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<?php

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/*
Plugin Name: HyperDB
Plugin URI: https://wordpress.org/plugins/hyperdb/
Description: An advanced database class that supports replication, failover, load balancing, and partitioning.
Author: Automattic
License: GPLv2 or later
Version: 1.5
*/

/** This file should be installed at ABSPATH/wp-content/db.php **/
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/** Load the wpdb class while preventing instantiation **/
$wpdb = true;
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if ( defined('WPDB_PATH') )
	require_once(WPDB_PATH);
else
	require_once( ABSPATH . WPINC . '/wp-db.php' );
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if ( defined('DB_CONFIG_FILE') && file_exists( DB_CONFIG_FILE ) ) {

	/** The config file was defined earlier. **/

} elseif ( file_exists( ABSPATH . 'db-config.php') ) {

	/** The config file resides in ABSPATH. **/
	define( 'DB_CONFIG_FILE', ABSPATH . 'db-config.php' );

} elseif ( file_exists( dirname(ABSPATH) . '/db-config.php' ) && ! file_exists( dirname(ABSPATH) . '/wp-settings.php' ) ) {

	/** The config file resides one level above ABSPATH but is not part of another install. **/
	define( 'DB_CONFIG_FILE', dirname(ABSPATH) . '/db-config.php' );

} else {

	/** Lacking a config file, revert to the standard database class. **/
	$wpdb = new wpdb( DB_USER, DB_PASSWORD, DB_NAME, DB_HOST );
	return;

}

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/**
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 * Common definitions
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 */
define( 'HYPERDB_LAG_OK', 1 );
define( 'HYPERDB_LAG_BEHIND', 2 );
define( 'HYPERDB_LAG_UNKNOWN', 3 );

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define( 'HYPERDB_CONN_HOST_ERROR', 2003 );   // Can't connect to MySQL server on '%s' (%d)
define( 'HYPERDB_SERVER_GONE_ERROR', 2006 ); // MySQL server has gone away

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class hyperdb extends wpdb {
	/**
	 * The last table that was queried
	 * @var string
	 */
	var $last_table;

	/**
	 * After any SQL_CALC_FOUND_ROWS query, the query "SELECT FOUND_ROWS()"
	 * is sent and the mysql result resource stored here. The next query
	 * for FOUND_ROWS() will retrieve this. We do this to prevent any
	 * intervening queries from making FOUND_ROWS() inaccessible. You may
	 * prevent this by adding "NO_SELECT_FOUND_ROWS" in a comment.
	 * @var resource
	 */
	var $last_found_rows_result;

	/**
	 * Whether to store queries in an array. Useful for debugging and profiling.
	 * @var bool
	 */
	var $save_queries = false;

	/**
	 * The current mysql link resource
	 * @var resource
	 */
	var $dbh;

	/**
	 * Associative array (dbhname => dbh) for established mysql connections
	 * @var array
	 */
	var $dbhs;

	/**
	 * The multi-dimensional array of datasets and servers
	 * @var array
	 */
	var $hyper_servers = array();

	/**
	 * Optional directory of tables and their datasets
	 * @var array
	 */
	var $hyper_tables = array();

	/**
	 * Optional directory of callbacks to determine datasets from queries
	 * @var array
	 */
	var $hyper_callbacks = array();

	/**
	 * Custom callback to save debug info in $this->queries
	 * @var callable
	 */
	var $save_query_callback = null;

	/**
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	 * Whether to use persistent connections
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	 * @var bool
	 */
	var $persistent = false;

	/**
	 * The maximum number of db links to keep open. The least-recently used
	 * link will be closed when the number of links exceeds this.
	 * @var int
	 */
	var $max_connections = 10;

	/**
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	 * Whether to check with fsockopen prior to connecting to mysql.
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	 * @var bool
	 */
	var $check_tcp_responsiveness = true;

	/**
	 * Minimum number of connections to try before bailing
	 * @var int
	 */
	var $min_tries = 3;

	/**
	 * Send Reads To Masters. This disables slave connections while true.
	 * Otherwise it is an array of written tables.
	 * @var array
	 */
	var $srtm = array();

	/**
	 * The log of db connections made and the time each one took
	 * @var array
	 */
	var $db_connections;

	/**
	 * The list of unclosed connections sorted by LRU
	 */
	var $open_connections = array();

	/**
	 * The last server used and the database name selected
	 * @var array
	 */
	var $last_used_server;

	/**
	 * Lookup array (dbhname => (server, db name) ) for re-selecting the db
	 * when a link is re-used.
	 * @var array
	 */
	var $used_servers = array();

