tity_Crisis::init()->check_response_for_idc( $response ) ) {
			return new WP_Error(
				'sync_error_idc',
				esc_html__( 'Sync has been blocked from WordPress.com because it would cause an identity crisis', 'jetpack-sync' )
			);
		}

		// Record last successful sync.
		update_option( self::LAST_SUCCESS_PREFIX . $queue_id, microtime( true ), false );

		return $response;
	}

	/**
	 * Kicks off the initial sync.
	 *
	 * @access public
	 * @static
	 *
	 * @return bool|null False if sync is not allowed.
	 */
	public static function do_initial_sync() {
		// Let's not sync if we are not supposed to.
		if ( ! self::sync_allowed() ) {
			return false;
		}

		// Don't start new sync if a full sync is in process.
		$full_sync_module = Modules::get_module( 'full-sync' );
		if ( $full_sync_module && $full_sync_module->is_started() && ! $full_sync_module->is_finished() ) {
			return false;
		}

		$initial_sync_config = array(
			'options'         => true,
			'functions'       => true,
			'constants'       => true,
			'users'           => array( get_current_user_id() ),
			'network_options' => true,
		);

		self::do_full_sync( $initial_sync_config );
	}

	/**
	 * Do an initial full sync only if one has not already been started.
	 *
	 * @return bool|null False if the initial full sync was already started, otherwise null.
	 */
	public static function do_only_first_initial_sync() {
		$full_sync_module = Modules::get_module( 'full-sync' );
		if ( $full_sync_module && $full_sync_module->is_started() ) {
			return false;
		}

		static::do_initial_sync();
	}

	/**
	 * Kicks off a full sync.
	 *
	 * @access public
	 * @static
	 *
	 * @param array $modules  The sync modules should be included in this full sync. All will be included if null.
	 * @return bool           True if full sync was successfully started.
	 */
	public static function do_full_sync( $modules = null ) {
		if ( ! self::sync_allowed() ) {
			return false;
		}

		$full_sync_module = Modules::get_module( 'full-sync' );

		if ( ! $full_sync_module ) {
			return false;
		}

		self::initialize_listener();

		$full_sync_module->start( $modules );

		return true;
	}

	/**
	 * Adds a cron schedule for regular syncing via cron, unless the schedule already exists.
	 *
	 * @access public
	 * @static
	 *
	 * @param array $schedules  The list of WordPress cron schedules prior to this filter.
	 * @return array            A list of WordPress cron schedules with the Jetpack sync interval added.
	 */
	public static function jetpack_cron_schedule( $schedules ) {
		if ( ! isset( $schedules[ self::DEFAULT_SYNC_CRON_INTERVAL_NAME ] ) ) {
			$minutes = (int) ( self::DEFAULT_SYNC_CRON_INTERVAL_VALUE / 60 );
			$display = ( 1 === $minutes ) ?
				__( 'Every minute', 'jetpack-sync' ) :
				/* translators: %d is an integer indicating the number of minutes. */
				sprintf( __( 'Every %d minutes', 'jetpack-sync' ), $minutes );
			$schedules[ self::DEFAULT_SYNC_CRON_INTERVAL_NAME ] = array(
				'interval' => self::DEFAULT_SYNC_CRON_INTERVAL_VALUE,
				'display'  => $display,
			);
		}
		return $schedules;
	}

	/**
	 * Starts an incremental sync via cron.
	 *
	 * @access public
	 * @static
	 */
	public static function do_cron_sync() {
		self::do_cron_sync_by_type( 'sync' );
	}

	/**
	 * Starts a full sync via cron.
	 *
	 * @access public
	 * @static
	 */
	public static function do_cron_full_sync() {
		self::do_cron_sync_by_type( 'full_sync' );
	}

	/**
	 * Try to send actions until we run out of things to send,
	 * or have to wait more than 15s before sending again,
	 * or we hit a lock or some other sending issue
	 *
	 * @access public
	 * @static
	 *
	 * @param string $type Sync type. Can be `sync` or `full_sync`.
	 */
	public static function do_cron_sync_by_type( $type ) {
		if ( ! self::sync_allowed() || ( 'sync' !== $type && 'full_sync' !== $type ) ) {
			return;
		}

		self::initialize_sender();

		$time_limit = Settings::get_setting( 'cron_sync_time_limit' );
		$start_time = time();
		$executions = 0;

		do {
			$next_sync_time = self::$sender->get_next_sync_time( $type );

			if ( $next_sync_time ) {
				$delay = $next_sync_time - time() + 1;
				if ( $delay > 15 ) {
					break;
				} elseif ( $delay > 0 ) {
					sleep( $delay );
				}
			}

