForPos->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 ) != 0 ) {
                                        # All replica DBs lagged. Switch to read-only mode
                                        $this->replLogger->error( "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;
                        }

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

                        $conn = $this->openConnection( $i, $domain );
                        if ( !$conn ) {
                                $this->connLogger->warning( __METHOD__ . ": Failed connecting to $i/$domain" );
                                unset( $nonErrorLoads[$i] );
                                unset( $currentLoads[$i] );
                                $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 ( !count( $nonErrorLoads ) ) {
                        $this->connLogger->error( "All servers down" );
                }

                if ( $i !== false ) {
                        # Replica DB connection successful.
                        # Wait for the session master pos for a short time.
                        if ( $this->mWaitForPos && $i > 0 ) {
                                $this->doWait( $i );
                        }
                        if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group === false ) {
                                $this->mReadIndex = $i;
                                # Record if the generic reader index is in "lagged replica DB" mode
                                if ( $laggedReplicaMode ) {
                                        $this->laggedReplicaMode = true;
                                }
                        }
                        $serverName = $this->getServerName( $i );
                        $this->connLogger->debug(
                                __METHOD__ . ": using server $serverName for group '$group'" );
                }

                return $i;
        }

        public function waitFor( $pos ) {
                $oldPos = $this->mWaitForPos;
                $this->mWaitForPos = $pos;

                // If a generic reader connection was already established, then wait now
                $i = $this->mReadIndex;
                if ( $i > 0 ) {
                        if ( !$this->doWait( $i ) ) {
                                $this->laggedReplicaMode = true;
                        }
                }

                // Restore the older position if it was higher
                $this->setWaitForPositionIfHigher( $oldPos );
        }

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

                $i = $this->mReadIndex;
                if ( $i <= 0 ) {
                        // Pick a generic replica DB if there isn't one yet
                        $readLoads = $this->mLoads;
                        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
                }

                // Restore the older position if it was higher
                $this->setWaitForPositionIfHigher( $oldPos );

                return $ok;
        }

        public function waitForAll( $pos, $timeout = null ) {
                $oldPos = $this->mWaitForPos;
                $this->mWaitForPos = $pos;
                $serverCount = count( $this->mServers );

                $ok = true;
                for ( $i = 1; $i < $serverCount; $i++ ) {
                        if ( $this->mLoads[$i] > 0 ) {
                                $ok = $this->doWait( $i, true, $timeout ) && $ok;
                        }
                }

                // Restore the older position if it was higher
                $this->setWaitForPositionIfHigher( $oldPos );

                return $ok;
        }

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

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

        /**
         * @param int $i
         * @return IDatabase|bool
         */
        public function getAnyOpenConnection( $i ) {
                foreach ( $this->mConns as $connsByServer ) {
                        if ( !empty( $connsByServer[$i] ) ) {
                                /** @var $serverConns IDatabase[] */
                                $serverConns = $connsByServer[$i];

                                return reset( $serverConns );
                        }
                }

                return false;
        }

        /**
         * Wait for a given replica DB to catch up to the master pos stored in $this
         * @param int $index Server index
         * @param bool $open Check the server even if a new connection has to be made
         * @param int $timeout Max seconds to wait; default is mWaitTimeout
         * @return bool
         */
        protected function doWait( $index, $open = false, $timeout = null ) {
                $close = false; // close the connection afterwards

                // 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->mWaitForPos )
                ) {
                        $this->replLogger->debug( __METHOD__ .
                                ": replica DB $server known to be caught up (pos >= $knownReachedPos)." );
                        return true;
                }

                // Find a connection to wait on, creating one if needed and allowed
                $conn = $this->getAnyOpenConnection( $index );
                if ( !$conn ) {
                        if ( !$open ) {
                                $this->replLogger->debug( __METHOD__ . ": no connection open for $server" );