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	/**
	 * Whether to save debug_backtrace in save_query_callback. You may wish
	 * to disable this, e.g. when tracing out-of-memory problems.
	 */
	var $save_backtrace = true;

	/**
	 * Maximum lag in seconds. Set null to disable. Requires callbacks.
	 * @var integer
	 */
	var $default_lag_threshold = null;

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	/**
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	 * Lookup array (dbhname => host:port)
	 * @var array
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	 */
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	var $dbh2host = array();

	/**
	 * Keeps track of the dbhname usage and errors.
	 */
	var $dbhname_heartbeats = array();

	/**
	 * Counter for how many queries have failed during the life of the $wpdb object
	 */
	var $num_failed_queries = 0;
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	/**
	 * Gets ready to make database connections
	 * @param array db class vars
	 */
	function __construct( $args = null ) {
		if ( is_array($args) )
			foreach ( get_class_vars(__CLASS__) as $var => $value )
				if ( isset($args[$var]) )
					$this->$var = $args[$var];
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		$this->use_mysqli = $this->should_use_mysqli();

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		$this->init_charset();
	}

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	/**
	 * Triggers __construct() for backwards compatibility with PHP4
	 */
	function hyperdb( $args = null ) {
		return $this->__construct($args);
	}

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	/**
	 * Sets $this->charset and $this->collate
	 */
	function init_charset() {
		if ( function_exists('is_multisite') && is_multisite() ) {
			$this->charset = 'utf8';
			if ( defined( 'DB_COLLATE' ) && DB_COLLATE )
				$this->collate = DB_COLLATE;
			else
				$this->collate = 'utf8_general_ci';
		} elseif ( defined( 'DB_COLLATE' ) ) {
			$this->collate = DB_COLLATE;
		}

		if ( defined( 'DB_CHARSET' ) )
			$this->charset = DB_CHARSET;
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	}

	/**
	 * Add the connection parameters for a database
	 */
	function add_database( $db ) {
		extract($db, EXTR_SKIP);
		isset($dataset) or $dataset = 'global';
		isset($read)    or $read = 1;
		isset($write)   or $write = 1;
		unset($db['dataset']);
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		if ( $read )
			$this->hyper_servers[ $dataset ][ 'read' ][ $read ][] = $db;
		if ( $write )
			$this->hyper_servers[ $dataset ][ 'write' ][ $write ][] = $db;
	}

	/**
	 * Specify the dateset where a table is found
	 */
	function add_table( $dataset, $table ) {
		$this->hyper_tables[ $table ] = $dataset;
	}

	/**
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	 * Add a callback to a group of callbacks.
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	 * The default group is 'dataset', used to examine
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	 * queries and determine dataset.
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	 */
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	function add_callback( $callback, $group = 'dataset' ) {
		$this->hyper_callbacks[ $group ][] = $callback;
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	}

	/**
	 * Find the first table name referenced in a query
	 * @param string query
	 * @return string table
	 */
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	function get_table_from_query( $q ) {
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		// Remove characters that can legally trail the table name
		$q = rtrim($q, ';/-#');
		// allow (select...) union [...] style queries. Use the first queries table name.
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		$q = ltrim($q, "\t (");
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		// Strip everything between parentheses except nested
		// selects and use only 1500 chars of the query
		$q = preg_replace( '/\((?!\s*select)[^(]*?\)/is', '()', substr( $q, 0, 1500 ) );

		// Refer to the previous query
		// wpdb doesn't implement last_table, so we run it first.
		if ( preg_match('/^\s*SELECT.*?\s+FOUND_ROWS\(\)/is', $q) )
			return $this->last_table;

		if( method_exists( get_parent_class( $this ), 'get_table_from_query' ) ) {
			// WPDB has added support for get_table_from_query, which should take precedence
			return parent::get_table_from_query( $q );
		}
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		// Quickly match most common queries
		if ( preg_match('/^\s*(?:'
				. 'SELECT.*?\s+FROM'
				. '|INSERT(?:\s+IGNORE)?(?:\s+INTO)?'
				. '|REPLACE(?:\s+INTO)?'
				. '|UPDATE(?:\s+IGNORE)?'
				. '|DELETE(?:\s+IGNORE)?(?:\s+FROM)?'
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				. ')\s+`?([\w-]+)`?/is', $q, $maybe) )
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			return $maybe[1];

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		// SHOW TABLE STATUS and SHOW TABLES
		if ( preg_match('/^\s*(?:'
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				. 'SHOW\s+TABLE\s+STATUS.+(?:LIKE\s+|WHERE\s+Name\s*=\s*)'
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				. '|SHOW\s+(?:FULL\s+)?TABLES.+(?:LIKE\s+|WHERE\s+Name\s*=\s*)'
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				. ')\W([\w-]+)\W/is', $q, $maybe) )
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			return $maybe[1];

