Database.php

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<?php
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 LogicException;
use InvalidArgumentException;
use UnexpectedValueException;
use Exception;
use RuntimeException;

abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAwareInterface {
    const DEADLOCK_TRIES = 4;
    const DEADLOCK_DELAY_MIN = 500000;
    const DEADLOCK_DELAY_MAX = 1500000;

    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;

    const ATTR_DB_LEVEL_LOCKING = 'db-level-locking';
    const ATTR_SCHEMAS_AS_TABLE_GROUPS = 'supports-schemas';

    const NEW_UNCONNECTED = 0;
    const NEW_CONNECTED = 1;

    protected $lastQuery = '';
    protected $lastWriteTime = false;
    protected $phpError = false;
    protected $server;
    protected $user;
    protected $password;
    protected $tableAliases = [];
    protected $indexAliases = [];
    protected $cliMode;
    protected $agent;
    protected $connectionParams = [];
    protected $srvCache;
    protected $connLogger;
    protected $queryLogger;
    protected $errorLogger;
    protected $deprecationLogger;

    protected $conn = null;
    protected $opened = false;

    protected $trxIdleCallbacks = [];
    protected $trxPreCommitCallbacks = [];
    protected $trxEndCallbacks = [];
    protected $trxRecurringCallbacks = [];
    protected $trxEndCallbacksSuppressed = false;

    protected $flags;
    protected $lbInfo = [];
    protected $schemaVars = false;
    protected $sessionVars = [];
    protected $preparedArgs;
    protected $htmlErrors;
    protected $delimiter = ';';
    protected $currentDomain;
    protected $affectedRowCount;

    protected $trxStatus = self::STATUS_TRX_NONE;
    protected $trxStatusCause;
    protected $trxStatusIgnoredCause;
    protected $trxLevel = 0;
    protected $trxShortId = '';
    private $trxTimestamp = null;
    private $trxReplicaLag = null;
    private $trxFname = null;
    private $trxDoneWrites = false;
    private $trxAutomatic = false;
    private $trxAtomicCounter = 0;
    private $trxAtomicLevels = [];
    private $trxAutomaticAtomic = false;
    private $trxWriteCallers = [];
    private $trxWriteDuration = 0.0;
    private $trxWriteQueryCount = 0;
    private $trxWriteAffectedRows = 0;
    private $trxWriteAdjDuration = 0.0;
    private $trxWriteAdjQueryCount = 0;
    private $rttEstimate = 0.0;

    private $namedLocksHeld = [];
    protected $sessionTempTables = [];

    private $lazyMasterHandle;

    protected $lastPing = 0.0;

    private $priorFlags = [];

    protected $profiler;
    protected $trxProfiler;

    protected $nonNativeInsertSelectBatchSize = 10000;

    private static $NOT_APPLICABLE = 'n/a';
    private static $SAVEPOINT_PREFIX = 'wikimedia_rdbms_atomic';

    const STATUS_TRX_ERROR = 1;
    const STATUS_TRX_OK = 2;
    const STATUS_TRX_NONE = 3;

    const TEMP_NORMAL = 1;
    const TEMP_PSEUDO_PERMANENT = 2;

    protected function __construct( array $params ) {
        foreach ( [ 'host', 'user', 'password', 'dbname', 'schema', 'tablePrefix' ] as $name ) {
            $this->connectionParams[$name] = $params[$name];
        }

        $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;
            }
        }
        // Disregard deprecated DBO_IGNORE flag (T189999)
        $this->flags &= ~self::DBO_IGNORE;

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

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

        $this->profiler = is_callable( $params['profiler'] ) ? $params['profiler'] : null;
        $this->trxProfiler = $params['trxProfiler'];
        $this->connLogger = $params['connLogger'];
        $this->queryLogger = $params['queryLogger'];
        $this->errorLogger = $params['errorLogger'];
        $this->deprecationLogger = $params['deprecationLogger'];

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

        // Set initial dummy domain until open() sets the final DB/prefix
        $this->currentDomain = new DatabaseDomain(
            $params['dbname'] != '' ? $params['dbname'] : null,
            $params['schema'] != '' ? $params['schema'] : null,
            $params['tablePrefix']
        );
    }

    final public function initConnection() {
        if ( $this->isOpen() ) {
            throw new LogicException( __METHOD__ . ': already connected.' );
        }
        // Establish the connection
        $this->doInitConnection();
    }

    protected function doInitConnection() {
        if ( strlen( $this->connectionParams['user'] ) ) {
            $this->open(
                $this->connectionParams['host'],
                $this->connectionParams['user'],
                $this->connectionParams['password'],
                $this->connectionParams['dbname'],
                $this->connectionParams['schema'],
                $this->connectionParams['tablePrefix']
            );
        } else {
            throw new InvalidArgumentException( "No database user provided." );
        }
    }

    abstract protected function open( $server, $user, $password, $dbName, $schema, $tablePrefix );

    final public static function factory( $dbType, $p = [], $connect = self::NEW_CONNECTED ) {
        $class = self::getClass( $dbType, $p['driver'] ?? null );

        if ( class_exists( $class ) && is_subclass_of( $class, IDatabase::class ) ) {
            // Resolve some defaults for b/c
            $p['host'] = $p['host'] ?? false;
            $p['user'] = $p['user'] ?? false;
            $p['password'] = $p['password'] ?? false;
            $p['dbname'] = $p['dbname'] ?? false;
            $p['flags'] = $p['flags'] ?? 0;
            $p['variables'] = $p['variables'] ?? [];
            $p['tablePrefix'] = $p['tablePrefix'] ?? '';
            $p['schema'] = $p['schema'] ?? null;
            $p['cliMode'] = $p['cliMode'] ?? ( PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg' );
            $p['agent'] = $p['agent'] ?? '';
            if ( !isset( $p['connLogger'] ) ) {
                $p['connLogger'] = new NullLogger();
            }
            if ( !isset( $p['queryLogger'] ) ) {
                $p['queryLogger'] = new NullLogger();
            }
            $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 );
                };
            }
            if ( !isset( $p['deprecationLogger'] ) ) {
                $p['deprecationLogger'] = function ( $msg ) {
                    trigger_error( $msg, E_USER_DEPRECATED );
                };
            }

            $conn = new $class( $p );
            if ( $connect == self::NEW_CONNECTED ) {
                $conn->initConnection();
            }
        } else {
            $conn = null;
        }

        return $conn;
    }

    final public static function attributesFromType( $dbType, $driver = null ) {
        static $defaults = [
            self::ATTR_DB_LEVEL_LOCKING => false,
            self::ATTR_SCHEMAS_AS_TABLE_GROUPS => false
        ];

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

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

    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 = $possibleDrivers;
            } elseif ( (string)$driver !== '' ) {
                if ( !isset( $possibleDrivers[$driver] ) ) {
                    throw new InvalidArgumentException( __METHOD__ .
                        " type '$dbType' does not support driver '{$driver}'" );
                }

