Expose $wgMaxArticleSize in siteinfo query api

[mediawiki.git] / includes / db / loadbalancer / LoadBalancer.php

<?php
/**
 * Database load balancing.
 *
 * 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
 */

/**
 * Database load balancing object
 *
 * @todo document
 * @ingroup Database
 */
class LoadBalancer {
        /** @var array[] Map of (server index => server config array) */
        private $mServers;
        /** @var array[] Map of (local/foreignUsed/foreignFree => server index => DatabaseBase array) */
        private $mConns;
        /** @var array Map of (server index => weight) */
        private $mLoads;
        /** @var array[] Map of (group => server index => weight) */
        private $mGroupLoads;
        /** @var bool Whether to disregard slave lag as a factor in slave selection */
        private $mAllowLagged;
        /** @var integer Seconds to spend waiting on slave lag to resolve */
        private $mWaitTimeout;
        /** @var array LBFactory information */
        private $mParentInfo;

        /** @var string The LoadMonitor subclass name */
        private $mLoadMonitorClass;
        /** @var LoadMonitor */
        private $mLoadMonitor;
        /** @var BagOStuff */
        private $srvCache;

        /** @var bool|DatabaseBase Database connection that caused a problem */
        private $mErrorConnection;
        /** @var integer The generic (not query grouped) slave 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 slave */
        private $laggedSlaveMode = false;
        /** @var bool Whether the generic reader fell back to a lagged slave */
        private $slavesDownMode = 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 TransactionProfiler */
        protected $trxProfiler;

        /** @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 = 10;
        /** @var integer Max time to wait for a slave to catch up (e.g. ChronologyProtector) */
        const POS_WAIT_TIMEOUT = 10;

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

        /**
         * @param array $params Array with keys:
         *  - servers : Required. Array of server info structures.
         *  - loadMonitor : Name of a class used to fetch server lag and load.
         *  - readOnlyReason : Reason the master DB is read-only if so [optional]
         * @throws MWException
         */
        public function __construct( array $params ) {
                if ( !isset( $params['servers'] ) ) {
                        throw new MWException( __CLASS__ . ': missing servers parameter' );
                }
                $this->mServers = $params['servers'];
                $this->mWaitTimeout = self::POS_WAIT_TIMEOUT;

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

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

                if ( isset( $params['loadMonitor'] ) ) {
                        $this->mLoadMonitorClass = $params['loadMonitor'];
                } else {
                        $master = reset( $params['servers'] );
                        if ( isset( $master['type'] ) && $master['type'] === 'mysql' ) {
                                $this->mLoadMonitorClass = 'LoadMonitorMySQL';
                        } else {
                                $this->mLoadMonitorClass = '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;
                                }
                        }
                }

                $this->srvCache = ObjectCache::getLocalServerInstance();

                if ( isset( $params['trxProfiler'] ) ) {
                        $this->trxProfiler = $params['trxProfiler'];
                } else {
                        $this->trxProfiler = new TransactionProfiler();
                }
        }

        /**
         * Get a LoadMonitor instance
         *
         * @return LoadMonitor
         */
        private function getLoadMonitor() {
                if ( !isset( $this->mLoadMonitor ) ) {
                        $class = $this->mLoadMonitorClass;
                        $this->mLoadMonitor = new $class( $this );
                }

                return $this->mLoadMonitor;
        }

        /**
         * Get or set arbitrary data used by the parent object, usually an LBFactory
         * @param mixed $x
         * @return mixed
         */
        public function parentInfo( $x = null ) {
                return wfSetVar( $this->mParentInfo, $x );
        }

        /**
         * @param array $loads
         * @param bool|string $wiki Wiki 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, $wiki = false, $maxLag = self::MAX_LAG ) {
                $lags = $this->getLagTimes( $wiki );

                # Unset excessively lagged servers
                foreach ( $lags as $i => $lag ) {
                        if ( $i != 0 ) {
                                $maxServerLag = $maxLag;
                                if ( isset( $this->mServers[$i]['max lag'] ) ) {
                                        $maxServerLag = min( $maxServerLag, $this->mServers[$i]['max lag'] );
                                }

                                $host = $this->getServerName( $i );
                                if ( $lag === false ) {
                                        wfDebugLog( 'replication', "Server $host (#$i) is not replicating?" );
                                        unset( $loads[$i] );
                                } elseif ( $lag > $maxServerLag ) {
                                        wfDebugLog( 'replication', "Server $host (#$i) has >= $lag seconds of lag" );
                                        unset( $loads[$i] );
                                }
                        }
                }