		// Big pattern for the rest of the table-related queries in MySQL 5.0
		if ( preg_match('/^\s*(?:'
				. '(?:EXPLAIN\s+(?:EXTENDED\s+)?)?SELECT.*?\s+FROM'
				. '|INSERT(?:\s+LOW_PRIORITY|\s+DELAYED|\s+HIGH_PRIORITY)?(?:\s+IGNORE)?(?:\s+INTO)?'
				. '|REPLACE(?:\s+LOW_PRIORITY|\s+DELAYED)?(?:\s+INTO)?'
				. '|UPDATE(?:\s+LOW_PRIORITY)?(?:\s+IGNORE)?'
				. '|DELETE(?:\s+LOW_PRIORITY|\s+QUICK|\s+IGNORE)*(?:\s+FROM)?'
				. '|DESCRIBE|DESC|EXPLAIN|HANDLER'
				. '|(?:LOCK|UNLOCK)\s+TABLE(?:S)?'
				. '|(?:RENAME|OPTIMIZE|BACKUP|RESTORE|CHECK|CHECKSUM|ANALYZE|OPTIMIZE|REPAIR).*\s+TABLE'
				. '|TRUNCATE(?:\s+TABLE)?'
				. '|CREATE(?:\s+TEMPORARY)?\s+TABLE(?:\s+IF\s+NOT\s+EXISTS)?'
				. '|ALTER(?:\s+IGNORE)?\s+TABLE'
				. '|DROP\s+TABLE(?:\s+IF\s+EXISTS)?'
				. '|CREATE(?:\s+\w+)?\s+INDEX.*\s+ON'
				. '|DROP\s+INDEX.*\s+ON'
				. '|LOAD\s+DATA.*INFILE.*INTO\s+TABLE'
				. '|(?:GRANT|REVOKE).*ON\s+TABLE'
				. '|SHOW\s+(?:.*FROM|.*TABLE)'
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				. ')\s+`?([\w-]+)`?/is', $q, $maybe) )
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			return $maybe[1];
	}

	/**
	 * Determine the likelihood that this query could alter anything
	 * @param string query
	 * @return bool
	 */
	function is_write_query( $q ) {
		// Quick and dirty: only SELECT statements are considered read-only.
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		$q = ltrim($q, "\r\n\t (");
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		return !preg_match('/^(?:SELECT|SHOW|DESCRIBE|DESC|EXPLAIN)\s/i', $q);
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	}

	/**
	 * Set a flag to prevent reading from slaves which might be lagging after a write
	 */
	function send_reads_to_masters() {
		$this->srtm = true;
	}

	/**
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	 * Callbacks are executed in the order in which they are registered until one
	 * of them returns something other than null.
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	 */
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	function run_callbacks( $group, $args = null) {
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		if ( ! isset( $this->hyper_callbacks[ $group ] ) || ! is_array( $this->hyper_callbacks[ $group ] ) )
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			return null;

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		if ( ! isset( $args ) ) {
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			$args = array( &$this );
		} elseif ( is_array( $args ) ) {
			$args[] = &$this;
		} else {
			$args = array( $args, &$this );
		}

		foreach ( $this->hyper_callbacks[ $group ] as $func ) {
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			$result = call_user_func_array($func, $args);
			if ( isset($result) )
				return $result;
		}
	}

	/**
	 * Figure out which database server should handle the query, and connect to it.
	 * @param string query
	 * @return resource mysql database connection
	 */
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	function db_connect( $query = '' ) {
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		if ( empty( $query ) )
			return false;

		$this->last_table = $this->table = $this->get_table_from_query($query);

		if ( isset($this->hyper_tables[$this->table]) ) {
			$dataset = $this->hyper_tables[$this->table];
			$this->callback_result = null;
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		} elseif ( null !== $this->callback_result = $this->run_callbacks( 'dataset', $query ) ) {
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			if ( is_array($this->callback_result) )
				extract( $this->callback_result, EXTR_OVERWRITE );
			else
				$dataset = $this->callback_result;
		}

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		if ( ! isset($dataset) )
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			$dataset = 'global';

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		if ( ! $dataset )
			return $this->log_and_bail("Unable to determine dataset (for table: $this->table)");
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		else
			$this->dataset = $dataset;