                $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;
    }

    protected static function getAttributes() {
        return [];
    }

    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;
    }

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

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

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

    public function tablePrefix( $prefix = null ) {
        $old = $this->currentDomain->getTablePrefix();
        if ( $prefix !== null ) {
            $this->currentDomain = new DatabaseDomain(
                $this->currentDomain->getDatabase(),
                $this->currentDomain->getSchema(),
                $prefix
            );
        }

        return $old;
    }

    public function dbSchema( $schema = null ) {
        if ( strlen( $schema ) && $this->getDBname() === null ) {
            throw new DBUnexpectedError( $this, "Cannot set schema to '$schema'; no database set." );
        }

        $old = $this->currentDomain->getSchema();
        if ( $schema !== null ) {
            $this->currentDomain = new DatabaseDomain(
                $this->currentDomain->getDatabase(),
                // DatabaseDomain uses null for unspecified schemas
                strlen( $schema ) ? $schema : null,
                $this->currentDomain->getTablePrefix()
            );
        }

        return (string)$old;
    }

    protected function relationSchemaQualifier() {
        return $this->dbSchema();
    }

    public function getLBInfo( $name = null ) {
        if ( is_null( $name ) ) {
            return $this->lbInfo;
        }

        if ( array_key_exists( $name, $this->lbInfo ) ) {
            return $this->lbInfo[$name];
        }

        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;
    }

    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 preCommitCallbacksPending() {
        return $this->trxLevel && $this->trxPreCommitCallbacks;
    }

    final protected function getTransactionRoundId() {
        // If transaction round participation is enabled, see if one is active
        if ( $this->getFlag( self::DBO_TRX ) ) {
            $id = $this->getLBInfo( 'trxRoundId' );

            return is_string( $id ) ? $id : null;
        }

        return null;
    }

    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:
                return $this->pingAndCalculateLastTrxApplyTime();
            default: // everything
                return $this->trxWriteDuration;
        }
    }

    private function pingAndCalculateLastTrxApplyTime() {
        $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;
    }

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

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

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

        return $fnames;
    }

    private function flatAtomicSectionList() {
        return array_reduce( $this->trxAtomicLevels, function ( $accum, $v ) {
            return $accum === null ? $v[0] : "$accum, " . $v[0];
        } );
    }

    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 (bool)( $this->flags & $flag );
    }

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

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

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

    abstract function indexInfo( $table, $index, $fname = __METHOD__ );

    abstract function strencode( $s );

    protected function installErrorHandler() {
        $this->phpError = false;
        $this->htmlErrors = ini_set( 'html_errors', '0' );
        set_error_handler( [ $this, 'connectionErrorLogger' ] );
    }

    protected function restoreErrorHandler() {
        restore_error_handler();
        if ( $this->htmlErrors !== false ) {
            ini_set( 'html_errors', $this->htmlErrors );
        }

        return $this->getLastPHPError();
    }

    protected function getLastPHPError() {
        if ( $this->phpError ) {
            $error = preg_replace( '!\[<a.*</a>\]!', '', $this->phpError );
            $error = preg_replace( '!^.*?:\s?(.*)$!', '$1', $error );

            return $error;
        }

        return false;
    }

    public function connectionErrorLogger( $errno, $errstr ) {
        $this->phpError = $errstr;
    }

    protected function getLogContext( array $extras = [] ) {
        return array_merge(
            [
                'db_server' => $this->server,
                'db_name' => $this->getDBname(),
                'db_user' => $this->user,
            ],
            $extras
        );
    }

    final public function close() {
        $exception = null; // error to throw after disconnecting

        $wasOpen = $this->opened;
        // This should mostly do nothing if the connection is already closed
        if ( $this->conn ) {
            // Roll back any dangling transaction first
            if ( $this->trxLevel ) {
                if ( $this->trxAtomicLevels ) {
                    // Cannot let incomplete atomic sections be committed
                    $levels = $this->flatAtomicSectionList();
                    $exception = new DBUnexpectedError(
                        $this,
                        __METHOD__ . ": atomic sections $levels are still open."
                    );
                } elseif ( $this->trxAutomatic ) {
                    // Only the connection manager can commit non-empty DBO_TRX transactions
                    // (empty ones we can silently roll back)
                    if ( $this->writesOrCallbacksPending() ) {
                        $exception = new DBUnexpectedError(
                            $this,
                            __METHOD__ .
                            ": mass commit/rollback of peer transaction required (DBO_TRX set)."
                        );
                    }
                } else {
                    // Manual transactions should have been committed or rolled
                    // back, even if empty.
                    $exception = new DBUnexpectedError(
                        $this,
                        __METHOD__ . ": transaction is still open (from {$this->trxFname})."
                    );
                }

                if ( $this->trxEndCallbacksSuppressed ) {
                    $exception = $exception ?: new DBUnexpectedError(
                        $this,
                        __METHOD__ . ': callbacks are suppressed; cannot properly commit.'
                    );
                }

                // Rollback the changes and run any callbacks as needed
                $this->rollback( __METHOD__, self::FLUSHING_INTERNAL );
            }

            // Close the actual connection in the binding handle
            $closed = $this->closeConnection();
        } else {
            $closed = true; // already closed; nothing to do
        }

        $this->conn = false;
        $this->opened = false;

        // Throw any unexpected errors after having disconnected
        if ( $exception instanceof Exception ) {
            throw $exception;
        }

        // Note that various subclasses call close() at the start of open(), which itself is
        // called by replaceLostConnection(). In that case, just because onTransactionResolution()
        // callbacks are pending does not mean that an exception should be thrown. Rather, they
        // will be executed after the reconnection step.
        if ( $wasOpen ) {
            // Sanity check that no callbacks are dangling
            $fnames = $this->pendingWriteAndCallbackCallers();
            if ( $fnames ) {
                throw new RuntimeException(
                    "Transaction callbacks are still pending:\n" . implode( ', ', $fnames )
                );
            }
        }

        return $closed;
    }

    protected function assertHasConnectionHandle() {
        if ( !$this->isOpen() ) {
            throw new DBUnexpectedError( $this, "DB connection was already closed." );
        }
    }

    protected function assertIsWritableMaster() {
        if ( $this->getLBInfo( 'replica' ) === true ) {
            throw new DBReadOnlyRoleError(
                $this,
                'Write operations are not allowed on replica database connections.'
            );
        }
        $reason = $this->getReadOnlyReason();
        if ( $reason !== false ) {
            throw new DBReadOnlyError( $this, "Database is read-only: $reason" );
        }
    }

    abstract protected function closeConnection();

    public function reportConnectionError( $error = 'Unknown error' ) {
        call_user_func( $this->deprecationLogger, 'Use of ' . __METHOD__ . ' is deprecated.' );
        throw new DBConnectionError( $this, $this->lastError() ?: $error );
    }

    abstract protected function doQuery( $sql );

    protected function isWriteQuery( $sql ) {
        // BEGIN and COMMIT queries are considered read queries here.
        // Database backends and drivers (MySQL, MariaDB, php-mysqli) generally
        // treat these as write queries, in that their results have "affected rows"
        // as meta data as from writes, instead of "num rows" as from reads.
        // But, we treat them as read queries because when reading data (from
        // either replica or master) we use transactions to enable repeatable-read
        // snapshots, which ensures we get consistent results from the same snapshot
        // for all queries within a request. Use cases:
        // - Treating these as writes would trigger ChronologyProtector (see method doc).
        // - We use this method to reject writes to replicas, but we need to allow
        //   use of transactions on replicas for read snapshots. This fine given
        //   that transactions by themselves don't make changes, only actual writes
        //   within the transaction matter, which we still detect.
        return !preg_match(
            '/^(?:SELECT|BEGIN|ROLLBACK|COMMIT|SAVEPOINT|RELEASE|SET|SHOW|EXPLAIN|\(SELECT)\b/i',
            $sql
        );
    }