                # Find out if all the slaves 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 slaves with zero load
                        # Do NOT use the master
                        # Instead, this function will return false, triggering read-only mode,
                        # and a lagged slave 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 index of the reader connection, which may be a slave
         * This takes into account load ratios and lag times. It should
         * always return a consistent index during a given invocation
         *
         * Side effect: opens connections to databases
         * @param string|bool $group Query group, or false for the generic reader
         * @param string|bool $wiki Wiki ID, or false for the current wiki
         * @throws MWException
         * @return bool|int|string
         */
        public function getReaderIndex( $group = false, $wiki = false ) {
                global $wgDBtype;

                # @todo FIXME: For now, only go through all this for mysql databases
                if ( $wgDBtype != 'mysql' ) {
                        return $this->getWriterIndex();
                }

                if ( count( $this->mServers ) == 1 ) {
                        # Skip the load balancing if there's only one server
                        return 0;
                } 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
                                wfDebugLog( 'connect', __METHOD__ . ": no loads for group $group\n" );

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

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

                # Scale the configured load ratios according to the dynamic load (if the load monitor supports it)
                $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $group, $wiki );

                $laggedSlaveMode = false;

                # No server found yet
                $i = false;
                $conn = false;
                # First try quickly looking through the available servers for a server that
                # meets our criteria
                $currentLoads = $nonErrorLoads;
                while ( count( $currentLoads ) ) {
                        if ( $this->mAllowLagged || $laggedSlaveMode ) {
                                $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, $wiki, $ago + 1 );
                                }
                                if ( $i === false ) {
                                        # Any server with less lag than it's 'max lag' param is preferable
                                        $i = $this->getRandomNonLagged( $currentLoads, $wiki );
                                }
                                if ( $i === false && count( $currentLoads ) != 0 ) {
                                        # All slaves lagged. Switch to read-only mode
                                        wfDebugLog( 'replication', "All slaves lagged. Switch to read-only mode" );
                                        $i = ArrayUtils::pickRandom( $currentLoads );
                                        $laggedSlaveMode = true;
                                }
                        }

                        if ( $i === false ) {
                                # pickRandom() returned false
                                # This is permanent and means the configuration or the load monitor
                                # wants us to return false.
                                wfDebugLog( 'connect', __METHOD__ . ": pickRandom() returned false" );

                                return false;
                        }

                        $serverName = $this->getServerName( $i );
                        wfDebugLog( 'connect', __METHOD__ . ": Using reader #$i: $serverName..." );

                        $conn = $this->openConnection( $i, $wiki );
                        if ( !$conn ) {
                                wfDebugLog( 'connect', __METHOD__ . ": Failed connecting to $i/$wiki" );
                                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 ( $wiki !== false ) {
                                $this->reuseConnection( $conn );
                        }

                        # Return this server
                        break;
                }

                # If all servers were down, quit now
                if ( !count( $nonErrorLoads ) ) {
                        wfDebugLog( 'connect', "All servers down" );
                }

                if ( $i !== false ) {
                        # Slave connection successful
                        # Wait for the session master pos for a short time
                        if ( $this->mWaitForPos && $i > 0 ) {
                                if ( !$this->doWait( $i ) ) {
                                        $this->mServers[$i]['slave pos'] = $conn->getSlavePos();
                                }
                        }
                        if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group === false ) {
                                $this->mReadIndex = $i;
                                # Record if the generic reader index is in "lagged slave" mode
                                if ( $laggedSlaveMode ) {
                                        $this->laggedSlaveMode = true;
                                }
                        }
                        $serverName = $this->getServerName( $i );
                        wfDebugLog( 'connect', __METHOD__ .
                                ": using server $serverName for group '$group'\n" );
                }

                return $i;
        }

        /**
         * Set the master wait position
         * If a DB_SLAVE connection has been opened already, waits
         * Otherwise sets a variable telling it to wait if such a connection is opened
         * @param DBMasterPos $pos
         */
        public function waitFor( $pos ) {
                $this->mWaitForPos = $pos;
                $i = $this->mReadIndex;

                if ( $i > 0 ) {
                        if ( !$this->doWait( $i ) ) {
                                $this->mServers[$i]['slave pos'] = $this->getAnyOpenConnection( $i )->getSlavePos();
                                $this->laggedSlaveMode = true;
                        }
                }
        }

