LoadBalancer.php

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<?php
namespace Wikimedia\Rdbms;
 
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use Wikimedia\ScopedCallback;
use BagOStuff;
use EmptyBagOStuff;
use WANObjectCache;
use ArrayUtils;
use UnexpectedValueException;
use InvalidArgumentException;
use RuntimeException;
use Exception;
 
class LoadBalancer implements ILoadBalancer {
    private $loadMonitor;
    private $chronologyCallback;
    private $srvCache;
    private $wanCache;
    private $profiler;
    private $trxProfiler;
    private $replLogger;
    private $connLogger;
    private $queryLogger;
    private $perfLogger;
    private $errorLogger;
    private $deprecationLogger;
 
    private $localDomain;
 
    private $conns;
 
    private $servers;
    private $loads;
    private $groupLoads;
    private $allowLagged;
    private $waitTimeout;
    private $loadMonitorConfig;
    private $localDomainIdAlias;
    private $maxLag;
 
    private $hostname;
    private $cliMode;
    private $agent;
 
    private $tableAliases = [];
    private $indexAliases = [];
    private $trxRecurringCallbacks = [];
 
    private $errorConnection;
    private $readIndex;
    private $waitForPos;
    private $laggedReplicaMode = false;
    private $allReplicasDownMode = false;
    private $lastError = 'Unknown error';
    private $readOnlyReason = false;
    private $connsOpened = 0;
    private $disabled = false;
    private $connectionAttempted = false;
 
    private $trxRoundId = false;
    private $trxRoundStage = self::ROUND_CURSORY;
 
    private $defaultGroup = null;
 
    const CONN_HELD_WARN_THRESHOLD = 10;
 
    const MAX_LAG_DEFAULT = 6;
    const MAX_WAIT_DEFAULT = 10;
    const TTL_CACHE_READONLY = 5;
 
    const KEY_LOCAL = 'local';
    const KEY_FOREIGN_FREE = 'foreignFree';
    const KEY_FOREIGN_INUSE = 'foreignInUse';
 
    const KEY_LOCAL_NOROUND = 'localAutoCommit';
    const KEY_FOREIGN_FREE_NOROUND = 'foreignFreeAutoCommit';
    const KEY_FOREIGN_INUSE_NOROUND = 'foreignInUseAutoCommit';
 
    const ROUND_CURSORY = 'cursory';
    const ROUND_FINALIZED = 'finalized';
    const ROUND_APPROVED = 'approved';
    const ROUND_COMMIT_CALLBACKS = 'commit-callbacks';
    const ROUND_ROLLBACK_CALLBACKS = 'rollback-callbacks';
    const ROUND_ERROR = 'error';
 
    public function __construct( array $params ) {
        if ( !isset( $params['servers'] ) ) {
            throw new InvalidArgumentException( __CLASS__ . ': missing servers parameter' );
        }
        $this->servers = $params['servers'];
        foreach ( $this->servers as $i => $server ) {
            if ( $i == 0 ) {
                $this->servers[$i]['master'] = true;
            } else {
                $this->servers[$i]['replica'] = true;
            }
        }
 
        $localDomain = isset( $params['localDomain'] )
            ? DatabaseDomain::newFromId( $params['localDomain'] )
            : DatabaseDomain::newUnspecified();
        $this->setLocalDomain( $localDomain );
 
        $this->waitTimeout = $params['waitTimeout'] ?? self::MAX_WAIT_DEFAULT;
 
        $this->readIndex = -1;
        $this->conns = [
            // Connection were transaction rounds may be applied
            self::KEY_LOCAL => [],
            self::KEY_FOREIGN_INUSE => [],
            self::KEY_FOREIGN_FREE => [],
            // Auto-committing counterpart connections that ignore transaction rounds
            self::KEY_LOCAL_NOROUND => [],
            self::KEY_FOREIGN_INUSE_NOROUND => [],
            self::KEY_FOREIGN_FREE_NOROUND => []
        ];
        $this->loads = [];
        $this->waitForPos = false;
        $this->allowLagged = false;
 
        if ( isset( $params['readOnlyReason'] ) && is_string( $params['readOnlyReason'] ) ) {
            $this->readOnlyReason = $params['readOnlyReason'];
        }
 
        $this->maxLag = $params['maxLag'] ?? self::MAX_LAG_DEFAULT;
 
        $this->loadMonitorConfig = $params['loadMonitor'] ?? [ 'class' => 'LoadMonitorNull' ];
        $this->loadMonitorConfig += [ 'lagWarnThreshold' => $this->maxLag ];
 
        foreach ( $params['servers'] as $i => $server ) {
            $this->loads[$i] = $server['load'];
            if ( isset( $server['groupLoads'] ) ) {
                foreach ( $server['groupLoads'] as $group => $ratio ) {
                    if ( !isset( $this->groupLoads[$group] ) ) {
                        $this->groupLoads[$group] = [];
                    }
                    $this->groupLoads[$group][$i] = $ratio;
                }
            }
        }
 
        $this->srvCache = $params['srvCache'] ?? new EmptyBagOStuff();
        $this->wanCache = $params['wanCache'] ?? WANObjectCache::newEmpty();
        $this->profiler = $params['profiler'] ?? null;
        $this->trxProfiler = $params['trxProfiler'] ?? new TransactionProfiler();
 
        $this->errorLogger = $params['errorLogger'] ?? function ( Exception $e ) {
            trigger_error( get_class( $e ) . ': ' . $e->getMessage(), E_USER_WARNING );
        };
        $this->deprecationLogger = $params['deprecationLogger'] ?? function ( $msg ) {
            trigger_error( $msg, E_USER_DEPRECATED );
        };
 
        foreach ( [ 'replLogger', 'connLogger', 'queryLogger', 'perfLogger' ] as $key ) {
            $this->$key = $params[$key] ?? new NullLogger();
        }
 
        $this->hostname = $params['hostname'] ?? ( gethostname() ?: 'unknown' );
        $this->cliMode = $params['cliMode'] ?? ( PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg' );
        $this->agent = $params['agent'] ?? '';
 
        if ( isset( $params['chronologyCallback'] ) ) {
            $this->chronologyCallback = $params['chronologyCallback'];
        }
 
        if ( isset( $params['roundStage'] ) ) {
            if ( $params['roundStage'] === self::STAGE_POSTCOMMIT_CALLBACKS ) {
                $this->trxRoundStage = self::ROUND_COMMIT_CALLBACKS;
            } elseif ( $params['roundStage'] === self::STAGE_POSTROLLBACK_CALLBACKS ) {
                $this->trxRoundStage = self::ROUND_ROLLBACK_CALLBACKS;
            }
        }
 
        $this->defaultGroup = $params['defaultGroup'] ?? null;
    }
 
    public function getLocalDomainID() {
        return $this->localDomain->getId();
    }
 
    public function resolveDomainID( $domain ) {
        return ( $domain !== false ) ? (string)$domain : $this->getLocalDomainID();
    }
 
    private function getLoadMonitor() {
        if ( !isset( $this->loadMonitor ) ) {
            $compat = [
                'LoadMonitor' => LoadMonitor::class,
                'LoadMonitorNull' => LoadMonitorNull::class,
                'LoadMonitorMySQL' => LoadMonitorMySQL::class,
            ];
 
            $class = $this->loadMonitorConfig['class'];
            if ( isset( $compat[$class] ) ) {
                $class = $compat[$class];
            }
 
            $this->loadMonitor = new $class(
                $this, $this->srvCache, $this->wanCache, $this->loadMonitorConfig );
            $this->loadMonitor->setLogger( $this->replLogger );
        }
 
        return $this->loadMonitor;
    }
 
    private function getRandomNonLagged( array $loads, $domain = false, $maxLag = INF ) {
        $lags = $this->getLagTimes( $domain );
 
        # Unset excessively lagged servers
        foreach ( $lags as $i => $lag ) {
            if ( $i != 0 ) {
                # How much lag this server nominally is allowed to have
                $maxServerLag = $this->servers[$i]['max lag'] ?? $this->maxLag; // default
                # Constrain that futher by $maxLag argument
                $maxServerLag = min( $maxServerLag, $maxLag );
 
