Merge "Add Russian grammar forms to support Wikiversity"

[lhc/web/wiklou.git] / includes / libs / rdbms / database / Database.php

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

use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use Wikimedia\ScopedCallback;
use Wikimedia\Timestamp\ConvertibleTimestamp;
use Wikimedia;
use BagOStuff;
use HashBagOStuff;
use InvalidArgumentException;
use Exception;
use RuntimeException;

/**
 * Relational database abstraction object
 *
 * @ingroup Database
 * @since 1.28
 */
abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAwareInterface {
        /** Number of times to re-try an operation in case of deadlock */
        const DEADLOCK_TRIES = 4;
        /** Minimum time to wait before retry, in microseconds */
        const DEADLOCK_DELAY_MIN = 500000;
        /** Maximum time to wait before retry */
        const DEADLOCK_DELAY_MAX = 1500000;

        /** How long before it is worth doing a dummy query to test the connection */
        const PING_TTL = 1.0;
        const PING_QUERY = 'SELECT 1 AS ping';

        const TINY_WRITE_SEC = 0.010;
        const SLOW_WRITE_SEC = 0.500;
        const SMALL_WRITE_ROWS = 100;

        /** @var string Whether lock granularity is on the level of the entire database */
        const ATTR_DB_LEVEL_LOCKING = 'db-level-locking';

        /** @var string SQL query */
        protected $lastQuery = '';
        /** @var float|bool UNIX timestamp of last write query */
        protected $lastWriteTime = false;
        /** @var string|bool */
        protected $phpError = false;
        /** @var string */
        protected $server;
        /** @var string */
        protected $user;
        /** @var string */
        protected $password;
        /** @var string */
        protected $dbName;
        /** @var array[] Map of (table => (dbname, schema, prefix) map) */
        protected $tableAliases = [];
        /** @var string[] Map of (index alias => index) */
        protected $indexAliases = [];
        /** @var bool Whether this PHP instance is for a CLI script */
        protected $cliMode;
        /** @var string Agent name for query profiling */
        protected $agent;

        /** @var BagOStuff APC cache */
        protected $srvCache;
        /** @var LoggerInterface */
        protected $connLogger;
        /** @var LoggerInterface */
        protected $queryLogger;
        /** @var callback Error logging callback */
        protected $errorLogger;

        /** @var resource|null Database connection */
        protected $conn = null;
        /** @var bool */
        protected $opened = false;

        /** @var array[] List of (callable, method name) */
        protected $trxIdleCallbacks = [];
        /** @var array[] List of (callable, method name) */
        protected $trxPreCommitCallbacks = [];
        /** @var array[] List of (callable, method name) */
        protected $trxEndCallbacks = [];
        /** @var callable[] Map of (name => callable) */
        protected $trxRecurringCallbacks = [];
        /** @var bool Whether to suppress triggering of transaction end callbacks */
        protected $trxEndCallbacksSuppressed = false;

        /** @var string */
        protected $tablePrefix = '';
        /** @var string */
        protected $schema = '';
        /** @var int */
        protected $flags;
        /** @var array */
        protected $lbInfo = [];
        /** @var array|bool */
        protected $schemaVars = false;
        /** @var array */
        protected $sessionVars = [];
        /** @var array|null */
        protected $preparedArgs;
        /** @var string|bool|null Stashed value of html_errors INI setting */
        protected $htmlErrors;
        /** @var string */
        protected $delimiter = ';';
        /** @var DatabaseDomain */
        protected $currentDomain;
        /** @var integer|null Rows affected by the last query to query() or its CRUD wrappers */
        protected $affectedRowCount;

        /**
         * Either 1 if a transaction is active or 0 otherwise.
         * The other Trx fields may not be meaningfull if this is 0.
         *
         * @var int
         */
        protected $trxLevel = 0;
        /**
         * Either a short hexidecimal string if a transaction is active or ""
         *
         * @var string
         * @see Database::trxLevel
         */
        protected $trxShortId = '';
        /**
         * The UNIX time that the transaction started. Callers can assume that if
         * snapshot isolation is used, then the data is *at least* up to date to that
         * point (possibly more up-to-date since the first SELECT defines the snapshot).
         *
         * @var float|null
         * @see Database::trxLevel
         */
        private $trxTimestamp = null;
        /** @var float Lag estimate at the time of BEGIN */
        private $trxReplicaLag = null;
        /**
         * Remembers the function name given for starting the most recent transaction via begin().
         * Used to provide additional context for error reporting.
         *
         * @var string
         * @see Database::trxLevel
         */
        private $trxFname = null;
        /**
         * Record if possible write queries were done in the last transaction started
         *
         * @var bool
         * @see Database::trxLevel
         */
        private $trxDoneWrites = false;
        /**
         * Record if the current transaction was started implicitly due to DBO_TRX being set.
         *
         * @var bool
         * @see Database::trxLevel
         */
        private $trxAutomatic = false;
        /**
         * Array of levels of atomicity within transactions
         *
         * @var array
         */
        private $trxAtomicLevels = [];
        /**
         * Record if the current transaction was started implicitly by Database::startAtomic
         *
         * @var bool
         */
        private $trxAutomaticAtomic = false;
        /**
         * Track the write query callers of the current transaction
         *
         * @var string[]
         */
        private $trxWriteCallers = [];
        /**
         * @var float Seconds spent in write queries for the current transaction
         */
        private $trxWriteDuration = 0.0;
        /**
         * @var int Number of write queries for the current transaction
         */
        private $trxWriteQueryCount = 0;
        /**
         * @var int Number of rows affected by write queries for the current transaction
         */
        private $trxWriteAffectedRows = 0;
        /**
         * @var float Like trxWriteQueryCount but excludes lock-bound, easy to replicate, queries
         */
        private $trxWriteAdjDuration = 0.0;
        /**
         * @var int Number of write queries counted in trxWriteAdjDuration
         */
        private $trxWriteAdjQueryCount = 0;
        /**
         * @var float RTT time estimate
         */
        private $rttEstimate = 0.0;

        /** @var array Map of (name => 1) for locks obtained via lock() */
        private $namedLocksHeld = [];
        /** @var array Map of (table name => 1) for TEMPORARY tables */
        protected $sessionTempTables = [];

        /** @var IDatabase|null Lazy handle to the master DB this server replicates from */
        private $lazyMasterHandle;

        /** @var float UNIX timestamp */
        protected $lastPing = 0.0;

        /** @var int[] Prior flags member variable values */
        private $priorFlags = [];

        /** @var object|string Class name or object With profileIn/profileOut methods */
        protected $profiler;
        /** @var TransactionProfiler */
        protected $trxProfiler;

        /** @var int */
        protected $nonNativeInsertSelectBatchSize = 10000;

        /**
         * Constructor and database handle and attempt to connect to the DB server
         *
         * IDatabase classes should not be constructed directly in external
         * code. Database::factory() should be used instead.
         *
         * @param array $params Parameters passed from Database::factory()
         */
        function __construct( array $params ) {
                $server = $params['host'];
                $user = $params['user'];
                $password = $params['password'];
                $dbName = $params['dbname'];

                $this->schema = $params['schema'];
                $this->tablePrefix = $params['tablePrefix'];

                $this->cliMode = $params['cliMode'];
                // Agent name is added to SQL queries in a comment, so make sure it can't break out
                $this->agent = str_replace( '/', '-', $params['agent'] );

                $this->flags = $params['flags'];
                if ( $this->flags & self::DBO_DEFAULT ) {
                        if ( $this->cliMode ) {
                                $this->flags &= ~self::DBO_TRX;
                        } else {
                                $this->flags |= self::DBO_TRX;
                        }
                }

                $this->sessionVars = $params['variables'];

                $this->srvCache = isset( $params['srvCache'] )
                        ? $params['srvCache']
                        : new HashBagOStuff();

                $this->profiler = $params['profiler'];
                $this->trxProfiler = $params['trxProfiler'];
                $this->connLogger = $params['connLogger'];
                $this->queryLogger = $params['queryLogger'];
                $this->errorLogger = $params['errorLogger'];

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

                // Set initial dummy domain until open() sets the final DB/prefix
                $this->currentDomain = DatabaseDomain::newUnspecified();

                if ( $user ) {
                        $this->open( $server, $user, $password, $dbName );
                } elseif ( $this->requiresDatabaseUser() ) {
                        throw new InvalidArgumentException( "No database user provided." );
                }

                // Set the domain object after open() sets the relevant fields
                if ( $this->dbName != '' ) {
                        // Domains with server scope but a table prefix are not used by IDatabase classes
                        $this->currentDomain = new DatabaseDomain( $this->dbName, null, $this->tablePrefix );
                }
        }

        /**
         * Construct a Database subclass instance given a database type and parameters
         *
         * This also connects to the database immediately upon object construction
         *
         * @param string $dbType A possible DB type (sqlite, mysql, postgres,...)
         * @param array $p Parameter map with keys:
         *   - host : The hostname of the DB server
         *   - user : The name of the database user the client operates under
         *   - password : The password for the database user
         *   - dbname : The name of the database to use where queries do not specify one.
         *      The database must exist or an error might be thrown. Setting this to the empty string
         *      will avoid any such errors and make the handle have no implicit database scope. This is
         *      useful for queries like SHOW STATUS, CREATE DATABASE, or DROP DATABASE. Note that a
         *      "database" in Postgres is rougly equivalent to an entire MySQL server. This the domain
         *      in which user names and such are defined, e.g. users are database-specific in Postgres.
         *   - schema : The database schema to use (if supported). A "schema" in Postgres is roughly
         *      equivalent to a "database" in MySQL. Note that MySQL and SQLite do not use schemas.
         *   - tablePrefix : Optional table prefix that is implicitly added on to all table names
         *      recognized in queries. This can be used in place of schemas for handle site farms.
         *   - flags : Optional bitfield of DBO_* constants that define connection, protocol,
         *      buffering, and transaction behavior. It is STRONGLY adviced to leave the DBO_DEFAULT
         *      flag in place UNLESS this this database simply acts as a key/value store.
         *   - driver: Optional name of a specific DB client driver. For MySQL, there is only the
         *      'mysqli' driver; the old one 'mysql' has been removed.
         *   - variables: Optional map of session variables to set after connecting. This can be
         *      used to adjust lock timeouts or encoding modes and the like.
         *   - connLogger: Optional PSR-3 logger interface instance.
         *   - queryLogger: Optional PSR-3 logger interface instance.
         *   - profiler: Optional class name or object with profileIn()/profileOut() methods.
         *      These will be called in query(), using a simplified version of the SQL that also
         *      includes the agent as a SQL comment.
         *   - trxProfiler: Optional TransactionProfiler instance.
         *   - errorLogger: Optional callback that takes an Exception and logs it.
         *   - cliMode: Whether to consider the execution context that of a CLI script.
         *   - agent: Optional name used to identify the end-user in query profiling/logging.
         *   - srvCache: Optional BagOStuff instance to an APC-style cache.
         *   - nonNativeInsertSelectBatchSize: Optional batch size for non-native INSERT SELECT emulation.
         * @return Database|null If the database driver or extension cannot be found
         * @throws InvalidArgumentException If the database driver or extension cannot be found
         * @since 1.18
         */
        final public static function factory( $dbType, $p = [] ) {
                $class = self::getClass( $dbType, isset( $p['driver'] ) ? $p['driver'] : null );

                if ( class_exists( $class ) && is_subclass_of( $class, IDatabase::class ) ) {
                        // Resolve some defaults for b/c
                        $p['host'] = isset( $p['host'] ) ? $p['host'] : false;
                        $p['user'] = isset( $p['user'] ) ? $p['user'] : false;
                        $p['password'] = isset( $p['password'] ) ? $p['password'] : false;
                        $p['dbname'] = isset( $p['dbname'] ) ? $p['dbname'] : false;
                        $p['flags'] = isset( $p['flags'] ) ? $p['flags'] : 0;
                        $p['variables'] = isset( $p['variables'] ) ? $p['variables'] : [];
                        $p['tablePrefix'] = isset( $p['tablePrefix'] ) ? $p['tablePrefix'] : '';
                        $p['schema'] = isset( $p['schema'] ) ? $p['schema'] : '';
                        $p['cliMode'] = isset( $p['cliMode'] )
                                ? $p['cliMode']
                                : ( PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg' );
                        $p['agent'] = isset( $p['agent'] ) ? $p['agent'] : '';
                        if ( !isset( $p['connLogger'] ) ) {
                                $p['connLogger'] = new NullLogger();
                        }
                        if ( !isset( $p['queryLogger'] ) ) {
                                $p['queryLogger'] = new NullLogger();
                        }
                        $p['profiler'] = isset( $p['profiler'] ) ? $p['profiler'] : null;
                        if ( !isset( $p['trxProfiler'] ) ) {
                                $p['trxProfiler'] = new TransactionProfiler();
                        }
                        if ( !isset( $p['errorLogger'] ) ) {
                                $p['errorLogger'] = function ( Exception $e ) {
                                        trigger_error( get_class( $e ) . ': ' . $e->getMessage(), E_USER_WARNING );
                                };
                        }

                        $conn = new $class( $p );
                } else {
                        $conn = null;
                }

                return $conn;
        }

        /**
         * @param string $dbType A possible DB type (sqlite, mysql, postgres,...)
         * @param string|null $driver Optional name of a specific DB client driver
         * @return array Map of (Database::ATTRIBUTE_* constant => value) for all such constants
         * @throws InvalidArgumentException
         * @since 1.31
         */
        final public static function attributesFromType( $dbType, $driver = null ) {
                static $defaults = [ self::ATTR_DB_LEVEL_LOCKING => false ];

                $class = self::getClass( $dbType, $driver );

                return call_user_func( [ $class, 'getAttributes' ] ) + $defaults;
        }

        /**
         * @param string $dbType A possible DB type (sqlite, mysql, postgres,...)
         * @param string|null $driver Optional name of a specific DB client driver
         * @return string Database subclass name to use
         * @throws InvalidArgumentException
         */
        private static function getClass( $dbType, $driver = null ) {
                // For database types with built-in support, the below maps type to IDatabase
                // implementations. For types with multipe driver implementations (PHP extensions),
                // an array can be used, keyed by extension name. In case of an array, the
                // optional 'driver' parameter can be used to force a specific driver. Otherwise,
                // we auto-detect the first available driver. For types without built-in support,
                // an class named "Database<Type>" us used, eg. DatabaseFoo for type 'foo'.
                static $builtinTypes = [
                        'mssql' => DatabaseMssql::class,
                        'mysql' => [ 'mysqli' => DatabaseMysqli::class ],
                        'sqlite' => DatabaseSqlite::class,
                        'postgres' => DatabasePostgres::class,
                ];

                $dbType = strtolower( $dbType );
                $class = false;

                if ( isset( $builtinTypes[$dbType] ) ) {
                        $possibleDrivers = $builtinTypes[$dbType];
                        if ( is_string( $possibleDrivers ) ) {
                                $class = $possibleDrivers;
                        } else {
                                if ( (string)$driver !== '' ) {
                                        if ( !isset( $possibleDrivers[$driver] ) ) {
                                                throw new InvalidArgumentException( __METHOD__ .
                                                        " type '$dbType' does not support driver '{$driver}'" );
                                        } else {
                                                $class = $possibleDrivers[$driver];
                                        }
                                } else {
                                        foreach ( $possibleDrivers as $posDriver => $possibleClass ) {
                                                if ( extension_loaded( $posDriver ) ) {
                                                        $class = $possibleClass;
                                                        break;
                                                }
                                        }
                                }
                        }
                } else {
                        $class = 'Database' . ucfirst( $dbType );
                }

                if ( $class === false ) {
                        throw new InvalidArgumentException( __METHOD__ .
                                " no viable database extension found for type '$dbType'" );
                }

                return $class;
        }

        /**
         * @return array Map of (Database::ATTRIBUTE_* constant => value
         * @since 1.31
         */
        protected static function getAttributes() {
                return [];
        }