        /**
         * Set the master wait position and wait for a "generic" slave to catch up to it
         *
         * This can be used a faster proxy for waitForAll()
         *
         * @param DBMasterPos $pos
         * @param int $timeout Max seconds to wait; default is mWaitTimeout
         * @return bool Success (able to connect and no timeouts reached)
         * @since 1.26
         */
        public function waitForOne( $pos, $timeout = null ) {
                $this->mWaitForPos = $pos;

                $i = $this->mReadIndex;
                if ( $i <= 0 ) {
                        // Pick a generic slave if there isn't one yet
                        $readLoads = $this->mLoads;
                        unset( $readLoads[$this->getWriterIndex()] ); // slaves 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
                }

                return $ok;
        }

        /**
         * Set the master wait position and wait for ALL slaves to catch up to it
         * @param DBMasterPos $pos
         * @param int $timeout Max seconds to wait; default is mWaitTimeout
         * @return bool Success (able to connect and no timeouts reached)
         */
        public function waitForAll( $pos, $timeout = null ) {
                $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;
                        }
                }

                return $ok;
        }

        /**
         * Get any open connection to a given server index, local or foreign
         * Returns false if there is no connection open
         *
         * @param int $i
         * @return DatabaseBase|bool False on failure
         */
        public function getAnyOpenConnection( $i ) {
                foreach ( $this->mConns as $conns ) {
                        if ( !empty( $conns[$i] ) ) {
                                return reset( $conns[$i] );
                        }
                }

                return false;
        }

        /**
         * Wait for a given slave 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 );
                /** @var DBMasterPos $knownReachedPos */
                $knownReachedPos = $this->srvCache->get( $key );
                if ( $knownReachedPos && $knownReachedPos->hasReached( $this->mWaitForPos ) ) {
                        wfDebugLog( 'replication', __METHOD__ .
                                ": slave $server known to be caught up (pos >= $knownReachedPos).\n" );
                        return true;
                }

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

                                return false;
                        } else {
                                $conn = $this->openConnection( $index, '' );
                                if ( !$conn ) {
                                        wfDebugLog( 'replication', __METHOD__ . ": failed to connect to $server\n" );

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

                wfDebugLog( 'replication', __METHOD__ . ": Waiting for slave $server to catch up...\n" );
                $timeout = $timeout ?: $this->mWaitTimeout;
                $result = $conn->masterPosWait( $this->mWaitForPos, $timeout );

                if ( $result == -1 || is_null( $result ) ) {
                        // Timed out waiting for slave, use master instead
                        $msg = __METHOD__ . ": Timed out waiting on $server pos {$this->mWaitForPos}";
                        wfDebugLog( 'replication', "$msg\n" );
                        wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );
                        $ok = false;
                } else {
                        wfDebugLog( 'replication', __METHOD__ . ": Done\n" );
                        $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;
        }

        /**
         * Get a connection by index
         * This is the main entry point for this class.
         *
         * @param int $i Server index
         * @param array|string|bool $groups Query group(s), or false for the generic reader
         * @param string|bool $wiki Wiki ID, or false for the current wiki
         *
         * @throws MWException
         * @return DatabaseBase
         */
        public function getConnection( $i, $groups = [], $wiki = false ) {
                if ( $i === null || $i === false ) {
                        throw new MWException( 'Attempt to call ' . __METHOD__ .
                                ' with invalid server index' );
                }

                if ( $wiki === wfWikiID() ) {
                        $wiki = false;
                }

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

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

                if ( $i == 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, $wiki );
                                if ( $groupIndex !== false ) {
                                        $i = $groupIndex;
                                        break;
                                }
                        }
                }

                # Operation-based index
                if ( $i == DB_SLAVE ) {
                        $this->mLastError = 'Unknown error'; // reset error string
                        # Try the general server pool if $groups are unavailable.
                        $i = in_array( false, $groups, true )
                                ? false // don't bother with this if that is what was tried above
                                : $this->getReaderIndex( false, $wiki );
                        # Couldn't find a working server in getReaderIndex()?
                        if ( $i === false ) {
                                $this->mLastError = 'No working slave server: ' . $this->mLastError;

                                return $this->reportConnectionError();
                        }
                }

                # Now we have an explicit index into the servers array
                $conn = $this->openConnection( $i, $wiki );
                if ( !$conn ) {
                        return $this->reportConnectionError();
                }