                $host = $this->getServerName( $i );
                if ( $lag === false && !is_infinite( $maxServerLag ) ) {
                    $this->replLogger->debug(
                        __METHOD__ .
                        ": server {host} is not replicating?", [ 'host' => $host ] );
                    unset( $loads[$i] );
                } elseif ( $lag > $maxServerLag ) {
                    $this->replLogger->debug(
                        __METHOD__ .
                        ": server {host} has {lag} seconds of lag (>= {maxlag})",
                        [ 'host' => $host, 'lag' => $lag, 'maxlag' => $maxServerLag ]
                    );
                    unset( $loads[$i] );
                }
            }
        }
 
        # Find out if all the replica DBs with non-zero load are lagged
        $sum = 0;
        foreach ( $loads as $load ) {
            $sum += $load;
        }
        if ( $sum == 0 ) {
            # No appropriate DB servers except maybe the master and some replica DBs with zero load
            # Do NOT use the master
            # Instead, this function will return false, triggering read-only mode,
            # and a lagged replica DB will be used instead.
            return false;
        }
 
        if ( count( $loads ) == 0 ) {
            return false;
        }
 
        # Return a random representative of the remainder
        return ArrayUtils::pickRandom( $loads );
    }
 
    public function getReaderIndex( $group = false, $domain = false ) {
        if ( count( $this->servers ) == 1 ) {
            // Skip the load balancing if there's only one server
            return $this->getWriterIndex();
        } elseif ( $group === false && $this->readIndex >= 0 ) {
            // Shortcut if the generic reader index was already cached
            return $this->readIndex;
        }
 
        if ( $group !== false ) {
            // Use the server weight array for this load group
            if ( isset( $this->groupLoads[$group] ) ) {
                $loads = $this->groupLoads[$group];
            } else {
                // No loads for this group, return false and the caller can use some other group
                $this->connLogger->info( __METHOD__ . ": no loads for group $group" );
 
                return false;
            }
        } else {
            // Use the generic load group
            $loads = $this->loads;
        }
 
        // Scale the configured load ratios according to each server's load and state
        $this->getLoadMonitor()->scaleLoads( $loads, $domain );
 
        // Pick a server to use, accounting for weights, load, lag, and "waitForPos"
        list( $i, $laggedReplicaMode ) = $this->pickReaderIndex( $loads, $domain );
        if ( $i === false ) {
            // Replica DB connection unsuccessful
            return false;
        }
 
        if ( $this->waitForPos && $i != $this->getWriterIndex() ) {
            // Before any data queries are run, wait for the server to catch up to the
            // specified position. This is used to improve session consistency. Note that
            // when LoadBalancer::waitFor() sets "waitForPos", the waiting triggers here,
            // so update laggedReplicaMode as needed for consistency.
            if ( !$this->doWait( $i ) ) {
                $laggedReplicaMode = true;
            }
        }
 
        if ( $this->readIndex <= 0 && $this->loads[$i] > 0 && $group === false ) {
            // Cache the generic reader index for future ungrouped DB_REPLICA handles
            $this->readIndex = $i;
            // Record if the generic reader index is in "lagged replica DB" mode
            if ( $laggedReplicaMode ) {
                $this->laggedReplicaMode = true;
            }
        }
 
        $serverName = $this->getServerName( $i );
        $this->connLogger->debug( __METHOD__ . ": using server $serverName for group '$group'" );
 
        return $i;
    }
 
    private function pickReaderIndex( array $loads, $domain = false ) {
        if ( $loads === [] ) {
            throw new InvalidArgumentException( "Empty server array given to LoadBalancer" );
        }
 
        $i = false;
        $laggedReplicaMode = false;
 
        // Quickly look through the available servers for a server that meets criteria...
        $currentLoads = $loads;
        while ( count( $currentLoads ) ) {
            if ( $this->allowLagged || $laggedReplicaMode ) {
                $i = ArrayUtils::pickRandom( $currentLoads );
            } else {
                $i = false;
                if ( $this->waitForPos && $this->waitForPos->asOfTime() ) {
                    // "chronologyCallback" sets "waitForPos" for session consistency.
                    // This triggers doWait() after connect, so it's especially good to
                    // avoid lagged servers so as to avoid excessive delay in that method.
                    $ago = microtime( true ) - $this->waitForPos->asOfTime();
                    // Aim for <= 1 second of waiting (being too picky can backfire)
                    $i = $this->getRandomNonLagged( $currentLoads, $domain, $ago + 1 );
                }
                if ( $i === false ) {
                    // Any server with less lag than it's 'max lag' param is preferable
                    $i = $this->getRandomNonLagged( $currentLoads, $domain );
                }
                if ( $i === false && count( $currentLoads ) != 0 ) {
                    // All replica DBs lagged. Switch to read-only mode
                    $this->replLogger->error(
                        __METHOD__ . ": all replica DBs lagged. Switch to read-only mode" );
                    $i = ArrayUtils::pickRandom( $currentLoads );
                    $laggedReplicaMode = true;
                }
            }
 
            if ( $i === false ) {
                // pickRandom() returned false.
                // This is permanent and means the configuration or the load monitor
                // wants us to return false.
                $this->connLogger->debug( __METHOD__ . ": pickRandom() returned false" );
 
                return [ false, false ];
            }
 
            $serverName = $this->getServerName( $i );
            $this->connLogger->debug( __METHOD__ . ": Using reader #$i: $serverName..." );
 
            $conn = $this->openConnection( $i, $domain );
            if ( !$conn ) {
                $this->connLogger->warning( __METHOD__ . ": Failed connecting to $i/$domain" );
                unset( $currentLoads[$i] ); // avoid this server next iteration
                $i = false;
                continue;
            }
 
            // Decrement reference counter, we are finished with this connection.
            // It will be incremented for the caller later.
            if ( $domain !== false ) {
                $this->reuseConnection( $conn );
            }
 
            // Return this server
            break;
        }
 
        // If all servers were down, quit now
        if ( $currentLoads === [] ) {
            $this->connLogger->error( __METHOD__ . ": all servers down" );
        }
 
        return [ $i, $laggedReplicaMode ];
    }
 
    public function waitFor( $pos ) {
        $oldPos = $this->waitForPos;
        try {
            $this->waitForPos = $pos;
            // If a generic reader connection was already established, then wait now
            $i = $this->readIndex;
            if ( ( $i > 0 ) && !$this->doWait( $i ) ) {
                $this->laggedReplicaMode = true;
            }
        } finally {
            // Restore the older position if it was higher since this is used for lag-protection
            $this->setWaitForPositionIfHigher( $oldPos );
        }
    }
 
    public function waitForOne( $pos, $timeout = null ) {
        $oldPos = $this->waitForPos;
        try {
            $this->waitForPos = $pos;
 
            $i = $this->readIndex;
            if ( $i <= 0 ) {
                // Pick a generic replica DB if there isn't one yet
                $readLoads = $this->loads;
                unset( $readLoads[$this->getWriterIndex()] ); // replica DBs only
                $readLoads = array_filter( $readLoads ); // with non-zero load
                $i = ArrayUtils::pickRandom( $readLoads );
            }
 
            if ( $i > 0 ) {
                $ok = $this->doWait( $i, true, $timeout );
            } else {
                $ok = true; // no applicable loads
            }
        } finally {
            # Restore the old position, as this is not used for lag-protection but for throttling
            $this->waitForPos = $oldPos;
        }
 
        return $ok;
    }
 
    public function waitForAll( $pos, $timeout = null ) {
        $timeout = $timeout ?: $this->waitTimeout;
 
        $oldPos = $this->waitForPos;
        try {
            $this->waitForPos = $pos;
            $serverCount = count( $this->servers );
 