                                return false;
                        } else {
                                $conn = $this->openConnection( $index, self::DOMAIN_ANY );
                                if ( !$conn ) {
                                        $this->replLogger->warning( __METHOD__ . ": failed to connect to $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 $server to catch up..." );
                $timeout = $timeout ?: $this->mWaitTimeout;
                $result = $conn->masterPosWait( $this->mWaitForPos, $timeout );

                if ( $result == -1 || is_null( $result ) ) {
                        // Timed out waiting for replica DB, use master instead
                        $this->replLogger->warning(
                                __METHOD__ . ": Timed out waiting on {host} pos {$this->mWaitForPos}",
                                [ 'host' => $server ]
                        );
                        $ok = false;
                } else {
                        $this->replLogger->info( __METHOD__ . ": Done" );
                        $ok = true;
                        // Remember that the DB reached this point
                        $this->srvCache->set( $key, $this->mWaitForPos, BagOStuff::TTL_DAY );
                }

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

                return $ok;
        }

        /**
         * @see ILoadBalancer::getConnection()
         *
         * @param int $i
         * @param array $groups
         * @param bool $domain
         * @return Database
         * @throws DBConnectionError
         */
        public function getConnection( $i, $groups = [], $domain = false ) {
                if ( $i === null || $i === false ) {
                        throw new InvalidArgumentException( 'Attempt to call ' . __METHOD__ .
                                ' with invalid server index' );
                }

                if ( $this->localDomain->equals( $domain ) || $domain === $this->localDomainIdAlias ) {
                        $domain = false; // local connection requested
                }

                $groups = ( $groups === false || $groups === [] )
                        ? [ false ] // check one "group": the generic pool
                        : (array)$groups;

                $masterOnly = ( $i == self::DB_MASTER || $i == $this->getWriterIndex() );
                $oldConnsOpened = $this->connsOpened; // connections open now

                if ( $i == self::DB_MASTER ) {
                        $i = $this->getWriterIndex();
                } else {
                        # Try to find an available server in any the query groups (in order)
                        foreach ( $groups as $group ) {
                                $groupIndex = $this->getReaderIndex( $group, $domain );
                                if ( $groupIndex !== false ) {
                                        $i = $groupIndex;
                                        break;
                                }
                        }
                }

                # Operation-based index
                if ( $i == self::DB_REPLICA ) {
                        $this->mLastError = 'Unknown error'; // reset error string
                        # Try the general server pool if $groups are unavailable.
                        $i = ( $groups === [ false ] )
                                ? false // don't bother with this if that is what was tried above
                                : $this->getReaderIndex( false, $domain );
                        # Couldn't find a working server in getReaderIndex()?
                        if ( $i === false ) {
                                $this->mLastError = 'No working replica DB server: ' . $this->mLastError;
                                // Throw an exception
                                $this->reportConnectionError();
                                return null; // not reached
                        }
                }

                # Now we have an explicit index into the servers array
                $conn = $this->openConnection( $i, $domain );
                if ( !$conn ) {
                        // Throw an exception
                        $this->reportConnectionError();
                        return null; // not reached
                }

                # Profile any new connections that happen
                if ( $this->connsOpened > $oldConnsOpened ) {
                        $host = $conn->getServer();
                        $dbname = $conn->getDBname();
                        $this->trxProfiler->recordConnection( $host, $dbname, $masterOnly );
                }

                if ( $masterOnly ) {
                        # Make master-requested DB handles inherit any read-only mode setting
                        $conn->setLBInfo( 'readOnlyReason', $this->getReadOnlyReason( $domain, $conn ) );
                }

                return $conn;
        }

        public function reuseConnection( $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 RuntimeException() )->getTraceAsString() );

                        return;
                }

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

                $domain = $conn->getDomainID();
                if ( !isset( $this->mConns['foreignUsed'][$serverIndex][$domain] ) ) {
                        throw new InvalidArgumentException( __METHOD__ .
                                ": connection $serverIndex/$domain not found; it may have already been freed." );
                } elseif ( $this->mConns['foreignUsed'][$serverIndex][$domain] !== $conn ) {
                        throw new InvalidArgumentException( __METHOD__ .
                                ": connection $serverIndex/$domain mismatched; it may have already been freed." );
                }
                $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
                if ( $refCount <= 0 ) {
                        $this->mConns['foreignFree'][$serverIndex][$domain] = $conn;
                        unset( $this->mConns['foreignUsed'][$serverIndex][$domain] );
                        if ( !$this->mConns['foreignUsed'][$serverIndex] ) {
                                unset( $this->mConns[ 'foreignUsed' ][$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( $db, $groups = [], $domain = false ) {
                $domain = ( $domain !== false ) ? $domain : $this->localDomain;

                return new DBConnRef( $this, $this->getConnection( $db, $groups, $domain ) );
        }

        public function getLazyConnectionRef( $db, $groups = [], $domain = false ) {
                $domain = ( $domain !== false ) ? $domain : $this->localDomain;

                return new DBConnRef( $this, [ $db, $groups, $domain ] );
        }

        public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false ) {
                $domain = ( $domain !== false ) ? $domain : $this->localDomain;

                return new MaintainableDBConnRef( $this, $this->getConnection( $db, $groups, $domain ) );
        }