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

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

                return $conn;
        }

        /**
         * Mark a foreign connection as being available for reuse under a different
         * DB name or prefix. This mechanism is reference-counted, and must be called
         * the same number of times as getConnection() to work.
         *
         * @param DatabaseBase $conn
         * @throws MWException
         */
        public function reuseConnection( $conn ) {
                $serverIndex = $conn->getLBInfo( 'serverIndex' );
                $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
                if ( $serverIndex === null || $refCount === null ) {
                        wfDebug( __METHOD__ . ": this connection was not opened as a foreign connection\n" );
                        /**
                         * This can happen in code like:
                         *   foreach ( $dbs as $db ) {
                         *     $conn = $lb->getConnection( DB_SLAVE, array(), $db );
                         *     ...
                         *     $lb->reuseConnection( $conn );
                         *   }
                         * When a connection to the local DB is opened in this way, reuseConnection()
                         * should be ignored
                         */
                        return;
                }

                $dbName = $conn->getDBname();
                $prefix = $conn->tablePrefix();
                if ( strval( $prefix ) !== '' ) {
                        $wiki = "$dbName-$prefix";
                } else {
                        $wiki = $dbName;
                }
                if ( $this->mConns['foreignUsed'][$serverIndex][$wiki] !== $conn ) {
                        throw new MWException( __METHOD__ . ": connection not found, has " .
                                "the connection been freed already?" );
                }
                $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
                if ( $refCount <= 0 ) {
                        $this->mConns['foreignFree'][$serverIndex][$wiki] = $conn;
                        unset( $this->mConns['foreignUsed'][$serverIndex][$wiki] );
                        wfDebug( __METHOD__ . ": freed connection $serverIndex/$wiki\n" );
                } else {
                        wfDebug( __METHOD__ . ": reference count for $serverIndex/$wiki reduced to $refCount\n" );
                }
        }

        /**
         * Get a database connection handle reference
         *
         * The handle's methods wrap simply wrap those of a DatabaseBase handle
         *
         * @see LoadBalancer::getConnection() for parameter information
         *
         * @param int $db
         * @param array|string|bool $groups Query group(s), or false for the generic reader
         * @param string|bool $wiki Wiki ID, or false for the current wiki
         * @return DBConnRef
         */
        public function getConnectionRef( $db, $groups = [], $wiki = false ) {
                return new DBConnRef( $this, $this->getConnection( $db, $groups, $wiki ) );
        }

        /**
         * Get a database connection handle reference without connecting yet
         *
         * The handle's methods wrap simply wrap those of a DatabaseBase handle
         *
         * @see LoadBalancer::getConnection() for parameter information
         *
         * @param int $db
         * @param array|string|bool $groups Query group(s), or false for the generic reader
         * @param string|bool $wiki Wiki ID, or false for the current wiki
         * @return DBConnRef
         */
        public function getLazyConnectionRef( $db, $groups = [], $wiki = false ) {
                return new DBConnRef( $this, [ $db, $groups, $wiki ] );
        }

        /**
         * Open a connection to the server given by the specified index
         * Index must be an actual index into the array.
         * If the server is already open, returns it.
         *
         * On error, returns false, and the connection which caused the
         * error will be available via $this->mErrorConnection.
         *
         * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
         *
         * @param int $i Server index
         * @param string|bool $wiki Wiki ID, or false for the current wiki
         * @return DatabaseBase|bool Returns false on errors
         */
        public function openConnection( $i, $wiki = false ) {
                if ( $wiki !== false ) {
                        $conn = $this->openForeignConnection( $i, $wiki );
                } elseif ( isset( $this->mConns['local'][$i][0] ) ) {
                        $conn = $this->mConns['local'][$i][0];
                } else {
                        $server = $this->mServers[$i];
                        $server['serverIndex'] = $i;
                        $conn = $this->reallyOpenConnection( $server, false );
                        $serverName = $this->getServerName( $i );
                        if ( $conn->isOpen() ) {
                                wfDebugLog( 'connect', "Connected to database $i at $serverName\n" );
                                $this->mConns['local'][$i][0] = $conn;
                        } else {
                                wfDebugLog( 'connect', "Failed to connect to database $i at $serverName\n" );
                                $this->mErrorConnection = $conn;
                                $conn = false;
                        }
                }