        /**
         * Set the PSR-3 logger interface to use for query logging. (The logger
         * interfaces for connection logging and error logging can be set with the
         * constructor.)
         *
         * @param LoggerInterface $logger
         */
        public function setLogger( LoggerInterface $logger ) {
                $this->queryLogger = $logger;
        }

        public function getServerInfo() {
                return $this->getServerVersion();
        }

        public function bufferResults( $buffer = null ) {
                $res = !$this->getFlag( self::DBO_NOBUFFER );
                if ( $buffer !== null ) {
                        $buffer
                                ? $this->clearFlag( self::DBO_NOBUFFER )
                                : $this->setFlag( self::DBO_NOBUFFER );
                }

                return $res;
        }

        /**
         * Turns on (false) or off (true) the automatic generation and sending
         * of a "we're sorry, but there has been a database error" page on
         * database errors. Default is on (false). When turned off, the
         * code should use lastErrno() and lastError() to handle the
         * situation as appropriate.
         *
         * Do not use this function outside of the Database classes.
         *
         * @param null|bool $ignoreErrors
         * @return bool The previous value of the flag.
         */
        protected function ignoreErrors( $ignoreErrors = null ) {
                $res = $this->getFlag( self::DBO_IGNORE );
                if ( $ignoreErrors !== null ) {
                        // setFlag()/clearFlag() do not allow DBO_IGNORE changes for sanity
                        if ( $ignoreErrors ) {
                                $this->flags |= self::DBO_IGNORE;
                        } else {
                                $this->flags &= ~self::DBO_IGNORE;
                        }
                }

                return $res;
        }

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

        public function trxTimestamp() {
                return $this->trxLevel ? $this->trxTimestamp : null;
        }

        public function tablePrefix( $prefix = null ) {
                $old = $this->tablePrefix;
                if ( $prefix !== null ) {
                        $this->tablePrefix = $prefix;
                        $this->currentDomain = ( $this->dbName != '' )
                                ? new DatabaseDomain( $this->dbName, null, $this->tablePrefix )
                                : DatabaseDomain::newUnspecified();
                }

                return $old;
        }

        public function dbSchema( $schema = null ) {
                $old = $this->schema;
                if ( $schema !== null ) {
                        $this->schema = $schema;
                }

                return $old;
        }

        public function getLBInfo( $name = null ) {
                if ( is_null( $name ) ) {
                        return $this->lbInfo;
                } else {
                        if ( array_key_exists( $name, $this->lbInfo ) ) {
                                return $this->lbInfo[$name];
                        } else {
                                return null;
                        }
                }
        }

        public function setLBInfo( $name, $value = null ) {
                if ( is_null( $value ) ) {
                        $this->lbInfo = $name;
                } else {
                        $this->lbInfo[$name] = $value;
                }
        }

        public function setLazyMasterHandle( IDatabase $conn ) {
                $this->lazyMasterHandle = $conn;
        }

        /**
         * @return IDatabase|null
         * @see setLazyMasterHandle()
         * @since 1.27
         */
        protected function getLazyMasterHandle() {
                return $this->lazyMasterHandle;
        }

        public function implicitGroupby() {
                return true;
        }

        public function implicitOrderby() {
                return true;
        }

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

        public function doneWrites() {
                return (bool)$this->lastWriteTime;
        }

        public function lastDoneWrites() {
                return $this->lastWriteTime ?: false;
        }

        public function writesPending() {
                return $this->trxLevel && $this->trxDoneWrites;
        }

        public function writesOrCallbacksPending() {
                return $this->trxLevel && (
                        $this->trxDoneWrites ||
                        $this->trxIdleCallbacks ||
                        $this->trxPreCommitCallbacks ||
                        $this->trxEndCallbacks
                );
        }

        public function pendingWriteQueryDuration( $type = self::ESTIMATE_TOTAL ) {
                if ( !$this->trxLevel ) {
                        return false;
                } elseif ( !$this->trxDoneWrites ) {
                        return 0.0;
                }

                switch ( $type ) {
                        case self::ESTIMATE_DB_APPLY:
                                $this->ping( $rtt );
                                $rttAdjTotal = $this->trxWriteAdjQueryCount * $rtt;
                                $applyTime = max( $this->trxWriteAdjDuration - $rttAdjTotal, 0 );
                                // For omitted queries, make them count as something at least
                                $omitted = $this->trxWriteQueryCount - $this->trxWriteAdjQueryCount;
                                $applyTime += self::TINY_WRITE_SEC * $omitted;

                                return $applyTime;
                        default: // everything
                                return $this->trxWriteDuration;
                }
        }

        public function pendingWriteCallers() {
                return $this->trxLevel ? $this->trxWriteCallers : [];
        }

        public function pendingWriteRowsAffected() {
                return $this->trxWriteAffectedRows;
        }

        /**
         * Get the list of method names that have pending write queries or callbacks
         * for this transaction
         *
         * @return array
         */
        protected function pendingWriteAndCallbackCallers() {
                if ( !$this->trxLevel ) {
                        return [];
                }

                $fnames = $this->trxWriteCallers;
                foreach ( [
                        $this->trxIdleCallbacks,
                        $this->trxPreCommitCallbacks,
                        $this->trxEndCallbacks
                ] as $callbacks ) {
                        foreach ( $callbacks as $callback ) {
                                $fnames[] = $callback[1];
                        }
                }

                return $fnames;
        }

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

        public function setFlag( $flag, $remember = self::REMEMBER_NOTHING ) {
                if ( ( $flag & self::DBO_IGNORE ) ) {
                        throw new \UnexpectedValueException( "Modifying DBO_IGNORE is not allowed." );
                }

                if ( $remember === self::REMEMBER_PRIOR ) {
                        array_push( $this->priorFlags, $this->flags );
                }
                $this->flags |= $flag;
        }

        public function clearFlag( $flag, $remember = self::REMEMBER_NOTHING ) {
                if ( ( $flag & self::DBO_IGNORE ) ) {
                        throw new \UnexpectedValueException( "Modifying DBO_IGNORE is not allowed." );
                }

                if ( $remember === self::REMEMBER_PRIOR ) {
                        array_push( $this->priorFlags, $this->flags );
                }
                $this->flags &= ~$flag;
        }

        public function restoreFlags( $state = self::RESTORE_PRIOR ) {
                if ( !$this->priorFlags ) {
                        return;
                }

                if ( $state === self::RESTORE_INITIAL ) {
                        $this->flags = reset( $this->priorFlags );
                        $this->priorFlags = [];
                } else {
                        $this->flags = array_pop( $this->priorFlags );
                }
        }

        public function getFlag( $flag ) {
                return !!( $this->flags & $flag );
        }

        /**
         * @param string $name Class field name
         * @return mixed
         * @deprecated Since 1.28
         */
        public function getProperty( $name ) {
                return $this->$name;
        }

        public function getDomainID() {
                return $this->currentDomain->getId();
        }

        final public function getWikiID() {
                return $this->getDomainID();
        }

        /**
         * Get information about an index into an object
         * @param string $table Table name
         * @param string $index Index name
         * @param string $fname Calling function name
         * @return mixed Database-specific index description class or false if the index does not exist
         */
        abstract function indexInfo( $table, $index, $fname = __METHOD__ );

        /**
         * Wrapper for addslashes()
         *
         * @param string $s String to be slashed.
         * @return string Slashed string.
         */
        abstract function strencode( $s );

        /**
         * Set a custom error handler for logging errors during database connection
         */
        protected function installErrorHandler() {
                $this->phpError = false;
                $this->htmlErrors = ini_set( 'html_errors', '0' );
                set_error_handler( [ $this, 'connectionErrorLogger' ] );
        }

        /**
         * Restore the previous error handler and return the last PHP error for this DB
         *
         * @return bool|string
         */
        protected function restoreErrorHandler() {
                restore_error_handler();
                if ( $this->htmlErrors !== false ) {
                        ini_set( 'html_errors', $this->htmlErrors );
                }

                return $this->getLastPHPError();
        }

        /**
         * @return string|bool Last PHP error for this DB (typically connection errors)
         */
        protected function getLastPHPError() {
                if ( $this->phpError ) {
                        $error = preg_replace( '!\[<a.*</a>\]!', '', $this->phpError );
                        $error = preg_replace( '!^.*?:\s?(.*)$!', '$1', $error );

                        return $error;
                }

                return false;
        }

        /**
         * Error handler for logging errors during database connection
         * This method should not be used outside of Database classes
         *
         * @param int $errno
         * @param string $errstr
         */
        public function connectionErrorLogger( $errno, $errstr ) {
                $this->phpError = $errstr;
        }

        /**
         * Create a log context to pass to PSR-3 logger functions.
         *
         * @param array $extras Additional data to add to context
         * @return array
         */
        protected function getLogContext( array $extras = [] ) {
                return array_merge(
                        [
                                'db_server' => $this->server,
                                'db_name' => $this->dbName,
                                'db_user' => $this->user,
                        ],
                        $extras
                );
        }

        public function close() {
                if ( $this->conn ) {
                        // Resolve any dangling transaction first
                        if ( $this->trxLevel() ) {
                                // Meaningful transactions should ideally have been resolved by now
                                if ( $this->writesOrCallbacksPending() ) {
                                        $this->queryLogger->warning(
                                                __METHOD__ . ": writes or callbacks still pending.",
                                                [ 'trace' => ( new RuntimeException() )->getTraceAsString() ]
                                        );
                                }
                                // Check if it is possible to properly commit and trigger callbacks
                                if ( $this->trxEndCallbacksSuppressed ) {
                                        throw new DBUnexpectedError(
                                                $this,
                                                __METHOD__ . ': callbacks are suppressed; cannot properly commit.'
                                        );
                                }
                                // Commit the changes and run any callbacks as needed
                                $this->commit( __METHOD__, self::FLUSHING_INTERNAL );
                        }
                        // Close the actual connection in the binding handle
                        $closed = $this->closeConnection();
                        $this->conn = false;
                        // Sanity check that no callbacks are dangling
                        if (
                                $this->trxIdleCallbacks || $this->trxPreCommitCallbacks || $this->trxEndCallbacks
                        ) {
                                throw new RuntimeException( "Transaction callbacks still pending." );
                        }
                } else {
                        $closed = true; // already closed; nothing to do
                }

                $this->opened = false;

                return $closed;
        }

        /**
         * Make sure isOpen() returns true as a sanity check
         *
         * @throws DBUnexpectedError
         */
        protected function assertOpen() {
                if ( !$this->isOpen() ) {
                        throw new DBUnexpectedError( $this, "DB connection was already closed." );
                }
        }

        /**
         * Closes underlying database connection
         * @since 1.20
         * @return bool Whether connection was closed successfully
         */
        abstract protected function closeConnection();

        public function reportConnectionError( $error = 'Unknown error' ) {
                $myError = $this->lastError();
                if ( $myError ) {
                        $error = $myError;
                }

                # New method
                throw new DBConnectionError( $this, $error );
        }

        /**
         * The DBMS-dependent part of query()
         *
         * @param string $sql SQL query.
         * @return ResultWrapper|bool Result object to feed to fetchObject,
         *   fetchRow, ...; or false on failure
         */
        abstract protected function doQuery( $sql );

        /**
         * Determine whether a query writes to the DB.
         * Should return true if unsure.
         *
         * @param string $sql
         * @return bool
         */
        protected function isWriteQuery( $sql ) {
                return !preg_match(
                        '/^(?:SELECT|BEGIN|ROLLBACK|COMMIT|SET|SHOW|EXPLAIN|\(SELECT)\b/i', $sql );
        }

        /**
         * @param string $sql
         * @return string|null
         */
        protected function getQueryVerb( $sql ) {
                return preg_match( '/^\s*([a-z]+)/i', $sql, $m ) ? strtoupper( $m[1] ) : null;
        }

        /**
         * Determine whether a SQL statement is sensitive to isolation level.
         * A SQL statement is considered transactable if its result could vary
         * depending on the transaction isolation level. Operational commands
         * such as 'SET' and 'SHOW' are not considered to be transactable.
         *
         * @param string $sql
         * @return bool
         */
        protected function isTransactableQuery( $sql ) {
                return !in_array(
                        $this->getQueryVerb( $sql ),
                        [ 'BEGIN', 'COMMIT', 'ROLLBACK', 'SHOW', 'SET', 'CREATE', 'ALTER' ],
                        true
                );
        }

        /**
         * @param string $sql A SQL query
         * @return bool Whether $sql is SQL for TEMPORARY table operation
         */
        protected function registerTempTableOperation( $sql ) {
                if ( preg_match(
                        '/^CREATE\s+TEMPORARY\s+TABLE\s+(?:IF\s+NOT\s+EXISTS\s+)?[`"\']?(\w+)[`"\']?/i',
                        $sql,
                        $matches
                ) ) {
                        $this->sessionTempTables[$matches[1]] = 1;

                        return true;
                } elseif ( preg_match(
                        '/^DROP\s+(?:TEMPORARY\s+)?TABLE\s+(?:IF\s+EXISTS\s+)?[`"\']?(\w+)[`"\']?/i',
                        $sql,
                        $matches
                ) ) {
                        $isTemp = isset( $this->sessionTempTables[$matches[1]] );
                        unset( $this->sessionTempTables[$matches[1]] );

                        return $isTemp;
                } elseif ( preg_match(
                        '/^TRUNCATE\s+(?:TEMPORARY\s+)?TABLE\s+(?:IF\s+EXISTS\s+)?[`"\']?(\w+)[`"\']?/i',
                        $sql,
                        $matches
                ) ) {
                        return isset( $this->sessionTempTables[$matches[1]] );
                } elseif ( preg_match(
                        '/^(?:INSERT\s+(?:\w+\s+)?INTO|UPDATE|DELETE\s+FROM)\s+[`"\']?(\w+)[`"\']?/i',
                        $sql,
                        $matches
                ) ) {
                        return isset( $this->sessionTempTables[$matches[1]] );
                }

                return false;
        }

        public function query( $sql, $fname = __METHOD__, $tempIgnore = false ) {
                $priorWritesPending = $this->writesOrCallbacksPending();
                $this->lastQuery = $sql;

                $isWrite = $this->isWriteQuery( $sql );
                if ( $isWrite ) {
                        $isNonTempWrite = !$this->registerTempTableOperation( $sql );
                } else {
                        $isNonTempWrite = false;
                }

                if ( $isWrite ) {
                        if ( $this->getLBInfo( 'replica' ) === true ) {
                                throw new DBError(
                                        $this,
                                        'Write operations are not allowed on replica database connections.'
                                );
                        }
                        # In theory, non-persistent writes are allowed in read-only mode, but due to things
                        # like https://bugs.mysql.com/bug.php?id=33669 that might not work anyway...
                        $reason = $this->getReadOnlyReason();
                        if ( $reason !== false ) {
                                throw new DBReadOnlyError( $this, "Database is read-only: $reason" );
                        }
                        # Set a flag indicating that writes have been done
                        $this->lastWriteTime = microtime( true );
                }

                # Add trace comment to the begin of the sql string, right after the operator.
                # Or, for one-word queries (like "BEGIN" or COMMIT") add it to the end (T44598)
                $commentedSql = preg_replace( '/\s|$/', " /* $fname {$this->agent} */ ", $sql, 1 );

                # Start implicit transactions that wrap the request if DBO_TRX is enabled
                if ( !$this->trxLevel && $this->getFlag( self::DBO_TRX )
                        && $this->isTransactableQuery( $sql )
                ) {
                        $this->begin( __METHOD__ . " ($fname)", self::TRANSACTION_INTERNAL );
                        $this->trxAutomatic = true;
                }

                # Keep track of whether the transaction has write queries pending
                if ( $this->trxLevel && !$this->trxDoneWrites && $isWrite ) {
                        $this->trxDoneWrites = true;
                        $this->trxProfiler->transactionWritingIn(
                                $this->server, $this->dbName, $this->trxShortId );
                }

                if ( $this->getFlag( self::DBO_DEBUG ) ) {
                        $this->queryLogger->debug( "{$this->dbName} {$commentedSql}" );
                }

                # Avoid fatals if close() was called
                $this->assertOpen();

                # Send the query to the server
                $ret = $this->doProfiledQuery( $sql, $commentedSql, $isNonTempWrite, $fname );