        /**
         * @see ILoadBalancer::openConnection()
         *
         * @param int $i
         * @param bool $domain
         * @return bool|Database
         * @throws DBAccessError
         */
        public function openConnection( $i, $domain = false ) {
                if ( $this->localDomain->equals( $domain ) || $domain === $this->localDomainIdAlias ) {
                        $domain = false; // local connection requested
                }

                if ( !$this->chronProtInitialized && $this->chronProt ) {
                        $this->connLogger->debug( __METHOD__ . ': calling initLB() before first connection.' );
                        // Load CP positions before connecting so that doWait() triggers later if needed
                        $this->chronProtInitialized = true;
                        $this->chronProt->initLB( $this );
                }

                if ( $domain !== false ) {
                        $conn = $this->openForeignConnection( $i, $domain );
                } elseif ( isset( $this->mConns['local'][$i][0] ) ) {
                        $conn = $this->mConns['local'][$i][0];
                } else {
                        if ( !isset( $this->mServers[$i] ) || !is_array( $this->mServers[$i] ) ) {
                                throw new InvalidArgumentException( "No server with index '$i'." );
                        }
                        // Open a new connection
                        $server = $this->mServers[$i];
                        $server['serverIndex'] = $i;
                        $conn = $this->reallyOpenConnection( $server, false );
                        $serverName = $this->getServerName( $i );
                        if ( $conn->isOpen() ) {
                                $this->connLogger->debug( "Connected to database $i at '$serverName'." );
                                $this->mConns['local'][$i][0] = $conn;
                        } else {
                                $this->connLogger->warning( "Failed to connect to database $i at '$serverName'." );
                                $this->errorConnection = $conn;
                                $conn = false;
                        }
                }

                if ( $conn instanceof IDatabase && !$conn->isOpen() ) {
                        // 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.
                        // See DatabaseMyslBase::ping() for how this can happen.
                        $this->errorConnection = $conn;
                        $conn = false;
                }

                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
         * @return Database
         */
        private function openForeignConnection( $i, $domain ) {
                $domainInstance = DatabaseDomain::newFromId( $domain );
                $dbName = $domainInstance->getDatabase();
                $prefix = $domainInstance->getTablePrefix();

                if ( isset( $this->mConns['foreignUsed'][$i][$domain] ) ) {
                        // Reuse an already-used connection
                        $conn = $this->mConns['foreignUsed'][$i][$domain];
                        $this->connLogger->debug( __METHOD__ . ": reusing connection $i/$domain" );
                } elseif ( isset( $this->mConns['foreignFree'][$i][$domain] ) ) {
                        // Reuse a free connection for the same domain
                        $conn = $this->mConns['foreignFree'][$i][$domain];
                        unset( $this->mConns['foreignFree'][$i][$domain] );
                        $this->mConns['foreignUsed'][$i][$domain] = $conn;
                        $this->connLogger->debug( __METHOD__ . ": reusing free connection $i/$domain" );
                } elseif ( !empty( $this->mConns['foreignFree'][$i] ) ) {
                        // Reuse a connection from another domain
                        $conn = reset( $this->mConns['foreignFree'][$i] );
                        $oldDomain = key( $this->mConns['foreignFree'][$i] );
                        // The empty string as a DB name means "don't care".
                        // DatabaseMysqlBase::open() already handle this on connection.
                        if ( strlen( $dbName ) && !$conn->selectDB( $dbName ) ) {
                                $this->mLastError = "Error selecting database '$dbName' on server " .
                                        $conn->getServer() . " from client host {$this->host}";
                                $this->errorConnection = $conn;
                                $conn = false;
                        } else {
                                $conn->tablePrefix( $prefix );
                                unset( $this->mConns['foreignFree'][$i][$oldDomain] );
                                $this->mConns['foreignUsed'][$i][$domain] = $conn;
                                $this->connLogger->debug( __METHOD__ .
                                        ": reusing free connection from $oldDomain for $domain" );
                        }
                } else {
                        if ( !isset( $this->mServers[$i] ) || !is_array( $this->mServers[$i] ) ) {
                                throw new InvalidArgumentException( "No server with index '$i'." );
                        }
                        // Open a new connection
                        $server = $this->mServers[$i];
                        $server['serverIndex'] = $i;
                        $server['foreignPoolRefCount'] = 0;
                        $server['foreign'] = true;
                        $conn = $this->reallyOpenConnection( $server, $dbName );
                        if ( !$conn->isOpen() ) {
                                $this->connLogger->warning( __METHOD__ . ": connection error for $i/$domain" );
                                $this->errorConnection = $conn;
                                $conn = false;
                        } else {
                                $conn->tablePrefix( $prefix );
                                $this->mConns['foreignUsed'][$i][$domain] = $conn;
                                $this->connLogger->debug( __METHOD__ . ": opened new connection for $i/$domain" );
                        }
                }