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		$this->run_callbacks( 'dataset_found', $dataset );

		if ( empty( $this->hyper_servers ) ) {
			if ( $this->is_mysql_connection( $this->dbh ) )
				return $this->dbh;
			if (
				!defined('DB_HOST')
				|| !defined('DB_USER')
				|| !defined('DB_PASSWORD')
				|| !defined('DB_NAME') )
				return $this->log_and_bail("We were unable to query because there was no database defined");
			$this->dbh = $this->ex_mysql_connect( DB_HOST, DB_USER, DB_PASSWORD, $this->persistent );
			if ( ! $this->is_mysql_connection( $this->dbh ) )
				return $this->log_and_bail("We were unable to connect to the database. (DB_HOST)");
			if ( ! $this->ex_mysql_select_db( DB_NAME, $this->dbh ) )
				return $this->log_and_bail("We were unable to select the database");
			if ( ! empty( $this->charset ) ) {
				$collation_query = "SET NAMES '$this->charset'";
				if ( !empty( $this->collate ) )
					$collation_query .= " COLLATE '$this->collate'";
				$this->ex_mysql_query( $collation_query, $this->dbh );
			}
			return $this->dbh;
		}

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		// Determine whether the query must be sent to the master (a writable server)
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		if ( !empty( $use_master ) || $this->srtm === true || isset($this->srtm[$this->table]) ) {
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			$use_master = true;
		} elseif ( $is_write = $this->is_write_query($query) ) {
			$use_master = true;
			if ( is_array($this->srtm) )
				$this->srtm[$this->table] = true;
		} elseif ( !isset($use_master) && is_array($this->srtm) && !empty($this->srtm) ) {
			// Detect queries that have a join in the srtm array.
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			$use_master = false;
			$query_match = substr( $query, 0, 1000 );
			foreach ( $this->srtm as $key => $value ) {
				if ( false !== stripos( $query_match, $key ) ) {
					$use_master = true;
					break;
				}
			}
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		} else {
			$use_master = false;
		}

		if ( $use_master ) {
			$this->dbhname = $dbhname = $dataset . '__w';
			$operation = 'write';
		} else {
			$this->dbhname = $dbhname = $dataset . '__r';
			$operation = 'read';
		}

		// Try to reuse an existing connection
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		while ( isset( $this->dbhs[$dbhname] ) && $this->is_mysql_connection( $this->dbhs[$dbhname] ) ) {
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			// Find the connection for incrementing counters
			foreach ( array_keys($this->db_connections) as $i )
				if ( $this->db_connections[$i]['dbhname'] == $dbhname )
					$conn =& $this->db_connections[$i];

			if ( isset($server['name']) ) {
				$name = $server['name'];
				// A callback has specified a database name so it's possible the existing connection selected a different one.
				if ( $name != $this->used_servers[$dbhname]['name'] ) {
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					if ( ! $this->ex_mysql_select_db( $name, $this->dbhs[$dbhname] ) ) {
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						// this can happen when the user varies and lacks permission on the $name database
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						if ( isset( $conn['disconnect (select failed)'] ) )
							++$conn['disconnect (select failed)'];
						else
							$conn['disconnect (select failed)'] = 1;

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						$this->disconnect($dbhname);
						break;
					}
					$this->used_servers[$dbhname]['name'] = $name;
				}
			} else {
				$name = $this->used_servers[$dbhname]['name'];
			}

			$this->current_host = $this->dbh2host[$dbhname];

			// Keep this connection at the top of the stack to prevent disconnecting frequently-used connections
			if ( $k = array_search($dbhname, $this->open_connections) ) {
				unset($this->open_connections[$k]);
				$this->open_connections[] = $dbhname;
			}

			$this->last_used_server = $this->used_servers[$dbhname];
			$this->last_connection = compact('dbhname', 'name');

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			if ( $this->should_mysql_ping() && ! $this->ex_mysql_ping( $this->dbhs[$dbhname] ) ) {
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				if ( isset( $conn['disconnect (ping failed)'] ) )
					++$conn['disconnect (ping failed)'];
				else
					$conn['disconnect (ping failed)'] = 1;

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				$this->disconnect($dbhname);
				break;
			}

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			if ( isset( $conn['queries'] ) )
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				++$conn['queries'];
			else
				$conn['queries'] = 1;
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			return $this->dbhs[$dbhname];
		}

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		if ( $use_master && defined( "MASTER_DB_DEAD" ) ) {
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			return $this->bail("We're updating the database, please try back in 5 minutes. If you are posting to your blog please hit the refresh button on your browser in a few minutes to post the data again. It will be posted as soon as the database is back online again.");
		}

		if ( empty($this->hyper_servers[$dataset][$operation]) )
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			return $this->log_and_bail("No databases available with $this->table ($dataset)");
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		// Put the groups in order by priority
		ksort($this->hyper_servers[$dataset][$operation]);