                # Try reconnecting if the connection was lost
                if ( false === $ret && $this->wasErrorReissuable() ) {
                        $recoverable = $this->canRecoverFromDisconnect( $sql, $priorWritesPending );
                        # Stash the last error values before anything might clear them
                        $lastError = $this->lastError();
                        $lastErrno = $this->lastErrno();
                        # Update state tracking to reflect transaction loss due to disconnection
                        $this->handleSessionLoss();
                        if ( $this->reconnect() ) {
                                $msg = __METHOD__ . ': lost connection to {dbserver}; reconnected';
                                $params = [ 'dbserver' => $this->getServer() ];
                                $this->connLogger->warning( $msg, $params );
                                $this->queryLogger->warning( $msg, $params +
                                        [ 'trace' => ( new RuntimeException() )->getTraceAsString() ] );

                                if ( $recoverable ) {
                                        # Should be safe to silently retry the query
                                        $ret = $this->doProfiledQuery( $sql, $commentedSql, $isNonTempWrite, $fname );
                                } else {
                                        # Callers may catch the exception and continue to use the DB
                                        $this->reportQueryError( $lastError, $lastErrno, $sql, $fname );
                                }
                        } else {
                                $msg = __METHOD__ . ': lost connection to {dbserver} permanently';
                                $this->connLogger->error( $msg, [ 'dbserver' => $this->getServer() ] );
                        }
                }

                if ( false === $ret ) {
                        # Deadlocks cause the entire transaction to abort, not just the statement.
                        # https://dev.mysql.com/doc/refman/5.7/en/innodb-error-handling.html
                        # https://www.postgresql.org/docs/9.1/static/explicit-locking.html
                        if ( $this->wasDeadlock() ) {
                                if ( $this->explicitTrxActive() || $priorWritesPending ) {
                                        $tempIgnore = false; // not recoverable
                                }
                                # Update state tracking to reflect transaction loss
                                $this->handleSessionLoss();
                        }

                        $this->reportQueryError(
                                $this->lastError(), $this->lastErrno(), $sql, $fname, $tempIgnore );
                }

                $res = $this->resultObject( $ret );

                return $res;
        }

        /**
         * Wrapper for query() that also handles profiling, logging, and affected row count updates
         *
         * @param string $sql Original SQL query
         * @param string $commentedSql SQL query with debugging/trace comment
         * @param bool $isWrite Whether the query is a (non-temporary) write operation
         * @param string $fname Name of the calling function
         * @return bool|ResultWrapper True for a successful write query, ResultWrapper
         *     object for a successful read query, or false on failure
         */
        private function doProfiledQuery( $sql, $commentedSql, $isWrite, $fname ) {
                $isMaster = !is_null( $this->getLBInfo( 'master' ) );
                # generalizeSQL() will probably cut down the query to reasonable
                # logging size most of the time. The substr is really just a sanity check.
                if ( $isMaster ) {
                        $queryProf = 'query-m: ' . substr( self::generalizeSQL( $sql ), 0, 255 );
                } else {
                        $queryProf = 'query: ' . substr( self::generalizeSQL( $sql ), 0, 255 );
                }

                # Include query transaction state
                $queryProf .= $this->trxShortId ? " [TRX#{$this->trxShortId}]" : "";

                $startTime = microtime( true );
                if ( $this->profiler ) {
                        call_user_func( [ $this->profiler, 'profileIn' ], $queryProf );
                }
                $this->affectedRowCount = null;
                $ret = $this->doQuery( $commentedSql );
                $this->affectedRowCount = $this->affectedRows();
                if ( $this->profiler ) {
                        call_user_func( [ $this->profiler, 'profileOut' ], $queryProf );
                }
                $queryRuntime = max( microtime( true ) - $startTime, 0.0 );

                unset( $queryProfSection ); // profile out (if set)

                if ( $ret !== false ) {
                        $this->lastPing = $startTime;
                        if ( $isWrite && $this->trxLevel ) {
                                $this->updateTrxWriteQueryTime( $sql, $queryRuntime, $this->affectedRows() );
                                $this->trxWriteCallers[] = $fname;
                        }
                }

                if ( $sql === self::PING_QUERY ) {
                        $this->rttEstimate = $queryRuntime;
                }

                $this->trxProfiler->recordQueryCompletion(
                        $queryProf, $startTime, $isWrite, $this->affectedRows()
                );
                $this->queryLogger->debug( $sql, [
                        'method' => $fname,
                        'master' => $isMaster,
                        'runtime' => $queryRuntime,
                ] );

                return $ret;
        }

        /**
         * Update the estimated run-time of a query, not counting large row lock times
         *
         * LoadBalancer can be set to rollback transactions that will create huge replication
         * lag. It bases this estimate off of pendingWriteQueryDuration(). Certain simple
         * queries, like inserting a row can take a long time due to row locking. This method
         * uses some simple heuristics to discount those cases.
         *
         * @param string $sql A SQL write query
         * @param float $runtime Total runtime, including RTT
         * @param int $affected Affected row count
         */
        private function updateTrxWriteQueryTime( $sql, $runtime, $affected ) {
                // Whether this is indicative of replica DB runtime (except for RBR or ws_repl)
                $indicativeOfReplicaRuntime = true;
                if ( $runtime > self::SLOW_WRITE_SEC ) {
                        $verb = $this->getQueryVerb( $sql );
                        // insert(), upsert(), replace() are fast unless bulky in size or blocked on locks
                        if ( $verb === 'INSERT' ) {
                                $indicativeOfReplicaRuntime = $this->affectedRows() > self::SMALL_WRITE_ROWS;
                        } elseif ( $verb === 'REPLACE' ) {
                                $indicativeOfReplicaRuntime = $this->affectedRows() > self::SMALL_WRITE_ROWS / 2;
                        }
                }

                $this->trxWriteDuration += $runtime;
                $this->trxWriteQueryCount += 1;
                $this->trxWriteAffectedRows += $affected;
                if ( $indicativeOfReplicaRuntime ) {
                        $this->trxWriteAdjDuration += $runtime;
                        $this->trxWriteAdjQueryCount += 1;
                }
        }

        /**
         * Determine whether or not it is safe to retry queries after a database
         * connection is lost
         *
         * @param string $sql SQL query
         * @param bool $priorWritesPending Whether there is a transaction open with
         *     possible write queries or transaction pre-commit/idle callbacks
         *     waiting on it to finish.
         * @return bool True if it is safe to retry the query, false otherwise
         */
        private function canRecoverFromDisconnect( $sql, $priorWritesPending ) {
                # Transaction dropped; this can mean lost writes, or REPEATABLE-READ snapshots.
                # Dropped connections also mean that named locks are automatically released.
                # Only allow error suppression in autocommit mode or when the lost transaction
                # didn't matter anyway (aside from DBO_TRX snapshot loss).
                if ( $this->namedLocksHeld ) {
                        return false; // possible critical section violation
                } elseif ( $sql === 'COMMIT' ) {
                        return !$priorWritesPending; // nothing written anyway? (T127428)
                } elseif ( $sql === 'ROLLBACK' ) {
                        return true; // transaction lost...which is also what was requested :)
                } elseif ( $this->explicitTrxActive() ) {
                        return false; // don't drop atomocity
                } elseif ( $priorWritesPending ) {
                        return false; // prior writes lost from implicit transaction
                }

                return true;
        }

        /**
         * Clean things up after transaction loss due to disconnection
         *
         * @return null|Exception
         */
        private function handleSessionLoss() {
                $this->trxLevel = 0;
                $this->trxIdleCallbacks = []; // T67263; transaction already lost
                $this->trxPreCommitCallbacks = []; // T67263; transaction already lost
                $this->sessionTempTables = [];
                $this->namedLocksHeld = [];

                // Note: if callback suppression is set then some *Callbacks arrays are not cleared here
                $e = null;
                try {
                        // Handle callbacks in trxEndCallbacks
                        $this->runOnTransactionIdleCallbacks( self::TRIGGER_ROLLBACK );
                } catch ( Exception $ex ) {
                        // Already logged; move on...
                        $e = $e ?: $ex;
                }
                try {
                        // Handle callbacks in trxRecurringCallbacks
                        $this->runTransactionListenerCallbacks( self::TRIGGER_ROLLBACK );
                } catch ( Exception $ex ) {
                        // Already logged; move on...
                        $e = $e ?: $ex;
                }

                return $e;
        }

        /**
         * Checks whether the cause of the error is detected to be a timeout.
         *
         * It returns false by default, and not all engines support detecting this yet.
         * If this returns false, it will be treated as a generic query error.
         *
         * @param string $error Error text
         * @param int $errno Error number
         * @return bool
         */
        protected function wasQueryTimeout( $error, $errno ) {
                return false;
        }

        public function reportQueryError( $error, $errno, $sql, $fname, $tempIgnore = false ) {
                if ( $this->ignoreErrors() || $tempIgnore ) {
                        $this->queryLogger->debug( "SQL ERROR (ignored): $error\n" );
                } else {
                        $sql1line = mb_substr( str_replace( "\n", "\\n", $sql ), 0, 5 * 1024 );
                        $this->queryLogger->error(
                                "{fname}\t{db_server}\t{errno}\t{error}\t{sql1line}",
                                $this->getLogContext( [
                                        'method' => __METHOD__,
                                        'errno' => $errno,
                                        'error' => $error,
                                        'sql1line' => $sql1line,
                                        'fname' => $fname,
                                ] )
                        );
                        $this->queryLogger->debug( "SQL ERROR: " . $error . "\n" );
                        $wasQueryTimeout = $this->wasQueryTimeout( $error, $errno );
                        if ( $wasQueryTimeout ) {
                                throw new DBQueryTimeoutError( $this, $error, $errno, $sql, $fname );
                        } else {
                                throw new DBQueryError( $this, $error, $errno, $sql, $fname );
                        }
                }
        }

        public function freeResult( $res ) {
        }

        public function selectField(
                $table, $var, $cond = '', $fname = __METHOD__, $options = [], $join_conds = []
        ) {
                if ( $var === '*' ) { // sanity
                        throw new DBUnexpectedError( $this, "Cannot use a * field: got '$var'" );
                }

                if ( !is_array( $options ) ) {
                        $options = [ $options ];
                }

                $options['LIMIT'] = 1;

                $res = $this->select( $table, $var, $cond, $fname, $options, $join_conds );
                if ( $res === false || !$this->numRows( $res ) ) {
                        return false;
                }

                $row = $this->fetchRow( $res );

                if ( $row !== false ) {
                        return reset( $row );
                } else {
                        return false;
                }
        }

        public function selectFieldValues(
                $table, $var, $cond = '', $fname = __METHOD__, $options = [], $join_conds = []
        ) {
                if ( $var === '*' ) { // sanity
                        throw new DBUnexpectedError( $this, "Cannot use a * field" );
                } elseif ( !is_string( $var ) ) { // sanity
                        throw new DBUnexpectedError( $this, "Cannot use an array of fields" );
                }

                if ( !is_array( $options ) ) {
                        $options = [ $options ];
                }

                $res = $this->select( $table, $var, $cond, $fname, $options, $join_conds );
                if ( $res === false ) {
                        return false;
                }

                $values = [];
                foreach ( $res as $row ) {
                        $values[] = $row->$var;
                }

                return $values;
        }

        /**
         * Returns an optional USE INDEX clause to go after the table, and a
         * string to go at the end of the query.
         *
         * @param array $options Associative array of options to be turned into
         *   an SQL query, valid keys are listed in the function.
         * @return array
         * @see Database::select()
         */
        protected function makeSelectOptions( $options ) {
                $preLimitTail = $postLimitTail = '';
                $startOpts = '';

                $noKeyOptions = [];

                foreach ( $options as $key => $option ) {
                        if ( is_numeric( $key ) ) {
                                $noKeyOptions[$option] = true;
                        }
                }

                $preLimitTail .= $this->makeGroupByWithHaving( $options );

                $preLimitTail .= $this->makeOrderBy( $options );

                if ( isset( $noKeyOptions['FOR UPDATE'] ) ) {
                        $postLimitTail .= ' FOR UPDATE';
                }

                if ( isset( $noKeyOptions['LOCK IN SHARE MODE'] ) ) {
                        $postLimitTail .= ' LOCK IN SHARE MODE';
                }

                if ( isset( $noKeyOptions['DISTINCT'] ) || isset( $noKeyOptions['DISTINCTROW'] ) ) {
                        $startOpts .= 'DISTINCT';
                }

                # Various MySQL extensions
                if ( isset( $noKeyOptions['STRAIGHT_JOIN'] ) ) {
                        $startOpts .= ' /*! STRAIGHT_JOIN */';
                }

                if ( isset( $noKeyOptions['HIGH_PRIORITY'] ) ) {
                        $startOpts .= ' HIGH_PRIORITY';
                }

                if ( isset( $noKeyOptions['SQL_BIG_RESULT'] ) ) {
                        $startOpts .= ' SQL_BIG_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_BUFFER_RESULT'] ) ) {
                        $startOpts .= ' SQL_BUFFER_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_SMALL_RESULT'] ) ) {
                        $startOpts .= ' SQL_SMALL_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_CALC_FOUND_ROWS'] ) ) {
                        $startOpts .= ' SQL_CALC_FOUND_ROWS';
                }

                if ( isset( $noKeyOptions['SQL_CACHE'] ) ) {
                        $startOpts .= ' SQL_CACHE';
                }

                if ( isset( $noKeyOptions['SQL_NO_CACHE'] ) ) {
                        $startOpts .= ' SQL_NO_CACHE';
                }

                if ( isset( $options['USE INDEX'] ) && is_string( $options['USE INDEX'] ) ) {
                        $useIndex = $this->useIndexClause( $options['USE INDEX'] );
                } else {
                        $useIndex = '';
                }
                if ( isset( $options['IGNORE INDEX'] ) && is_string( $options['IGNORE INDEX'] ) ) {
                        $ignoreIndex = $this->ignoreIndexClause( $options['IGNORE INDEX'] );
                } else {
                        $ignoreIndex = '';
                }

                return [ $startOpts, $useIndex, $preLimitTail, $postLimitTail, $ignoreIndex ];
        }

        /**
         * Returns an optional GROUP BY with an optional HAVING
         *
         * @param array $options Associative array of options
         * @return string
         * @see Database::select()
         * @since 1.21
         */
        protected function makeGroupByWithHaving( $options ) {
                $sql = '';
                if ( isset( $options['GROUP BY'] ) ) {
                        $gb = is_array( $options['GROUP BY'] )
                                ? implode( ',', $options['GROUP BY'] )
                                : $options['GROUP BY'];
                        $sql .= ' GROUP BY ' . $gb;
                }
                if ( isset( $options['HAVING'] ) ) {
                        $having = is_array( $options['HAVING'] )
                                ? $this->makeList( $options['HAVING'], self::LIST_AND )
                                : $options['HAVING'];
                        $sql .= ' HAVING ' . $having;
                }

                return $sql;
        }

        /**
         * Returns an optional ORDER BY
         *
         * @param array $options Associative array of options
         * @return string
         * @see Database::select()
         * @since 1.21
         */
        protected function makeOrderBy( $options ) {
                if ( isset( $options['ORDER BY'] ) ) {
                        $ob = is_array( $options['ORDER BY'] )
                                ? implode( ',', $options['ORDER BY'] )
                                : $options['ORDER BY'];

                        return ' ORDER BY ' . $ob;
                }

                return '';
        }

        public function select( $table, $vars, $conds = '', $fname = __METHOD__,
                $options = [], $join_conds = [] ) {
                $sql = $this->selectSQLText( $table, $vars, $conds, $fname, $options, $join_conds );

                return $this->query( $sql, $fname );
        }

        public function selectSQLText( $table, $vars, $conds = '', $fname = __METHOD__,
                $options = [], $join_conds = []
        ) {
                if ( is_array( $vars ) ) {
                        $vars = implode( ',', $this->fieldNamesWithAlias( $vars ) );
                }

