LoadBalancer.php

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<?php
class LoadBalancer {
    private $mServers;
    private $mConns;
    private $mLoads;
    private $mGroupLoads;
    private $mAllowLagged;
    private $mWaitTimeout;
    private $mParentInfo;

    private $mLoadMonitorClass;
    private $mLoadMonitor;
    private $srvCache;

    private $mErrorConnection;
    private $mReadIndex;
    private $mWaitForPos;
    private $laggedSlaveMode = false;
    private $slavesDownMode = false;
    private $mLastError = 'Unknown error';
    private $readOnlyReason = false;
    private $connsOpened = 0;

    protected $trxProfiler;

    const CONN_HELD_WARN_THRESHOLD = 10;
    const MAX_LAG = 10;
    const POS_WAIT_TIMEOUT = 10;

    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();
        }
    }

    private function getLoadMonitor() {
        if ( !isset( $this->mLoadMonitor ) ) {
            $class = $this->mLoadMonitorClass;
            $this->mLoadMonitor = new $class( $this );
        }

        return $this->mLoadMonitor;
    }

    public function parentInfo( $x = null ) {
        return wfSetVar( $this->mParentInfo, $x );
    }

    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 );
    }

    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;
    }

    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;
            }
        }
    }

    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;
    }

    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;
    }

    public function getAnyOpenConnection( $i ) {
        foreach ( $this->mConns as $conns ) {
            if ( !empty( $conns[$i] ) ) {
                return reset( $conns[$i] );
            }
        }

        return false;
    }

    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 );
        $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;
    }

    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;
    }

    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" );
            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" );
        }
    }

    public function getConnectionRef( $db, $groups = [], $wiki = false ) {
        return new DBConnRef( $this, $this->getConnection( $db, $groups, $wiki ) );
    }

    public function getLazyConnectionRef( $db, $groups = [], $wiki = false ) {
        return new DBConnRef( $this, [ $db, $groups, $wiki ] );
    }

    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;
    }

    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;
    }

    private function isOpen( $index ) {
        if ( !is_integer( $index ) ) {
            return false;
        }

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

    protected function reallyOpenConnection( $server, $dbNameOverride = false ) {
        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;
    }

    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 */
    }

    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 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;
    }

    public function closeAll() {
        foreach ( $this->mConns as $conns2 ) {
            foreach ( $conns2 as $conns3 ) {
                foreach ( $conns3 as $conn ) {
                    $conn->close();
                }
            }
        }
        $this->mConns = [
            'local' => [],
            'foreignFree' => [],
            'foreignUsed' => [],
        ];
        $this->connsOpened = 0;
    }

    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();
        }
    }

    public function commitAll( $fname = __METHOD__ ) {
        foreach ( $this->mConns as $conns2 ) {
            foreach ( $conns2 as $conns3 ) {
                foreach ( $conns3 as $conn ) {
                    if ( $conn->trxLevel() ) {
                        $conn->commit( $fname, 'flush' );
                    }
                }
            }
        }
    }

    public function commitMasterChanges( $fname = __METHOD__ ) {
        $masterIndex = $this->getWriterIndex();
        foreach ( $this->mConns as $conns2 ) {
            if ( empty( $conns2[$masterIndex] ) ) {
                continue;
            }
            foreach ( $conns2[$masterIndex] as $conn ) {
                if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
                    $conn->commit( $fname, 'flush' );
                }
            }
        }
    }

    public function rollbackMasterChanges( $fname = __METHOD__ ) {
        $failedServers = [];

        $masterIndex = $this->getWriterIndex();
        foreach ( $this->mConns as $conns2 ) {
            if ( empty( $conns2[$masterIndex] ) ) {
                continue;
            }
            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 ) ) );
        }
    }

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

    public function hasMasterChanges() {
        $masterIndex = $this->getWriterIndex();
        foreach ( $this->mConns as $conns2 ) {
            if ( empty( $conns2[$masterIndex] ) ) {
                continue;
            }
            foreach ( $conns2[$masterIndex] as $conn ) {
                if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
                    return true;
                }
            }
        }
        return false;
    }

    public function lastMasterChangeTimestamp() {
        $lastTime = false;
        $masterIndex = $this->getWriterIndex();
        foreach ( $this->mConns as $conns2 ) {
            if ( empty( $conns2[$masterIndex] ) ) {
                continue;
            }
            foreach ( $conns2[$masterIndex] as $conn ) {
                $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 = [];

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

        return $fnames;
    }

    public function waitTimeout( $value = null ) {
        return wfSetVar( $this->mWaitTimeout, $value );
    }

    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;
    }

    public function laggedSlaveUsed() {
        return $this->laggedSlaveMode;
    }

    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;
    }

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

        return $this->mAllowLagged;
    }

    public function pingAll() {
        $success = true;
        foreach ( $this->mConns as $conns2 ) {
            foreach ( $conns2 as $conns3 ) {
                foreach ( $conns3 as $conn ) {
                    if ( !$conn->ping() ) {
                        $success = false;
                    }
                }
            }
        }

        return $success;
    }

    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 );
                }
            }
        }
    }

    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 ( $lag > $maxLag ) {
                $maxLag = $lag;
                $host = $this->mServers[$i]['host'];
                $maxIndex = $i;
            }
        }

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

    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 );
    }

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

    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 ) {
            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;
    }

    public function clearLagTimeCache() {
        $this->getLoadMonitor()->clearCaches();
    }
}