			// Explicitly only allow 1 do_full_sync call until issue with Immediate Full Sync is resolved.
			// For more context see p1HpG7-9pe-p2.
			if ( 'full_sync' === $type && $executions >= 1 ) {
				break;
			}

			/**
			 * Only try to sync once if Dedicated Sync is enabled. Dedicated Sync has its own requeueing mechanism
			 * that will re-run it if there are items in the queue at the end.
			 */
			if ( 'sync' === $type && $executions >= 1 && Settings::is_dedicated_sync_enabled() ) {
				break;
			}

			$result = 'full_sync' === $type ? self::$sender->do_full_sync() : self::$sender->do_sync();

			// # of send actions performed.
			++$executions;

		} while ( $result && ! is_wp_error( $result ) && ( $start_time + $time_limit ) > time() );

		return $executions;
	}

	/**
	 * Initialize the sync listener.
	 *
	 * @access public
	 * @static
	 */
	public static function initialize_listener() {
		self::$listener = Listener::get_instance();
	}

	/**
	 * Initializes the sync sender.
	 *
	 * @access public
	 * @static
	 */
	public static function initialize_sender() {
		self::$sender = Sender::get_instance();
		add_filter( 'jetpack_sync_send_data', array( __CLASS__, 'send_data' ), 10, 8 );
	}

	/**
	 * Initializes sync for WooCommerce.
	 *
	 * @access public
	 * @static
	 */
	public static function initialize_woocommerce() {
		if ( false === class_exists( 'WooCommerce' ) ) {
			return;
		}
		add_filter( 'jetpack_sync_modules', array( __CLASS__, 'add_woocommerce_sync_module' ) );

		if ( ! class_exists( CustomOrdersTableController::class ) ) {
			return;
		}
		$cot_controller = wc_get_container()->get( CustomOrdersTableController::class );
		if ( $cot_controller->custom_orders_table_usage_is_enabled() ) {
			add_filter( 'jetpack_sync_modules', array( __CLASS__, 'add_woocommerce_hpos_order_sync_module' ) );
		}
	}

	/**
	 * Initializes sync for Instant Search.
	 *
	 * @access public
	 * @static
	 */
	public static function initialize_search() {
		if ( false === class_exists( 'Automattic\\Jetpack\\Search\\Module_Control' ) ) {
			return;
		}
		$search_module = new \Automattic\Jetpack\Search\Module_Control();
		if ( $search_module->is_instant_search_enabled() ) {
			add_filter( 'jetpack_sync_modules', array( __CLASS__, 'add_search_sync_module' ) );
		}
	}

	/**
	 * Add Search updates to Sync Filters.
	 *
	 * @access public
	 * @static
	 *
	 * @param array $sync_modules The list of sync modules declared prior to this filter.
	 * @return array A list of sync modules that now includes Search's modules.
	 */
	public static function add_search_sync_module( $sync_modules ) {
		$sync_modules[] = 'Automattic\\Jetpack\\Sync\\Modules\\Search';
		return $sync_modules;
	}

	/**
	 * Adds Woo's sync modules to existing modules for sending.
	 *
	 * @access public
	 * @static
	 *
	 * @param array $sync_modules The list of sync modules declared prior to this filter.
	 * @return array A list of sync modules that now includes Woo's modules.
	 */
	public static function add_woocommerce_sync_module( $sync_modules ) {
		$sync_modules[] = 'Automattic\\Jetpack\\Sync\\Modules\\WooCommerce';
		return $sync_modules;
	}

	/**
	 * Adds Woo's HPOS sync modules to existing modules for sending.
	 *
	 * @param array $sync_modules The list of sync modules declared prior to this filter.
	 *
	 * @access public
	 * @static
	 *
	 * @return array A list of sync modules that now includes Woo's HPOS modules.
	 */
	public static function add_woocommerce_hpos_order_sync_module( $sync_modules ) {
		$sync_modules[] = WooCommerce_HPOS_Orders::class;
		return $sync_modules;
	}

	/**
	 * Initializes sync for WP Super Cache.
	 *
	 * @access public
	 * @static
	 */
	public static function initialize_wp_super_cache() {
		if ( false === function_exists( 'wp_cache_is_enabled' ) ) {
			return;
		}
		add_filter( 'jetpack_sync_modules', array( __CLASS__, 'add_wp_super_cache_sync_module' ) );
	}

	/**
	 * Adds WP Super Cache's sync modules to existing modules for sending.
	 *
	 * @access public
	 * @static
	 *
	 * @param array $sync_modules The list of sync modules declared prior to this filer.
	 * @return array A list of sync modules that now includes WP Super Cache's modules.
	 */
	public static function add_wp_super_cache_sync_module( $sync_modules ) {
		$sync_modules[] = 'Automattic\\Jetpack\\Sync\\Modules\\WP_Super_Cache';
		return $sync_modules;
	}