                // Increment reference count
                if ( $conn instanceof IDatabase ) {
                        $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
                        $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
                }

                return $conn;
        }

        /**
         * Test if the specified index represents an open connection
         *
         * @param int $index Server index
         * @access private
         * @return bool
         */
        private function isOpen( $index ) {
                if ( !is_integer( $index ) ) {
                        return false;
                }

                return (bool)$this->getAnyOpenConnection( $index );
        }

        /**
         * Really opens a connection. Uncached.
         * Returns a Database object whether or not the connection was successful.
         * @access private
         *
         * @param array $server
         * @param string|bool $dbNameOverride Use "" to not select any database
         * @return Database
         * @throws DBAccessError
         * @throws InvalidArgumentException
         */
        protected function reallyOpenConnection( array $server, $dbNameOverride = false ) {
                if ( $this->disabled ) {
                        throw new DBAccessError();
                }

                if ( $dbNameOverride !== false ) {
                        $server['dbname'] = $dbNameOverride;
                }

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

                // Log when many connection are made on requests
                if ( ++$this->connsOpened >= self::CONN_HELD_WARN_THRESHOLD ) {
                        $this->perfLogger->warning( __METHOD__ . ": " .
                                "{$this->connsOpened}+ connections made (master=$masterName)" );
                }

                $server['srvCache'] = $this->srvCache;
                // Set loggers and profilers
                $server['connLogger'] = $this->connLogger;
                $server['queryLogger'] = $this->queryLogger;
                $server['errorLogger'] = $this->errorLogger;
                $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'] = isset( $server['flags'] ) ? $server['flags'] : IDatabase::DBO_DEFAULT;

                // Create a live connection object
                try {
                        $db = Database::factory( $server['type'], $server );
                } 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 );

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

                return $db;
        }

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

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

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

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

        public function getWriterIndex() {
                return 0;
        }

        public function haveIndex( $i ) {
                return array_key_exists( $i, $this->mServers );
        }

        public function isNonZeroLoad( $i ) {
                return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
        }

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

        public function getServerName( $i ) {
                if ( isset( $this->mServers[$i]['hostName'] ) ) {
                        $name = $this->mServers[$i]['hostName'];
                } elseif ( isset( $this->mServers[$i]['host'] ) ) {
                        $name = $this->mServers[$i]['host'];
                } else {
                        $name = '';
                }

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

        public function getServerInfo( $i ) {
                if ( isset( $this->mServers[$i] ) ) {
                        return $this->mServers[$i];
                } else {
                        return false;
                }
        }

        public function setServerInfo( $i, array $serverInfo ) {
                $this->mServers[$i] = $serverInfo;
        }

        public function getMasterPos() {
                # If this entire request was served from a replica DB without opening a connection to the
                # master (however unlikely that may be), then we can fetch the position from the replica DB.
                $masterConn = $this->getAnyOpenConnection( $this->getWriterIndex() );
                if ( !$masterConn ) {
                        $serverCount = count( $this->mServers );
                        for ( $i = 1; $i < $serverCount; $i++ ) {
                                $conn = $this->getAnyOpenConnection( $i );
                                if ( $conn ) {
                                        return $conn->getReplicaPos();
                                }
                        }
                } else {
                        return $masterConn->getMasterPos();
                }