		// Make a list of at least $this->min_tries connections to try, repeating as necessary.
		$servers = array();
		do {
			foreach ( $this->hyper_servers[$dataset][$operation] as $group => $items ) {
				$keys = array_keys($items);
				shuffle($keys);
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				foreach ( $keys as $key )
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					$servers[] = compact('group', 'key');
			}
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			if ( !$tries_remaining = count( $servers ) )
				return $this->log_and_bail("No database servers were found to match the query ($this->table, $dataset)");

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			if ( !isset( $unique_servers ) )
				$unique_servers = $tries_remaining;

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		} while ( $tries_remaining < $this->min_tries );

		// Connect to a database server
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		do {
			$unique_lagged_slaves = array();
			$success = false;
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			foreach ( $servers as $group_key ) {
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				--$tries_remaining;

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				// If all servers are lagged, we need to start ignoring the lag and retry
				if ( count( $unique_lagged_slaves ) == $unique_servers )
					break;
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				// $group, $key
				extract($group_key, EXTR_OVERWRITE);
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				// $host, $user, $password, $name, $read, $write [, $lag_threshold, $timeout ]
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				extract($this->hyper_servers[$dataset][$operation][$group][$key], EXTR_OVERWRITE);
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				$port = null;
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				// Split host:port into $host and $port
				if ( strpos($host, ':') )
					list($host, $port) = explode(':', $host);

				// Overlay $server if it was extracted from a callback
				if ( isset($server) && is_array($server) )
					extract($server, EXTR_OVERWRITE);

				// Split again in case $server had host:port
				if ( strpos($host, ':') )
					list($host, $port) = explode(':', $host);

				// Make sure there's always a port number
				if ( empty($port) )
					$port = 3306;

				// Use a default timeout of 200ms
				if ( !isset($timeout) )
					$timeout = 0.2;

				// Get the minimum group here, in case $server rewrites it
				if ( !isset( $min_group ) || $min_group > $group )
					$min_group = $group;

				// Can be used by the lag callbacks
				$this->lag_cache_key = "$host:$port";
				$this->lag_threshold = isset($lag_threshold) ? $lag_threshold : $this->default_lag_threshold;

				// Check for a lagged slave, if applicable
				if ( !$use_master && !$write && !isset( $ignore_slave_lag )
					&& isset($this->lag_threshold) && !isset( $server['host'] )
					&& ( $lagged_status = $this->get_lag_cache() ) === HYPERDB_LAG_BEHIND
				) {
					// If it is the last lagged slave and it is with the best preference we will ignore its lag
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					if ( !isset( $unique_lagged_slaves["$host:$port"] )
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						&& $unique_servers == count( $unique_lagged_slaves ) + 1
						&& $group == $min_group )
					{
						$this->lag_threshold = null;
					} else {
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						$unique_lagged_slaves["$host:$port"] = $this->lag;
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						continue;
					}
				}
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				$this->timer_start();
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				// Connect if necessary or possible
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				$server_state = null;
				if ( $use_master || ! $tries_remaining ||
					'up' == $server_state = $this->get_server_state( $host, $port, $timeout ) )
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				{
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					$this->set_connect_timeout( 'pre_connect', $use_master, $tries_remaining );
					$this->dbhs[$dbhname] = $this->ex_mysql_connect( "$host:$port", $user, $password, $this->persistent );
					$this->set_connect_timeout( 'post_connect', $use_master, $tries_remaining );
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				} else {
					$this->dbhs[$dbhname] = false;
				}
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				$elapsed = $this->timer_stop();

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				if ( $this->is_mysql_connection( $this->dbhs[$dbhname] ) ) {
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					/**
					 * If we care about lag, disconnect lagged slaves and try to find others.
					 * We don't disconnect if it is the last lagged slave and it is with the best preference.
					 */
					if ( !$use_master && !$write && !isset( $ignore_slave_lag )
						&& isset($this->lag_threshold) && !isset( $server['host'] )
						&& $lagged_status !== HYPERDB_LAG_OK
						&& ( $lagged_status = $this->get_lag() ) === HYPERDB_LAG_BEHIND
						&& !(
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							!isset( $unique_lagged_slaves["$host:$port"] )
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							&& $unique_servers == count( $unique_lagged_slaves ) + 1
							&& $group == $min_group
						)
					) {
						$success = false;
						$unique_lagged_slaves["$host:$port"] = $this->lag;
						$this->disconnect( $dbhname );
						$this->dbhs[$dbhname] = false;
						$msg = "Replication lag of {$this->lag}s on $host:$port ($dbhname)";
						$this->print_error( $msg );
						continue;
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					} elseif ( $this->ex_mysql_select_db( $name, $this->dbhs[$dbhname] ) ) {
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						$success = true;
						$this->current_host = "$host:$port";
						$this->dbh2host[$dbhname] = "$host:$port";
						$queries = 1;
						$lag = isset( $this->lag ) ? $this->lag : 0;
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						$this->last_connection = compact('dbhname', 'host', 'port', 'user', 'name', 'tcp', 'elapsed', 'success', 'queries', 'lag');
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						$this->db_connections[] = $this->last_connection;
						$this->open_connections[] = $dbhname;
						break;
					}
				}
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				if ( 'down' == $server_state )
					continue; // don't flood the logs if already down