                if ( $conn && !$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->mErrorConnection = $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 wiki. 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->mErrorConnection.
         *
         * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
         *
         * @param int $i Server index
         * @param string $wiki Wiki ID to open
         * @return DatabaseBase
         */
        private function openForeignConnection( $i, $wiki ) {
                list( $dbName, $prefix ) = wfSplitWikiID( $wiki );
                if ( isset( $this->mConns['foreignUsed'][$i][$wiki] ) ) {
                        // Reuse an already-used connection
                        $conn = $this->mConns['foreignUsed'][$i][$wiki];
                        wfDebug( __METHOD__ . ": reusing connection $i/$wiki\n" );
                } elseif ( isset( $this->mConns['foreignFree'][$i][$wiki] ) ) {
                        // Reuse a free connection for the same wiki
                        $conn = $this->mConns['foreignFree'][$i][$wiki];
                        unset( $this->mConns['foreignFree'][$i][$wiki] );
                        $this->mConns['foreignUsed'][$i][$wiki] = $conn;
                        wfDebug( __METHOD__ . ": reusing free connection $i/$wiki\n" );
                } elseif ( !empty( $this->mConns['foreignFree'][$i] ) ) {
                        // Reuse a connection from another wiki
                        $conn = reset( $this->mConns['foreignFree'][$i] );
                        $oldWiki = key( $this->mConns['foreignFree'][$i] );

                        // The empty string as a DB name means "don't care".
                        // DatabaseMysqlBase::open() already handle this on connection.
                        if ( $dbName !== '' && !$conn->selectDB( $dbName ) ) {
                                $this->mLastError = "Error selecting database $dbName on server " .
                                        $conn->getServer() . " from client host " . wfHostname() . "\n";
                                $this->mErrorConnection = $conn;
                                $conn = false;
                        } else {
                                $conn->tablePrefix( $prefix );
                                unset( $this->mConns['foreignFree'][$i][$oldWiki] );
                                $this->mConns['foreignUsed'][$i][$wiki] = $conn;
                                wfDebug( __METHOD__ . ": reusing free connection from $oldWiki for $wiki\n" );
                        }
                } else {
                        // 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() ) {
                                wfDebug( __METHOD__ . ": error opening connection for $i/$wiki\n" );
                                $this->mErrorConnection = $conn;
                                $conn = false;
                        } else {
                                $conn->tablePrefix( $prefix );
                                $this->mConns['foreignUsed'][$i][$wiki] = $conn;
                                wfDebug( __METHOD__ . ": opened new connection for $i/$wiki\n" );
                        }
                }

                // Increment reference count
                if ( $conn ) {
                        $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 bool $dbNameOverride
         * @throws MWException
         * @return DatabaseBase
         */
        protected function reallyOpenConnection( $server, $dbNameOverride = false ) {
                if ( $this->disabled ) {
                        throw new DBAccessError();
                }

                if ( !is_array( $server ) ) {
                        throw new MWException( 'You must update your load-balancing configuration. ' .
                                'See DefaultSettings.php entry for $wgDBservers.' );
                }

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

                // Let the handle know what the cluster master is (e.g. "db1052")
                $masterName = $this->getServerName( 0 );
                $server['clusterMasterHost'] = $masterName;

                // Log when many connection are made on requests
                if ( ++$this->connsOpened >= self::CONN_HELD_WARN_THRESHOLD ) {
                        wfDebugLog( 'DBPerformance', __METHOD__ . ": " .
                                "{$this->connsOpened}+ connections made (master=$masterName)\n" .
                                wfBacktrace( true ) );
                }

                # Create object
                try {
                        $db = DatabaseBase::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( DB_MASTER, [], $db->getWikiID() )
                );
                $db->setTransactionProfiler( $this->trxProfiler );

                return $db;
        }

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

                if ( !is_object( $conn ) ) {
                        // No last connection, probably due to all servers being too busy
                        wfLogDBError(
                                "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 );
                } else {
                        $context['db_server'] = $conn->getProperty( 'mServer' );
                        wfLogDBError(
                                "Connection error: {last_error} ({db_server})",
                                $context
                        );

                        // throws DBConnectionError
                        $conn->reportConnectionError( "{$this->mLastError} ({$context['db_server']})" );
                }

                return false; /* not reached */
        }

        /**
         * @return int
         * @since 1.26
         */
        public function getWriterIndex() {
                return 0;
        }

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

        /**
         * Returns true if the specified index is valid and has non-zero load
         *
         * @param string $i
         * @return bool
         */
        public function isNonZeroLoad( $i ) {
                return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
        }