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

    protected function isTransactableQuery( $sql ) {
        return !in_array(
            $this->getQueryVerb( $sql ),
            [ 'BEGIN', 'ROLLBACK', 'COMMIT', 'SET', 'SHOW', 'CREATE', 'ALTER' ],
            true
        );
    }

    protected function registerTempTableWrite( $sql, $pseudoPermanent ) {
        static $qt = '[`"\']?(\w+)[`"\']?'; // quoted table

        if ( preg_match(
            '/^CREATE\s+TEMPORARY\s+TABLE\s+(?:IF\s+NOT\s+EXISTS\s+)?' . $qt . '/i',
            $sql,
            $matches
        ) ) {
            $type = $pseudoPermanent ? self::TEMP_PSEUDO_PERMANENT : self::TEMP_NORMAL;
            $this->sessionTempTables[$matches[1]] = $type;

            return $type;
        } elseif ( preg_match(
            '/^DROP\s+(?:TEMPORARY\s+)?TABLE\s+(?:IF\s+EXISTS\s+)?' . $qt . '/i',
            $sql,
            $matches
        ) ) {
            $type = $this->sessionTempTables[$matches[1]] ?? null;
            unset( $this->sessionTempTables[$matches[1]] );

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

        return null;
    }

    public function query( $sql, $fname = __METHOD__, $flags = 0 ) {
        $this->assertTransactionStatus( $sql, $fname );
        $this->assertHasConnectionHandle();

        $flags = (int)$flags; // b/c; this field used to be a bool
        $ignoreErrors = $this->hasFlags( $flags, self::QUERY_SILENCE_ERRORS );

        $priorTransaction = $this->trxLevel;
        $priorWritesPending = $this->writesOrCallbacksPending();
        $this->lastQuery = $sql;

        if ( $this->isWriteQuery( $sql ) ) {
            # 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...
            $this->assertIsWritableMaster();
            # Do not treat temporary table writes as "meaningful writes" that need committing.
            # Profile them as reads. Integration tests can override this behavior via $flags.
            $pseudoPermanent = $this->hasFlags( $flags, self::QUERY_PSEUDO_PERMANENT );
            $tableType = $this->registerTempTableWrite( $sql, $pseudoPermanent );
            $isEffectiveWrite = ( $tableType !== self::TEMP_NORMAL );
            # DBConnRef uses QUERY_REPLICA_ROLE to enforce the replica role for raw SQL queries
            if ( $isEffectiveWrite && $this->hasFlags( $flags, self::QUERY_REPLICA_ROLE ) ) {
                throw new DBReadOnlyRoleError( $this, "Cannot write; target role is DB_REPLICA" );
            }
        } else {
            $isEffectiveWrite = false;
        }

        # 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 );

        # Send the query to the server and fetch any corresponding errors
        $ret = $this->attemptQuery( $sql, $commentedSql, $isEffectiveWrite, $fname );
        $lastError = $this->lastError();
        $lastErrno = $this->lastErrno();

        $recoverableSR = false; // recoverable statement rollback?
        $recoverableCL = false; // recoverable connection loss?

        if ( $ret === false && $this->wasConnectionLoss() ) {
            # Check if no meaningful session state was lost
            $recoverableCL = $this->canRecoverFromDisconnect( $sql, $priorWritesPending );
            # Update session state tracking and try to restore the connection
            $reconnected = $this->replaceLostConnection( __METHOD__ );
            # Silently resend the query to the server if it is safe and possible
            if ( $recoverableCL && $reconnected ) {
                $ret = $this->attemptQuery( $sql, $commentedSql, $isEffectiveWrite, $fname );
                $lastError = $this->lastError();
                $lastErrno = $this->lastErrno();

                if ( $ret === false && $this->wasConnectionLoss() ) {
                    # Query probably causes disconnects; reconnect and do not re-run it
                    $this->replaceLostConnection( __METHOD__ );
                } else {
                    $recoverableCL = false; // connection does not need recovering
                    $recoverableSR = $this->wasKnownStatementRollbackError();
                }
            }
        } else {
            $recoverableSR = $this->wasKnownStatementRollbackError();
        }

        if ( $ret === false ) {
            if ( $priorTransaction ) {
                if ( $recoverableSR ) {
                    # We're ignoring an error that caused just the current query to be aborted.
                    # But log the cause so we can log a deprecation notice if a caller actually
                    # does ignore it.
                    $this->trxStatusIgnoredCause = [ $lastError, $lastErrno, $fname ];
                } elseif ( !$recoverableCL ) {
                    # Either the query was aborted or all queries after BEGIN where aborted.
                    # In the first case, the only options going forward are (a) ROLLBACK, or
                    # (b) ROLLBACK TO SAVEPOINT (if one was set). If the later case, the only
                    # option is ROLLBACK, since the snapshots would have been released.
                    $this->trxStatus = self::STATUS_TRX_ERROR;
                    $this->trxStatusCause =
                        $this->getQueryExceptionAndLog( $lastError, $lastErrno, $sql, $fname );
                    $ignoreErrors = false; // cannot recover
                    $this->trxStatusIgnoredCause = null;
                }
            }

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

        return $this->resultObject( $ret );
    }

    private function attemptQuery( $sql, $commentedSql, $isEffectiveWrite, $fname ) {
        $this->beginIfImplied( $sql, $fname );

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

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

        $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 );
        $ps = $this->profiler ? ( $this->profiler )( $queryProf ) : null;
        $this->affectedRowCount = null;
        $ret = $this->doQuery( $commentedSql );
        $this->affectedRowCount = $this->affectedRows();
        unset( $ps ); // profile out (if set)
        $queryRuntime = max( microtime( true ) - $startTime, 0.0 );

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

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

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

        return $ret;
    }

    private function beginIfImplied( $sql, $fname ) {
        if (
            !$this->trxLevel &&
            $this->getFlag( self::DBO_TRX ) &&
            $this->isTransactableQuery( $sql )
        ) {
            $this->begin( __METHOD__ . " ($fname)", self::TRANSACTION_INTERNAL );
            $this->trxAutomatic = true;
        }
    }

    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;
        }
    }

    private function assertTransactionStatus( $sql, $fname ) {
        $verb = $this->getQueryVerb( $sql );
        if ( $verb === 'USE' ) {
            throw new DBUnexpectedError( $this, "Got USE query; use selectDomain() instead." );
        }

        if ( $verb === 'ROLLBACK' ) { // transaction/savepoint
            return;
        }

        if ( $this->trxStatus < self::STATUS_TRX_OK ) {
            throw new DBTransactionStateError(
                $this,
                "Cannot execute query from $fname while transaction status is ERROR.",
                [],
                $this->trxStatusCause
            );
        } elseif ( $this->trxStatus === self::STATUS_TRX_OK && $this->trxStatusIgnoredCause ) {
            list( $iLastError, $iLastErrno, $iFname ) = $this->trxStatusIgnoredCause;
            call_user_func( $this->deprecationLogger,
                "Caller from $fname ignored an error originally raised from $iFname: " .
                "[$iLastErrno] $iLastError"
            );
            $this->trxStatusIgnoredCause = null;
        }
    }

    public function assertNoOpenTransactions() {
        if ( $this->explicitTrxActive() ) {
            throw new DBTransactionError(
                $this,
                "Explicit transaction still active. A caller may have caught an error. "
                . "Open transactions: " . $this->flatAtomicSectionList()
            );
        }
    }

