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Editing: class.jetpack-sync-queue.php

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<?php

/**
 * A buffer of items from the queue that can be checked out
 */
class Jetpack_Sync_Queue_Buffer {
	public $id;
	public $items_with_ids;

public function __construct( $id, $items_with_ids ) {
		$this->id             = $id;
		$this->items_with_ids = $items_with_ids;
	}

public function get_items() {
		return array_combine( $this->get_item_ids(), $this->get_item_values() );
	}

public function get_item_values() {
		return Jetpack_Sync_Utils::get_item_values( $this->items_with_ids );
	}

public function get_item_ids() {
		return Jetpack_Sync_Utils::get_item_ids( $this->items_with_ids );
	}
}

/**
 * A persistent queue that can be flushed in increments of N items,
 * and which blocks reads until checked-out buffers are checked in or
 * closed. This uses raw SQL for two reasons: speed, and not triggering
 * tons of added_option callbacks.
 */
class Jetpack_Sync_Queue {
	public $id;
	private $row_iterator;

function __construct( $id ) {
		$this->id           = str_replace( '-', '_', $id ); // necessary to ensure we don't have ID collisions in the SQL
		$this->row_iterator = 0;
		$this->random_int = mt_rand( 1, 1000000 );
	}

function add( $item ) {
		global $wpdb;
		$added = false;
		// this basically tries to add the option until enough time has elapsed that
		// it has a unique (microtime-based) option key
		while ( ! $added ) {
			$rows_added = $wpdb->query( $wpdb->prepare(
				"INSERT INTO $wpdb->options (option_name, option_value, autoload) VALUES (%s, %s,%s)",
				$this->get_next_data_row_option_name(),
				serialize( $item ),
				'no'
			) );
			$added      = ( 0 !== $rows_added );
		}
	}

// Attempts to insert all the items in a single SQL query. May be subject to query size limits!
	function add_all( $items ) {
		global $wpdb;
		$base_option_name = $this->get_next_data_row_option_name();

$query = "INSERT INTO $wpdb->options (option_name, option_value, autoload) VALUES ";

$rows = array();

for ( $i = 0; $i < count( $items ); $i += 1 ) {
			$option_name  = esc_sql( $base_option_name . '-' . $i );
			$option_value = esc_sql( serialize( $items[ $i ] ) );
			$rows[]       = "('$option_name', '$option_value', 'no')";
		}

$rows_added = $wpdb->query( $query . join( ',', $rows ) );

if ( count( $items ) === $rows_added ) {
			return new WP_Error( 'row_count_mismatch', "The number of rows inserted didn't match the size of the input array" );
		}
	}

// Peek at the front-most item on the queue without checking it out
	function peek( $count = 1 ) {
		$items = $this->fetch_items( $count );
		if ( $items ) {
			return Jetpack_Sync_Utils::get_item_values( $items );
		}

return array();
	}

// lag is the difference in time between the age of the oldest item
	// (aka first or frontmost item) and the current time
	function lag() {
		global $wpdb;

$first_item_name = $wpdb->get_var( $wpdb->prepare(
			"SELECT option_name FROM $wpdb->options WHERE option_name LIKE %s ORDER BY option_name ASC LIMIT 1",
			"jpsq_{$this->id}-%"
		) );

if ( ! $first_item_name ) {
			return 0;
		}

// break apart the item name to get the timestamp
		$matches = null;
		if ( preg_match( '/^jpsq_' . $this->id . '-(\d+\.\d+)-/', $first_item_name, $matches ) ) {
			return microtime( true ) - floatval( $matches[1] );
		} else {
			return 0;
		}
	}

function reset() {
		global $wpdb;
		$this->delete_checkout_id();
		$wpdb->query( $wpdb->prepare(
			"DELETE FROM $wpdb->options WHERE option_name LIKE %s", "jpsq_{$this->id}-%"
		) );
	}

function size() {
		global $wpdb;

return (int) $wpdb->get_var( $wpdb->prepare(
			"SELECT count(*) FROM $wpdb->options WHERE option_name LIKE %s", "jpsq_{$this->id}-%"
		) );
	}

// we use this peculiar implementation because it's much faster than count(*)
	function has_any_items() {
		global $wpdb;
		$value = $wpdb->get_var( $wpdb->prepare(
			"SELECT exists( SELECT option_name FROM $wpdb->options WHERE option_name LIKE %s )", "jpsq_{$this->id}-%"
		) );

return ( $value === '1' );
	}

function checkout( $buffer_size ) {
		if ( $this->get_checkout_id() ) {
			return new WP_Error( 'unclosed_buffer', 'There is an unclosed buffer' );
		}