        /**
         * Get the number of defined servers (not the number of open connections)
         *
         * @return int
         */
        public function getServerCount() {
                return count( $this->mServers );
        }

        /**
         * Get the host name or IP address of the server with the specified index
         * Prefer a readable name if available.
         * @param string $i
         * @return string
         */
        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';
        }

        /**
         * Return the server info structure for a given index, or false if the index is invalid.
         * @param int $i
         * @return array|bool
         */
        public function getServerInfo( $i ) {
                if ( isset( $this->mServers[$i] ) ) {
                        return $this->mServers[$i];
                } else {
                        return false;
                }
        }

        /**
         * Sets the server info structure for the given index. Entry at index $i
         * is created if it doesn't exist
         * @param int $i
         * @param array $serverInfo
         */
        public function setServerInfo( $i, array $serverInfo ) {
                $this->mServers[$i] = $serverInfo;
        }

        /**
         * Get the current master position for chronology control purposes
         * @return mixed
         */
        public function getMasterPos() {
                # If this entire request was served from a slave without opening a connection to the
                # master (however unlikely that may be), then we can fetch the position from the slave.
                $masterConn = $this->getAnyOpenConnection( 0 );
                if ( !$masterConn ) {
                        $serverCount = count( $this->mServers );
                        for ( $i = 1; $i < $serverCount; $i++ ) {
                                $conn = $this->getAnyOpenConnection( $i );
                                if ( $conn ) {
                                        return $conn->getSlavePos();
                                }
                        }
                } else {
                        return $masterConn->getMasterPos();
                }

                return false;
        }

        /**
         * Disable this load balancer. All connections are closed, and any attempt to
         * open a new connection will result in a DBAccessError.
         *
         * @since 1.27
         */
        public function disable() {
                $this->closeAll();
                $this->disabled = true;
        }

        /**
         * Close all open connections
         */
        public function closeAll() {
                foreach ( $this->mConns as $conns2 ) {
                        foreach ( $conns2 as $conns3 ) {
                                /** @var DatabaseBase $conn */
                                foreach ( $conns3 as $conn ) {
                                        $conn->close();
                                }
                        }
                }
                $this->mConns = [
                        'local' => [],
                        'foreignFree' => [],
                        'foreignUsed' => [],
                ];
                $this->connsOpened = 0;
        }

        /**
         * Close a connection
         * Using this function makes sure the LoadBalancer knows the connection is closed.
         * If you use $conn->close() directly, the load balancer won't update its state.
         * @param DatabaseBase $conn
         */
        public function closeConnection( $conn ) {
                $done = false;
                foreach ( $this->mConns as $i1 => $conns2 ) {
                        foreach ( $conns2 as $i2 => $conns3 ) {
                                foreach ( $conns3 as $i3 => $candidateConn ) {
                                        if ( $conn === $candidateConn ) {
                                                $conn->close();
                                                unset( $this->mConns[$i1][$i2][$i3] );
                                                --$this->connsOpened;
                                                $done = true;
                                                break;
                                        }
                                }
                        }
                }
                if ( !$done ) {
                        $conn->close();
                }
        }

        /**
         * Commit transactions on all open connections
         * @param string $fname Caller name
         */
        public function commitAll( $fname = __METHOD__ ) {
                foreach ( $this->mConns as $conns2 ) {
                        foreach ( $conns2 as $conns3 ) {
                                /** @var DatabaseBase[] $conns3 */
                                foreach ( $conns3 as $conn ) {
                                        if ( $conn->trxLevel() ) {
                                                $conn->commit( $fname, 'flush' );
                                        }
                                }
                        }
                }
        }

        /**
         * Issue COMMIT only on master, only if queries were done on connection
         * @param string $fname Caller name
         */
        public function commitMasterChanges( $fname = __METHOD__ ) {
                $masterIndex = $this->getWriterIndex();
                foreach ( $this->mConns as $conns2 ) {
                        if ( empty( $conns2[$masterIndex] ) ) {
                                continue;
                        }
                        /** @var DatabaseBase $conn */
                        foreach ( $conns2[$masterIndex] as $conn ) {
                                if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
                                        $conn->commit( $fname, 'flush' );
                                }
                        }
                }
        }

        /**
         * Issue ROLLBACK only on master, only if queries were done on connection
         * @param string $fname Caller name
         * @throws DBExpectedError
         * @since 1.23
         */
        public function rollbackMasterChanges( $fname = __METHOD__ ) {
                $failedServers = [];