    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 ( $this->sessionTempTables ) {
            return false; // tables might be queried latter
        } 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 and explicit snapshots
        } elseif ( $priorWritesPending ) {
            return false; // prior writes lost from implicit transaction
        }

        return true;
    }

    private function handleSessionLossPreconnect() {
        // Clean up tracking of session-level things...
        // https://dev.mysql.com/doc/refman/5.7/en/implicit-commit.html
        // https://www.postgresql.org/docs/9.2/static/sql-createtable.html (ignoring ON COMMIT)
        $this->sessionTempTables = [];
        // https://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_get-lock
        // https://www.postgresql.org/docs/9.4/static/functions-admin.html#FUNCTIONS-ADVISORY-LOCKS
        $this->namedLocksHeld = [];
        // Session loss implies transaction loss
        $this->trxLevel = 0;
        $this->trxAtomicCounter = 0;
        $this->trxIdleCallbacks = []; // T67263; transaction already lost
        $this->trxPreCommitCallbacks = []; // T67263; transaction already lost
        // @note: leave trxRecurringCallbacks in place
        if ( $this->trxDoneWrites ) {
            $this->trxProfiler->transactionWritingOut(
                $this->server,
                $this->getDomainID(),
                $this->trxShortId,
                $this->pendingWriteQueryDuration( self::ESTIMATE_TOTAL ),
                $this->trxWriteAffectedRows
            );
        }
    }

    private function handleSessionLossPostconnect() {
        try {
            // Handle callbacks in trxEndCallbacks, e.g. onTransactionResolution().
            // If callback suppression is set then the array will remain unhandled.
            $this->runOnTransactionIdleCallbacks( self::TRIGGER_ROLLBACK );
        } catch ( Exception $ex ) {
            // Already logged; move on...
        }
        try {
            // Handle callbacks in trxRecurringCallbacks, e.g. setTransactionListener()
            $this->runTransactionListenerCallbacks( self::TRIGGER_ROLLBACK );
        } catch ( Exception $ex ) {
            // Already logged; move on...
        }
    }

    protected function wasQueryTimeout( $error, $errno ) {
        return false;
    }

    public function reportQueryError( $error, $errno, $sql, $fname, $ignoreErrors = false ) {
        if ( $ignoreErrors ) {
            $this->queryLogger->debug( "SQL ERROR (ignored): $error\n" );
        } else {
            $exception = $this->getQueryExceptionAndLog( $error, $errno, $sql, $fname );

            throw $exception;
        }
    }

    private function getQueryExceptionAndLog( $error, $errno, $sql, $fname ) {
        $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,
                'trace' => ( new RuntimeException() )->getTraceAsString()
            ] )
        );
        $this->queryLogger->debug( "SQL ERROR: " . $error . "\n" );
        $wasQueryTimeout = $this->wasQueryTimeout( $error, $errno );
        if ( $wasQueryTimeout ) {
            $e = new DBQueryTimeoutError( $this, $error, $errno, $sql, $fname );
        } else {
            $e = new DBQueryError( $this, $error, $errno, $sql, $fname );
        }

        return $e;
    }

    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, [ 'value' => $var ], $cond, $fname, $options, $join_conds );
        if ( $res === false ) {
            return false;
        }

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

        return $values;
    }

    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 ];
    }

    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;
    }

    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 ) ) {
            $fields = implode( ',', $this->fieldNamesWithAlias( $vars ) );
        } else {
            $fields = $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 (
            $this->selectOptionsIncludeLocking( $options ) &&
            $this->selectFieldsOrOptionsAggregate( $vars, $options )
        ) {
            // Some DB types (postgres/oracle) disallow FOR UPDATE with aggregate
            // functions. Discourage use of such queries to encourage compatibility.
            call_user_func(
                $this->deprecationLogger,
                __METHOD__ . ": aggregation used with a locking SELECT ($fname)."
            );
        }

        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 === '' || $conds === '*' ) {
            $sql = "SELECT $startOpts $fields $from $useIndex $ignoreIndex $preLimitTail";
        } elseif ( is_string( $conds ) ) {
            $sql = "SELECT $startOpts $fields $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'],
                $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, $var = '*', $conds = '', $fname = __METHOD__, $options = [], $join_conds = []
    ) {
        $conds = $this->normalizeConditions( $conds, $fname );
        $column = $this->extractSingleFieldFromList( $var );
        if ( is_string( $column ) && !in_array( $column, [ '*', '1' ] ) ) {
            $conds[] = "$column IS NOT NULL";
        }

        $res = $this->select(
            $table, [ 'rowcount' => 'COUNT(*)' ], $conds, $fname, $options, $join_conds
        );
        $row = $res ? $this->fetchRow( $res ) : [];

        return isset( $row['rowcount'] ) ? (int)$row['rowcount'] : 0;
    }

    public function selectRowCount(
        $tables, $var = '*', $conds = '', $fname = __METHOD__, $options = [], $join_conds = []
    ) {
        $conds = $this->normalizeConditions( $conds, $fname );
        $column = $this->extractSingleFieldFromList( $var );
        if ( is_string( $column ) && !in_array( $column, [ '*', '1' ] ) ) {
            $conds[] = "$column IS NOT NULL";
        }

        $res = $this->select(
            [
                'tmp_count' => $this->buildSelectSubquery(
                    $tables,
                    '1',
                    $conds,
                    $fname,
                    $options,
                    $join_conds
                )
            ],
            [ 'rowcount' => 'COUNT(*)' ],
            [],
            $fname
        );
        $row = $res ? $this->fetchRow( $res ) : [];

        return isset( $row['rowcount'] ) ? (int)$row['rowcount'] : 0;
    }

    private function selectOptionsIncludeLocking( $options ) {
        $options = (array)$options;
        foreach ( [ 'FOR UPDATE', 'LOCK IN SHARE MODE' ] as $lock ) {
            if ( in_array( $lock, $options, true ) ) {
                return true;
            }
        }

        return false;
    }

    private function selectFieldsOrOptionsAggregate( $fields, $options ) {
        foreach ( (array)$options as $key => $value ) {
            if ( is_string( $key ) ) {
                if ( preg_match( '/^(?:GROUP BY|HAVING)$/i', $key ) ) {
                    return true;
                }
            } elseif ( is_string( $value ) ) {
                if ( preg_match( '/^(?:DISTINCT|DISTINCTROW)$/i', $value ) ) {
                    return true;
                }
            }
        }