                $options = (array)$options;
                $useIndexes = ( isset( $options['USE INDEX'] ) && is_array( $options['USE INDEX'] ) )
                        ? $options['USE INDEX']
                        : [];
                $ignoreIndexes = (
                        isset( $options['IGNORE INDEX'] ) &&
                        is_array( $options['IGNORE INDEX'] )
                )
                        ? $options['IGNORE INDEX']
                        : [];

                if ( is_array( $table ) ) {
                        $from = ' FROM ' .
                                $this->tableNamesWithIndexClauseOrJOIN(
                                        $table, $useIndexes, $ignoreIndexes, $join_conds );
                } elseif ( $table != '' ) {
                        $from = ' FROM ' .
                                $this->tableNamesWithIndexClauseOrJOIN(
                                        [ $table ], $useIndexes, $ignoreIndexes, [] );
                } else {
                        $from = '';
                }

                list( $startOpts, $useIndex, $preLimitTail, $postLimitTail, $ignoreIndex ) =
                        $this->makeSelectOptions( $options );

                if ( is_array( $conds ) ) {
                        $conds = $this->makeList( $conds, self::LIST_AND );
                }

                if ( $conds === null || $conds === false ) {
                        $this->queryLogger->warning(
                                __METHOD__
                                . ' called from '
                                . $fname
                                . ' with incorrect parameters: $conds must be a string or an array'
                        );
                        $conds = '';
                }

                if ( $conds === '' ) {
                        $sql = "SELECT $startOpts $vars $from $useIndex $ignoreIndex $preLimitTail";
                } elseif ( is_string( $conds ) ) {
                        $sql = "SELECT $startOpts $vars $from $useIndex $ignoreIndex " .
                                "WHERE $conds $preLimitTail";
                } else {
                        throw new DBUnexpectedError( $this, __METHOD__ . ' called with incorrect parameters' );
                }

                if ( isset( $options['LIMIT'] ) ) {
                        $sql = $this->limitResult( $sql, $options['LIMIT'],
                                isset( $options['OFFSET'] ) ? $options['OFFSET'] : false );
                }
                $sql = "$sql $postLimitTail";

                if ( isset( $options['EXPLAIN'] ) ) {
                        $sql = 'EXPLAIN ' . $sql;
                }

                return $sql;
        }

        public function selectRow( $table, $vars, $conds, $fname = __METHOD__,
                $options = [], $join_conds = []
        ) {
                $options = (array)$options;
                $options['LIMIT'] = 1;
                $res = $this->select( $table, $vars, $conds, $fname, $options, $join_conds );

                if ( $res === false ) {
                        return false;
                }

                if ( !$this->numRows( $res ) ) {
                        return false;
                }

                $obj = $this->fetchObject( $res );

                return $obj;
        }

        public function estimateRowCount(
                $table, $vars = '*', $conds = '', $fname = __METHOD__, $options = [], $join_conds = []
        ) {
                $rows = 0;
                $res = $this->select(
                        $table, [ 'rowcount' => 'COUNT(*)' ], $conds, $fname, $options, $join_conds
                );

                if ( $res ) {
                        $row = $this->fetchRow( $res );
                        $rows = ( isset( $row['rowcount'] ) ) ? (int)$row['rowcount'] : 0;
                }

                return $rows;
        }

        public function selectRowCount(
                $tables, $vars = '*', $conds = '', $fname = __METHOD__, $options = [], $join_conds = []
        ) {
                $rows = 0;
                $sql = $this->selectSQLText( $tables, '1', $conds, $fname, $options, $join_conds );
                // The identifier quotes is primarily for MSSQL.
                $rowCountCol = $this->addIdentifierQuotes( "rowcount" );
                $tableName = $this->addIdentifierQuotes( "tmp_count" );
                $res = $this->query( "SELECT COUNT(*) AS $rowCountCol FROM ($sql) $tableName", $fname );

                if ( $res ) {
                        $row = $this->fetchRow( $res );
                        $rows = ( isset( $row['rowcount'] ) ) ? (int)$row['rowcount'] : 0;
                }

                return $rows;
        }

        /**
         * Removes most variables from an SQL query and replaces them with X or N for numbers.
         * It's only slightly flawed. Don't use for anything important.
         *
         * @param string $sql A SQL Query
         *
         * @return string
         */
        protected static function generalizeSQL( $sql ) {
                # This does the same as the regexp below would do, but in such a way
                # as to avoid crashing php on some large strings.
                # $sql = preg_replace( "/'([^\\\\']|\\\\.)*'|\"([^\\\\\"]|\\\\.)*\"/", "'X'", $sql );

                $sql = str_replace( "\\\\", '', $sql );
                $sql = str_replace( "\\'", '', $sql );
                $sql = str_replace( "\\\"", '', $sql );
                $sql = preg_replace( "/'.*'/s", "'X'", $sql );
                $sql = preg_replace( '/".*"/s', "'X'", $sql );

                # All newlines, tabs, etc replaced by single space
                $sql = preg_replace( '/\s+/', ' ', $sql );

                # All numbers => N,
                # except the ones surrounded by characters, e.g. l10n
                $sql = preg_replace( '/-?\d+(,-?\d+)+/s', 'N,...,N', $sql );
                $sql = preg_replace( '/(?<![a-zA-Z])-?\d+(?![a-zA-Z])/s', 'N', $sql );

                return $sql;
        }

        public function fieldExists( $table, $field, $fname = __METHOD__ ) {
                $info = $this->fieldInfo( $table, $field );

                return (bool)$info;
        }

        public function indexExists( $table, $index, $fname = __METHOD__ ) {
                if ( !$this->tableExists( $table ) ) {
                        return null;
                }

                $info = $this->indexInfo( $table, $index, $fname );
                if ( is_null( $info ) ) {
                        return null;
                } else {
                        return $info !== false;
                }
        }

        public function tableExists( $table, $fname = __METHOD__ ) {
                $tableRaw = $this->tableName( $table, 'raw' );
                if ( isset( $this->sessionTempTables[$tableRaw] ) ) {
                        return true; // already known to exist
                }

                $table = $this->tableName( $table );
                $ignoreErrors = true;
                $res = $this->query( "SELECT 1 FROM $table LIMIT 1", $fname, $ignoreErrors );

                return (bool)$res;
        }

        public function indexUnique( $table, $index ) {
                $indexInfo = $this->indexInfo( $table, $index );

                if ( !$indexInfo ) {
                        return null;
                }

                return !$indexInfo[0]->Non_unique;
        }

        /**
         * Helper for Database::insert().
         *
         * @param array $options
         * @return string
         */
        protected function makeInsertOptions( $options ) {
                return implode( ' ', $options );
        }

        public function insert( $table, $a, $fname = __METHOD__, $options = [] ) {
                # No rows to insert, easy just return now
                if ( !count( $a ) ) {
                        return true;
                }

                $table = $this->tableName( $table );

                if ( !is_array( $options ) ) {
                        $options = [ $options ];
                }

                $fh = null;
                if ( isset( $options['fileHandle'] ) ) {
                        $fh = $options['fileHandle'];
                }
                $options = $this->makeInsertOptions( $options );

                if ( isset( $a[0] ) && is_array( $a[0] ) ) {
                        $multi = true;
                        $keys = array_keys( $a[0] );
                } else {
                        $multi = false;
                        $keys = array_keys( $a );
                }

                $sql = 'INSERT ' . $options .
                        " INTO $table (" . implode( ',', $keys ) . ') VALUES ';

                if ( $multi ) {
                        $first = true;
                        foreach ( $a as $row ) {
                                if ( $first ) {
                                        $first = false;
                                } else {
                                        $sql .= ',';
                                }
                                $sql .= '(' . $this->makeList( $row ) . ')';
                        }
                } else {
                        $sql .= '(' . $this->makeList( $a ) . ')';
                }

                if ( $fh !== null && false === fwrite( $fh, $sql ) ) {
                        return false;
                } elseif ( $fh !== null ) {
                        return true;
                }

                return (bool)$this->query( $sql, $fname );
        }

        /**
         * Make UPDATE options array for Database::makeUpdateOptions
         *
         * @param array $options
         * @return array
         */
        protected function makeUpdateOptionsArray( $options ) {
                if ( !is_array( $options ) ) {
                        $options = [ $options ];
                }

                $opts = [];

                if ( in_array( 'IGNORE', $options ) ) {
                        $opts[] = 'IGNORE';
                }

                return $opts;
        }

        /**
         * Make UPDATE options for the Database::update function
         *
         * @param array $options The options passed to Database::update
         * @return string
         */
        protected function makeUpdateOptions( $options ) {
                $opts = $this->makeUpdateOptionsArray( $options );

                return implode( ' ', $opts );
        }

        public function update( $table, $values, $conds, $fname = __METHOD__, $options = [] ) {
                $table = $this->tableName( $table );
                $opts = $this->makeUpdateOptions( $options );
                $sql = "UPDATE $opts $table SET " . $this->makeList( $values, self::LIST_SET );

                if ( $conds !== [] && $conds !== '*' ) {
                        $sql .= " WHERE " . $this->makeList( $conds, self::LIST_AND );
                }

                return (bool)$this->query( $sql, $fname );
        }

        public function makeList( $a, $mode = self::LIST_COMMA ) {
                if ( !is_array( $a ) ) {
                        throw new DBUnexpectedError( $this, __METHOD__ . ' called with incorrect parameters' );
                }

                $first = true;
                $list = '';

                foreach ( $a as $field => $value ) {
                        if ( !$first ) {
                                if ( $mode == self::LIST_AND ) {
                                        $list .= ' AND ';
                                } elseif ( $mode == self::LIST_OR ) {
                                        $list .= ' OR ';
                                } else {
                                        $list .= ',';
                                }
                        } else {
                                $first = false;
                        }

                        if ( ( $mode == self::LIST_AND || $mode == self::LIST_OR ) && is_numeric( $field ) ) {
                                $list .= "($value)";
                        } elseif ( $mode == self::LIST_SET && is_numeric( $field ) ) {
                                $list .= "$value";
                        } elseif (
                                ( $mode == self::LIST_AND || $mode == self::LIST_OR ) && is_array( $value )
                        ) {
                                // Remove null from array to be handled separately if found
                                $includeNull = false;
                                foreach ( array_keys( $value, null, true ) as $nullKey ) {
                                        $includeNull = true;
                                        unset( $value[$nullKey] );
                                }
                                if ( count( $value ) == 0 && !$includeNull ) {
                                        throw new InvalidArgumentException(
                                                __METHOD__ . ": empty input for field $field" );
                                } elseif ( count( $value ) == 0 ) {
                                        // only check if $field is null
                                        $list .= "$field IS NULL";
                                } else {
                                        // IN clause contains at least one valid element
                                        if ( $includeNull ) {
                                                // Group subconditions to ensure correct precedence
                                                $list .= '(';
                                        }
                                        if ( count( $value ) == 1 ) {
                                                // Special-case single values, as IN isn't terribly efficient
                                                // Don't necessarily assume the single key is 0; we don't
                                                // enforce linear numeric ordering on other arrays here.
                                                $value = array_values( $value )[0];
                                                $list .= $field . " = " . $this->addQuotes( $value );
                                        } else {
                                                $list .= $field . " IN (" . $this->makeList( $value ) . ") ";
                                        }
                                        // if null present in array, append IS NULL
                                        if ( $includeNull ) {
                                                $list .= " OR $field IS NULL)";
                                        }
                                }
                        } elseif ( $value === null ) {
                                if ( $mode == self::LIST_AND || $mode == self::LIST_OR ) {
                                        $list .= "$field IS ";
                                } elseif ( $mode == self::LIST_SET ) {
                                        $list .= "$field = ";
                                }
                                $list .= 'NULL';
                        } else {
                                if (
                                        $mode == self::LIST_AND || $mode == self::LIST_OR || $mode == self::LIST_SET
                                ) {
                                        $list .= "$field = ";
                                }
                                $list .= $mode == self::LIST_NAMES ? $value : $this->addQuotes( $value );
                        }
                }

                return $list;
        }

        public function makeWhereFrom2d( $data, $baseKey, $subKey ) {
                $conds = [];

                foreach ( $data as $base => $sub ) {
                        if ( count( $sub ) ) {
                                $conds[] = $this->makeList(
                                        [ $baseKey => $base, $subKey => array_keys( $sub ) ],
                                        self::LIST_AND );
                        }
                }

                if ( $conds ) {
                        return $this->makeList( $conds, self::LIST_OR );
                } else {
                        // Nothing to search for...
                        return false;
                }
        }

        public function aggregateValue( $valuedata, $valuename = 'value' ) {
                return $valuename;
        }

        public function bitNot( $field ) {
                return "(~$field)";
        }

        public function bitAnd( $fieldLeft, $fieldRight ) {
                return "($fieldLeft & $fieldRight)";
        }

        public function bitOr( $fieldLeft, $fieldRight ) {
                return "($fieldLeft | $fieldRight)";
        }

        public function buildConcat( $stringList ) {
                return 'CONCAT(' . implode( ',', $stringList ) . ')';
        }

        public function buildGroupConcatField(
                $delim, $table, $field, $conds = '', $join_conds = []
        ) {
                $fld = "GROUP_CONCAT($field SEPARATOR " . $this->addQuotes( $delim ) . ')';

                return '(' . $this->selectSQLText( $table, $fld, $conds, null, [], $join_conds ) . ')';
        }

        public function buildSubstring( $input, $startPosition, $length = null ) {
                $this->assertBuildSubstringParams( $startPosition, $length );
                $functionBody = "$input FROM $startPosition";
                if ( $length !== null ) {
                        $functionBody .= " FOR $length";
                }
                return 'SUBSTRING(' . $functionBody . ')';
        }

        /**
         * Check type and bounds for parameters to self::buildSubstring()
         *
         * All supported databases have substring functions that behave the same for
         * positive $startPosition and non-negative $length, but behaviors differ when
         * given 0 or negative $startPosition or negative $length. The simplest
         * solution to that is to just forbid those values.
         *
         * @param int $startPosition
         * @param int|null $length
         * @since 1.31
         */
        protected function assertBuildSubstringParams( $startPosition, $length ) {
                if ( !is_int( $startPosition ) || $startPosition <= 0 ) {
                        throw new InvalidArgumentException(
                                '$startPosition must be a positive integer'
                        );
                }
                if ( !( is_int( $length ) && $length >= 0 || $length === null ) ) {
                        throw new InvalidArgumentException(
                                '$length must be null or an integer greater than or equal to 0'
                        );
                }
        }

        public function buildStringCast( $field ) {
                return $field;
        }

        public function buildIntegerCast( $field ) {
                return 'CAST( ' . $field . ' AS INTEGER )';
        }

        public function databasesAreIndependent() {
                return false;
        }

        public function selectDB( $db ) {
                # Stub. Shouldn't cause serious problems if it's not overridden, but
                # if your database engine supports a concept similar to MySQL's
                # databases you may as well.
                $this->dbName = $db;

                return true;
        }

        public function getDBname() {
                return $this->dbName;
        }

        public function getServer() {
                return $this->server;
        }

        public function tableName( $name, $format = 'quoted' ) {
                # Skip the entire process when we have a string quoted on both ends.
                # Note that we check the end so that we will still quote any use of
                # use of `database`.table. But won't break things if someone wants
                # to query a database table with a dot in the name.
                if ( $this->isQuotedIdentifier( $name ) ) {
                        return $name;
                }

                # Lets test for any bits of text that should never show up in a table
                # name. Basically anything like JOIN or ON which are actually part of
                # SQL queries, but may end up inside of the table value to combine
                # sql. Such as how the API is doing.
                # Note that we use a whitespace test rather than a \b test to avoid
                # any remote case where a word like on may be inside of a table name
                # surrounded by symbols which may be considered word breaks.
                if ( preg_match( '/(^|\s)(DISTINCT|JOIN|ON|AS)(\s|$)/i', $name ) !== 0 ) {
                        return $name;
                }

                # Split database and table into proper variables.
                list( $database, $schema, $prefix, $table ) = $this->qualifiedTableComponents( $name );