                $masterIndex = $this->getWriterIndex();
                foreach ( $this->mConns as $conns2 ) {
                        if ( empty( $conns2[$masterIndex] ) ) {
                                continue;
                        }
                        /** @var DatabaseBase $conn */
                        foreach ( $conns2[$masterIndex] as $conn ) {
                                if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
                                        try {
                                                $conn->rollback( $fname, 'flush' );
                                        } catch ( DBError $e ) {
                                                MWExceptionHandler::logException( $e );
                                                $failedServers[] = $conn->getServer();
                                        }
                                }
                        }
                }

                if ( $failedServers ) {
                        throw new DBExpectedError( null, "Rollback failed on server(s) " .
                                implode( ', ', array_unique( $failedServers ) ) );
                }
        }

        /**
         * @return bool Whether a master connection is already open
         * @since 1.24
         */
        public function hasMasterConnection() {
                return $this->isOpen( $this->getWriterIndex() );
        }

        /**
         * Determine if there are pending changes in a transaction by this thread
         * @since 1.23
         * @return bool
         */
        public function hasMasterChanges() {
                $masterIndex = $this->getWriterIndex();
                foreach ( $this->mConns as $conns2 ) {
                        if ( empty( $conns2[$masterIndex] ) ) {
                                continue;
                        }
                        /** @var DatabaseBase $conn */
                        foreach ( $conns2[$masterIndex] as $conn ) {
                                if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
                                        return true;
                                }
                        }
                }
                return false;
        }

        /**
         * Get the timestamp of the latest write query done by this thread
         * @since 1.25
         * @return float|bool UNIX timestamp or false
         */
        public function lastMasterChangeTimestamp() {
                $lastTime = false;
                $masterIndex = $this->getWriterIndex();
                foreach ( $this->mConns as $conns2 ) {
                        if ( empty( $conns2[$masterIndex] ) ) {
                                continue;
                        }
                        /** @var DatabaseBase $conn */
                        foreach ( $conns2[$masterIndex] as $conn ) {
                                $lastTime = max( $lastTime, $conn->lastDoneWrites() );
                        }
                }
                return $lastTime;
        }

        /**
         * Check if this load balancer object had any recent or still
         * pending writes issued against it by this PHP thread
         *
         * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
         * @return bool
         * @since 1.25
         */
        public function hasOrMadeRecentMasterChanges( $age = null ) {
                $age = ( $age === null ) ? $this->mWaitTimeout : $age;

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

        /**
         * Get the list of callers that have pending master changes
         *
         * @return array
         * @since 1.27
         */
        public function pendingMasterChangeCallers() {
                $fnames = [];

                $masterIndex = $this->getWriterIndex();
                foreach ( $this->mConns as $conns2 ) {
                        if ( empty( $conns2[$masterIndex] ) ) {
                                continue;
                        }
                        /** @var DatabaseBase $conn */
                        foreach ( $conns2[$masterIndex] as $conn ) {
                                $fnames = array_merge( $fnames, $conn->pendingWriteCallers() );
                        }
                }

                return $fnames;
        }

        /**
         * @param mixed $value
         * @return mixed
         */
        public function waitTimeout( $value = null ) {
                return wfSetVar( $this->mWaitTimeout, $value );
        }

        /**
         * @note This method will trigger a DB connection if not yet done
         *
         * @param string|bool $wiki Wiki ID, or false for the current wiki
         * @return bool Whether the generic connection for reads is highly "lagged"
         */
        public function getLaggedSlaveMode( $wiki = false ) {
                // No-op if there is only one DB (also avoids recursion)
                if ( !$this->laggedSlaveMode && $this->getServerCount() > 1 ) {
                        try {
                                // See if laggedSlaveMode gets set
                                $conn = $this->getConnection( DB_SLAVE, false, $wiki );
                                $this->reuseConnection( $conn );
                        } catch ( DBConnectionError $e ) {
                                // Avoid expensive re-connect attempts and failures
                                $this->slavesDownMode = true;
                                $this->laggedSlaveMode = true;
                        }
                }

                return $this->laggedSlaveMode;
        }

        /**
         * @note This method will never cause a new DB connection
         * @return bool Whether any generic connection used for reads was highly "lagged"
         * @since 1.27
         */
        public function laggedSlaveUsed() {
                return $this->laggedSlaveMode;
        }