                return false;
        }

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

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

                $this->mConns = [
                        'local' => [],
                        'foreignFree' => [],
                        'foreignUsed' => [],
                ];
                $this->connsOpened = 0;
        }

        public function closeConnection( IDatabase $conn ) {
                $serverIndex = $conn->getLBInfo( 'serverIndex' ); // second index level of mConns
                foreach ( $this->mConns as $type => $connsByServer ) {
                        if ( !isset( $connsByServer[$serverIndex] ) ) {
                                continue;
                        }

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

                $conn->close();
        }

        public function commitAll( $fname = __METHOD__ ) {
                $failures = [];

                $restore = ( $this->trxRoundId !== false );
                $this->trxRoundId = false;
                $this->forEachOpenConnection(
                        function ( IDatabase $conn ) use ( $fname, $restore, &$failures ) {
                                try {
                                        $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
                                } catch ( DBError $e ) {
                                        call_user_func( $this->errorLogger, $e );
                                        $failures[] = "{$conn->getServer()}: {$e->getMessage()}";
                                }
                                if ( $restore && $conn->getLBInfo( 'master' ) ) {
                                        $this->undoTransactionRoundFlags( $conn );
                                }
                        }
                );

                if ( $failures ) {
                        throw new DBExpectedError(
                                null,
                                "Commit failed on server(s) " . implode( "\n", array_unique( $failures ) )
                        );
                }
        }

        public function finalizeMasterChanges() {
                $this->forEachOpenMasterConnection( function ( Database $conn ) {
                        // Any error should cause all DB transactions to be rolled back together
                        $conn->setTrxEndCallbackSuppression( false );
                        $conn->runOnTransactionPreCommitCallbacks();
                        // Defer post-commit callbacks until COMMIT finishes for all DBs
                        $conn->setTrxEndCallbackSuppression( true );
                } );
        }

        public function approveMasterChanges( array $options ) {
                $limit = isset( $options['maxWriteDuration'] ) ? $options['maxWriteDuration'] : 0;
                $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).
                        if ( $conn->explicitTrxActive() ) {
                                throw new DBTransactionError(
                                        $conn,
                                        "Explicit transaction still active. A caller may have caught an error."
                                );
                        }
                        // 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 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."
                                );
                        }
                } );
        }

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

                $failures = [];
                $this->forEachOpenMasterConnection(
                        function ( Database $conn ) use ( $fname, &$failures ) {
                                $conn->setTrxEndCallbackSuppression( true );
                                try {
                                        $conn->flushSnapshot( $fname );
                                } catch ( DBError $e ) {
                                        call_user_func( $this->errorLogger, $e );
                                        $failures[] = "{$conn->getServer()}: {$e->getMessage()}";
                                }
                                $conn->setTrxEndCallbackSuppression( false );
                                $this->applyTransactionRoundFlags( $conn );
                        }
                );

                if ( $failures ) {
                        throw new DBExpectedError(
                                null,
                                "$fname: Flush failed on server(s) " . implode( "\n", array_unique( $failures ) )
                        );
                }
        }

        public function commitMasterChanges( $fname = __METHOD__ ) {
                $failures = [];

                /** @noinspection PhpUnusedLocalVariableInspection */
                $scope = $this->getScopedPHPBehaviorForCommit(); // try to ignore client aborts

                $restore = ( $this->trxRoundId !== false );
                $this->trxRoundId = false;
                $this->forEachOpenMasterConnection(
                        function ( IDatabase $conn ) use ( $fname, $restore, &$failures ) {
                                try {
                                        if ( $conn->writesOrCallbacksPending() ) {
                                                $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
                                        } elseif ( $restore ) {
                                                $conn->flushSnapshot( $fname );
                                        }
                                } catch ( DBError $e ) {
                                        call_user_func( $this->errorLogger, $e );
                                        $failures[] = "{$conn->getServer()}: {$e->getMessage()}";
                                }
                                if ( $restore ) {
                                        $this->undoTransactionRoundFlags( $conn );
                                }
                        }
                );