                # Quote $table and apply the prefix if not quoted.
                # $tableName might be empty if this is called from Database::replaceVars()
                $tableName = "{$prefix}{$table}";
                if ( $format === 'quoted'
                        && !$this->isQuotedIdentifier( $tableName )
                        && $tableName !== ''
                ) {
                        $tableName = $this->addIdentifierQuotes( $tableName );
                }

                # Quote $schema and $database and merge them with the table name if needed
                $tableName = $this->prependDatabaseOrSchema( $schema, $tableName, $format );
                $tableName = $this->prependDatabaseOrSchema( $database, $tableName, $format );

                return $tableName;
        }

        /**
         * Get the table components needed for a query given the currently selected database
         *
         * @param string $name Table name in the form of db.schema.table, db.table, or table
         * @return array (DB name or "" for default, schema name, table prefix, table name)
         */
        protected function qualifiedTableComponents( $name ) {
                # We reverse the explode so that database.table and table both output the correct table.
                $dbDetails = explode( '.', $name, 3 );
                if ( count( $dbDetails ) == 3 ) {
                        list( $database, $schema, $table ) = $dbDetails;
                        # We don't want any prefix added in this case
                        $prefix = '';
                } elseif ( count( $dbDetails ) == 2 ) {
                        list( $database, $table ) = $dbDetails;
                        # We don't want any prefix added in this case
                        $prefix = '';
                        # In dbs that support it, $database may actually be the schema
                        # but that doesn't affect any of the functionality here
                        $schema = '';
                } else {
                        list( $table ) = $dbDetails;
                        if ( isset( $this->tableAliases[$table] ) ) {
                                $database = $this->tableAliases[$table]['dbname'];
                                $schema = is_string( $this->tableAliases[$table]['schema'] )
                                        ? $this->tableAliases[$table]['schema']
                                        : $this->schema;
                                $prefix = is_string( $this->tableAliases[$table]['prefix'] )
                                        ? $this->tableAliases[$table]['prefix']
                                        : $this->tablePrefix;
                        } else {
                                $database = '';
                                $schema = $this->schema; # Default schema
                                $prefix = $this->tablePrefix; # Default prefix
                        }
                }

                return [ $database, $schema, $prefix, $table ];
        }

        /**
         * @param string|null $namespace Database or schema
         * @param string $relation Name of table, view, sequence, etc...
         * @param string $format One of (raw, quoted)
         * @return string Relation name with quoted and merged $namespace as needed
         */
        private function prependDatabaseOrSchema( $namespace, $relation, $format ) {
                if ( strlen( $namespace ) ) {
                        if ( $format === 'quoted' && !$this->isQuotedIdentifier( $namespace ) ) {
                                $namespace = $this->addIdentifierQuotes( $namespace );
                        }
                        $relation = $namespace . '.' . $relation;
                }

                return $relation;
        }

        public function tableNames() {
                $inArray = func_get_args();
                $retVal = [];

                foreach ( $inArray as $name ) {
                        $retVal[$name] = $this->tableName( $name );
                }

                return $retVal;
        }

        public function tableNamesN() {
                $inArray = func_get_args();
                $retVal = [];

                foreach ( $inArray as $name ) {
                        $retVal[] = $this->tableName( $name );
                }

                return $retVal;
        }

        /**
         * Get an aliased table name
         * e.g. tableName AS newTableName
         *
         * @param string $name Table name, see tableName()
         * @param string|bool $alias Alias (optional)
         * @return string SQL name for aliased table. Will not alias a table to its own name
         */
        protected function tableNameWithAlias( $name, $alias = false ) {
                if ( !$alias || $alias == $name ) {
                        return $this->tableName( $name );
                } else {
                        return $this->tableName( $name ) . ' ' . $this->addIdentifierQuotes( $alias );
                }
        }

        /**
         * Gets an array of aliased table names
         *
         * @param array $tables [ [alias] => table ]
         * @return string[] See tableNameWithAlias()
         */
        protected function tableNamesWithAlias( $tables ) {
                $retval = [];
                foreach ( $tables as $alias => $table ) {
                        if ( is_numeric( $alias ) ) {
                                $alias = $table;
                        }
                        $retval[] = $this->tableNameWithAlias( $table, $alias );
                }

                return $retval;
        }

        /**
         * Get an aliased field name
         * e.g. fieldName AS newFieldName
         *
         * @param string $name Field name
         * @param string|bool $alias Alias (optional)
         * @return string SQL name for aliased field. Will not alias a field to its own name
         */
        protected function fieldNameWithAlias( $name, $alias = false ) {
                if ( !$alias || (string)$alias === (string)$name ) {
                        return $name;
                } else {
                        return $name . ' AS ' . $this->addIdentifierQuotes( $alias ); // PostgreSQL needs AS
                }
        }

        /**
         * Gets an array of aliased field names
         *
         * @param array $fields [ [alias] => field ]
         * @return string[] See fieldNameWithAlias()
         */
        protected function fieldNamesWithAlias( $fields ) {
                $retval = [];
                foreach ( $fields as $alias => $field ) {
                        if ( is_numeric( $alias ) ) {
                                $alias = $field;
                        }
                        $retval[] = $this->fieldNameWithAlias( $field, $alias );
                }

                return $retval;
        }

        /**
         * Get the aliased table name clause for a FROM clause
         * which might have a JOIN and/or USE INDEX or IGNORE INDEX clause
         *
         * @param array $tables ( [alias] => table )
         * @param array $use_index Same as for select()
         * @param array $ignore_index Same as for select()
         * @param array $join_conds Same as for select()
         * @return string
         */
        protected function tableNamesWithIndexClauseOrJOIN(
                $tables, $use_index = [], $ignore_index = [], $join_conds = []
        ) {
                $ret = [];
                $retJOIN = [];
                $use_index = (array)$use_index;
                $ignore_index = (array)$ignore_index;
                $join_conds = (array)$join_conds;

                foreach ( $tables as $alias => $table ) {
                        if ( !is_string( $alias ) ) {
                                // No alias? Set it equal to the table name
                                $alias = $table;
                        }

                        if ( is_array( $table ) ) {
                                // A parenthesized group
                                if ( count( $table ) > 1 ) {
                                        $joinedTable = '('
                                                . $this->tableNamesWithIndexClauseOrJOIN( $table, $use_index, $ignore_index, $join_conds )
                                                . ')';
                                } else {
                                        // Degenerate case
                                        $innerTable = reset( $table );
                                        $innerAlias = key( $table );
                                        $joinedTable = $this->tableNameWithAlias(
                                                $innerTable,
                                                is_string( $innerAlias ) ? $innerAlias : $innerTable
                                        );
                                }
                        } else {
                                $joinedTable = $this->tableNameWithAlias( $table, $alias );
                        }

                        // Is there a JOIN clause for this table?
                        if ( isset( $join_conds[$alias] ) ) {
                                list( $joinType, $conds ) = $join_conds[$alias];
                                $tableClause = $joinType;
                                $tableClause .= ' ' . $joinedTable;
                                if ( isset( $use_index[$alias] ) ) { // has USE INDEX?
                                        $use = $this->useIndexClause( implode( ',', (array)$use_index[$alias] ) );
                                        if ( $use != '' ) {
                                                $tableClause .= ' ' . $use;
                                        }
                                }
                                if ( isset( $ignore_index[$alias] ) ) { // has IGNORE INDEX?
                                        $ignore = $this->ignoreIndexClause(
                                                implode( ',', (array)$ignore_index[$alias] ) );
                                        if ( $ignore != '' ) {
                                                $tableClause .= ' ' . $ignore;
                                        }
                                }
                                $on = $this->makeList( (array)$conds, self::LIST_AND );
                                if ( $on != '' ) {
                                        $tableClause .= ' ON (' . $on . ')';
                                }

                                $retJOIN[] = $tableClause;
                        } elseif ( isset( $use_index[$alias] ) ) {
                                // Is there an INDEX clause for this table?
                                $tableClause = $joinedTable;
                                $tableClause .= ' ' . $this->useIndexClause(
                                                implode( ',', (array)$use_index[$alias] )
                                        );

                                $ret[] = $tableClause;
                        } elseif ( isset( $ignore_index[$alias] ) ) {
                                // Is there an INDEX clause for this table?
                                $tableClause = $joinedTable;
                                $tableClause .= ' ' . $this->ignoreIndexClause(
                                                implode( ',', (array)$ignore_index[$alias] )
                                        );

                                $ret[] = $tableClause;
                        } else {
                                $tableClause = $joinedTable;

                                $ret[] = $tableClause;
                        }
                }

                // We can't separate explicit JOIN clauses with ',', use ' ' for those
                $implicitJoins = $ret ? implode( ',', $ret ) : "";
                $explicitJoins = $retJOIN ? implode( ' ', $retJOIN ) : "";

                // Compile our final table clause
                return implode( ' ', [ $implicitJoins, $explicitJoins ] );
        }

        /**
         * Allows for index remapping in queries where this is not consistent across DBMS
         *
         * @param string $index
         * @return string
         */
        protected function indexName( $index ) {
                return isset( $this->indexAliases[$index] )
                        ? $this->indexAliases[$index]
                        : $index;
        }

        public function addQuotes( $s ) {
                if ( $s instanceof Blob ) {
                        $s = $s->fetch();
                }
                if ( $s === null ) {
                        return 'NULL';
                } elseif ( is_bool( $s ) ) {
                        return (int)$s;
                } else {
                        # This will also quote numeric values. This should be harmless,
                        # and protects against weird problems that occur when they really
                        # _are_ strings such as article titles and string->number->string
                        # conversion is not 1:1.
                        return "'" . $this->strencode( $s ) . "'";
                }
        }

        /**
         * Quotes an identifier using `backticks` or "double quotes" depending on the database type.
         * MySQL uses `backticks` while basically everything else uses double quotes.
         * Since MySQL is the odd one out here the double quotes are our generic
         * and we implement backticks in DatabaseMysqlBase.
         *
         * @param string $s
         * @return string
         */
        public function addIdentifierQuotes( $s ) {
                return '"' . str_replace( '"', '""', $s ) . '"';
        }

        /**
         * Returns if the given identifier looks quoted or not according to
         * the database convention for quoting identifiers .
         *
         * @note Do not use this to determine if untrusted input is safe.
         *   A malicious user can trick this function.
         * @param string $name
         * @return bool
         */
        public function isQuotedIdentifier( $name ) {
                return $name[0] == '"' && substr( $name, -1, 1 ) == '"';
        }

        /**
         * @param string $s
         * @param string $escapeChar
         * @return string
         */
        protected function escapeLikeInternal( $s, $escapeChar = '`' ) {
                return str_replace( [ $escapeChar, '%', '_' ],
                        [ "{$escapeChar}{$escapeChar}", "{$escapeChar}%", "{$escapeChar}_" ],
                        $s );
        }

        public function buildLike() {
                $params = func_get_args();

                if ( count( $params ) > 0 && is_array( $params[0] ) ) {
                        $params = $params[0];
                }

                $s = '';

                // We use ` instead of \ as the default LIKE escape character, since addQuotes()
                // may escape backslashes, creating problems of double escaping. The `
                // character has good cross-DBMS compatibility, avoiding special operators
                // in MS SQL like ^ and %
                $escapeChar = '`';

                foreach ( $params as $value ) {
                        if ( $value instanceof LikeMatch ) {
                                $s .= $value->toString();
                        } else {
                                $s .= $this->escapeLikeInternal( $value, $escapeChar );
                        }
                }

                return ' LIKE ' . $this->addQuotes( $s ) . ' ESCAPE ' . $this->addQuotes( $escapeChar ) . ' ';
        }

        public function anyChar() {
                return new LikeMatch( '_' );
        }

        public function anyString() {
                return new LikeMatch( '%' );
        }

        public function nextSequenceValue( $seqName ) {
                return null;
        }

        /**
         * USE INDEX clause. Unlikely to be useful for anything but MySQL. This
         * is only needed because a) MySQL must be as efficient as possible due to
         * its use on Wikipedia, and b) MySQL 4.0 is kind of dumb sometimes about
         * which index to pick. Anyway, other databases might have different
         * indexes on a given table. So don't bother overriding this unless you're
         * MySQL.
         * @param string $index
         * @return string
         */
        public function useIndexClause( $index ) {
                return '';
        }

        /**
         * IGNORE INDEX clause. Unlikely to be useful for anything but MySQL. This
         * is only needed because a) MySQL must be as efficient as possible due to
         * its use on Wikipedia, and b) MySQL 4.0 is kind of dumb sometimes about
         * which index to pick. Anyway, other databases might have different
         * indexes on a given table. So don't bother overriding this unless you're
         * MySQL.
         * @param string $index
         * @return string
         */
        public function ignoreIndexClause( $index ) {
                return '';
        }

        public function replace( $table, $uniqueIndexes, $rows, $fname = __METHOD__ ) {
                if ( count( $rows ) == 0 ) {
                        return;
                }

                // Single row case
                if ( !is_array( reset( $rows ) ) ) {
                        $rows = [ $rows ];
                }

                try {
                        $this->startAtomic( $fname );
                        $affectedRowCount = 0;
                        foreach ( $rows as $row ) {
                                // Delete rows which collide with this one
                                $indexWhereClauses = [];
                                foreach ( $uniqueIndexes as $index ) {
                                        $indexColumns = (array)$index;
                                        $indexRowValues = array_intersect_key( $row, array_flip( $indexColumns ) );
                                        if ( count( $indexRowValues ) != count( $indexColumns ) ) {
                                                throw new DBUnexpectedError(
                                                        $this,
                                                        'New record does not provide all values for unique key (' .
                                                        implode( ', ', $indexColumns ) . ')'
                                                );
                                        } elseif ( in_array( null, $indexRowValues, true ) ) {
                                                throw new DBUnexpectedError(
                                                        $this,
                                                        'New record has a null value for unique key (' .
                                                        implode( ', ', $indexColumns ) . ')'
                                                );
                                        }
                                        $indexWhereClauses[] = $this->makeList( $indexRowValues, LIST_AND );
                                }

                                if ( $indexWhereClauses ) {
                                        $this->delete( $table, $this->makeList( $indexWhereClauses, LIST_OR ), $fname );
                                        $affectedRowCount += $this->affectedRows();
                                }

                                // Now insert the row
                                $this->insert( $table, $row, $fname );
                                $affectedRowCount += $this->affectedRows();
                        }
                        $this->endAtomic( $fname );
                        $this->affectedRowCount = $affectedRowCount;
                } catch ( Exception $e ) {
                        $this->rollback( $fname, self::FLUSHING_INTERNAL );
                        throw $e;
                }
        }

        /**
         * REPLACE query wrapper for MySQL and SQLite, which have a native REPLACE
         * statement.
         *
         * @param string $table Table name
         * @param array|string $rows Row(s) to insert
         * @param string $fname Caller function name
         *
         * @return ResultWrapper
         */
        protected function nativeReplace( $table, $rows, $fname ) {
                $table = $this->tableName( $table );

                # Single row case
                if ( !is_array( reset( $rows ) ) ) {
                        $rows = [ $rows ];
                }

                $sql = "REPLACE INTO $table (" . implode( ',', array_keys( $rows[0] ) ) . ') VALUES ';
                $first = true;

                foreach ( $rows as $row ) {
                        if ( $first ) {
                                $first = false;
                        } else {
                                $sql .= ',';
                        }

                        $sql .= '(' . $this->makeList( $row ) . ')';
                }

                return $this->query( $sql, $fname );
        }

        public function upsert( $table, array $rows, array $uniqueIndexes, array $set,
                $fname = __METHOD__
        ) {
                if ( !count( $rows ) ) {
                        return true; // nothing to do
                }

                if ( !is_array( reset( $rows ) ) ) {
                        $rows = [ $rows ];
                }

                if ( count( $uniqueIndexes ) ) {
                        $clauses = []; // list WHERE clauses that each identify a single row
                        foreach ( $rows as $row ) {
                                foreach ( $uniqueIndexes as $index ) {
                                        $index = is_array( $index ) ? $index : [ $index ]; // columns
                                        $rowKey = []; // unique key to this row
                                        foreach ( $index as $column ) {
                                                $rowKey[$column] = $row[$column];
                                        }
                                        $clauses[] = $this->makeList( $rowKey, self::LIST_AND );
                                }
                        }
                        $where = [ $this->makeList( $clauses, self::LIST_OR ) ];
                } else {
                        $where = false;
                }