	/**
	 * Sanitizes the name of sync's cron schedule.
	 *
	 * @access public
	 * @static
	 *
	 * @param string $schedule The name of a WordPress cron schedule.
	 * @return string The sanitized name of sync's cron schedule.
	 */
	public static function sanitize_filtered_sync_cron_schedule( $schedule ) {
		$schedule  = sanitize_key( $schedule );
		$schedules = wp_get_schedules();

		// Make sure that the schedule has actually been registered using the `cron_intervals` filter.
		if ( isset( $schedules[ $schedule ] ) ) {
			return $schedule;
		}

		return self::DEFAULT_SYNC_CRON_INTERVAL_NAME;
	}

	/**
	 * Allows offsetting of start times for sync cron jobs.
	 *
	 * @access public
	 * @static
	 *
	 * @param string $schedule The name of a cron schedule.
	 * @param string $hook     The hook that this method is responding to.
	 * @return int The offset for the sync cron schedule.
	 */
	public static function get_start_time_offset( $schedule = '', $hook = '' ) {
		$start_time_offset = is_multisite()
			? wp_rand( 0, ( 2 * self::DEFAULT_SYNC_CRON_INTERVAL_VALUE ) )
			: 0;

		/**
		 * Allows overriding the offset that the sync cron jobs will first run. This can be useful when scheduling
		 * cron jobs across multiple sites in a network.
		 *
		 * @since 1.6.3
		 * @since-jetpack 4.5.0
		 *
		 * @param int    $start_time_offset
		 * @param string $hook
		 * @param string $schedule
		 */
		return (int) apply_filters(
			'jetpack_sync_cron_start_time_offset',
			$start_time_offset,
			$hook,
			$schedule
		);
	}

	/**
	 * Decides if a sync cron should be scheduled.
	 *
	 * @access public
	 * @static
	 *
	 * @param string $schedule The name of a cron schedule.
	 * @param string $hook     The hook that this method is responding to.
	 */
	public static function maybe_schedule_sync_cron( $schedule, $hook ) {
		if ( ! $hook ) {
			return;
		}
		$schedule = self::sanitize_filtered_sync_cron_schedule( $schedule );

		$start_time = time() + self::get_start_time_offset( $schedule, $hook );
		if ( ! wp_next_scheduled( $hook ) ) {
			// Schedule a job to send pending queue items once a minute.
			wp_schedule_event( $start_time, $schedule, $hook );
		} elseif ( wp_get_schedule( $hook ) !== $schedule ) {
			// If the schedule has changed, update the schedule.
			wp_clear_scheduled_hook( $hook );
			wp_schedule_event( $start_time, $schedule, $hook );
		}
	}

	/**
	 * Clears Jetpack sync cron jobs.
	 *
	 * @access public
	 * @static
	 */
	public static function clear_sync_cron_jobs() {
		wp_clear_scheduled_hook( 'jetpack_sync_cron' );
		wp_clear_scheduled_hook( 'jetpack_sync_full_cron' );
	}

	/**
	 * Initializes Jetpack sync cron jobs.
	 *
	 * @access public
	 * @static
	 */
	public static function init_sync_cron_jobs() {
		add_filter( 'cron_schedules', array( __CLASS__, 'jetpack_cron_schedule' ) ); // phpcs:ignore WordPress.WP.CronInterval.ChangeDetected

		add_action( 'jetpack_sync_cron', array( __CLASS__, 'do_cron_sync' ) );
		add_action( 'jetpack_sync_full_cron', array( __CLASS__, 'do_cron_full_sync' ) );

		/**
		 * Allows overriding of the default incremental sync cron schedule which defaults to once every 5 minutes.
		 *
		 * @since 1.6.3
		 * @since-jetpack 4.3.2
		 *
		 * @param string self::DEFAULT_SYNC_CRON_INTERVAL_NAME
		 */
		$incremental_sync_cron_schedule = apply_filters( 'jetpack_sync_incremental_sync_interval', self::DEFAULT_SYNC_CRON_INTERVAL_NAME );
		self::maybe_schedule_sync_cron( $incremental_sync_cron_schedule, 'jetpack_sync_cron' );

		/**
		 * Allows overriding of the full sync cron schedule which defaults to once every 5 minutes.
		 *
		 * @since 1.6.3
		 * @since-jetpack 4.3.2
		 *
		 * @param string self::DEFAULT_SYNC_CRON_INTERVAL_NAME
		 */
		$full_sync_cron_schedule = apply_filters( 'jetpack_sync_full_sync_interval', self::DEFAULT_SYNC_CRON_INTERVAL_NAME );
		self::maybe_schedule_sync_cron( $full_sync_cron_schedule, 'jetpack_sync_full_cron' );
	}