            $ok = true;
            for ( $i = 1; $i < $serverCount; $i++ ) {
                if ( $this->loads[$i] > 0 ) {
                    $start = microtime( true );
                    $ok = $this->doWait( $i, true, $timeout ) && $ok;
                    $timeout -= intval( microtime( true ) - $start );
                    if ( $timeout <= 0 ) {
                        break; // timeout reached
                    }
                }
            }
        } finally {
            # Restore the old position, as this is not used for lag-protection but for throttling
            $this->waitForPos = $oldPos;
        }
 
        return $ok;
    }
 
    private function setWaitForPositionIfHigher( $pos ) {
        if ( !$pos ) {
            return;
        }
 
        if ( !$this->waitForPos || $pos->hasReached( $this->waitForPos ) ) {
            $this->waitForPos = $pos;
        }
    }
 
    public function getAnyOpenConnection( $i, $flags = 0 ) {
        $i = ( $i === self::DB_MASTER ) ? $this->getWriterIndex() : $i;
        $autocommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );
 
        foreach ( $this->conns as $connsByServer ) {
            if ( $i === self::DB_REPLICA ) {
                $indexes = array_keys( $connsByServer );
            } else {
                $indexes = isset( $connsByServer[$i] ) ? [ $i ] : [];
            }
 
            foreach ( $indexes as $index ) {
                foreach ( $connsByServer[$index] as $conn ) {
                    if ( !$conn->isOpen() ) {
                        continue; // some sort of error occured?
                    }
                    if ( !$autocommit || $conn->getLBInfo( 'autoCommitOnly' ) ) {
                        return $conn;
                    }
                }
            }
        }
 
        return false;
    }
 
    protected function doWait( $index, $open = false, $timeout = null ) {
        $timeout = max( 1, intval( $timeout ?: $this->waitTimeout ) );
 
        // Check if we already know that the DB has reached this point
        $server = $this->getServerName( $index );
        $key = $this->srvCache->makeGlobalKey( __CLASS__, 'last-known-pos', $server, 'v1' );
        $knownReachedPos = $this->srvCache->get( $key );
        if (
            $knownReachedPos instanceof DBMasterPos &&
            $knownReachedPos->hasReached( $this->waitForPos )
        ) {
            $this->replLogger->debug(
                __METHOD__ .
                ': replica DB {dbserver} known to be caught up (pos >= $knownReachedPos).',
                [ 'dbserver' => $server ]
            );
            return true;
        }
 
        // Find a connection to wait on, creating one if needed and allowed
        $close = false; // close the connection afterwards
        $conn = $this->getAnyOpenConnection( $index );
        if ( !$conn ) {
            if ( !$open ) {
                $this->replLogger->debug(
                    __METHOD__ . ': no connection open for {dbserver}',
                    [ 'dbserver' => $server ]
                );
 
                return false;
            } else {
                $conn = $this->openConnection( $index, self::DOMAIN_ANY );
                if ( !$conn ) {
                    $this->replLogger->warning(
                        __METHOD__ . ': failed to connect to {dbserver}',
                        [ 'dbserver' => $server ]
                    );
 
                    return false;
                }
                // Avoid connection spam in waitForAll() when connections
                // are made just for the sake of doing this lag check.
                $close = true;
            }
        }
 
        $this->replLogger->info(
            __METHOD__ .
            ': waiting for replica DB {dbserver} to catch up...',
            [ 'dbserver' => $server ]
        );
 
        $result = $conn->masterPosWait( $this->waitForPos, $timeout );
 
        if ( $result === null ) {
            $this->replLogger->warning(
                __METHOD__ . ': Errored out waiting on {host} pos {pos}',
                [
                    'host' => $server,
                    'pos' => $this->waitForPos,
                    'trace' => ( new RuntimeException() )->getTraceAsString()
                ]
            );
            $ok = false;
        } elseif ( $result == -1 ) {
            $this->replLogger->warning(
                __METHOD__ . ': Timed out waiting on {host} pos {pos}',
                [
                    'host' => $server,
                    'pos' => $this->waitForPos,
                    'trace' => ( new RuntimeException() )->getTraceAsString()
                ]
            );
            $ok = false;
        } else {
            $this->replLogger->debug( __METHOD__ . ": done waiting" );
            $ok = true;
            // Remember that the DB reached this point
            $this->srvCache->set( $key, $this->waitForPos, BagOStuff::TTL_DAY );
        }
 
        if ( $close ) {
            $this->closeConnection( $conn );
        }
 
        return $ok;
    }
 
    public function getConnection( $i, $groups = [], $domain = false, $flags = 0 ) {
        if ( $i === null || $i === false ) {
            throw new InvalidArgumentException( 'Attempt to call ' . __METHOD__ .
                ' with invalid server index' );
        }
 
        if ( $this->localDomain->equals( $domain ) || $domain === $this->localDomainIdAlias ) {
            $domain = false; // local connection requested
        }
 
        if ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) === self::CONN_TRX_AUTOCOMMIT ) {
            // Assuming all servers are of the same type (or similar), which is overwhelmingly
            // the case, use the master server information to get the attributes. The information
            // for $i cannot be used since it might be DB_REPLICA, which might require connection
            // attempts in order to be resolved into a real server index.
            $attributes = $this->getServerAttributes( $this->getWriterIndex() );
            if ( $attributes[Database::ATTR_DB_LEVEL_LOCKING] ) {
                // Callers sometimes want to (a) escape REPEATABLE-READ stateness without locking
                // rows (e.g. FOR UPDATE) or (b) make small commits during a larger transactions
                // to reduce lock contention. None of these apply for sqlite and using separate
                // connections just causes self-deadlocks.
                $flags &= ~self::CONN_TRX_AUTOCOMMIT;
                $this->connLogger->info( __METHOD__ .
                    ': ignoring CONN_TRX_AUTOCOMMIT to avoid deadlocks.' );
            }
        }
 
        // Check one "group" per default: the generic pool
        $defaultGroups = $this->defaultGroup ? [ $this->defaultGroup ] : [ false ];
 
        $groups = ( $groups === false || $groups === [] )
            ? $defaultGroups
            : (array)$groups;
 
        $masterOnly = ( $i == self::DB_MASTER || $i == $this->getWriterIndex() );
        $oldConnsOpened = $this->connsOpened; // connections open now
 
        if ( $i == self::DB_MASTER ) {
            $i = $this->getWriterIndex();
        } elseif ( $i == self::DB_REPLICA ) {
            # Try to find an available server in any the query groups (in order)
            foreach ( $groups as $group ) {
                $groupIndex = $this->getReaderIndex( $group, $domain );
                if ( $groupIndex !== false ) {
                    $i = $groupIndex;
                    break;
                }
            }
        }
 
        # Operation-based index
        if ( $i == self::DB_REPLICA ) {
            $this->lastError = 'Unknown error'; // reset error string
            # Try the general server pool if $groups are unavailable.
            $i = ( $groups === [ false ] )
                ? false // don't bother with this if that is what was tried above
                : $this->getReaderIndex( false, $domain );
            # Couldn't find a working server in getReaderIndex()?
            if ( $i === false ) {
                $this->lastError = 'No working replica DB server: ' . $this->lastError;
                // Throw an exception
                $this->reportConnectionError();
                return null; // not reached
            }
        }
 
        # Now we have an explicit index into the servers array
        $conn = $this->openConnection( $i, $domain, $flags );
        if ( !$conn ) {
            // Throw an exception
            $this->reportConnectionError();
            return null; // not reached
        }
 