$buffer_id = uniqid();

$result = $this->set_checkout_id( $buffer_id );

if ( ! $result || is_wp_error( $result ) ) {
			return $result;
		}

$items = $this->fetch_items( $buffer_size );

if ( count( $items ) === 0 ) {
			return false;
		}

$buffer = new Jetpack_Sync_Queue_Buffer( $buffer_id, array_slice( $items, 0, $buffer_size ) );

return $buffer;
	}

// this checks out rows until it either empties the queue or hits a certain memory limit
	// it loads the sizes from the DB first so that it doesn't accidentally
	// load more data into memory than it needs to.
	// The only way it will load more items than $max_size is if a single queue item
	// exceeds the memory limit, but in that case it will send that item by itself.
	function checkout_with_memory_limit( $max_memory, $max_buffer_size = 500 ) {
		if ( $this->get_checkout_id() ) {
			return new WP_Error( 'unclosed_buffer', 'There is an unclosed buffer' );
		}

$buffer_id = uniqid();

$result = $this->set_checkout_id( $buffer_id );

if ( ! $result || is_wp_error( $result ) ) {
			return $result;
		}

// get the map of buffer_id -> memory_size
		global $wpdb;

$items_with_size = $wpdb->get_results(
			$wpdb->prepare(
				"SELECT option_name AS id, LENGTH(option_value) AS value_size FROM $wpdb->options WHERE option_name LIKE %s ORDER BY option_name ASC LIMIT %d",
				"jpsq_{$this->id}-%",
				$max_buffer_size
			),
			OBJECT
		);

if ( count( $items_with_size ) === 0 ) {
			return false;
		}

$total_memory = 0;

$min_item_id = $max_item_id = $items_with_size[0]->id;

foreach ( $items_with_size as $id => $item_with_size ) {
			$total_memory += $item_with_size->value_size;

// if this is the first item and it exceeds memory, allow loop to continue
			// we will exit on the next iteration instead
			if ( $total_memory > $max_memory && $id > 0 ) {
				break;
			}

$max_item_id = $item_with_size->id;
		}

$query = $wpdb->prepare( 
			"SELECT option_name AS id, option_value AS value FROM $wpdb->options WHERE option_name >= %s and option_name <= %s ORDER BY option_name ASC",
			$min_item_id,
			$max_item_id
		);

$items = $wpdb->get_results( $query, OBJECT );
		foreach ( $items as $item ) {
			$item->value = maybe_unserialize( $item->value );
		}

if ( count( $items ) === 0 ) {
			$this->delete_checkout_id();

return false;
		}

$buffer = new Jetpack_Sync_Queue_Buffer( $buffer_id, $items );

return $buffer;
	}

function checkin( $buffer ) {
		$is_valid = $this->validate_checkout( $buffer );

if ( is_wp_error( $is_valid ) ) {
			return $is_valid;
		}

$this->delete_checkout_id();

return true;
	}

function close( $buffer, $ids_to_remove = null ) {
		$is_valid = $this->validate_checkout( $buffer );

if ( is_wp_error( $is_valid ) ) {
			return $is_valid;
		}

$this->delete_checkout_id();

// by default clear all items in the buffer
		if ( is_null( $ids_to_remove ) ) {
			$ids_to_remove = $buffer->get_item_ids();
		}

global $wpdb;

if ( count( $ids_to_remove ) > 0 ) {
			$sql   = "DELETE FROM $wpdb->options WHERE option_name IN (" . implode( ', ', array_fill( 0, count( $ids_to_remove ), '%s' ) ) . ')';
			$query = call_user_func_array( array( $wpdb, 'prepare' ), array_merge( array( $sql ), $ids_to_remove ) );
			$wpdb->query( $query );
		}

return true;
	}

function flush_all() {
		$items = Jetpack_Sync_Utils::get_item_values( $this->fetch_items() );
		$this->reset();

return $items;
	}

function get_all() {
		return $this->fetch_items();
	}

// use with caution, this could allow multiple processes to delete
	// and send from the queue at the same time
	function force_checkin() {
		$this->delete_checkout_id();
	}

// used to lock checkouts from the queue.
	// tries to wait up to $timeout seconds for the queue to be empty
	function lock( $timeout = 30 ) {
		$tries = 0;

while ( $this->has_any_items() && $tries < $timeout ) {
			sleep( 1 );
			$tries += 1;
		}

if ( $tries === 30 ) {
			return new WP_Error( 'lock_timeout', 'Timeout waiting for sync queue to empty' );
		}

if ( $this->get_checkout_id() ) {
			return new WP_Error( 'unclosed_buffer', 'There is an unclosed buffer' );
		}