				if ( HYPERDB_CONN_HOST_ERROR == $this->ex_mysql_errno() &&
					( 'up' == $server_state || ! $tries_remaining ) )
				{
					$this->mark_server_as_down( $host, $port );
					$server_state = 'down';
				}

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				$success = false;
				$this->last_connection = compact('dbhname', 'host', 'port', 'user', 'name', 'tcp', 'elapsed', 'success');
				$this->db_connections[] = $this->last_connection;
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				$msg = date( "Y-m-d H:i:s" ) . " Can't select $dbhname - \n";
				$msg .= "'referrer' => '{$_SERVER['HTTP_HOST']}{$_SERVER['REQUEST_URI']}',\n";
				$msg .= "'server' => {$server},\n";
				$msg .= "'host' => {$host},\n";
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				$msg .= "'error' => " . $this->ex_mysql_error() . ",\n";
				$msg .= "'errno' => " . $this->ex_mysql_errno() . ",\n";
				$msg .= "'server_state' => $server_state\n";
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				$msg .= "'lagged_status' => " . ( isset( $lagged_status ) ? $lagged_status : HYPERDB_LAG_UNKNOWN );

				$this->print_error( $msg );
			}

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			if ( ! $success || ! isset( $this->dbhs[$dbhname] ) || ! $this->is_mysql_connection( $this->dbhs[$dbhname] ) ) {
				if ( !isset( $ignore_slave_lag ) && count( $unique_lagged_slaves ) ) {
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					// Lagged slaves were not used. Ignore the lag for this connection attempt and retry.
					$ignore_slave_lag = true;
					$tries_remaining = count( $servers );
					continue;
				}
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				$error_details = array(
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					'host' => $host,
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					'port' => $port,
					'operation' => $operation,
					'table' => $this->table,
					'dataset' => $dataset,
					'dbhname' => $dbhname
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				);
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				$this->run_callbacks( 'db_connection_error', $error_details );
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				return $this->bail( "Unable to connect to $host:$port to $operation table '$this->table' ($dataset)" );
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			}

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			break;
		} while ( true );

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		if ( !isset( $charset ) )
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			$charset = null;

		if ( !isset( $collate ) )
			$collate = null;

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		$this->set_charset($this->dbhs[$dbhname], $charset, $collate);
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		$this->dbh = $this->dbhs[$dbhname]; // needed by $wpdb->_real_escape()
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		$this->last_used_server = compact('host', 'user', 'name', 'read', 'write');

		$this->used_servers[$dbhname] = $this->last_used_server;

		while ( !$this->persistent && count($this->open_connections) > $this->max_connections ) {
			$oldest_connection = array_shift($this->open_connections);
			if ( $this->dbhs[$oldest_connection] != $this->dbhs[$dbhname] )
				$this->disconnect($oldest_connection);
		}

		return $this->dbhs[$dbhname];
	}

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	/**
	 * Sets the connection's character set.
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	 * @param resource $dbh     The resource given by ex_mysql_connect
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	 * @param string   $charset The character set (optional)
	 * @param string   $collate The collation (optional)
	 */
	function set_charset($dbh, $charset = null, $collate = null) {
		if ( !isset($charset) )
			$charset = $this->charset;
		if ( !isset($collate) )
			$collate = $this->collate;
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		if ( ! $this->has_cap( 'collation', $dbh ) || empty( $charset ) )
			return;

		if ( ! in_array( strtolower( $charset ), array( 'utf8', 'utf8mb4', 'latin1' ) ) )
			wp_die( "$charset charset isn't supported in HyperDB for security reasons" );

		if ( $this->is_mysql_set_charset_callable() && $this->has_cap( 'set_charset', $dbh ) ) {
			$this->ex_mysql_set_charset( $charset, $dbh );
			$this->real_escape = true;
		} else {
			$query = $this->prepare( 'SET NAMES %s', $charset );
			if ( ! empty( $collate ) )
				$query .= $this->prepare( ' COLLATE %s', $collate );
			$this->ex_mysql_query( $query, $dbh );
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		}
	}