        # Profile any new connections that happen
        if ( $this->connsOpened > $oldConnsOpened ) {
            $host = $conn->getServer();
            $dbname = $conn->getDBname();
            $this->trxProfiler->recordConnection( $host, $dbname, $masterOnly );
        }
 
        if ( $masterOnly ) {
            # Make master-requested DB handles inherit any read-only mode setting
            $conn->setLBInfo( 'readOnlyReason', $this->getReadOnlyReason( $domain, $conn ) );
        }
 
        return $conn;
    }
 
    public function reuseConnection( IDatabase $conn ) {
        $serverIndex = $conn->getLBInfo( 'serverIndex' );
        $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
        if ( $serverIndex === null || $refCount === null ) {
            return;
        } elseif ( $conn instanceof DBConnRef ) {
            // DBConnRef already handles calling reuseConnection() and only passes the live
            // Database instance to this method. Any caller passing in a DBConnRef is broken.
            $this->connLogger->error(
                __METHOD__ . ": got DBConnRef instance.\n" .
                ( new RuntimeException() )->getTraceAsString() );
 
            return;
        }
 
        if ( $this->disabled ) {
            return; // DBConnRef handle probably survived longer than the LoadBalancer
        }
 
        if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
            $connFreeKey = self::KEY_FOREIGN_FREE_NOROUND;
            $connInUseKey = self::KEY_FOREIGN_INUSE_NOROUND;
        } else {
            $connFreeKey = self::KEY_FOREIGN_FREE;
            $connInUseKey = self::KEY_FOREIGN_INUSE;
        }
 
        $domain = $conn->getDomainID();
        if ( !isset( $this->conns[$connInUseKey][$serverIndex][$domain] ) ) {
            throw new InvalidArgumentException( __METHOD__ .
                ": connection $serverIndex/$domain not found; it may have already been freed." );
        } elseif ( $this->conns[$connInUseKey][$serverIndex][$domain] !== $conn ) {
            throw new InvalidArgumentException( __METHOD__ .
                ": connection $serverIndex/$domain mismatched; it may have already been freed." );
        }
 
        $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
        if ( $refCount <= 0 ) {
            $this->conns[$connFreeKey][$serverIndex][$domain] = $conn;
            unset( $this->conns[$connInUseKey][$serverIndex][$domain] );
            if ( !$this->conns[$connInUseKey][$serverIndex] ) {
                unset( $this->conns[$connInUseKey][$serverIndex] ); // clean up
            }
            $this->connLogger->debug( __METHOD__ . ": freed connection $serverIndex/$domain" );
        } else {
            $this->connLogger->debug( __METHOD__ .
                ": reference count for $serverIndex/$domain reduced to $refCount" );
        }
    }
 
    public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
        $domain = $this->resolveDomainID( $domain );
        $role = $this->getRoleFromIndex( $i );
 
        return new DBConnRef( $this, $this->getConnection( $i, $groups, $domain, $flags ), $role );
    }
 
    public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
        $domain = $this->resolveDomainID( $domain );
        $role = $this->getRoleFromIndex( $i );
 
        return new DBConnRef( $this, [ $i, $groups, $domain, $flags ], $role );
    }
 
    public function getMaintenanceConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ) {
        $domain = $this->resolveDomainID( $domain );
        $role = $this->getRoleFromIndex( $i );
 
        return new MaintainableDBConnRef(
            $this, $this->getConnection( $i, $groups, $domain, $flags ), $role );
    }
 
    private function getRoleFromIndex( $i ) {
        return ( $i === self::DB_MASTER || $i === $this->getWriterIndex() )
            ? self::DB_MASTER
            : self::DB_REPLICA;
    }
 
    public function openConnection( $i, $domain = false, $flags = 0 ) {
        if ( $this->localDomain->equals( $domain ) || $domain === $this->localDomainIdAlias ) {
            $domain = false; // local connection requested
        }
 
        if ( !$this->connectionAttempted && $this->chronologyCallback ) {
            $this->connLogger->debug( __METHOD__ . ': calling initLB() before first connection.' );
            // Load any "waitFor" positions before connecting so that doWait() is triggered
            $this->connectionAttempted = true;
            ( $this->chronologyCallback )( $this );
        }
 
        // Check if an auto-commit connection is being requested. If so, it will not reuse the
        // main set of DB connections but rather its own pool since:
        // a) those are usually set to implicitly use transaction rounds via DBO_TRX
        // b) those must support the use of explicit transaction rounds via beginMasterChanges()
        $autoCommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );
 
        if ( $domain !== false ) {
            // Connection is to a foreign domain
            $conn = $this->openForeignConnection( $i, $domain, $flags );
        } else {
            // Connection is to the local domain
            $conn = $this->openLocalConnection( $i, $flags );
        }
 
        if ( $conn instanceof IDatabase && !$conn->isOpen() ) {
            // Connection was made but later unrecoverably lost for some reason.
            // Do not return a handle that will just throw exceptions on use,
            // but let the calling code (e.g. getReaderIndex) try another server.
            // See DatabaseMyslBase::ping() for how this can happen.
            $this->errorConnection = $conn;
            $conn = false;
        }
 
        if ( $autoCommit && $conn instanceof IDatabase ) {
            if ( $conn->trxLevel() ) { // sanity
                throw new DBUnexpectedError(
                    $conn,
                    __METHOD__ . ': CONN_TRX_AUTOCOMMIT handle has a transaction.'
                );
            }
 
            $conn->clearFlag( $conn::DBO_TRX ); // auto-commit mode
        }
 
        return $conn;
    }
 
    private function openLocalConnection( $i, $flags = 0 ) {
        $autoCommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );
 
        $connKey = $autoCommit ? self::KEY_LOCAL_NOROUND : self::KEY_LOCAL;
        if ( isset( $this->conns[$connKey][$i][0] ) ) {
            $conn = $this->conns[$connKey][$i][0];
        } else {
            if ( !isset( $this->servers[$i] ) || !is_array( $this->servers[$i] ) ) {
                throw new InvalidArgumentException( "No server with index '$i'." );
            }
            // Open a new connection
            $server = $this->servers[$i];
            $server['serverIndex'] = $i;
            $server['autoCommitOnly'] = $autoCommit;
            $conn = $this->reallyOpenConnection( $server, $this->localDomain );
            $host = $this->getServerName( $i );
            if ( $conn->isOpen() ) {
                $this->connLogger->debug(
                    __METHOD__ . ": connected to database $i at '$host'." );
                $this->conns[$connKey][$i][0] = $conn;
            } else {
                $this->connLogger->warning(
                    __METHOD__ . ": failed to connect to database $i at '$host'." );
                $this->errorConnection = $conn;
                $conn = false;
            }
        }
 
        // Final sanity check to make sure the right domain is selected
        if (
            $conn instanceof IDatabase &&
            !$this->localDomain->isCompatible( $conn->getDomainID() )
        ) {
            throw new UnexpectedValueException(
                "Got connection to '{$conn->getDomainID()}', " .
                "but expected local domain ('{$this->localDomain}')." );
        }
 
        return $conn;
    }
 
    private function openForeignConnection( $i, $domain, $flags = 0 ) {
        $domainInstance = DatabaseDomain::newFromId( $domain );
        $autoCommit = ( ( $flags & self::CONN_TRX_AUTOCOMMIT ) == self::CONN_TRX_AUTOCOMMIT );
 
        if ( $autoCommit ) {
            $connFreeKey = self::KEY_FOREIGN_FREE_NOROUND;
            $connInUseKey = self::KEY_FOREIGN_INUSE_NOROUND;
        } else {
            $connFreeKey = self::KEY_FOREIGN_FREE;
            $connInUseKey = self::KEY_FOREIGN_INUSE;
        }
 