// hopefully this means we can acquire a checkout?
		$result = $this->set_checkout_id( 'lock' );

if ( ! $result || is_wp_error( $result ) ) {
			return $result;
		}

return true;
	}

function unlock() {
		return $this->delete_checkout_id();
	}

private function get_checkout_id() {
		global $wpdb;
		$checkout_value = $wpdb->get_var( 
			$wpdb->prepare(
				"SELECT option_value FROM $wpdb->options WHERE option_name = %s", 
				$this->get_lock_option_name()
			)
		);

if ( $checkout_value ) {
			list( $checkout_id, $timestamp ) = explode( ':', $checkout_value );
			if ( intval( $timestamp ) > time() ) {
				return $checkout_id;
			}
		}

return false;
	}

private function set_checkout_id( $checkout_id ) {
		global $wpdb;

$expires = time() + Jetpack_Sync_Defaults::$default_sync_queue_lock_timeout;
		$updated_num = $wpdb->query(
			$wpdb->prepare(
				"UPDATE $wpdb->options SET option_value = %s WHERE option_name = %s", 
				"$checkout_id:$expires",
				$this->get_lock_option_name()
			)
		);

if ( ! $updated_num ) {
			$updated_num = $wpdb->query(
				$wpdb->prepare(
					"INSERT INTO $wpdb->options ( option_name, option_value, autoload ) VALUES ( %s, %s, 'no' )", 
					$this->get_lock_option_name(),
					"$checkout_id:$expires"
				)
			);
		}

return $updated_num;
	}

private function delete_checkout_id() {
		global $wpdb;
		// rather than delete, which causes fragmentation, we update in place
		return $wpdb->query(
			$wpdb->prepare( 
				"UPDATE $wpdb->options SET option_value = %s WHERE option_name = %s", 
				"0:0",
				$this->get_lock_option_name() 
			) 
		);

}

private function get_lock_option_name() {
		return "jpsq_{$this->id}_checkout";
	}

private function get_next_data_row_option_name() {
		// this option is specifically chosen to, as much as possible, preserve time order
		// and minimise the possibility of collisions between multiple processes working
		// at the same time
		// TODO: confirm we only need to support PHP 5.05+ (otherwise we'll need to emulate microtime as float, and avoid PHP_INT_MAX)
		// @see: http://php.net/manual/en/function.microtime.php
		$timestamp = sprintf( '%.6f', microtime( true ) );

// row iterator is used to avoid collisions where we're writing data waaay fast in a single process
		if ( $this->row_iterator === PHP_INT_MAX ) {
			$this->row_iterator = 0;
		} else {
			$this->row_iterator += 1;
		}

return 'jpsq_' . $this->id . '-' . $timestamp . '-' . $this->random_int . '-' . $this->row_iterator;
	}

private function fetch_items( $limit = null ) {
		global $wpdb;

if ( $limit ) {
			$query_sql = $wpdb->prepare( "SELECT option_name AS id, option_value AS value FROM $wpdb->options WHERE option_name LIKE %s ORDER BY option_name ASC LIMIT %d", "jpsq_{$this->id}-%", $limit );
		} else {
			$query_sql = $wpdb->prepare( "SELECT option_name AS id, option_value AS value FROM $wpdb->options WHERE option_name LIKE %s ORDER BY option_name ASC", "jpsq_{$this->id}-%" );
		}

$items = $wpdb->get_results( $query_sql, OBJECT );
		foreach ( $items as $item ) {
			$item->value = maybe_unserialize( $item->value );
		}

return $items;
	}

private function validate_checkout( $buffer ) {
		if ( ! $buffer instanceof Jetpack_Sync_Queue_Buffer ) {
			return new WP_Error( 'not_a_buffer', 'You must checkin an instance of Jetpack_Sync_Queue_Buffer' );
		}

$checkout_id = $this->get_checkout_id();

if ( ! $checkout_id ) {
			return new WP_Error( 'buffer_not_checked_out', 'There are no checked out buffers' );
		}

if ( $checkout_id != $buffer->id ) {
			return new WP_Error( 'buffer_mismatch', 'The buffer you checked in was not checked out' );
		}

return true;
	}
}

class Jetpack_Sync_Utils {

static function get_item_values( $items ) {
		return array_map( array( __CLASS__, 'get_item_value' ), $items );
	}

static function get_item_ids( $items ) {
		return array_map( array( __CLASS__, 'get_item_id' ), $items );
	}

static private function get_item_value( $item ) {
		return $item->value;
	}

static private function get_item_id( $item ) {
		return $item->id;
	}
}

Editing: class.jetpack-sync-queue.php

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