                if ( $failures ) {
                        throw new DBExpectedError(
                                null,
                                "$fname: Commit failed on server(s) " . implode( "\n", array_unique( $failures ) )
                        );
                }
        }

        public function runMasterPostTrxCallbacks( $type ) {
                $e = null; // first exception
                $this->forEachOpenMasterConnection( function ( Database $conn ) use ( $type, &$e ) {
                        $conn->setTrxEndCallbackSuppression( false );
                        if ( $conn->writesOrCallbacksPending() ) {
                                // This happens if onTransactionIdle() callbacks leave callbacks on *another* DB
                                // (which finished its callbacks already). Warn and recover in this case. Let the
                                // callbacks run in the final commitMasterChanges() in LBFactory::shutdown().
                                $this->queryLogger->error( __METHOD__ . ": found writes/callbacks pending." );
                                return;
                        } elseif ( $conn->trxLevel() ) {
                                // This happens for single-DB setups where DB_REPLICA uses the master DB,
                                // thus leaving an implicit read-only transaction open at this point. It
                                // also happens if onTransactionIdle() callbacks leave implicit transactions
                                // open on *other* DBs (which is slightly improper). Let these COMMIT on the
                                // next call to commitMasterChanges(), possibly in LBFactory::shutdown().
                                return;
                        }
                        try {
                                $conn->runOnTransactionIdleCallbacks( $type );
                        } catch ( Exception $ex ) {
                                $e = $e ?: $ex;
                        }
                        try {
                                $conn->runTransactionListenerCallbacks( $type );
                        } catch ( Exception $ex ) {
                                $e = $e ?: $ex;
                        }
                } );

                return $e;
        }

        public function rollbackMasterChanges( $fname = __METHOD__ ) {
                $restore = ( $this->trxRoundId !== false );
                $this->trxRoundId = false;
                $this->forEachOpenMasterConnection(
                        function ( IDatabase $conn ) use ( $fname, $restore ) {
                                if ( $conn->writesOrCallbacksPending() ) {
                                        $conn->rollback( $fname, $conn::FLUSHING_ALL_PEERS );
                                }
                                if ( $restore ) {
                                        $this->undoTransactionRoundFlags( $conn );
                                }
                        }
                );
        }

        public function suppressTransactionEndCallbacks() {
                $this->forEachOpenMasterConnection( function ( Database $conn ) {
                        $conn->setTrxEndCallbackSuppression( true );
                } );
        }

        /**
         * @param IDatabase $conn
         */
        private function applyTransactionRoundFlags( IDatabase $conn ) {
                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 config has explicitly requested DBO_TRX be either on or off by not
                        // setting DBO_DEFAULT, then respect that. Forcing no transactions is useful
                        // for things like blob stores (ExternalStore) which want auto-commit mode.
                }
        }

        /**
         * @param IDatabase $conn
         */
        private function undoTransactionRoundFlags( IDatabase $conn ) {
                if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
                        $conn->restoreFlags( $conn::RESTORE_PRIOR );
                }
        }

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

        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->mWaitTimeout : $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( $domain = false ) {
                // No-op if there is only one DB (also avoids recursion)
                if ( !$this->laggedReplicaMode && $this->getServerCount() > 1 ) {
                        try {
                                // See if laggedReplicaMode gets set
                                $conn = $this->getConnection( self::DB_REPLICA, false, $domain );
                                $this->reuseConnection( $conn );
                        } catch ( DBConnectionError $e ) {
                                // Avoid expensive re-connect attempts and failures
                                $this->allReplicasDownMode = true;
                                $this->laggedReplicaMode = true;
                        }
                }

                return $this->laggedReplicaMode;
        }

        /**
         * @param bool $domain
         * @return bool
         * @deprecated 1.28; use getLaggedReplicaMode()
         */
        public function getLaggedSlaveMode( $domain = false ) {
                return $this->getLaggedReplicaMode( $domain );
        }

        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->getLaggedReplicaMode( $domain ) ) {
                        if ( $this->allReplicasDownMode ) {
                                return 'The database has been automatically locked ' .
                                        'until the replica database servers become available';
                        } else {
                                return 'The database has been automatically locked ' .
                                        'while the replica database servers catch up to the master.';
                        }
                } elseif ( $this->masterRunningReadOnly( $domain, $conn ) ) {
                        return 'The database master is running in read-only mode.';
                }