        $conn = null;
 
        if ( isset( $this->conns[$connInUseKey][$i][$domain] ) ) {
            // Reuse an in-use connection for the same domain
            $conn = $this->conns[$connInUseKey][$i][$domain];
            $this->connLogger->debug( __METHOD__ . ": reusing connection $i/$domain" );
        } elseif ( isset( $this->conns[$connFreeKey][$i][$domain] ) ) {
            // Reuse a free connection for the same domain
            $conn = $this->conns[$connFreeKey][$i][$domain];
            unset( $this->conns[$connFreeKey][$i][$domain] );
            $this->conns[$connInUseKey][$i][$domain] = $conn;
            $this->connLogger->debug( __METHOD__ . ": reusing free connection $i/$domain" );
        } elseif ( !empty( $this->conns[$connFreeKey][$i] ) ) {
            // Reuse a free connection from another domain if possible
            foreach ( $this->conns[$connFreeKey][$i] as $oldDomain => $conn ) {
                if ( $domainInstance->getDatabase() !== null ) {
                    // Check if changing the database will require a new connection.
                    // In that case, leave the connection handle alone and keep looking.
                    // This prevents connections from being closed mid-transaction and can
                    // also avoid overhead if the same database will later be requested.
                    if (
                        $conn->databasesAreIndependent() &&
                        $conn->getDBname() !== $domainInstance->getDatabase()
                    ) {
                        continue;
                    }
                    // Select the new database, schema, and prefix
                    $conn->selectDomain( $domainInstance );
                } else {
                    // Stay on the current database, but update the schema/prefix
                    $conn->dbSchema( $domainInstance->getSchema() );
                    $conn->tablePrefix( $domainInstance->getTablePrefix() );
                }
                unset( $this->conns[$connFreeKey][$i][$oldDomain] );
                // Note that if $domain is an empty string, getDomainID() might not match it
                $this->conns[$connInUseKey][$i][$conn->getDomainID()] = $conn;
                $this->connLogger->debug( __METHOD__ .
                    ": reusing free connection from $oldDomain for $domain" );
                break;
            }
        }
 
        if ( !$conn ) {
            if ( !isset( $this->servers[$i] ) || !is_array( $this->servers[$i] ) ) {
                throw new InvalidArgumentException( "No server with index '$i'." );
            }
            // Open a new connection
            $server = $this->servers[$i];
            $server['serverIndex'] = $i;
            $server['foreignPoolRefCount'] = 0;
            $server['foreign'] = true;
            $server['autoCommitOnly'] = $autoCommit;
            $conn = $this->reallyOpenConnection( $server, $domainInstance );
            if ( !$conn->isOpen() ) {
                $this->connLogger->warning( __METHOD__ . ": connection error for $i/$domain" );
                $this->errorConnection = $conn;
                $conn = false;
            } else {
                // Note that if $domain is an empty string, getDomainID() might not match it
                $this->conns[$connInUseKey][$i][$conn->getDomainID()] = $conn;
                $this->connLogger->debug( __METHOD__ . ": opened new connection for $i/$domain" );
            }
        }
 
        if ( $conn instanceof IDatabase ) {
            // Final sanity check to make sure the right domain is selected
            if ( !$domainInstance->isCompatible( $conn->getDomainID() ) ) {
                throw new UnexpectedValueException(
                    "Got connection to '{$conn->getDomainID()}', but expected '$domain'." );
            }
            // Increment reference count
            $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
            $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
        }
 
        return $conn;
    }
 
    public function getServerAttributes( $i ) {
        return Database::attributesFromType(
            $this->getServerType( $i ),
            $this->servers[$i]['driver'] ?? null
        );
    }
 
    private function isOpen( $index ) {
        if ( !is_int( $index ) ) {
            return false;
        }
 
        return (bool)$this->getAnyOpenConnection( $index );
    }
 
    protected function reallyOpenConnection( array $server, DatabaseDomain $domain ) {
        if ( $this->disabled ) {
            throw new DBAccessError();
        }
 
        if ( $domain->getDatabase() === null ) {
            // The database domain does not specify a DB name and some database systems require a
            // valid DB specified on connection. The $server configuration array contains a default
            // DB name to use for connections in such cases.
            if ( $server['type'] === 'mysql' ) {
                // For MySQL, DATABASE and SCHEMA are synonyms, connections need not specify a DB,
                // and the DB name in $server might not exist due to legacy reasons (the default
                // domain used to ignore the local LB domain, even when mismatched).
                $server['dbname'] = null;
            }
        } else {
            $server['dbname'] = $domain->getDatabase();
        }
 
        if ( $domain->getSchema() !== null ) {
            $server['schema'] = $domain->getSchema();
        }
 
        // It is always possible to connect with any prefix, even the empty string
        $server['tablePrefix'] = $domain->getTablePrefix();
 
        // Let the handle know what the cluster master is (e.g. "db1052")
        $masterName = $this->getServerName( $this->getWriterIndex() );
        $server['clusterMasterHost'] = $masterName;
 
        // Log when many connection are made on requests
        if ( ++$this->connsOpened >= self::CONN_HELD_WARN_THRESHOLD ) {
            $this->perfLogger->warning( __METHOD__ . ": " .
                "{$this->connsOpened}+ connections made (master=$masterName)" );
        }
 
        $server['srvCache'] = $this->srvCache;
        // Set loggers and profilers
        $server['connLogger'] = $this->connLogger;
        $server['queryLogger'] = $this->queryLogger;
        $server['errorLogger'] = $this->errorLogger;
        $server['deprecationLogger'] = $this->deprecationLogger;
        $server['profiler'] = $this->profiler;
        $server['trxProfiler'] = $this->trxProfiler;
        // Use the same agent and PHP mode for all DB handles
        $server['cliMode'] = $this->cliMode;
        $server['agent'] = $this->agent;
        // Use DBO_DEFAULT flags by default for LoadBalancer managed databases. Assume that the
        // application calls LoadBalancer::commitMasterChanges() before the PHP script completes.
        $server['flags'] = $server['flags'] ?? IDatabase::DBO_DEFAULT;
 
        // Create a live connection object
        try {
            $db = Database::factory( $server['type'], $server );
        } catch ( DBConnectionError $e ) {
            // FIXME: This is probably the ugliest thing I have ever done to
            // PHP. I'm half-expecting it to segfault, just out of disgust. -- TS
            $db = $e->db;
        }
 
        $db->setLBInfo( $server );
        $db->setLazyMasterHandle(
            $this->getLazyConnectionRef( self::DB_MASTER, [], $db->getDomainID() )
        );
        $db->setTableAliases( $this->tableAliases );
        $db->setIndexAliases( $this->indexAliases );
 
        if ( $server['serverIndex'] === $this->getWriterIndex() ) {
            if ( $this->trxRoundId !== false ) {
                $this->applyTransactionRoundFlags( $db );
            }
            foreach ( $this->trxRecurringCallbacks as $name => $callback ) {
                $db->setTransactionListener( $name, $callback );
            }
        }
 
        return $db;
    }
 
    private function reportConnectionError() {
        $conn = $this->errorConnection; // the connection which caused the error
        $context = [
            'method' => __METHOD__,
            'last_error' => $this->lastError,
        ];
 
        if ( $conn instanceof IDatabase ) {
            $context['db_server'] = $conn->getServer();
            $this->connLogger->warning(
                __METHOD__ . ": connection error: {last_error} ({db_server})",
                $context
            );
 
            throw new DBConnectionError( $conn, "{$this->lastError} ({$context['db_server']})" );
        } else {
            // No last connection, probably due to all servers being too busy
            $this->connLogger->error(
                __METHOD__ .
                ": LB failure with no last connection. Connection error: {last_error}",
                $context
            );
 
            // If all servers were busy, "lastError" will contain something sensible
            throw new DBConnectionError( null, $this->lastError );
        }
    }
 
    public function getWriterIndex() {
        return 0;
    }
 
    public function haveIndex( $i ) {
        return array_key_exists( $i, $this->servers );
    }
 
    public function isNonZeroLoad( $i ) {
        return array_key_exists( $i, $this->servers ) && $this->loads[$i] != 0;
    }
 
    public function getServerCount() {
        return count( $this->servers );
    }
 
    public function getServerName( $i ) {
        $name = $this->servers[$i]['hostName'] ?? $this->servers[$i]['host'] ?? '';
 
        return ( $name != '' ) ? $name : 'localhost';
    }
 
    public function getServerInfo( $i ) {
        return $this->servers[$i] ?? false;
    }
 
    public function getServerType( $i ) {
        return $this->servers[$i]['type'] ?? 'unknown';
    }
 
    public function getMasterPos() {
        # If this entire request was served from a replica DB without opening a connection to the
        # master (however unlikely that may be), then we can fetch the position from the replica DB.
        $masterConn = $this->getAnyOpenConnection( $this->getWriterIndex() );
        if ( !$masterConn ) {
            $serverCount = count( $this->servers );
            for ( $i = 1; $i < $serverCount; $i++ ) {
                $conn = $this->getAnyOpenConnection( $i );
                if ( $conn ) {
                    return $conn->getReplicaPos();
                }
            }
        } else {
            return $masterConn->getMasterPos();
        }
 
        return false;
    }
 
    public function disable() {
        $this->closeAll();
        $this->disabled = true;
    }
 
    public function closeAll() {
        $fname = __METHOD__;
        $this->forEachOpenConnection( function ( IDatabase $conn ) use ( $fname ) {
            $host = $conn->getServer();
            $this->connLogger->debug(
                $fname . ": closing connection to database '$host'." );
            $conn->close();
        } );
 