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	/*
 	 * Force addslashes() for the escapes.
 	 *
 	 * HyperDB makes connections when a query is made
 	 * which is why we can't use mysql_real_escape_string() for escapes.
 	 * This is also the reason why we don't allow certain charsets. See set_charset().
 	 */
	function _real_escape( $string ) {
		$escaped = addslashes( $string );
		if ( method_exists( get_parent_class( $this ), 'add_placeholder_escape' ) ) {
			$escaped = $this->add_placeholder_escape( $escaped );
		}
		return $escaped;
	}
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	/**
	 * Disconnect and remove connection from open connections list
	 * @param string $tdbhname
	 */
	function disconnect($dbhname) {
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		if ( false !== $k = array_search($dbhname, $this->open_connections) )
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			unset($this->open_connections[$k]);

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		foreach ( array_keys( $this->db_connections ) as $i )
			if ( $this->db_connections[$i]['dbhname'] == $dbhname )
				unset( $this->db_connections[$i] );

		if ( $this->is_mysql_connection( $this->dbhs[$dbhname] ) )
			$this->ex_mysql_close( $this->dbhs[$dbhname] );
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		unset($this->dbhs[$dbhname]);
	}

	/**
	 * Kill cached query results
	 */
	function flush() {
		$this->last_error = '';
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		$this->last_errno = 0;
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		$this->num_rows = 0;
		parent::flush();
	}

	/**
	 * Basic query. See docs for more details.
	 * @param string $query
	 * @return int number of rows
	 */
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	function query( $query ) {
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		// some queries are made before the plugins have been loaded, and thus cannot be filtered with this method
		if ( function_exists('apply_filters') )
			$query = apply_filters('query', $query);

		// initialise return
		$return_val = 0;
		$this->flush();

		// Log how the function was called
		$this->func_call = "\$db->query(\"$query\")";

		// Keep track of the last query for debug..
		$this->last_query = $query;

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		if ( preg_match( '/^\s*SELECT\s+FOUND_ROWS(\s*)/i', $query ) ) {
			if ( $this->is_mysql_result( $this->last_found_rows_result ) ) {
				$this->result = $this->last_found_rows_result;
				$elapsed = 0;
			} else {
				$this->print_error( "Attempted SELECT FOUND_ROWS() without prior SQL_CALC_FOUND_ROWS." );
				return false;
			}
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		} else {
			$this->dbh = $this->db_connect( $query );

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			if ( ! $this->is_mysql_connection( $this->dbh ) ) {
				$this->check_current_query = true;
				$this->last_error = 'Database connection failed';
				$this->num_failed_queries++;
				do_action( 'sql_query_log', $query, false, $this->last_error );
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				return false;
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			}

			$query_comment = $this->run_callbacks( 'get_query_comment', $query );
			if ( ! empty( $query_comment ) )
				$query = rtrim( $query, ";\t \n\r" ) . ' /* ' . $query_comment . ' */';

			// If we're writing to the database, make sure the query will write safely.
			if ( $this->check_current_query && method_exists( $this, 'check_ascii' ) && ! $this->check_ascii( $query ) ) {
				$stripped_query = $this->strip_invalid_text_from_query( $query );
				if ( $stripped_query !== $query ) {
					$this->insert_id = 0;
					$this->last_error = 'Invalid query';
					$this->num_failed_queries++;
					do_action( 'sql_query_log', $query, false, $this->last_error );
					return false;
				}
			}

			$this->check_current_query = true;

			// Inject setup and teardown statements
			$statement_before_query = $this->run_callbacks( 'statement_before_query' );
			$statement_after_query = $this->run_callbacks( 'statement_after_query' );
			$query_for_log = $query;
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			$this->timer_start();
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			if ( $statement_before_query ) {
				$query_for_log = "$statement_before_query; $query_for_log";
				$this->ex_mysql_query( $statement_before_query, $this->dbh );
			}

			$this->result = $this->ex_mysql_query( $query, $this->dbh );

			if ( $statement_after_query ) {
				$query_for_log = "$query_for_log; $statement_after_query";
				$this->ex_mysql_query( $statement_after_query, $this->dbh );
			}
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			$elapsed = $this->timer_stop();
			++$this->num_queries;

			if ( preg_match('/^\s*SELECT\s+SQL_CALC_FOUND_ROWS\s/i', $query) ) {
				if ( false === strpos($query, "NO_SELECT_FOUND_ROWS") ) {
					$this->timer_start();
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					$this->last_found_rows_result = $this->ex_mysql_query( "SELECT FOUND_ROWS()", $this->dbh );
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					$elapsed += $this->timer_stop();
					++$this->num_queries;
					$query .= "; SELECT FOUND_ROWS()";
				}
			} else {
				$this->last_found_rows_result = null;
			}