                $affectedRowCount = 0;
                try {
                        $this->startAtomic( $fname );
                        # Update any existing conflicting row(s)
                        if ( $where !== false ) {
                                $ok = $this->update( $table, $set, $where, $fname );
                                $affectedRowCount += $this->affectedRows();
                        } else {
                                $ok = true;
                        }
                        # Now insert any non-conflicting row(s)
                        $ok = $this->insert( $table, $rows, $fname, [ 'IGNORE' ] ) && $ok;
                        $affectedRowCount += $this->affectedRows();
                        $this->endAtomic( $fname );
                        $this->affectedRowCount = $affectedRowCount;
                } catch ( Exception $e ) {
                        $this->rollback( $fname, self::FLUSHING_INTERNAL );
                        throw $e;
                }

                return $ok;
        }

        public function deleteJoin( $delTable, $joinTable, $delVar, $joinVar, $conds,
                $fname = __METHOD__
        ) {
                if ( !$conds ) {
                        throw new DBUnexpectedError( $this, __METHOD__ . ' called with empty $conds' );
                }

                $delTable = $this->tableName( $delTable );
                $joinTable = $this->tableName( $joinTable );
                $sql = "DELETE FROM $delTable WHERE $delVar IN (SELECT $joinVar FROM $joinTable ";
                if ( $conds != '*' ) {
                        $sql .= 'WHERE ' . $this->makeList( $conds, self::LIST_AND );
                }
                $sql .= ')';

                $this->query( $sql, $fname );
        }

        public function textFieldSize( $table, $field ) {
                $table = $this->tableName( $table );
                $sql = "SHOW COLUMNS FROM $table LIKE \"$field\";";
                $res = $this->query( $sql, __METHOD__ );
                $row = $this->fetchObject( $res );

                $m = [];

                if ( preg_match( '/\((.*)\)/', $row->Type, $m ) ) {
                        $size = $m[1];
                } else {
                        $size = -1;
                }

                return $size;
        }

        public function delete( $table, $conds, $fname = __METHOD__ ) {
                if ( !$conds ) {
                        throw new DBUnexpectedError( $this, __METHOD__ . ' called with no conditions' );
                }

                $table = $this->tableName( $table );
                $sql = "DELETE FROM $table";

                if ( $conds != '*' ) {
                        if ( is_array( $conds ) ) {
                                $conds = $this->makeList( $conds, self::LIST_AND );
                        }
                        $sql .= ' WHERE ' . $conds;
                }

                return $this->query( $sql, $fname );
        }

        final public function insertSelect(
                $destTable, $srcTable, $varMap, $conds,
                $fname = __METHOD__, $insertOptions = [], $selectOptions = [], $selectJoinConds = []
        ) {
                static $hints = [ 'NO_AUTO_COLUMNS' ];

                $insertOptions = (array)$insertOptions;
                $selectOptions = (array)$selectOptions;

                if ( $this->cliMode && $this->isInsertSelectSafe( $insertOptions, $selectOptions ) ) {
                        // For massive migrations with downtime, we don't want to select everything
                        // into memory and OOM, so do all this native on the server side if possible.
                        return $this->nativeInsertSelect(
                                $destTable,
                                $srcTable,
                                $varMap,
                                $conds,
                                $fname,
                                array_diff( $insertOptions, $hints ),
                                $selectOptions,
                                $selectJoinConds
                        );
                }

                return $this->nonNativeInsertSelect(
                        $destTable,
                        $srcTable,
                        $varMap,
                        $conds,
                        $fname,
                        array_diff( $insertOptions, $hints ),
                        $selectOptions,
                        $selectJoinConds
                );
        }

        /**
         * @param array $insertOptions INSERT options
         * @param array $selectOptions SELECT options
         * @return bool Whether an INSERT SELECT with these options will be replication safe
         * @since 1.31
         */
        protected function isInsertSelectSafe( array $insertOptions, array $selectOptions ) {
                return true;
        }

        /**
         * Implementation of insertSelect() based on select() and insert()
         *
         * @see IDatabase::insertSelect()
         * @since 1.30
         * @param string $destTable
         * @param string|array $srcTable
         * @param array $varMap
         * @param array $conds
         * @param string $fname
         * @param array $insertOptions
         * @param array $selectOptions
         * @param array $selectJoinConds
         * @return bool
         */
        protected function nonNativeInsertSelect( $destTable, $srcTable, $varMap, $conds,
                $fname = __METHOD__,
                $insertOptions = [], $selectOptions = [], $selectJoinConds = []
        ) {
                // For web requests, do a locking SELECT and then INSERT. This puts the SELECT burden
                // on only the master (without needing row-based-replication). It also makes it easy to
                // know how big the INSERT is going to be.
                $fields = [];
                foreach ( $varMap as $dstColumn => $sourceColumnOrSql ) {
                        $fields[] = $this->fieldNameWithAlias( $sourceColumnOrSql, $dstColumn );
                }
                $selectOptions[] = 'FOR UPDATE';
                $res = $this->select(
                        $srcTable, implode( ',', $fields ), $conds, $fname, $selectOptions, $selectJoinConds
                );
                if ( !$res ) {
                        return false;
                }

                try {
                        $affectedRowCount = 0;
                        $this->startAtomic( $fname );
                        $rows = [];
                        $ok = true;
                        foreach ( $res as $row ) {
                                $rows[] = (array)$row;

                                // Avoid inserts that are too huge
                                if ( count( $rows ) >= $this->nonNativeInsertSelectBatchSize ) {
                                        $ok = $this->insert( $destTable, $rows, $fname, $insertOptions );
                                        if ( !$ok ) {
                                                break;
                                        }
                                        $affectedRowCount += $this->affectedRows();
                                        $rows = [];
                                }
                        }
                        if ( $rows && $ok ) {
                                $ok = $this->insert( $destTable, $rows, $fname, $insertOptions );
                                if ( $ok ) {
                                        $affectedRowCount += $this->affectedRows();
                                }
                        }
                        if ( $ok ) {
                                $this->endAtomic( $fname );
                                $this->affectedRowCount = $affectedRowCount;
                        } else {
                                $this->rollback( $fname, self::FLUSHING_INTERNAL );
                        }
                        return $ok;
                } catch ( Exception $e ) {
                        $this->rollback( $fname, self::FLUSHING_INTERNAL );
                        throw $e;
                }
        }

        /**
         * Native server-side implementation of insertSelect() for situations where
         * we don't want to select everything into memory
         *
         * @see IDatabase::insertSelect()
         * @param string $destTable
         * @param string|array $srcTable
         * @param array $varMap
         * @param array $conds
         * @param string $fname
         * @param array $insertOptions
         * @param array $selectOptions
         * @param array $selectJoinConds
         * @return bool
         */
        protected function nativeInsertSelect( $destTable, $srcTable, $varMap, $conds,
                $fname = __METHOD__,
                $insertOptions = [], $selectOptions = [], $selectJoinConds = []
        ) {
                $destTable = $this->tableName( $destTable );

                if ( !is_array( $insertOptions ) ) {
                        $insertOptions = [ $insertOptions ];
                }

                $insertOptions = $this->makeInsertOptions( $insertOptions );

                $selectSql = $this->selectSQLText(
                        $srcTable,
                        array_values( $varMap ),
                        $conds,
                        $fname,
                        $selectOptions,
                        $selectJoinConds
                );

                $sql = "INSERT $insertOptions" .
                        " INTO $destTable (" . implode( ',', array_keys( $varMap ) ) . ') ' .
                        $selectSql;

                return $this->query( $sql, $fname );
        }

        /**
         * Construct a LIMIT query with optional offset. This is used for query
         * pages. The SQL should be adjusted so that only the first $limit rows
         * are returned. If $offset is provided as well, then the first $offset
         * rows should be discarded, and the next $limit rows should be returned.
         * If the result of the query is not ordered, then the rows to be returned
         * are theoretically arbitrary.
         *
         * $sql is expected to be a SELECT, if that makes a difference.
         *
         * The version provided by default works in MySQL and SQLite. It will very
         * likely need to be overridden for most other DBMSes.
         *
         * @param string $sql SQL query we will append the limit too
         * @param int $limit The SQL limit
         * @param int|bool $offset The SQL offset (default false)
         * @throws DBUnexpectedError
         * @return string
         */
        public function limitResult( $sql, $limit, $offset = false ) {
                if ( !is_numeric( $limit ) ) {
                        throw new DBUnexpectedError( $this,
                                "Invalid non-numeric limit passed to limitResult()\n" );
                }

                return "$sql LIMIT "
                . ( ( is_numeric( $offset ) && $offset != 0 ) ? "{$offset}," : "" )
                . "{$limit} ";
        }

        public function unionSupportsOrderAndLimit() {
                return true; // True for almost every DB supported
        }

        public function unionQueries( $sqls, $all ) {
                $glue = $all ? ') UNION ALL (' : ') UNION (';

                return '(' . implode( $glue, $sqls ) . ')';
        }

        public function unionConditionPermutations(
                $table, $vars, array $permute_conds, $extra_conds = '', $fname = __METHOD__,
                $options = [], $join_conds = []
        ) {
                // First, build the Cartesian product of $permute_conds
                $conds = [ [] ];
                foreach ( $permute_conds as $field => $values ) {
                        if ( !$values ) {
                                // Skip empty $values
                                continue;
                        }
                        $values = array_unique( $values ); // For sanity
                        $newConds = [];
                        foreach ( $conds as $cond ) {
                                foreach ( $values as $value ) {
                                        $cond[$field] = $value;
                                        $newConds[] = $cond; // Arrays are by-value, not by-reference, so this works
                                }
                        }
                        $conds = $newConds;
                }

                $extra_conds = $extra_conds === '' ? [] : (array)$extra_conds;

                // If there's just one condition and no subordering, hand off to
                // selectSQLText directly.
                if ( count( $conds ) === 1 &&
                        ( !isset( $options['INNER ORDER BY'] ) || !$this->unionSupportsOrderAndLimit() )
                ) {
                        return $this->selectSQLText(
                                $table, $vars, $conds[0] + $extra_conds, $fname, $options, $join_conds
                        );
                }

                // Otherwise, we need to pull out the order and limit to apply after
                // the union. Then build the SQL queries for each set of conditions in
                // $conds. Then union them together (using UNION ALL, because the
                // product *should* already be distinct).
                $orderBy = $this->makeOrderBy( $options );
                $limit = isset( $options['LIMIT'] ) ? $options['LIMIT'] : null;
                $offset = isset( $options['OFFSET'] ) ? $options['OFFSET'] : false;
                $all = empty( $options['NOTALL'] ) && !in_array( 'NOTALL', $options );
                if ( !$this->unionSupportsOrderAndLimit() ) {
                        unset( $options['ORDER BY'], $options['LIMIT'], $options['OFFSET'] );
                } else {
                        if ( array_key_exists( 'INNER ORDER BY', $options ) ) {
                                $options['ORDER BY'] = $options['INNER ORDER BY'];
                        }
                        if ( $limit !== null && is_numeric( $offset ) && $offset != 0 ) {
                                // We need to increase the limit by the offset rather than
                                // using the offset directly, otherwise it'll skip incorrectly
                                // in the subqueries.
                                $options['LIMIT'] = $limit + $offset;
                                unset( $options['OFFSET'] );
                        }
                }

                $sqls = [];
                foreach ( $conds as $cond ) {
                        $sqls[] = $this->selectSQLText(
                                $table, $vars, $cond + $extra_conds, $fname, $options, $join_conds
                        );
                }
                $sql = $this->unionQueries( $sqls, $all ) . $orderBy;
                if ( $limit !== null ) {
                        $sql = $this->limitResult( $sql, $limit, $offset );
                }

                return $sql;
        }

        public function conditional( $cond, $trueVal, $falseVal ) {
                if ( is_array( $cond ) ) {
                        $cond = $this->makeList( $cond, self::LIST_AND );
                }

                return " (CASE WHEN $cond THEN $trueVal ELSE $falseVal END) ";
        }

        public function strreplace( $orig, $old, $new ) {
                return "REPLACE({$orig}, {$old}, {$new})";
        }

        public function getServerUptime() {
                return 0;
        }

        public function wasDeadlock() {
                return false;
        }

        public function wasLockTimeout() {
                return false;
        }

        public function wasErrorReissuable() {
                return false;
        }

        public function wasReadOnlyError() {
                return false;
        }

        /**
         * Do not use this method outside of Database/DBError classes
         *
         * @param int|string $errno
         * @return bool Whether the given query error was a connection drop
         */
        public function wasConnectionError( $errno ) {
                return false;
        }

        public function deadlockLoop() {
                $args = func_get_args();
                $function = array_shift( $args );
                $tries = self::DEADLOCK_TRIES;

                $this->begin( __METHOD__ );

                $retVal = null;
                /** @var Exception $e */
                $e = null;
                do {
                        try {
                                $retVal = call_user_func_array( $function, $args );
                                break;
                        } catch ( DBQueryError $e ) {
                                if ( $this->wasDeadlock() ) {
                                        // Retry after a randomized delay
                                        usleep( mt_rand( self::DEADLOCK_DELAY_MIN, self::DEADLOCK_DELAY_MAX ) );
                                } else {
                                        // Throw the error back up
                                        throw $e;
                                }
                        }
                } while ( --$tries > 0 );

                if ( $tries <= 0 ) {
                        // Too many deadlocks; give up
                        $this->rollback( __METHOD__ );
                        throw $e;
                } else {
                        $this->commit( __METHOD__ );

                        return $retVal;
                }
        }

        public function masterPosWait( DBMasterPos $pos, $timeout ) {
                # Real waits are implemented in the subclass.
                return 0;
        }

        public function getReplicaPos() {
                # Stub
                return false;
        }

        public function getMasterPos() {
                # Stub
                return false;
        }

        public function serverIsReadOnly() {
                return false;
        }

        final public function onTransactionResolution( callable $callback, $fname = __METHOD__ ) {
                if ( !$this->trxLevel ) {
                        throw new DBUnexpectedError( $this, "No transaction is active." );
                }
                $this->trxEndCallbacks[] = [ $callback, $fname ];
        }

        final public function onTransactionIdle( callable $callback, $fname = __METHOD__ ) {
                $this->trxIdleCallbacks[] = [ $callback, $fname ];
                if ( !$this->trxLevel ) {
                        $this->runOnTransactionIdleCallbacks( self::TRIGGER_IDLE );
                }
        }

        final public function onTransactionPreCommitOrIdle( callable $callback, $fname = __METHOD__ ) {
                if ( $this->trxLevel || $this->getFlag( self::DBO_TRX ) ) {
                        // As long as DBO_TRX is set, writes will accumulate until the load balancer issues
                        // an implicit commit of all peer databases. This is true even if a transaction has
                        // not yet been triggered by writes; make sure $callback runs *after* any such writes.
                        $this->trxPreCommitCallbacks[] = [ $callback, $fname ];
                } else {
                        // No transaction is active nor will start implicitly, so make one for this callback
                        $this->startAtomic( __METHOD__ );
                        try {
                                call_user_func( $callback );
                                $this->endAtomic( __METHOD__ );
                        } catch ( Exception $e ) {
                                $this->rollback( __METHOD__, self::FLUSHING_INTERNAL );
                                throw $e;
                        }
                }
        }

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

        /**
         * Whether to disable running of post-COMMIT/ROLLBACK callbacks
         *
         * This method should not be used outside of Database/LoadBalancer
         *
         * @param bool $suppress
         * @since 1.28
         */
        final public function setTrxEndCallbackSuppression( $suppress ) {
                $this->trxEndCallbacksSuppressed = $suppress;
        }

        /**
         * Actually run and consume any "on transaction idle/resolution" callbacks.
         *
         * This method should not be used outside of Database/LoadBalancer
         *
         * @param int $trigger IDatabase::TRIGGER_* constant
         * @since 1.20
         * @throws Exception
         */
        public function runOnTransactionIdleCallbacks( $trigger ) {
                if ( $this->trxEndCallbacksSuppressed ) {
                        return;
                }