        $this->conns = [
            self::KEY_LOCAL => [],
            self::KEY_FOREIGN_INUSE => [],
            self::KEY_FOREIGN_FREE => [],
            self::KEY_LOCAL_NOROUND => [],
            self::KEY_FOREIGN_INUSE_NOROUND => [],
            self::KEY_FOREIGN_FREE_NOROUND => []
        ];
        $this->connsOpened = 0;
    }
 
    public function closeConnection( IDatabase $conn ) {
        if ( $conn instanceof DBConnRef ) {
            // Avoid calling close() but still leaving the handle in the pool
            throw new RuntimeException( __METHOD__ . ': got DBConnRef instance.' );
        }
 
        $serverIndex = $conn->getLBInfo( 'serverIndex' );
        foreach ( $this->conns as $type => $connsByServer ) {
            if ( !isset( $connsByServer[$serverIndex] ) ) {
                continue;
            }
 
            foreach ( $connsByServer[$serverIndex] as $i => $trackedConn ) {
                if ( $conn === $trackedConn ) {
                    $host = $this->getServerName( $i );
                    $this->connLogger->debug(
                        __METHOD__ . ": closing connection to database $i at '$host'." );
                    unset( $this->conns[$type][$serverIndex][$i] );
                    --$this->connsOpened;
                    break 2;
                }
            }
        }
 
        $conn->close();
    }
 
    public function commitAll( $fname = __METHOD__ ) {
        $this->commitMasterChanges( $fname );
        $this->flushMasterSnapshots( $fname );
        $this->flushReplicaSnapshots( $fname );
    }
 
    public function finalizeMasterChanges() {
        $this->assertTransactionRoundStage( [ self::ROUND_CURSORY, self::ROUND_FINALIZED ] );
 
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        // Loop until callbacks stop adding callbacks on other connections
        $total = 0;
        do {
            $count = 0; // callbacks execution attempts
            $this->forEachOpenMasterConnection( function ( Database $conn ) use ( &$count ) {
                // Run any pre-commit callbacks while leaving the post-commit ones suppressed.
                // Any error should cause all (peer) transactions to be rolled back together.
                $count += $conn->runOnTransactionPreCommitCallbacks();
            } );
            $total += $count;
        } while ( $count > 0 );
        // Defer post-commit callbacks until after COMMIT/ROLLBACK happens on all handles
        $this->forEachOpenMasterConnection( function ( Database $conn ) {
            $conn->setTrxEndCallbackSuppression( true );
        } );
        $this->trxRoundStage = self::ROUND_FINALIZED;
 
        return $total;
    }
 
    public function approveMasterChanges( array $options ) {
        $this->assertTransactionRoundStage( self::ROUND_FINALIZED );
 
        $limit = $options['maxWriteDuration'] ?? 0;
 
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $limit ) {
            // If atomic sections or explicit transactions are still open, some caller must have
            // caught an exception but failed to properly rollback any changes. Detect that and
            // throw and error (causing rollback).
            $conn->assertNoOpenTransactions();
            // Assert that the time to replicate the transaction will be sane.
            // If this fails, then all DB transactions will be rollback back together.
            $time = $conn->pendingWriteQueryDuration( $conn::ESTIMATE_DB_APPLY );
            if ( $limit > 0 && $time > $limit ) {
                throw new DBTransactionSizeError(
                    $conn,
                    "Transaction spent $time second(s) in writes, exceeding the limit of $limit.",
                    [ $time, $limit ]
                );
            }
            // If a connection sits idle while slow queries execute on another, that connection
            // may end up dropped before the commit round is reached. Ping servers to detect this.
            if ( $conn->writesOrCallbacksPending() && !$conn->ping() ) {
                throw new DBTransactionError(
                    $conn,
                    "A connection to the {$conn->getDBname()} database was lost before commit."
                );
            }
        } );
        $this->trxRoundStage = self::ROUND_APPROVED;
    }
 
    public function beginMasterChanges( $fname = __METHOD__ ) {
        if ( $this->trxRoundId !== false ) {
            throw new DBTransactionError(
                null,
                "$fname: Transaction round '{$this->trxRoundId}' already started."
            );
        }
        $this->assertTransactionRoundStage( self::ROUND_CURSORY );
 
        // Clear any empty transactions (no writes/callbacks) from the implicit round
        $this->flushMasterSnapshots( $fname );
 
        $this->trxRoundId = $fname;
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        // Mark applicable handles as participating in this explicit transaction round.
        // For each of these handles, any writes and callbacks will be tied to a single
        // transaction. The (peer) handles will reject begin()/commit() calls unless they
        // are part of an en masse commit or an en masse rollback.
        $this->forEachOpenMasterConnection( function ( Database $conn ) {
            $this->applyTransactionRoundFlags( $conn );
        } );
        $this->trxRoundStage = self::ROUND_CURSORY;
    }
 
    public function commitMasterChanges( $fname = __METHOD__ ) {
        $this->assertTransactionRoundStage( self::ROUND_APPROVED );
 
        $failures = [];
 
        $scope = ScopedCallback::newScopedIgnoreUserAbort(); // try to ignore client aborts
 
        $restore = ( $this->trxRoundId !== false );
        $this->trxRoundId = false;
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        // Commit any writes and clear any snapshots as well (callbacks require AUTOCOMMIT).
        // Note that callbacks should already be suppressed due to finalizeMasterChanges().
        $this->forEachOpenMasterConnection(
            function ( IDatabase $conn ) use ( $fname, &$failures ) {
                try {
                    $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
                } catch ( DBError $e ) {
                    ( $this->errorLogger )( $e );
                    $failures[] = "{$conn->getServer()}: {$e->getMessage()}";
                }
            }
        );
        if ( $failures ) {
            throw new DBTransactionError(
                null,
                "$fname: Commit failed on server(s) " . implode( "\n", array_unique( $failures ) )
            );
        }
        if ( $restore ) {
            // Unmark handles as participating in this explicit transaction round
            $this->forEachOpenMasterConnection( function ( Database $conn ) {
                $this->undoTransactionRoundFlags( $conn );
            } );
        }
        $this->trxRoundStage = self::ROUND_COMMIT_CALLBACKS;
    }
 
    public function runMasterTransactionIdleCallbacks() {
        if ( $this->trxRoundStage === self::ROUND_COMMIT_CALLBACKS ) {
            $type = IDatabase::TRIGGER_COMMIT;
        } elseif ( $this->trxRoundStage === self::ROUND_ROLLBACK_CALLBACKS ) {
            $type = IDatabase::TRIGGER_ROLLBACK;
        } else {
            throw new DBTransactionError(
                null,
                "Transaction should be in the callback stage (not '{$this->trxRoundStage}')"
            );
        }
 
        $oldStage = $this->trxRoundStage;
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
 
        // Now that the COMMIT/ROLLBACK step is over, enable post-commit callback runs
        $this->forEachOpenMasterConnection( function ( Database $conn ) {
            $conn->setTrxEndCallbackSuppression( false );
        } );
 