        $regex = '/^(?:COUNT|MIN|MAX|SUM|GROUP_CONCAT|LISTAGG|ARRAY_AGG)\s*\\(/i';
        foreach ( (array)$fields as $field ) {
            if ( is_string( $field ) && preg_match( $regex, $field ) ) {
                return true;
            }
        }

        return false;
    }

    final protected function normalizeConditions( $conds, $fname ) {
        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 ( !is_array( $conds ) ) {
            $conds = ( $conds === '' ) ? [] : [ $conds ];
        }

        return $conds;
    }

    final protected function extractSingleFieldFromList( $var ) {
        if ( is_array( $var ) ) {
            if ( !$var ) {
                $column = null;
            } elseif ( count( $var ) == 1 ) {
                $column = $var[0] ?? reset( $var );
            } else {
                throw new DBUnexpectedError( $this, __METHOD__ . ': got multiple columns.' );
            }
        } else {
            $column = $var;
        }

        return $column;
    }

    public function lockForUpdate(
        $table, $conds = '', $fname = __METHOD__, $options = [], $join_conds = []
    ) {
        if ( !$this->trxLevel && !$this->getFlag( self::DBO_TRX ) ) {
            throw new DBUnexpectedError(
                $this,
                __METHOD__ . ': no transaction is active nor is DBO_TRX set'
            );
        }

        $options = (array)$options;
        $options[] = 'FOR UPDATE';

        return $this->selectRowCount( $table, '*', $conds, $fname, $options, $join_conds );
    }

    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;
        }
    }

    abstract public function tableExists( $table, $fname = __METHOD__ );

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

        if ( !$indexInfo ) {
            return null;
        }

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

    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 ];
        }

        $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 ) . ')';
        }

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

        return true;
    }

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

        $opts = [];

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

        return $opts;
    }

    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 );
        }

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

        return true;
    }

    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 . ')';
    }

    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 ) {
        // In theory this should work for any standards-compliant
        // SQL implementation, although it may not be the best way to do it.
        return "CAST( $field AS CHARACTER )";
    }

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

    public function buildSelectSubquery(
        $table, $vars, $conds = '', $fname = __METHOD__,
        $options = [], $join_conds = []
    ) {
        return new Subquery(
            $this->selectSQLText( $table, $vars, $conds, $fname, $options, $join_conds )
        );
    }

    public function databasesAreIndependent() {
        return false;
    }

    final public function selectDB( $db ) {
        $this->selectDomain( new DatabaseDomain(
            $db,
            $this->currentDomain->getSchema(),
            $this->currentDomain->getTablePrefix()
        ) );

        return true;
    }

    final public function selectDomain( $domain ) {
        $this->doSelectDomain( DatabaseDomain::newFromId( $domain ) );
    }

    protected function doSelectDomain( DatabaseDomain $domain ) {
        $this->currentDomain = $domain;
    }

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

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

    public function tableName( $name, $format = 'quoted' ) {
        if ( $name instanceof Subquery ) {
            throw new DBUnexpectedError(
                $this,
                __METHOD__ . ': got Subquery instance when expecting a string.'
            );
        }

        # 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 ) {
            $this->queryLogger->warning(
                __METHOD__ . ": use of subqueries is not supported this way.",
                [ 'trace' => ( new RuntimeException() )->getTraceAsString() ]
            );

            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;
    }

    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->relationSchemaQualifier();
                $prefix = is_string( $this->tableAliases[$table]['prefix'] )
                    ? $this->tableAliases[$table]['prefix']
                    : $this->tablePrefix();
            } else {
                $database = '';
                $schema = $this->relationSchemaQualifier(); # Default schema
                $prefix = $this->tablePrefix(); # Default prefix
            }
        }

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

    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;
    }

    protected function tableNameWithAlias( $table, $alias = false ) {
        if ( is_string( $table ) ) {
            $quotedTable = $this->tableName( $table );
        } elseif ( $table instanceof Subquery ) {
            $quotedTable = (string)$table;
        } else {
            throw new InvalidArgumentException( "Table must be a string or Subquery." );
        }

        if ( $alias === false || $alias === $table ) {
            if ( $table instanceof Subquery ) {
                throw new InvalidArgumentException( "Subquery table missing alias." );
            }

            return $quotedTable;
        } else {
            return $quotedTable . ' ' . $this->addIdentifierQuotes( $alias );
        }
    }

    protected function tableNamesWithAlias( $tables ) {
        $retval = [];
        foreach ( $tables as $alias => $table ) {
            if ( is_numeric( $alias ) ) {
                $alias = $table;
            }
            $retval[] = $this->tableNameWithAlias( $table, $alias );
        }

        return $retval;
    }

    protected function fieldNameWithAlias( $name, $alias = false ) {
        if ( !$alias || (string)$alias === (string)$name ) {
            return $name;
        } else {
            return $name . ' AS ' . $this->addIdentifierQuotes( $alias ); // PostgreSQL needs AS
        }
    }

    protected function fieldNamesWithAlias( $fields ) {
        $retval = [];
        foreach ( $fields as $alias => $field ) {
            if ( is_numeric( $alias ) ) {
                $alias = $field;
            }
            $retval[] = $this->fieldNameWithAlias( $field, $alias );
        }

        return $retval;
    }

    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 = implode( ',', $ret );
        $explicitJoins = implode( ' ', $retJOIN );

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

    protected function indexName( $index ) {
        return $this->indexAliases[$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 ) . "'";
        }
    }

    public function addIdentifierQuotes( $s ) {
        return '"' . str_replace( '"', '""', $s ) . '"';
    }

    public function isQuotedIdentifier( $name ) {
        return $name[0] == '"' && substr( $name, -1, 1 ) == '"';
    }

    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;
    }

    public function useIndexClause( $index ) {
        return '';
    }

    public function ignoreIndexClause( $index ) {
        return '';
    }

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

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

        try {
            $this->startAtomic( $fname, self::ATOMIC_CANCELABLE );
            $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->cancelAtomic( $fname );
            throw $e;
        }
    }

    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 ) . ')';
        }

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

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

        $uniqueIndexes = (array)$uniqueIndexes;
        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, self::ATOMIC_CANCELABLE );
            # Update any existing conflicting row(s)
            if ( $where !== false ) {
                $this->update( $table, $set, $where, $fname );
                $affectedRowCount += $this->affectedRows();
            }
            # Now insert any non-conflicting row(s)
            $this->insert( $table, $rows, $fname, [ 'IGNORE' ] );
            $affectedRowCount += $this->affectedRows();
            $this->endAtomic( $fname );
            $this->affectedRowCount = $affectedRowCount;
        } catch ( Exception $e ) {
            $this->cancelAtomic( $fname );
            throw $e;
        }

        return true;
    }

    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;
        }

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

        return true;
    }

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

        return true;
    }

    protected function isInsertSelectSafe( array $insertOptions, array $selectOptions ) {
        return true;
    }

    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;
        }

        try {
            $affectedRowCount = 0;
            $this->startAtomic( $fname, self::ATOMIC_CANCELABLE );
            $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->cancelAtomic( $fname );
            }
        } catch ( Exception $e ) {
            $this->cancelAtomic( $fname );
            throw $e;
        }
    }

    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;