        /**
         * @note This method may trigger a DB connection if not yet done
         * @param string|bool $wiki Wiki ID, or false for the current wiki
         * @return string|bool Reason the master is read-only or false if it is not
         * @since 1.27
         */
        public function getReadOnlyReason( $wiki = false ) {
                if ( $this->readOnlyReason !== false ) {
                        return $this->readOnlyReason;
                } elseif ( $this->getLaggedSlaveMode( $wiki ) ) {
                        if ( $this->slavesDownMode ) {
                                return 'The database has been automatically locked ' .
                                        'until the slave database servers become available';
                        } else {
                                return 'The database has been automatically locked ' .
                                        'while the slave database servers catch up to the master.';
                        }
                }

                return false;
        }

        /**
         * Disables/enables lag checks
         * @param null|bool $mode
         * @return bool
         */
        public function allowLagged( $mode = null ) {
                if ( $mode === null ) {
                        return $this->mAllowLagged;
                }
                $this->mAllowLagged = $mode;

                return $this->mAllowLagged;
        }

        /**
         * @return bool
         */
        public function pingAll() {
                $success = true;
                foreach ( $this->mConns as $conns2 ) {
                        foreach ( $conns2 as $conns3 ) {
                                /** @var DatabaseBase[] $conns3 */
                                foreach ( $conns3 as $conn ) {
                                        if ( !$conn->ping() ) {
                                                $success = false;
                                        }
                                }
                        }
                }

                return $success;
        }

        /**
         * Call a function with each open connection object
         * @param callable $callback
         * @param array $params
         */
        public function forEachOpenConnection( $callback, array $params = [] ) {
                foreach ( $this->mConns as $conns2 ) {
                        foreach ( $conns2 as $conns3 ) {
                                foreach ( $conns3 as $conn ) {
                                        $mergedParams = array_merge( [ $conn ], $params );
                                        call_user_func_array( $callback, $mergedParams );
                                }
                        }
                }
        }

        /**
         * Get the hostname and lag time of the most-lagged slave
         *
         * This is useful for maintenance scripts that need to throttle their updates.
         * May attempt to open connections to slaves on the default DB. If there is
         * no lag, the maximum lag will be reported as -1.
         *
         * @param bool|string $wiki Wiki ID, or false for the default database
         * @return array ( host, max lag, index of max lagged host )
         */
        public function getMaxLag( $wiki = false ) {
                $maxLag = -1;
                $host = '';
                $maxIndex = 0;

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

                $lagTimes = $this->getLagTimes( $wiki );
                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 ];
        }

        /**
         * Get an estimate of replication lag (in seconds) for each server
         *
         * Results are cached for a short time in memcached/process cache
         *
         * Values may be "false" if replication is too broken to estimate
         *
         * @param string|bool $wiki
         * @return int[] Map of (server index => float|int|bool)
         */
        public function getLagTimes( $wiki = false ) {
                if ( $this->getServerCount() <= 1 ) {
                        return [ 0 => 0 ]; // no replication = no lag
                }

                # Send the request to the load monitor
                return $this->getLoadMonitor()->getLagTimes( array_keys( $this->mServers ), $wiki );
        }

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

        /**
         * Wait for a slave 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 Slave DB
         * @param DBMasterPos|bool $pos Master position; default: current position
         * @param integer $timeout Timeout in seconds
         * @return bool Success
         * @since 1.27
         */
        public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 ) {
                if ( $this->getServerCount() == 1 || !$conn->getLBInfo( 'slave' ) ) {
                        return true; // server is not a slave DB
                }

                $pos = $pos ?: $this->getConnection( DB_MASTER )->getMasterPos();
                if ( !( $pos instanceof DBMasterPos ) ) {
                        return false; // something is misconfigured
                }

                $result = $conn->masterPosWait( $pos, $timeout );
                if ( $result == -1 || is_null( $result ) ) {
                        $msg = __METHOD__ . ": Timed out waiting on {$conn->getServer()} pos {$pos}";
                        wfDebugLog( 'replication', "$msg\n" );
                        wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );
                        $ok = false;
                } else {
                        wfDebugLog( 'replication', __METHOD__ . ": Done\n" );
                        $ok = true;
                }

                return $ok;
        }

        /**
         * Clear the cache for slag lag delay times
         *
         * This is only used for testing
         */
        public function clearLagTimeCache() {
                $this->getLoadMonitor()->clearCaches();
        }
}