        $e = null; // first exception
        $fname = __METHOD__;
        // Loop until callbacks stop adding callbacks on other connections
        do {
            // Run any pending callbacks for each connection...
            $count = 0; // callback execution attempts
            $this->forEachOpenMasterConnection(
                function ( Database $conn ) use ( $type, &$e, &$count ) {
                    if ( $conn->trxLevel() ) {
                        return; // retry in the next iteration, after commit() is called
                    }
                    try {
                        $count += $conn->runOnTransactionIdleCallbacks( $type );
                    } catch ( Exception $ex ) {
                        $e = $e ?: $ex;
                    }
                }
            );
            // Clear out any active transactions left over from callbacks...
            $this->forEachOpenMasterConnection( function ( Database $conn ) use ( &$e, $fname ) {
                if ( $conn->writesPending() ) {
                    // A callback from another handle wrote to this one and DBO_TRX is set
                    $this->queryLogger->warning( $fname . ": found writes pending." );
                    $fnames = implode( ', ', $conn->pendingWriteAndCallbackCallers() );
                    $this->queryLogger->warning(
                        $fname . ": found writes pending ($fnames).",
                        [
                            'db_server' => $conn->getServer(),
                            'db_name' => $conn->getDBname()
                        ]
                    );
                } elseif ( $conn->trxLevel() ) {
                    // A callback from another handle read from this one and DBO_TRX is set,
                    // which can easily happen if there is only one DB (no replicas)
                    $this->queryLogger->debug( $fname . ": found empty transaction." );
                }
                try {
                    $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
                } catch ( Exception $ex ) {
                    $e = $e ?: $ex;
                }
            } );
        } while ( $count > 0 );
 
        $this->trxRoundStage = $oldStage;
 
        return $e;
    }
 
    public function runMasterTransactionListenerCallbacks() {
        if ( $this->trxRoundStage === self::ROUND_COMMIT_CALLBACKS ) {
            $type = IDatabase::TRIGGER_COMMIT;
        } elseif ( $this->trxRoundStage === self::ROUND_ROLLBACK_CALLBACKS ) {
            $type = IDatabase::TRIGGER_ROLLBACK;
        } else {
            throw new DBTransactionError(
                null,
                "Transaction should be in the callback stage (not '{$this->trxRoundStage}')"
            );
        }
 
        $e = null;
 
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        $this->forEachOpenMasterConnection( function ( Database $conn ) use ( $type, &$e ) {
            try {
                $conn->runTransactionListenerCallbacks( $type );
            } catch ( Exception $ex ) {
                $e = $e ?: $ex;
            }
        } );
        $this->trxRoundStage = self::ROUND_CURSORY;
 
        return $e;
    }
 
    public function rollbackMasterChanges( $fname = __METHOD__ ) {
        $restore = ( $this->trxRoundId !== false );
        $this->trxRoundId = false;
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $fname ) {
            $conn->rollback( $fname, $conn::FLUSHING_ALL_PEERS );
        } );
        if ( $restore ) {
            // Unmark handles as participating in this explicit transaction round
            $this->forEachOpenMasterConnection( function ( Database $conn ) {
                $this->undoTransactionRoundFlags( $conn );
            } );
        }
        $this->trxRoundStage = self::ROUND_ROLLBACK_CALLBACKS;
    }
 
    private function assertTransactionRoundStage( $stage ) {
        $stages = (array)$stage;
 
        if ( !in_array( $this->trxRoundStage, $stages, true ) ) {
            $stageList = implode(
                '/',
                array_map( function ( $v ) {
                    return "'$v'";
                }, $stages )
            );
            throw new DBTransactionError(
                null,
                "Transaction round stage must be $stageList (not '{$this->trxRoundStage}')"
            );
        }
    }
 
    private function applyTransactionRoundFlags( Database $conn ) {
        if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
            return; // transaction rounds do not apply to these connections
        }
 
        if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
            // DBO_TRX is controlled entirely by CLI mode presence with DBO_DEFAULT.
            // Force DBO_TRX even in CLI mode since a commit round is expected soon.
            $conn->setFlag( $conn::DBO_TRX, $conn::REMEMBER_PRIOR );
        }
 
        if ( $conn->getFlag( $conn::DBO_TRX ) ) {
            $conn->setLBInfo( 'trxRoundId', $this->trxRoundId );
        }
    }
 
    private function undoTransactionRoundFlags( Database $conn ) {
        if ( $conn->getLBInfo( 'autoCommitOnly' ) ) {
            return; // transaction rounds do not apply to these connections
        }
 
        if ( $conn->getFlag( $conn::DBO_TRX ) ) {
            $conn->setLBInfo( 'trxRoundId', false );
        }
 
        if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
            $conn->restoreFlags( $conn::RESTORE_PRIOR );
        }
    }
 
    public function flushReplicaSnapshots( $fname = __METHOD__ ) {
        $this->forEachOpenReplicaConnection( function ( IDatabase $conn ) use ( $fname ) {
            $conn->flushSnapshot( $fname );
        } );
    }
 
    public function flushMasterSnapshots( $fname = __METHOD__ ) {
        $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( $fname ) {
            $conn->flushSnapshot( $fname );
        } );
    }
 
    public function getTransactionRoundStage() {
        return $this->trxRoundStage;
    }
 
    public function hasMasterConnection() {
        return $this->isOpen( $this->getWriterIndex() );
    }
 
    public function hasMasterChanges() {
        $pending = 0;
        $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$pending ) {
            $pending |= $conn->writesOrCallbacksPending();
        } );
 
        return (bool)$pending;
    }
 
    public function lastMasterChangeTimestamp() {
        $lastTime = false;
        $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$lastTime ) {
            $lastTime = max( $lastTime, $conn->lastDoneWrites() );
        } );
 
        return $lastTime;
    }
 
    public function hasOrMadeRecentMasterChanges( $age = null ) {
        $age = ( $age === null ) ? $this->waitTimeout : $age;
 
        return ( $this->hasMasterChanges()
            || $this->lastMasterChangeTimestamp() > microtime( true ) - $age );
    }
 
    public function pendingMasterChangeCallers() {
        $fnames = [];
        $this->forEachOpenMasterConnection( function ( IDatabase $conn ) use ( &$fnames ) {
            $fnames = array_merge( $fnames, $conn->pendingWriteCallers() );
        } );
 
        return $fnames;
    }
 
    public function getLaggedReplicaMode( $domain = false ) {
        // No-op if there is only one DB (also avoids recursion)
        if ( !$this->laggedReplicaMode && $this->getServerCount() > 1 ) {
            try {
                // See if laggedReplicaMode gets set
                $conn = $this->getConnection( self::DB_REPLICA, false, $domain );
                $this->reuseConnection( $conn );
            } catch ( DBConnectionError $e ) {
                // Avoid expensive re-connect attempts and failures
                $this->allReplicasDownMode = true;
                $this->laggedReplicaMode = true;
            }
        }
 
        return $this->laggedReplicaMode;
    }
 
    public function laggedReplicaUsed() {
        return $this->laggedReplicaMode;
    }
 
    public function laggedSlaveUsed() {
        return $this->laggedReplicaUsed();
    }
 
    public function getReadOnlyReason( $domain = false, IDatabase $conn = null ) {
        if ( $this->readOnlyReason !== false ) {
            return $this->readOnlyReason;
        } elseif ( $this->getLaggedReplicaMode( $domain ) ) {
            if ( $this->allReplicasDownMode ) {
                return 'The database has been automatically locked ' .
                    'until the replica database servers become available';
            } else {
                return 'The database has been automatically locked ' .
                    'while the replica database servers catch up to the master.';
            }
        } elseif ( $this->masterRunningReadOnly( $domain, $conn ) ) {
            return 'The database master is running in read-only mode.';
        }
 
        return false;
    }
 
    private function masterRunningReadOnly( $domain, IDatabase $conn = null ) {
        $cache = $this->wanCache;
        $masterServer = $this->getServerName( $this->getWriterIndex() );
 
        return (bool)$cache->getWithSetCallback(
            $cache->makeGlobalKey( __CLASS__, 'server-read-only', $masterServer ),
            self::TTL_CACHE_READONLY,
            function () use ( $domain, $conn ) {
                $old = $this->trxProfiler->setSilenced( true );
                try {
                    $dbw = $conn ?: $this->getConnection( self::DB_MASTER, [], $domain );
                    $readOnly = (int)$dbw->serverIsReadOnly();
                    if ( !$conn ) {
                        $this->reuseConnection( $dbw );
                    }
                } catch ( DBError $e ) {
                    $readOnly = 0;
                }
                $this->trxProfiler->setSilenced( $old );
                return $readOnly;
            },
            [ 'pcTTL' => $cache::TTL_PROC_LONG, 'busyValue' => 0 ]
        );
    }
 