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			$this->dbhname_heartbeats[$this->dbhname]['last_used'] = microtime( true );

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			if ( $this->save_queries ) {
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				if ( is_callable($this->save_query_callback) ) {
					$saved_query = call_user_func_array( $this->save_query_callback, array( $query_for_log, $elapsed, $this->save_backtrace ? debug_backtrace( DEBUG_BACKTRACE_IGNORE_ARGS ) : null, &$this ) );
					if ( $saved_query !== null ) {
						$this->queries[] = $saved_query;
					}
				} else {
					$this->queries[] = array( $query_for_log, $elapsed, $this->get_caller() );
				}
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			}
		}

		// If there is an error then take note of it
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		if ( $this->last_error = $this->ex_mysql_error( $this->dbh ) ) {
			$this->last_errno = $this->ex_mysql_errno( $this->dbh );
			$this->dbhname_heartbeats[$this->dbhname]['last_errno'] = $this->last_errno;
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			$this->print_error($this->last_error);
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			$this->num_failed_queries++;
			do_action( 'sql_query_log', $query, false, $this->last_error );
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			return false;
		}

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		if ( preg_match('/^\s*(insert|delete|update|replace|alter)\s/i',$query) ) {
			$this->rows_affected = $this->ex_mysql_affected_rows( $this->dbh );
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			// Take note of the insert_id
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			if ( preg_match('/^\s*(insert|replace)\s/i',$query) ) {
				$this->insert_id = $this->ex_mysql_insert_id( $this->dbh );
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			}
			// Return number of rows affected
			$return_val = $this->rows_affected;
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		} else if ( is_bool( $this->result ) ) {
			$return_val = $this->result;
			$this->result = null;
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		} else {
			$i = 0;
			$this->col_info = array();
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			while ( $i < $this->ex_mysql_num_fields( $this->result ) ) {
				$this->col_info[$i] = $this->ex_mysql_fetch_field( $this->result );
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				$i++;
			}
			$num_rows = 0;
			$this->last_result = array();
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			while ( $row = $this->ex_mysql_fetch_object( $this->result ) ) {
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				$this->last_result[$num_rows] = $row;
				$num_rows++;
			}

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			$this->ex_mysql_free_result( $this->result );
			$this->result = null;
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			// Log number of rows the query returned
			$this->num_rows = $num_rows;

			// Return number of rows selected
			$return_val = $this->num_rows;
		}

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		do_action( 'sql_query_log', $query, $return_val, $this->last_error );
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		return $return_val;
	}

	/**
	 * Whether or not MySQL database is at least the required minimum version.
	 * The additional argument allows the caller to check a specific database.
	 *
	 * @since 2.5.0
	 * @uses $wp_version
	 *
	 * @return WP_Error
	 */
	function check_database_version( $dbh_or_table = false ) {
		global $wp_version;
		// Make sure the server has MySQL 4.1.2
		$mysql_version = preg_replace( '|[^0-9\.]|', '', $this->db_version( $dbh_or_table ) );
		if ( version_compare($mysql_version, '4.1.2', '<') )
			return new WP_Error( 'database_version', sprintf(__('<strong>ERROR</strong>: WordPress %s requires MySQL 4.1.2 or higher'), $wp_version) );
	}

	/**
	 * This function is called when WordPress is generating the table schema to determine wether or not the current database
	 * supports or needs the collation statements.
	 * The additional argument allows the caller to check a specific database.
	 * @return bool
	 */
	function supports_collation( $dbh_or_table = false ) {
		return $this->has_cap( 'collation', $dbh_or_table );
	}

	/**
	 * Generic function to determine if a database supports a particular feature
	 * The additional argument allows the caller to check a specific database.
	 * @param string $db_cap the feature
	 * @param false|string|resource $dbh_or_table the databaese (the current database, the database housing the specified table, or the database of the mysql resource)
	 * @return bool
	 */
	function has_cap( $db_cap, $dbh_or_table = false ) {
		$version = $this->db_version( $dbh_or_table );

		switch ( strtolower( $db_cap ) ) :
		case 'collation' :
		case 'group_concat' :
		case 'subqueries' :
			return version_compare($version, '4.1', '>=');
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		case 'set_charset' :
			return version_compare($version, '5.0.7', '>=');
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		endswitch;

		return false;
	}

	/**
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