                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 {
                                        $dbw = $conn ?: $this->getConnection( self::DB_MASTER, [], $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->mAllowLagged;
                }
                $this->mAllowLagged = $mode;

                return $this->mAllowLagged;
        }

        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->mConns as $connsByServer ) {
                        foreach ( $connsByServer as $serverConns ) {
                                foreach ( $serverConns as $conn ) {
                                        $mergedParams = array_merge( [ $conn ], $params );
                                        call_user_func_array( $callback, $mergedParams );
                                }
                        }
                }
        }

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

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

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

                if ( $this->getServerCount() <= 1 ) {
                        return [ $host, $maxLag, $maxIndex ]; // no replication = no lag
                }

                $lagTimes = $this->getLagTimes( $domain );
                foreach ( $lagTimes as $i => $lag ) {
                        if ( $this->mLoads[$i] > 0 && $lag > $maxLag ) {
                                $maxLag = $lag;
                                $host = $this->mServers[$i]['host'];
                                $maxIndex = $i;
                        }
                }

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

        public function getLagTimes( $domain = false ) {
                if ( $this->getServerCount() <= 1 ) {
                        return [ $this->getWriterIndex() => 0 ]; // no replication = no lag
                }

                $knownLagTimes = []; // map of (server index => 0 seconds)
                $indexesWithLag = [];
                foreach ( $this->mServers 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;
        }

        public function safeGetLag( IDatabase $conn ) {
                if ( $this->getServerCount() <= 1 ) {
                        return 0;
                } else {
                        return $conn->getLag();
                }
        }

        /**
         * @param IDatabase $conn
         * @param DBMasterPos|bool $pos
         * @param int $timeout
         * @return bool
         */
        public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 ) {
                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
                        $masterConn = $this->getAnyOpenConnection( $this->getWriterIndex() );
                        if ( $masterConn ) {
                                $pos = $masterConn->getMasterPos();
                        } else {
                                $masterConn = $this->openConnection( $this->getWriterIndex(), self::DOMAIN_ANY );
                                $pos = $masterConn->getMasterPos();
                                $this->closeConnection( $masterConn );
                        }
                }

                if ( $pos instanceof DBMasterPos ) {
                        $result = $conn->masterPosWait( $pos, $timeout );
                        if ( $result == -1 || is_null( $result ) ) {
                                $msg = __METHOD__ . ": Timed out waiting on {$conn->getServer()} pos {$pos}";
                                $this->replLogger->warning( "$msg" );
                                $ok = false;
                        } else {
                                $this->replLogger->info( __METHOD__ . ": Done" );
                                $ok = true;
                        }
                } else {
                        $ok = false; // something is misconfigured
                        $this->replLogger->error( "Could not get master pos for {$conn->getServer()}." );
                }

                return $ok;
        }

        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 setDomainPrefix( $prefix ) {
                if ( $this->mConns['foreignUsed'] ) {
                        // Do not switch connections to explicit foreign domains unless marked as free
                        $domains = [];
                        foreach ( $this->mConns['foreignUsed'] as $i => $connsByDomain ) {
                                $domains = array_merge( $domains, array_keys( $connsByDomain ) );
                        }
                        $domains = implode( ', ', $domains );
                        throw new DBUnexpectedError( null,
                                "Foreign domain connections are still in use ($domains)." );
                }

                $this->localDomain = new DatabaseDomain(
                        $this->localDomain->getDatabase(),
                        null,
                        $prefix
                );

                $this->forEachOpenConnection( function ( IDatabase $db ) use ( $prefix ) {
                        $db->tablePrefix( $prefix );
                } );
        }

        /**
         * Make PHP ignore user aborts/disconnects until the returned
         * value leaves scope. This returns null and does nothing in CLI mode.
         *
         * @return ScopedCallback|null
         */
        final protected function getScopedPHPBehaviorForCommit() {
                if ( PHP_SAPI != 'cli' ) { // https://bugs.php.net/bug.php?id=47540
                        $old = ignore_user_abort( true ); // avoid half-finished operations
                        return new ScopedCallback( function () use ( $old ) {
                                ignore_user_abort( $old );
                        } );
                }

                return null;
        }

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

class_alias( LoadBalancer::class, 'LoadBalancer' );