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

    public function limitResult( $sql, $limit, $offset = false ) {
        if ( !is_numeric( $limit ) ) {
            throw new DBUnexpectedError( $this,
                "Invalid non-numeric limit passed to limitResult()\n" );
        }
        // This version works in MySQL and SQLite. It will very likely need to be
        // overridden for most other RDBMS subclasses.
        return "$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 = $options['LIMIT'] ?? null;
        $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 wasConnectionLoss() {
        return $this->wasConnectionError( $this->lastErrno() );
    }

    public function wasReadOnlyError() {
        return false;
    }

    public function wasErrorReissuable() {
        return (
            $this->wasDeadlock() ||
            $this->wasLockTimeout() ||
            $this->wasConnectionLoss()
        );
    }

    public function wasConnectionError( $errno ) {
        return false;
    }

    protected function wasKnownStatementRollbackError() {
        return false; // don't know; it could have caused a transaction rollback
    }

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

        $this->begin( __METHOD__ );

        $retVal = null;
        $e = null;
        do {
            try {
                $retVal = $function( ...$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, $this->currentAtomicSectionId() ];
    }

    final public function onTransactionCommitOrIdle( callable $callback, $fname = __METHOD__ ) {
        if ( !$this->trxLevel && $this->getTransactionRoundId() ) {
            // Start an implicit transaction similar to how query() does
            $this->begin( __METHOD__, self::TRANSACTION_INTERNAL );
            $this->trxAutomatic = true;
        }

        $this->trxIdleCallbacks[] = [ $callback, $fname, $this->currentAtomicSectionId() ];
        if ( !$this->trxLevel ) {
            $this->runOnTransactionIdleCallbacks( self::TRIGGER_IDLE );
        }
    }

    final public function onTransactionIdle( callable $callback, $fname = __METHOD__ ) {
        $this->onTransactionCommitOrIdle( $callback, $fname );
    }

    final public function onTransactionPreCommitOrIdle( callable $callback, $fname = __METHOD__ ) {
        if ( !$this->trxLevel && $this->getTransactionRoundId() ) {
            // Start an implicit transaction similar to how query() does
            $this->begin( __METHOD__, self::TRANSACTION_INTERNAL );
            $this->trxAutomatic = true;
        }

        if ( $this->trxLevel ) {
            $this->trxPreCommitCallbacks[] = [ $callback, $fname, $this->currentAtomicSectionId() ];
        } else {
            // No transaction is active nor will start implicitly, so make one for this callback
            $this->startAtomic( __METHOD__, self::ATOMIC_CANCELABLE );
            try {
                $callback( $this );
                $this->endAtomic( __METHOD__ );
            } catch ( Exception $e ) {
                $this->cancelAtomic( __METHOD__ );
                throw $e;
            }
        }
    }

    private function currentAtomicSectionId() {
        if ( $this->trxLevel && $this->trxAtomicLevels ) {
            $levelInfo = end( $this->trxAtomicLevels );

            return $levelInfo[1];
        }

        return null;
    }

    private function reassignCallbacksForSection(
        AtomicSectionIdentifier $old, AtomicSectionIdentifier $new
    ) {
        foreach ( $this->trxPreCommitCallbacks as $key => $info ) {
            if ( $info[2] === $old ) {
                $this->trxPreCommitCallbacks[$key][2] = $new;
            }
        }
        foreach ( $this->trxIdleCallbacks as $key => $info ) {
            if ( $info[2] === $old ) {
                $this->trxIdleCallbacks[$key][2] = $new;
            }
        }
        foreach ( $this->trxEndCallbacks as $key => $info ) {
            if ( $info[2] === $old ) {
                $this->trxEndCallbacks[$key][2] = $new;
            }
        }
    }

    private function modifyCallbacksForCancel( array $sectionIds ) {
        // Cancel the "on commit" callbacks owned by this savepoint
        $this->trxIdleCallbacks = array_filter(
            $this->trxIdleCallbacks,
            function ( $entry ) use ( $sectionIds ) {
                return !in_array( $entry[2], $sectionIds, true );
            }
        );
        $this->trxPreCommitCallbacks = array_filter(
            $this->trxPreCommitCallbacks,
            function ( $entry ) use ( $sectionIds ) {
                return !in_array( $entry[2], $sectionIds, true );
            }
        );
        // Make "on resolution" callbacks owned by this savepoint to perceive a rollback
        foreach ( $this->trxEndCallbacks as $key => $entry ) {
            if ( in_array( $entry[2], $sectionIds, true ) ) {
                $callback = $entry[0];
                $this->trxEndCallbacks[$key][0] = function () use ( $callback ) {
                    return $callback( self::TRIGGER_ROLLBACK, $this );
                };
            }
        }
    }

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

    final public function setTrxEndCallbackSuppression( $suppress ) {
        $this->trxEndCallbacksSuppressed = $suppress;
    }

    public function runOnTransactionIdleCallbacks( $trigger ) {
        if ( $this->trxLevel ) { // sanity
            throw new DBUnexpectedError( $this, __METHOD__ . ': a transaction is still open.' );
        }

        if ( $this->trxEndCallbacksSuppressed ) {
            return 0;
        }

        $count = 0;
        $autoTrx = $this->getFlag( self::DBO_TRX ); // automatic begin() enabled?
        $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 $callback ) {
                ++$count;
                list( $phpCallback ) = $callback;
                $this->clearFlag( self::DBO_TRX ); // make each query its own transaction
                try {
                    // @phan-suppress-next-line PhanParamTooManyCallable
                    call_user_func( $phpCallback, $trigger, $this );
                } 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 );
                    }
                } finally {
                    if ( $autoTrx ) {
                        $this->setFlag( self::DBO_TRX ); // restore automatic begin()
                    } else {
                        $this->clearFlag( self::DBO_TRX ); // restore auto-commit
                    }
                }
            }
        } while ( count( $this->trxIdleCallbacks ) );

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

        return $count;
    }

    public function runOnTransactionPreCommitCallbacks() {
        $count = 0;

        $e = null; // first exception
        do { // callbacks may add callbacks :)
            $callbacks = $this->trxPreCommitCallbacks;
            $this->trxPreCommitCallbacks = []; // consumed (and recursion guard)
            foreach ( $callbacks as $callback ) {
                try {
                    ++$count;
                    list( $phpCallback ) = $callback;
                    $phpCallback( $this );
                } catch ( Exception $ex ) {
                    ( $this->errorLogger )( $ex );
                    $e = $e ?: $ex;
                }
            }
        } while ( count( $this->trxPreCommitCallbacks ) );

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

        return $count;
    }

    public function runTransactionListenerCallbacks( $trigger ) {
        if ( $this->trxEndCallbacksSuppressed ) {
            return;
        }