    public function allowLagged( $mode = null ) {
        if ( $mode === null ) {
            return $this->allowLagged;
        }
        $this->allowLagged = $mode;
 
        return $this->allowLagged;
    }
 
    public function pingAll() {
        $success = true;
        $this->forEachOpenConnection( function ( IDatabase $conn ) use ( &$success ) {
            if ( !$conn->ping() ) {
                $success = false;
            }
        } );
 
        return $success;
    }
 
    public function forEachOpenConnection( $callback, array $params = [] ) {
        foreach ( $this->conns as $connsByServer ) {
            foreach ( $connsByServer as $serverConns ) {
                foreach ( $serverConns as $conn ) {
                    $callback( $conn, ...$params );
                }
            }
        }
    }
 
    public function forEachOpenMasterConnection( $callback, array $params = [] ) {
        $masterIndex = $this->getWriterIndex();
        foreach ( $this->conns as $connsByServer ) {
            if ( isset( $connsByServer[$masterIndex] ) ) {
                foreach ( $connsByServer[$masterIndex] as $conn ) {
                    $callback( $conn, ...$params );
                }
            }
        }
    }
 
    public function forEachOpenReplicaConnection( $callback, array $params = [] ) {
        foreach ( $this->conns as $connsByServer ) {
            foreach ( $connsByServer as $i => $serverConns ) {
                if ( $i === $this->getWriterIndex() ) {
                    continue; // skip master
                }
                foreach ( $serverConns as $conn ) {
                    $callback( $conn, ...$params );
                }
            }
        }
    }
 
    public function getMaxLag( $domain = false ) {
        $maxLag = -1;
        $host = '';
        $maxIndex = 0;
 
        if ( $this->getServerCount() <= 1 ) {
            return [ $host, $maxLag, $maxIndex ]; // no replication = no lag
        }
 
        $lagTimes = $this->getLagTimes( $domain );
        foreach ( $lagTimes as $i => $lag ) {
            if ( $this->loads[$i] > 0 && $lag > $maxLag ) {
                $maxLag = $lag;
                $host = $this->servers[$i]['host'];
                $maxIndex = $i;
            }
        }
 
        return [ $host, $maxLag, $maxIndex ];
    }
 
    public function getLagTimes( $domain = false ) {
        if ( $this->getServerCount() <= 1 ) {
            return [ $this->getWriterIndex() => 0 ]; // no replication = no lag
        }
 
        $knownLagTimes = []; // map of (server index => 0 seconds)
        $indexesWithLag = [];
        foreach ( $this->servers as $i => $server ) {
            if ( empty( $server['is static'] ) ) {
                $indexesWithLag[] = $i; // DB server might have replication lag
            } else {
                $knownLagTimes[$i] = 0; // DB server is a non-replicating and read-only archive
            }
        }
 
        return $this->getLoadMonitor()->getLagTimes( $indexesWithLag, $domain ) + $knownLagTimes;
    }
 
    public function safeGetLag( IDatabase $conn ) {
        if ( $this->getServerCount() <= 1 ) {
            return 0;
        } else {
            return $conn->getLag();
        }
    }
 
    public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = null ) {
        $timeout = max( 1, $timeout ?: $this->waitTimeout );
 
        if ( $this->getServerCount() <= 1 || !$conn->getLBInfo( 'replica' ) ) {
            return true; // server is not a replica DB
        }
 
        if ( !$pos ) {
            // Get the current master position, opening a connection if needed
            $masterConn = $this->getAnyOpenConnection( $this->getWriterIndex() );
            if ( $masterConn ) {
                $pos = $masterConn->getMasterPos();
            } else {
                $masterConn = $this->openConnection( $this->getWriterIndex(), self::DOMAIN_ANY );
                if ( !$masterConn ) {
                    throw new DBReplicationWaitError(
                        null,
                        "Could not obtain a master database connection to get the position"
                    );
                }
                $pos = $masterConn->getMasterPos();
                $this->closeConnection( $masterConn );
            }
        }
 
        if ( $pos instanceof DBMasterPos ) {
            $result = $conn->masterPosWait( $pos, $timeout );
            if ( $result == -1 || is_null( $result ) ) {
                $msg = __METHOD__ . ': timed out waiting on {host} pos {pos}';
                $this->replLogger->warning( $msg, [
                    'host' => $conn->getServer(),
                    'pos' => $pos,
                    'trace' => ( new RuntimeException() )->getTraceAsString()
                ] );
                $ok = false;
            } else {
                $this->replLogger->debug( __METHOD__ . ': done waiting' );
                $ok = true;
            }
        } else {
            $ok = false; // something is misconfigured
            $this->replLogger->error(
                __METHOD__ . ': could not get master pos for {host}',
                [
                    'host' => $conn->getServer(),
                    'trace' => ( new RuntimeException() )->getTraceAsString()
                ]
            );
        }
 
        return $ok;
    }
 
    public function setTransactionListener( $name, callable $callback = null ) {
        if ( $callback ) {
            $this->trxRecurringCallbacks[$name] = $callback;
        } else {
            unset( $this->trxRecurringCallbacks[$name] );
        }
        $this->forEachOpenMasterConnection(
            function ( IDatabase $conn ) use ( $name, $callback ) {
                $conn->setTransactionListener( $name, $callback );
            }
        );
    }
 
    public function setTableAliases( array $aliases ) {
        $this->tableAliases = $aliases;
    }
 
    public function setIndexAliases( array $aliases ) {
        $this->indexAliases = $aliases;
    }
 
    public function setDomainPrefix( $prefix ) {
        $this->setLocalDomainPrefix( $prefix );
    }
 
    public function setLocalDomainPrefix( $prefix ) {
        // Find connections to explicit foreign domains still marked as in-use...
        $domainsInUse = [];
        $this->forEachOpenConnection( function ( IDatabase $conn ) use ( &$domainsInUse ) {
            // Once reuseConnection() is called on a handle, its reference count goes from 1 to 0.
            // Until then, it is still in use by the caller (explicitly or via DBConnRef scope).
            if ( $conn->getLBInfo( 'foreignPoolRefCount' ) > 0 ) {
                $domainsInUse[] = $conn->getDomainID();
            }
        } );
 
        // Do not switch connections to explicit foreign domains unless marked as safe
        if ( $domainsInUse ) {
            $domains = implode( ', ', $domainsInUse );
            throw new DBUnexpectedError( null,
                "Foreign domain connections are still in use ($domains)." );
        }
 
        $this->setLocalDomain( new DatabaseDomain(
            $this->localDomain->getDatabase(),
            $this->localDomain->getSchema(),
            $prefix
        ) );
 
        // Update the prefix for all local connections...
        $this->forEachOpenConnection( function ( IDatabase $db ) use ( $prefix ) {
            if ( !$db->getLBInfo( 'foreign' ) ) {
                $db->tablePrefix( $prefix );
            }
        } );
    }
 
    public function redefineLocalDomain( $domain ) {
        $this->closeAll();
 
        $this->setLocalDomain( DatabaseDomain::newFromId( $domain ) );
    }
 
    private function setLocalDomain( DatabaseDomain $domain ) {
        $this->localDomain = $domain;
        // In case a caller assumes that the domain ID is simply <db>-<prefix>, which is almost
        // always true, gracefully handle the case when they fail to account for escaping.
        if ( $this->localDomain->getTablePrefix() != '' ) {
            $this->localDomainIdAlias =
                $this->localDomain->getDatabase() . '-' . $this->localDomain->getTablePrefix();
        } else {
            $this->localDomainIdAlias = $this->localDomain->getDatabase();
        }
    }
 
    function __destruct() {
        // Avoid connection leaks for sanity
        $this->disable();
    }
}
 
class_alias( LoadBalancer::class, 'LoadBalancer' );