                $autoTrx = $this->getFlag( self::DBO_TRX ); // automatic begin() enabled?
                /** @var Exception $e */
                $e = null; // first exception
                do { // callbacks may add callbacks :)
                        $callbacks = array_merge(
                                $this->trxIdleCallbacks,
                                $this->trxEndCallbacks // include "transaction resolution" callbacks
                        );
                        $this->trxIdleCallbacks = []; // consumed (and recursion guard)
                        $this->trxEndCallbacks = []; // consumed (recursion guard)
                        foreach ( $callbacks as $callback ) {
                                try {
                                        list( $phpCallback ) = $callback;
                                        $this->clearFlag( self::DBO_TRX ); // make each query its own transaction
                                        call_user_func_array( $phpCallback, [ $trigger ] );
                                        if ( $autoTrx ) {
                                                $this->setFlag( self::DBO_TRX ); // restore automatic begin()
                                        } else {
                                                $this->clearFlag( self::DBO_TRX ); // restore auto-commit
                                        }
                                } catch ( Exception $ex ) {
                                        call_user_func( $this->errorLogger, $ex );
                                        $e = $e ?: $ex;
                                        // Some callbacks may use startAtomic/endAtomic, so make sure
                                        // their transactions are ended so other callbacks don't fail
                                        if ( $this->trxLevel() ) {
                                                $this->rollback( __METHOD__, self::FLUSHING_INTERNAL );
                                        }
                                }
                        }
                } while ( count( $this->trxIdleCallbacks ) );

                if ( $e instanceof Exception ) {
                        throw $e; // re-throw any first exception
                }
        }

        /**
         * Actually run and consume any "on transaction pre-commit" callbacks.
         *
         * This method should not be used outside of Database/LoadBalancer
         *
         * @since 1.22
         * @throws Exception
         */
        public function runOnTransactionPreCommitCallbacks() {
                $e = null; // first exception
                do { // callbacks may add callbacks :)
                        $callbacks = $this->trxPreCommitCallbacks;
                        $this->trxPreCommitCallbacks = []; // consumed (and recursion guard)
                        foreach ( $callbacks as $callback ) {
                                try {
                                        list( $phpCallback ) = $callback;
                                        call_user_func( $phpCallback );
                                } catch ( Exception $ex ) {
                                        call_user_func( $this->errorLogger, $ex );
                                        $e = $e ?: $ex;
                                }
                        }
                } while ( count( $this->trxPreCommitCallbacks ) );

                if ( $e instanceof Exception ) {
                        throw $e; // re-throw any first exception
                }
        }

        /**
         * Actually run any "transaction listener" callbacks.
         *
         * This method should not be used outside of Database/LoadBalancer
         *
         * @param int $trigger IDatabase::TRIGGER_* constant
         * @throws Exception
         * @since 1.20
         */
        public function runTransactionListenerCallbacks( $trigger ) {
                if ( $this->trxEndCallbacksSuppressed ) {
                        return;
                }

                /** @var Exception $e */
                $e = null; // first exception

                foreach ( $this->trxRecurringCallbacks as $phpCallback ) {
                        try {
                                $phpCallback( $trigger, $this );
                        } catch ( Exception $ex ) {
                                call_user_func( $this->errorLogger, $ex );
                                $e = $e ?: $ex;
                        }
                }

                if ( $e instanceof Exception ) {
                        throw $e; // re-throw any first exception
                }
        }

        final public function startAtomic( $fname = __METHOD__ ) {
                if ( !$this->trxLevel ) {
                        $this->begin( $fname, self::TRANSACTION_INTERNAL );
                        // If DBO_TRX is set, a series of startAtomic/endAtomic pairs will result
                        // in all changes being in one transaction to keep requests transactional.
                        if ( !$this->getFlag( self::DBO_TRX ) ) {
                                $this->trxAutomaticAtomic = true;
                        }
                }

                $this->trxAtomicLevels[] = $fname;
        }

        final public function endAtomic( $fname = __METHOD__ ) {
                if ( !$this->trxLevel ) {
                        throw new DBUnexpectedError( $this, "No atomic transaction is open (got $fname)." );
                }
                if ( !$this->trxAtomicLevels ||
                        array_pop( $this->trxAtomicLevels ) !== $fname
                ) {
                        throw new DBUnexpectedError( $this, "Invalid atomic section ended (got $fname)." );
                }

                if ( !$this->trxAtomicLevels && $this->trxAutomaticAtomic ) {
                        $this->commit( $fname, self::FLUSHING_INTERNAL );
                }
        }

        final public function doAtomicSection( $fname, callable $callback ) {
                $this->startAtomic( $fname );
                try {
                        $res = call_user_func_array( $callback, [ $this, $fname ] );
                } catch ( Exception $e ) {
                        $this->rollback( $fname, self::FLUSHING_INTERNAL );
                        throw $e;
                }
                $this->endAtomic( $fname );

                return $res;
        }

        final public function begin( $fname = __METHOD__, $mode = self::TRANSACTION_EXPLICIT ) {
                // Protect against mismatched atomic section, transaction nesting, and snapshot loss
                if ( $this->trxLevel ) {
                        if ( $this->trxAtomicLevels ) {
                                $levels = implode( ', ', $this->trxAtomicLevels );
                                $msg = "$fname: Got explicit BEGIN while atomic section(s) $levels are open.";
                                throw new DBUnexpectedError( $this, $msg );
                        } elseif ( !$this->trxAutomatic ) {
                                $msg = "$fname: Explicit transaction already active (from {$this->trxFname}).";
                                throw new DBUnexpectedError( $this, $msg );
                        } else {
                                // @TODO: make this an exception at some point
                                $msg = "$fname: Implicit transaction already active (from {$this->trxFname}).";
                                $this->queryLogger->error( $msg );
                                return; // join the main transaction set
                        }
                } elseif ( $this->getFlag( self::DBO_TRX ) && $mode !== self::TRANSACTION_INTERNAL ) {
                        // @TODO: make this an exception at some point
                        $msg = "$fname: Implicit transaction expected (DBO_TRX set).";
                        $this->queryLogger->error( $msg );
                        return; // let any writes be in the main transaction
                }

                // Avoid fatals if close() was called
                $this->assertOpen();

                $this->doBegin( $fname );
                $this->trxTimestamp = microtime( true );
                $this->trxFname = $fname;
                $this->trxDoneWrites = false;
                $this->trxAutomaticAtomic = false;
                $this->trxAtomicLevels = [];
                $this->trxShortId = sprintf( '%06x', mt_rand( 0, 0xffffff ) );
                $this->trxWriteDuration = 0.0;
                $this->trxWriteQueryCount = 0;
                $this->trxWriteAffectedRows = 0;
                $this->trxWriteAdjDuration = 0.0;
                $this->trxWriteAdjQueryCount = 0;
                $this->trxWriteCallers = [];
                // First SELECT after BEGIN will establish the snapshot in REPEATABLE-READ.
                // Get an estimate of the replica DB lag before then, treating estimate staleness
                // as lag itself just to be safe
                $status = $this->getApproximateLagStatus();
                $this->trxReplicaLag = $status['lag'] + ( microtime( true ) - $status['since'] );
                // T147697: make explicitTrxActive() return true until begin() finishes. This way, no
                // caller will think its OK to muck around with the transaction just because startAtomic()
                // has not yet completed (e.g. setting trxAtomicLevels).
                $this->trxAutomatic = ( $mode === self::TRANSACTION_INTERNAL );
        }

        /**
         * Issues the BEGIN command to the database server.
         *
         * @see Database::begin()
         * @param string $fname
         */
        protected function doBegin( $fname ) {
                $this->query( 'BEGIN', $fname );
                $this->trxLevel = 1;
        }

        final public function commit( $fname = __METHOD__, $flush = '' ) {
                if ( $this->trxLevel && $this->trxAtomicLevels ) {
                        // There are still atomic sections open. This cannot be ignored
                        $levels = implode( ', ', $this->trxAtomicLevels );
                        throw new DBUnexpectedError(
                                $this,
                                "$fname: Got COMMIT while atomic sections $levels are still open."
                        );
                }

                if ( $flush === self::FLUSHING_INTERNAL || $flush === self::FLUSHING_ALL_PEERS ) {
                        if ( !$this->trxLevel ) {
                                return; // nothing to do
                        } elseif ( !$this->trxAutomatic ) {
                                throw new DBUnexpectedError(
                                        $this,
                                        "$fname: Flushing an explicit transaction, getting out of sync."
                                );
                        }
                } else {
                        if ( !$this->trxLevel ) {
                                $this->queryLogger->error(
                                        "$fname: No transaction to commit, something got out of sync." );
                                return; // nothing to do
                        } elseif ( $this->trxAutomatic ) {
                                // @TODO: make this an exception at some point
                                $msg = "$fname: Explicit commit of implicit transaction.";
                                $this->queryLogger->error( $msg );
                                return; // wait for the main transaction set commit round
                        }
                }

                // Avoid fatals if close() was called
                $this->assertOpen();

                $this->runOnTransactionPreCommitCallbacks();
                $writeTime = $this->pendingWriteQueryDuration( self::ESTIMATE_DB_APPLY );
                $this->doCommit( $fname );
                if ( $this->trxDoneWrites ) {
                        $this->lastWriteTime = microtime( true );
                        $this->trxProfiler->transactionWritingOut(
                                $this->server,
                                $this->dbName,
                                $this->trxShortId,
                                $writeTime,
                                $this->trxWriteAffectedRows
                        );
                }

                $this->runOnTransactionIdleCallbacks( self::TRIGGER_COMMIT );
                $this->runTransactionListenerCallbacks( self::TRIGGER_COMMIT );
        }

        /**
         * Issues the COMMIT command to the database server.
         *
         * @see Database::commit()
         * @param string $fname
         */
        protected function doCommit( $fname ) {
                if ( $this->trxLevel ) {
                        $this->query( 'COMMIT', $fname );
                        $this->trxLevel = 0;
                }
        }

        final public function rollback( $fname = __METHOD__, $flush = '' ) {
                if ( $flush === self::FLUSHING_INTERNAL || $flush === self::FLUSHING_ALL_PEERS ) {
                        if ( !$this->trxLevel ) {
                                return; // nothing to do
                        }
                } else {
                        if ( !$this->trxLevel ) {
                                $this->queryLogger->error(
                                        "$fname: No transaction to rollback, something got out of sync." );
                                return; // nothing to do
                        } elseif ( $this->getFlag( self::DBO_TRX ) ) {
                                throw new DBUnexpectedError(
                                        $this,
                                        "$fname: Expected mass rollback of all peer databases (DBO_TRX set)."
                                );
                        }
                }

                // Avoid fatals if close() was called
                $this->assertOpen();

                $this->doRollback( $fname );
                $this->trxAtomicLevels = [];
                if ( $this->trxDoneWrites ) {
                        $this->trxProfiler->transactionWritingOut(
                                $this->server,
                                $this->dbName,
                                $this->trxShortId
                        );
                }

                $this->trxIdleCallbacks = []; // clear
                $this->trxPreCommitCallbacks = []; // clear
                try {
                        $this->runOnTransactionIdleCallbacks( self::TRIGGER_ROLLBACK );
                } catch ( Exception $e ) {
                        // already logged; finish and let LoadBalancer move on during mass-rollback
                }
                try {
                        $this->runTransactionListenerCallbacks( self::TRIGGER_ROLLBACK );
                } catch ( Exception $e ) {
                        // already logged; let LoadBalancer move on during mass-rollback
                }

                $this->affectedRowCount = 0; // for the sake of consistency
        }

        /**
         * Issues the ROLLBACK command to the database server.
         *
         * @see Database::rollback()
         * @param string $fname
         */
        protected function doRollback( $fname ) {
                if ( $this->trxLevel ) {
                        # Disconnects cause rollback anyway, so ignore those errors
                        $ignoreErrors = true;
                        $this->query( 'ROLLBACK', $fname, $ignoreErrors );
                        $this->trxLevel = 0;
                }
        }

        public function flushSnapshot( $fname = __METHOD__ ) {
                if ( $this->writesOrCallbacksPending() || $this->explicitTrxActive() ) {
                        // This only flushes transactions to clear snapshots, not to write data
                        $fnames = implode( ', ', $this->pendingWriteAndCallbackCallers() );
                        throw new DBUnexpectedError(
                                $this,
                                "$fname: Cannot flush snapshot because writes are pending ($fnames)."
                        );
                }

                $this->commit( $fname, self::FLUSHING_INTERNAL );
        }

        public function explicitTrxActive() {
                return $this->trxLevel && ( $this->trxAtomicLevels || !$this->trxAutomatic );
        }

        public function duplicateTableStructure(
                $oldName, $newName, $temporary = false, $fname = __METHOD__
        ) {
                throw new RuntimeException( __METHOD__ . ' is not implemented in descendant class' );
        }

        public function listTables( $prefix = null, $fname = __METHOD__ ) {
                throw new RuntimeException( __METHOD__ . ' is not implemented in descendant class' );
        }

        public function listViews( $prefix = null, $fname = __METHOD__ ) {
                throw new RuntimeException( __METHOD__ . ' is not implemented in descendant class' );
        }

        public function timestamp( $ts = 0 ) {
                $t = new ConvertibleTimestamp( $ts );
                // Let errors bubble up to avoid putting garbage in the DB
                return $t->getTimestamp( TS_MW );
        }

        public function timestampOrNull( $ts = null ) {
                if ( is_null( $ts ) ) {
                        return null;
                } else {
                        return $this->timestamp( $ts );
                }
        }

        public function affectedRows() {
                return ( $this->affectedRowCount === null )
                        ? $this->fetchAffectedRowCount() // default to driver value
                        : $this->affectedRowCount;
        }

        /**
         * @return int Number of retrieved rows according to the driver
         */
        abstract protected function fetchAffectedRowCount();

        /**
         * Take the result from a query, and wrap it in a ResultWrapper if
         * necessary. Boolean values are passed through as is, to indicate success
         * of write queries or failure.
         *
         * Once upon a time, Database::query() returned a bare MySQL result
         * resource, and it was necessary to call this function to convert it to
         * a wrapper. Nowadays, raw database objects are never exposed to external
         * callers, so this is unnecessary in external code.
         *
         * @param bool|ResultWrapper|resource|object $result
         * @return bool|ResultWrapper
         */
        protected function resultObject( $result ) {
                if ( !$result ) {
                        return false;
                } elseif ( $result instanceof ResultWrapper ) {
                        return $result;
                } elseif ( $result === true ) {
                        // Successful write query
                        return $result;
                } else {
                        return new ResultWrapper( $this, $result );
                }
        }

        public function ping( &$rtt = null ) {
                // Avoid hitting the server if it was hit recently
                if ( $this->isOpen() && ( microtime( true ) - $this->lastPing ) < self::PING_TTL ) {
                        if ( !func_num_args() || $this->rttEstimate > 0 ) {
                                $rtt = $this->rttEstimate;
                                return true; // don't care about $rtt
                        }
                }

                // This will reconnect if possible or return false if not
                $this->clearFlag( self::DBO_TRX, self::REMEMBER_PRIOR );
                $ok = ( $this->query( self::PING_QUERY, __METHOD__, true ) !== false );
                $this->restoreFlags( self::RESTORE_PRIOR );

                if ( $ok ) {
                        $rtt = $this->rttEstimate;
                }

                return $ok;
        }

        /**
         * Close existing database connection and open a new connection
         *
         * @return bool True if new connection is opened successfully, false if error
         */
        protected function reconnect() {
                $this->closeConnection();
                $this->opened = false;
                $this->conn = false;
                try {
                        $this->open( $this->server, $this->user, $this->password, $this->dbName );
                        $this->lastPing = microtime( true );
                        $ok = true;
                } catch ( DBConnectionError $e ) {
                        $ok = false;
                }

                return $ok;
        }

        public function getSessionLagStatus() {
                return $this->getTransactionLagStatus() ?: $this->getApproximateLagStatus();
        }