        $e = null; // first exception

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

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

    protected function doSavepoint( $identifier, $fname ) {
        $this->query( 'SAVEPOINT ' . $this->addIdentifierQuotes( $identifier ), $fname );
    }

    protected function doReleaseSavepoint( $identifier, $fname ) {
        $this->query( 'RELEASE SAVEPOINT ' . $this->addIdentifierQuotes( $identifier ), $fname );
    }

    protected function doRollbackToSavepoint( $identifier, $fname ) {
        $this->query( 'ROLLBACK TO SAVEPOINT ' . $this->addIdentifierQuotes( $identifier ), $fname );
    }

    private function nextSavepointId( $fname ) {
        $savepointId = self::$SAVEPOINT_PREFIX . ++$this->trxAtomicCounter;
        if ( strlen( $savepointId ) > 30 ) {
            // 30 == Oracle's identifier length limit (pre 12c)
            // With a 22 character prefix, that puts the highest number at 99999999.
            throw new DBUnexpectedError(
                $this,
                'There have been an excessively large number of atomic sections in a transaction'
                . " started by $this->trxFname (at $fname)"
            );
        }

        return $savepointId;
    }

    final public function startAtomic(
        $fname = __METHOD__, $cancelable = self::ATOMIC_NOT_CANCELABLE
    ) {
        $savepointId = $cancelable === self::ATOMIC_CANCELABLE ? self::$NOT_APPLICABLE : null;

        if ( !$this->trxLevel ) {
            $this->begin( $fname, self::TRANSACTION_INTERNAL ); // sets trxAutomatic
            // 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 ) ) {
                // Since writes could happen in between the topmost atomic sections as part
                // of the transaction, those sections will need savepoints.
                $savepointId = $this->nextSavepointId( $fname );
                $this->doSavepoint( $savepointId, $fname );
            } else {
                $this->trxAutomaticAtomic = true;
            }
        } elseif ( $cancelable === self::ATOMIC_CANCELABLE ) {
            $savepointId = $this->nextSavepointId( $fname );
            $this->doSavepoint( $savepointId, $fname );
        }

        $sectionId = new AtomicSectionIdentifier;
        $this->trxAtomicLevels[] = [ $fname, $sectionId, $savepointId ];
        $this->queryLogger->debug( 'startAtomic: entering level ' .
            ( count( $this->trxAtomicLevels ) - 1 ) . " ($fname)" );

        return $sectionId;
    }

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

        // Check if the current section matches $fname
        $pos = count( $this->trxAtomicLevels ) - 1;
        list( $savedFname, $sectionId, $savepointId ) = $this->trxAtomicLevels[$pos];
        $this->queryLogger->debug( "endAtomic: leaving level $pos ($fname)" );

        if ( $savedFname !== $fname ) {
            throw new DBUnexpectedError(
                $this,
                "Invalid atomic section ended (got $fname but expected $savedFname)."
            );
        }

        // Remove the last section (no need to re-index the array)
        array_pop( $this->trxAtomicLevels );

        if ( !$this->trxAtomicLevels && $this->trxAutomaticAtomic ) {
            $this->commit( $fname, self::FLUSHING_INTERNAL );
        } elseif ( $savepointId !== null && $savepointId !== self::$NOT_APPLICABLE ) {
            $this->doReleaseSavepoint( $savepointId, $fname );
        }

        // Hoist callback ownership for callbacks in the section that just ended;
        // all callbacks should have an owner that is present in trxAtomicLevels.
        $currentSectionId = $this->currentAtomicSectionId();
        if ( $currentSectionId ) {
            $this->reassignCallbacksForSection( $sectionId, $currentSectionId );
        }
    }

    final public function cancelAtomic(
        $fname = __METHOD__, AtomicSectionIdentifier $sectionId = null
    ) {
        if ( !$this->trxLevel || !$this->trxAtomicLevels ) {
            throw new DBUnexpectedError( $this, "No atomic section is open (got $fname)." );
        }

        $excisedFnames = [];
        if ( $sectionId !== null ) {
            // Find the (last) section with the given $sectionId
            $pos = -1;
            foreach ( $this->trxAtomicLevels as $i => list( $asFname, $asId, $spId ) ) {
                if ( $asId === $sectionId ) {
                    $pos = $i;
                }
            }
            if ( $pos < 0 ) {
                throw new DBUnexpectedError( $this, "Atomic section not found (for $fname)" );
            }
            // Remove all descendant sections and re-index the array
            $excisedIds = [];
            $len = count( $this->trxAtomicLevels );
            for ( $i = $pos + 1; $i < $len; ++$i ) {
                $excisedFnames[] = $this->trxAtomicLevels[$i][0];
                $excisedIds[] = $this->trxAtomicLevels[$i][1];
            }
            $this->trxAtomicLevels = array_slice( $this->trxAtomicLevels, 0, $pos + 1 );
            $this->modifyCallbacksForCancel( $excisedIds );
        }

        // Check if the current section matches $fname
        $pos = count( $this->trxAtomicLevels ) - 1;
        list( $savedFname, $savedSectionId, $savepointId ) = $this->trxAtomicLevels[$pos];

        if ( $excisedFnames ) {
            $this->queryLogger->debug( "cancelAtomic: canceling level $pos ($savedFname) " .
                "and descendants " . implode( ', ', $excisedFnames ) );
        } else {
            $this->queryLogger->debug( "cancelAtomic: canceling level $pos ($savedFname)" );
        }

        if ( $savedFname !== $fname ) {
            throw new DBUnexpectedError(
                $this,
                "Invalid atomic section ended (got $fname but expected $savedFname)."
            );
        }

        // Remove the last section (no need to re-index the array)
        array_pop( $this->trxAtomicLevels );
        $this->modifyCallbacksForCancel( [ $savedSectionId ] );

        if ( $savepointId !== null ) {
            // Rollback the transaction to the state just before this atomic section
            if ( $savepointId === self::$NOT_APPLICABLE ) {
                $this->rollback( $fname, self::FLUSHING_INTERNAL );
            } else {
                $this->doRollbackToSavepoint( $savepointId, $fname );
                $this->trxStatus = self::STATUS_TRX_OK; // no exception; recovered
                $this->trxStatusIgnoredCause = null;
            }
        } elseif ( $this->trxStatus > self::STATUS_TRX_ERROR ) {
            // Put the transaction into an error state if it's not already in one
            $this->trxStatus = self::STATUS_TRX_ERROR;
            $this->trxStatusCause = new DBUnexpectedError(
                $this,
                "Uncancelable atomic section canceled (got $fname)."
            );
        }

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

    final public function doAtomicSection(
        $fname, callable $callback, $cancelable = self::ATOMIC_NOT_CANCELABLE
    ) {
        $sectionId = $this->startAtomic( $fname, $cancelable );
        try {
            $res = $callback( $this, $fname );
        } catch ( Exception $e ) {
            $this->cancelAtomic( $fname, $sectionId );

            throw $e;
        }
        $this->endAtomic( $fname );

        return $res;
    }

    final public function begin( $fname = __METHOD__, $mode = self::TRANSACTION_EXPLICIT ) {
        static $modes = [ self::TRANSACTION_EXPLICIT, self::TRANSACTION_INTERNAL ];
        if ( !in_array( $mode, $modes, true ) ) {
            throw new DBUnexpectedError( $this, "$fname: invalid mode parameter '$mode'." );
        }

        // Protect against mismatched atomic section, transaction nesting, and snapshot loss
        if ( $this->trxLevel ) {
            if ( $this->trxAtomicLevels ) {
                $levels = $this->flatAtomicSectionList();
                $msg = "$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 {
                $msg = "$fname: Implicit transaction already active (from {$this->trxFname}).";
                throw new DBUnexpectedError( $this, $msg );
            }
        } elseif ( $this->getFlag( self::DBO_TRX ) && $mode !== self::TRANSACTION_INTERNAL ) {
            $msg = "$fname: Implicit transaction expected (DBO_TRX set).";
            throw new DBUnexpectedError( $this, $msg );
        }