        /**
         * Get the replica DB lag when the current transaction started
         *
         * This is useful when transactions might use snapshot isolation
         * (e.g. REPEATABLE-READ in innodb), so the "real" lag of that data
         * is this lag plus transaction duration. If they don't, it is still
         * safe to be pessimistic. This returns null if there is no transaction.
         *
         * @return array|null ('lag': seconds or false on error, 'since': UNIX timestamp of BEGIN)
         * @since 1.27
         */
        protected function getTransactionLagStatus() {
                return $this->trxLevel
                        ? [ 'lag' => $this->trxReplicaLag, 'since' => $this->trxTimestamp() ]
                        : null;
        }

        /**
         * Get a replica DB lag estimate for this server
         *
         * @return array ('lag': seconds or false on error, 'since': UNIX timestamp of estimate)
         * @since 1.27
         */
        protected function getApproximateLagStatus() {
                return [
                        'lag'   => $this->getLBInfo( 'replica' ) ? $this->getLag() : 0,
                        'since' => microtime( true )
                ];
        }

        /**
         * Merge the result of getSessionLagStatus() for several DBs
         * using the most pessimistic values to estimate the lag of
         * any data derived from them in combination
         *
         * This is information is useful for caching modules
         *
         * @see WANObjectCache::set()
         * @see WANObjectCache::getWithSetCallback()
         *
         * @param IDatabase $db1
         * @param IDatabase $db2 [optional]
         * @return array Map of values:
         *   - lag: highest lag of any of the DBs or false on error (e.g. replication stopped)
         *   - since: oldest UNIX timestamp of any of the DB lag estimates
         *   - pending: whether any of the DBs have uncommitted changes
         * @throws DBError
         * @since 1.27
         */
        public static function getCacheSetOptions( IDatabase $db1, IDatabase $db2 = null ) {
                $res = [ 'lag' => 0, 'since' => INF, 'pending' => false ];
                foreach ( func_get_args() as $db ) {
                        /** @var IDatabase $db */
                        $status = $db->getSessionLagStatus();
                        if ( $status['lag'] === false ) {
                                $res['lag'] = false;
                        } elseif ( $res['lag'] !== false ) {
                                $res['lag'] = max( $res['lag'], $status['lag'] );
                        }
                        $res['since'] = min( $res['since'], $status['since'] );
                        $res['pending'] = $res['pending'] ?: $db->writesPending();
                }

                return $res;
        }

        public function getLag() {
                return 0;
        }

        public function maxListLen() {
                return 0;
        }

        public function encodeBlob( $b ) {
                return $b;
        }

        public function decodeBlob( $b ) {
                if ( $b instanceof Blob ) {
                        $b = $b->fetch();
                }
                return $b;
        }

        public function setSessionOptions( array $options ) {
        }

        public function sourceFile(
                $filename,
                callable $lineCallback = null,
                callable $resultCallback = null,
                $fname = false,
                callable $inputCallback = null
        ) {
                Wikimedia\suppressWarnings();
                $fp = fopen( $filename, 'r' );
                Wikimedia\restoreWarnings();

                if ( false === $fp ) {
                        throw new RuntimeException( "Could not open \"{$filename}\".\n" );
                }

                if ( !$fname ) {
                        $fname = __METHOD__ . "( $filename )";
                }

                try {
                        $error = $this->sourceStream(
                                $fp, $lineCallback, $resultCallback, $fname, $inputCallback );
                } catch ( Exception $e ) {
                        fclose( $fp );
                        throw $e;
                }

                fclose( $fp );

                return $error;
        }

        public function setSchemaVars( $vars ) {
                $this->schemaVars = $vars;
        }

        public function sourceStream(
                $fp,
                callable $lineCallback = null,
                callable $resultCallback = null,
                $fname = __METHOD__,
                callable $inputCallback = null
        ) {
                $delimiterReset = new ScopedCallback(
                        function ( $delimiter ) {
                                $this->delimiter = $delimiter;
                        },
                        [ $this->delimiter ]
                );
                $cmd = '';

                while ( !feof( $fp ) ) {
                        if ( $lineCallback ) {
                                call_user_func( $lineCallback );
                        }

                        $line = trim( fgets( $fp ) );

                        if ( $line == '' ) {
                                continue;
                        }

                        if ( '-' == $line[0] && '-' == $line[1] ) {
                                continue;
                        }

                        if ( $cmd != '' ) {
                                $cmd .= ' ';
                        }

                        $done = $this->streamStatementEnd( $cmd, $line );

                        $cmd .= "$line\n";

                        if ( $done || feof( $fp ) ) {
                                $cmd = $this->replaceVars( $cmd );

                                if ( $inputCallback ) {
                                        $callbackResult = call_user_func( $inputCallback, $cmd );

                                        if ( is_string( $callbackResult ) || !$callbackResult ) {
                                                $cmd = $callbackResult;
                                        }
                                }

                                if ( $cmd ) {
                                        $res = $this->query( $cmd, $fname );

                                        if ( $resultCallback ) {
                                                call_user_func( $resultCallback, $res, $this );
                                        }

                                        if ( false === $res ) {
                                                $err = $this->lastError();

                                                return "Query \"{$cmd}\" failed with error code \"$err\".\n";
                                        }
                                }
                                $cmd = '';
                        }
                }

                ScopedCallback::consume( $delimiterReset );
                return true;
        }

        /**
         * Called by sourceStream() to check if we've reached a statement end
         *
         * @param string &$sql SQL assembled so far
         * @param string &$newLine New line about to be added to $sql
         * @return bool Whether $newLine contains end of the statement
         */
        public function streamStatementEnd( &$sql, &$newLine ) {
                if ( $this->delimiter ) {
                        $prev = $newLine;
                        $newLine = preg_replace(
                                '/' . preg_quote( $this->delimiter, '/' ) . '$/', '', $newLine );
                        if ( $newLine != $prev ) {
                                return true;
                        }
                }

                return false;
        }

        /**
         * Database independent variable replacement. Replaces a set of variables
         * in an SQL statement with their contents as given by $this->getSchemaVars().
         *
         * Supports '{$var}' `{$var}` and / *$var* / (without the spaces) style variables.
         *
         * - '{$var}' should be used for text and is passed through the database's
         *   addQuotes method.
         * - `{$var}` should be used for identifiers (e.g. table and database names).
         *   It is passed through the database's addIdentifierQuotes method which
         *   can be overridden if the database uses something other than backticks.
         * - / *_* / or / *$wgDBprefix* / passes the name that follows through the
         *   database's tableName method.
         * - / *i* / passes the name that follows through the database's indexName method.
         * - In all other cases, / *$var* / is left unencoded. Except for table options,
         *   its use should be avoided. In 1.24 and older, string encoding was applied.
         *
         * @param string $ins SQL statement to replace variables in
         * @return string The new SQL statement with variables replaced
         */
        protected function replaceVars( $ins ) {
                $vars = $this->getSchemaVars();
                return preg_replace_callback(
                        '!
                                /\* (\$wgDBprefix|[_i]) \*/ (\w*) | # 1-2. tableName, indexName
                                \'\{\$ (\w+) }\'                  | # 3. addQuotes
                                `\{\$ (\w+) }`                    | # 4. addIdentifierQuotes
                                /\*\$ (\w+) \*/                     # 5. leave unencoded
                        !x',
                        function ( $m ) use ( $vars ) {
                                // Note: Because of <https://bugs.php.net/bug.php?id=51881>,
                                // check for both nonexistent keys *and* the empty string.
                                if ( isset( $m[1] ) && $m[1] !== '' ) {
                                        if ( $m[1] === 'i' ) {
                                                return $this->indexName( $m[2] );
                                        } else {
                                                return $this->tableName( $m[2] );
                                        }
                                } elseif ( isset( $m[3] ) && $m[3] !== '' && array_key_exists( $m[3], $vars ) ) {
                                        return $this->addQuotes( $vars[$m[3]] );
                                } elseif ( isset( $m[4] ) && $m[4] !== '' && array_key_exists( $m[4], $vars ) ) {
                                        return $this->addIdentifierQuotes( $vars[$m[4]] );
                                } elseif ( isset( $m[5] ) && $m[5] !== '' && array_key_exists( $m[5], $vars ) ) {
                                        return $vars[$m[5]];
                                } else {
                                        return $m[0];
                                }
                        },
                        $ins
                );
        }

        /**
         * Get schema variables. If none have been set via setSchemaVars(), then
         * use some defaults from the current object.
         *
         * @return array
         */
        protected function getSchemaVars() {
                if ( $this->schemaVars ) {
                        return $this->schemaVars;
                } else {
                        return $this->getDefaultSchemaVars();
                }
        }

        /**
         * Get schema variables to use if none have been set via setSchemaVars().
         *
         * Override this in derived classes to provide variables for tables.sql
         * and SQL patch files.
         *
         * @return array
         */
        protected function getDefaultSchemaVars() {
                return [];
        }

        public function lockIsFree( $lockName, $method ) {
                // RDBMs methods for checking named locks may or may not count this thread itself.
                // In MySQL, IS_FREE_LOCK() returns 0 if the thread already has the lock. This is
                // the behavior choosen by the interface for this method.
                return !isset( $this->namedLocksHeld[$lockName] );
        }

        public function lock( $lockName, $method, $timeout = 5 ) {
                $this->namedLocksHeld[$lockName] = 1;

                return true;
        }

        public function unlock( $lockName, $method ) {
                unset( $this->namedLocksHeld[$lockName] );

                return true;
        }

        public function getScopedLockAndFlush( $lockKey, $fname, $timeout ) {
                if ( $this->writesOrCallbacksPending() ) {
                        // This only flushes transactions to clear snapshots, not to write data
                        $fnames = implode( ', ', $this->pendingWriteAndCallbackCallers() );
                        throw new DBUnexpectedError(
                                $this,
                                "$fname: Cannot flush pre-lock snapshot because writes are pending ($fnames)."
                        );
                }

                if ( !$this->lock( $lockKey, $fname, $timeout ) ) {
                        return null;
                }

                $unlocker = new ScopedCallback( function () use ( $lockKey, $fname ) {
                        if ( $this->trxLevel() ) {
                                // There is a good chance an exception was thrown, causing any early return
                                // from the caller. Let any error handler get a chance to issue rollback().
                                // If there isn't one, let the error bubble up and trigger server-side rollback.
                                $this->onTransactionResolution(
                                        function () use ( $lockKey, $fname ) {
                                                $this->unlock( $lockKey, $fname );
                                        },
                                        $fname
                                );
                        } else {
                                $this->unlock( $lockKey, $fname );
                        }
                } );

                $this->commit( $fname, self::FLUSHING_INTERNAL );

                return $unlocker;
        }

        public function namedLocksEnqueue() {
                return false;
        }

        public function tableLocksHaveTransactionScope() {
                return true;
        }

        final public function lockTables( array $read, array $write, $method ) {
                if ( $this->writesOrCallbacksPending() ) {
                        throw new DBUnexpectedError( $this, "Transaction writes or callbacks still pending." );
                }

                if ( $this->tableLocksHaveTransactionScope() ) {
                        $this->startAtomic( $method );
                }

                return $this->doLockTables( $read, $write, $method );
        }

        /**
         * Helper function for lockTables() that handles the actual table locking
         *
         * @param array $read Array of tables to lock for read access
         * @param array $write Array of tables to lock for write access
         * @param string $method Name of caller
         * @return true
         */
        protected function doLockTables( array $read, array $write, $method ) {
                return true;
        }

        final public function unlockTables( $method ) {
                if ( $this->tableLocksHaveTransactionScope() ) {
                        $this->endAtomic( $method );

                        return true; // locks released on COMMIT/ROLLBACK
                }

                return $this->doUnlockTables( $method );
        }

        /**
         * Helper function for unlockTables() that handles the actual table unlocking
         *
         * @param string $method Name of caller
         * @return true
         */
        protected function doUnlockTables( $method ) {
                return true;
        }

        /**
         * Delete a table
         * @param string $tableName
         * @param string $fName
         * @return bool|ResultWrapper
         * @since 1.18
         */
        public function dropTable( $tableName, $fName = __METHOD__ ) {
                if ( !$this->tableExists( $tableName, $fName ) ) {
                        return false;
                }
                $sql = "DROP TABLE " . $this->tableName( $tableName ) . " CASCADE";

                return $this->query( $sql, $fName );
        }

        public function getInfinity() {
                return 'infinity';
        }

        public function encodeExpiry( $expiry ) {
                return ( $expiry == '' || $expiry == 'infinity' || $expiry == $this->getInfinity() )
                        ? $this->getInfinity()
                        : $this->timestamp( $expiry );
        }

        public function decodeExpiry( $expiry, $format = TS_MW ) {
                if ( $expiry == '' || $expiry == 'infinity' || $expiry == $this->getInfinity() ) {
                        return 'infinity';
                }

                return ConvertibleTimestamp::convert( $format, $expiry );
        }

        public function setBigSelects( $value = true ) {
                // no-op
        }

        public function isReadOnly() {
                return ( $this->getReadOnlyReason() !== false );
        }

        /**
         * @return string|bool Reason this DB is read-only or false if it is not
         */
        protected function getReadOnlyReason() {
                $reason = $this->getLBInfo( 'readOnlyReason' );

                return is_string( $reason ) ? $reason : false;
        }

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

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

        /**
         * @return bool Whether a DB user is required to access the DB
         * @since 1.28
         */
        protected function requiresDatabaseUser() {
                return true;
        }

        /**
         * Get the underlying binding connection handle
         *
         * Makes sure the connection resource is set (disconnects and ping() failure can unset it).
         * This catches broken callers than catch and ignore disconnection exceptions.
         * Unlike checking isOpen(), this is safe to call inside of open().
         *
         * @return resource|object
         * @throws DBUnexpectedError
         * @since 1.26
         */
        protected function getBindingHandle() {
                if ( !$this->conn ) {
                        throw new DBUnexpectedError(
                                $this,
                                'DB connection was already closed or the connection dropped.'
                        );
                }

                return $this->conn;
        }

        /**
         * @since 1.19
         * @return string
         */
        public function __toString() {
                return (string)$this->conn;
        }

        /**
         * Make sure that copies do not share the same client binding handle
         * @throws DBConnectionError
         */
        public function __clone() {
                $this->connLogger->warning(
                        "Cloning " . static::class . " is not recomended; forking connection:\n" .
                        ( new RuntimeException() )->getTraceAsString()
                );

                if ( $this->isOpen() ) {
                        // Open a new connection resource without messing with the old one
                        $this->opened = false;
                        $this->conn = false;
                        $this->trxEndCallbacks = []; // don't copy
                        $this->handleSessionLoss(); // no trx or locks anymore
                        $this->open( $this->server, $this->user, $this->password, $this->dbName );
                        $this->lastPing = microtime( true );
                }
        }

        /**
         * Called by serialize. Throw an exception when DB connection is serialized.
         * This causes problems on some database engines because the connection is
         * not restored on unserialize.
         */
        public function __sleep() {
                throw new RuntimeException( 'Database serialization may cause problems, since ' .
                        'the connection is not restored on wakeup.' );
        }

        /**
         * Run a few simple sanity checks and close dangling connections
         */
        public function __destruct() {
                if ( $this->trxLevel && $this->trxDoneWrites ) {
                        trigger_error( "Uncommitted DB writes (transaction from {$this->trxFname})." );
                }

                $danglingWriters = $this->pendingWriteAndCallbackCallers();
                if ( $danglingWriters ) {
                        $fnames = implode( ', ', $danglingWriters );
                        trigger_error( "DB transaction writes or callbacks still pending ($fnames)." );
                }

                if ( $this->conn ) {
                        // Avoid connection leaks for sanity. Normally, resources close at script completion.
                        // The connection might already be closed in zend/hhvm by now, so suppress warnings.
                        Wikimedia\suppressWarnings();
                        $this->closeConnection();
                        Wikimedia\restoreWarnings();
                        $this->conn = false;
                        $this->opened = false;
                }
        }
}

class_alias( Database::class, 'DatabaseBase' ); // b/c for old name
class_alias( Database::class, 'Database' ); // b/c global alias