        $this->assertHasConnectionHandle();

        $this->doBegin( $fname );
        $this->trxStatus = self::STATUS_TRX_OK;
        $this->trxStatusIgnoredCause = null;
        $this->trxAtomicCounter = 0;
        $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 replication lag before any such queries.
        $this->trxReplicaLag = null; // clear cached value first
        $this->trxReplicaLag = $this->getApproximateLagStatus()['lag'];
        // 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 );
    }

    protected function doBegin( $fname ) {
        $this->query( 'BEGIN', $fname );
        $this->trxLevel = 1;
    }

    final public function commit( $fname = __METHOD__, $flush = self::FLUSHING_ONE ) {
        static $modes = [ self::FLUSHING_ONE, self::FLUSHING_ALL_PEERS, self::FLUSHING_INTERNAL ];
        if ( !in_array( $flush, $modes, true ) ) {
            throw new DBUnexpectedError( $this, "$fname: invalid flush parameter '$flush'." );
        }

        if ( $this->trxLevel && $this->trxAtomicLevels ) {
            // There are still atomic sections open; this cannot be ignored
            $levels = $this->flatAtomicSectionList();
            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."
                );
            }
        } elseif ( !$this->trxLevel ) {
            $this->queryLogger->error(
                "$fname: No transaction to commit, something got out of sync." );
            return; // nothing to do
        } elseif ( $this->trxAutomatic ) {
            throw new DBUnexpectedError(
                $this,
                "$fname: Expected mass commit of all peer transactions (DBO_TRX set)."
            );
        }

        $this->assertHasConnectionHandle();

        $this->runOnTransactionPreCommitCallbacks();

        $writeTime = $this->pendingWriteQueryDuration( self::ESTIMATE_DB_APPLY );
        $this->doCommit( $fname );
        $this->trxStatus = self::STATUS_TRX_NONE;

        if ( $this->trxDoneWrites ) {
            $this->lastWriteTime = microtime( true );
            $this->trxProfiler->transactionWritingOut(
                $this->server,
                $this->getDomainID(),
                $this->trxShortId,
                $writeTime,
                $this->trxWriteAffectedRows
            );
        }

        // With FLUSHING_ALL_PEERS, callbacks will be explicitly run later
        if ( $flush !== self::FLUSHING_ALL_PEERS ) {
            $this->runOnTransactionIdleCallbacks( self::TRIGGER_COMMIT );
            $this->runTransactionListenerCallbacks( self::TRIGGER_COMMIT );
        }
    }

    protected function doCommit( $fname ) {
        if ( $this->trxLevel ) {
            $this->query( 'COMMIT', $fname );
            $this->trxLevel = 0;
        }
    }

    final public function rollback( $fname = __METHOD__, $flush = '' ) {
        $trxActive = $this->trxLevel;

        if ( $flush !== self::FLUSHING_INTERNAL
            && $flush !== self::FLUSHING_ALL_PEERS
            && $this->getFlag( self::DBO_TRX )
        ) {
            throw new DBUnexpectedError(
                $this,
                "$fname: Expected mass rollback of all peer transactions (DBO_TRX set)."
            );
        }

        if ( $trxActive ) {
            $this->assertHasConnectionHandle();

            $this->doRollback( $fname );
            $this->trxStatus = self::STATUS_TRX_NONE;
            $this->trxAtomicLevels = [];
            // Estimate the RTT via a query now that trxStatus is OK
            $writeTime = $this->pingAndCalculateLastTrxApplyTime();

            if ( $this->trxDoneWrites ) {
                $this->trxProfiler->transactionWritingOut(
                    $this->server,
                    $this->getDomainID(),
                    $this->trxShortId,
                    $writeTime,
                    $this->trxWriteAffectedRows
                );
            }
        }

        // Clear any commit-dependant callbacks. They might even be present
        // only due to transaction rounds, with no SQL transaction being active
        $this->trxIdleCallbacks = [];
        $this->trxPreCommitCallbacks = [];

        // With FLUSHING_ALL_PEERS, callbacks will be explicitly run later
        if ( $trxActive && $flush !== self::FLUSHING_ALL_PEERS ) {
            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
        }
    }

    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;
    }

    abstract protected function fetchAffectedRowCount();

    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;
    }

    protected function replaceLostConnection( $fname ) {
        $this->closeConnection();
        $this->opened = false;
        $this->conn = false;

        $this->handleSessionLossPreconnect();

        try {
            $this->open(
                $this->server,
                $this->user,
                $this->password,
                $this->getDBname(),
                $this->dbSchema(),
                $this->tablePrefix()
            );
            $this->lastPing = microtime( true );
            $ok = true;

            $this->connLogger->warning(
                $fname . ': lost connection to {dbserver}; reconnected',
                [
                    'dbserver' => $this->getServer(),
                    'trace' => ( new RuntimeException() )->getTraceAsString()
                ]
            );
        } catch ( DBConnectionError $e ) {
            $ok = false;

            $this->connLogger->error(
                $fname . ': lost connection to {dbserver} permanently',
                [ 'dbserver' => $this->getServer() ]
            );
        }

        $this->handleSessionLossPostconnect();

        return $ok;
    }

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

    final protected function getRecordedTransactionLagStatus() {
        return ( $this->trxLevel && $this->trxReplicaLag !== null )
            ? [ 'lag' => $this->trxReplicaLag, 'since' => $this->trxTimestamp() ]
            : null;
    }

    protected function getApproximateLagStatus() {
        return [
            'lag'   => $this->getLBInfo( 'replica' ) ? $this->getLag() : 0,
            'since' => microtime( true )
        ];
    }

    public static function getCacheSetOptions( IDatabase $db1, IDatabase $db2 = null ) {
        $res = [ 'lag' => 0, 'since' => INF, 'pending' => false ];
        foreach ( func_get_args() as $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 ( $fp === false ) {
            throw new RuntimeException( "Could not open \"{$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 = $inputCallback( $cmd );

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

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

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

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

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

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

    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;
    }

    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
        );
    }

    protected function getSchemaVars() {
        if ( $this->schemaVars ) {
            return $this->schemaVars;
        } else {
            return $this->getDefaultSchemaVars();
        }
    }

    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 );
    }

    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 );
    }

    protected function doUnlockTables( $method ) {
        return true;
    }

    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 );
    }

    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;
    }

    protected function hasFlags( $field, $flags ) {
        return ( ( $field & $flags ) === $flags );
    }

    protected function getBindingHandle() {
        if ( !$this->conn ) {
            throw new DBUnexpectedError(
                $this,
                'DB connection was already closed or the connection dropped.'
            );
        }

        return $this->conn;
    }

    public function __toString() {
        return (string)$this->conn;
    }

    public function __clone() {
        $this->connLogger->warning(
            "Cloning " . static::class . " is not recommended; 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->handleSessionLossPreconnect(); // no trx or locks anymore
            $this->open(
                $this->server,
                $this->user,
                $this->password,
                $this->getDBname(),
                $this->dbSchema(),
                $this->tablePrefix()
            );
            $this->lastPing = microtime( true );
        }
    }

    public function __sleep() {
        throw new RuntimeException( 'Database serialization may cause problems, since ' .
            'the connection is not restored on wakeup.' );
    }

    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' );

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