	/**
	 * Perform maintenance when a plugin upgrade occurs.
	 *
	 * @access public
	 * @static
	 *
	 * @param string $new_version New version of the plugin.
	 * @param string $old_version Old version of the plugin.
	 */
	public static function cleanup_on_upgrade( $new_version = '', $old_version = '' ) {
		if ( wp_next_scheduled( 'jetpack_sync_send_db_checksum' ) ) {
			wp_clear_scheduled_hook( 'jetpack_sync_send_db_checksum' );
		}

		$is_new_sync_upgrade = version_compare( $old_version, '4.2', '>=' );
		if ( ! empty( $old_version ) && $is_new_sync_upgrade && version_compare( $old_version, '4.5', '<' ) ) {
			self::clear_sync_cron_jobs();
			Settings::update_settings(
				array(
					'render_filtered_content' => Defaults::$default_render_filtered_content,
				)
			);
		}

		Health::on_jetpack_upgraded();
	}

	/**
	 * Get syncing status for the given fields.
	 *
	 * @access public
	 * @static
	 *
	 * @param string|null $fields A comma-separated string of the fields to include in the array from the JSON response.
	 * @return array An associative array with the status report.
	 */
	public static function get_sync_status( $fields = null ) {
		self::initialize_sender();

		$sync_module = Modules::get_module( 'full-sync' );
		$queue       = self::$sender->get_sync_queue();

		// _get_cron_array can be false
		$cron_timestamps = ( _get_cron_array() ) ? array_keys( _get_cron_array() ) : array();
		$next_cron       = ( ! empty( $cron_timestamps ) ) ? $cron_timestamps[0] - time() : '';

		$checksums = array();
		$debug     = array();

		if ( ! empty( $fields ) ) {
			$store         = new Replicastore();
			$fields_params = array_map( 'trim', explode( ',', $fields ) );

			if ( in_array( 'posts_checksum', $fields_params, true ) ) {
				$checksums['posts_checksum'] = $store->posts_checksum();
			}
			if ( in_array( 'comments_checksum', $fields_params, true ) ) {
				$checksums['comments_checksum'] = $store->comments_checksum();
			}
			if ( in_array( 'post_meta_checksum', $fields_params, true ) ) {
				$checksums['post_meta_checksum'] = $store->post_meta_checksum();
			}
			if ( in_array( 'comment_meta_checksum', $fields_params, true ) ) {
				$checksums['comment_meta_checksum'] = $store->comment_meta_checksum();
			}

			if ( in_array( 'debug_details', $fields_params, true ) ) {
				$debug = self::get_debug_details();
			}
		}

		$full_sync_status = ( $sync_module ) ? $sync_module->get_status() : array();

		$full_queue = self::$sender->get_full_sync_queue();

		$result = array_merge(
			$full_sync_status,
			$checksums,
			$debug,
			array(
				'cron_size'            => count( $cron_timestamps ),
				'next_cron'            => $next_cron,
				'queue_size'           => $queue->size(),
				'queue_lag'            => $queue->lag(),
				'queue_next_sync'      => ( self::$sender->get_next_sync_time( 'sync' ) - microtime( true ) ),
				'full_queue_next_sync' => ( self::$sender->get_next_sync_time( 'full_sync' ) - microtime( true ) ),
			)
		);

		// Verify $sync_module is not false.
		if ( ( $sync_module ) && ! str_contains( get_class( $sync_module ), 'Full_Sync_Immediately' ) ) {
			$result['full_queue_size'] = $full_queue->size();
			$result['full_queue_lag']  = $full_queue->lag();
		}
		return $result;
	}

	/**
	 * Reset Sync locks.
	 *
	 * @access public
	 * @static
	 * @since 1.43.0
	 *
	 * @param bool $unlock_queues Whether to unlock Sync queues. Defaults to true.
	 */
	public static function reset_sync_locks( $unlock_queues = true ) {
		// Next sync locks.
		delete_option( Sender::NEXT_SYNC_TIME_OPTION_NAME . '_sync' );
		delete_option( Sender::NEXT_SYNC_TIME_OPTION_NAME . '_full_sync' );
		delete_option( Sender::NEXT_SYNC_TIME_OPTION_NAME . '_full-sync-enqueue' );
		// Retry after locks.
		delete_option( self::RETRY_AFTER_PREFIX . 'sync' );
		delete_option( self::RETRY_AFTER_PREFIX . 'full_sync' );
		// Dedicated sync locks.
		\Jetpack_Options::delete_raw_option( Dedicated_Sender::DEDICATED_SYNC_REQUEST_LOCK_OPTION_NAME );
		delete_transient( Dedicated_Sender::DEDICATED_SYNC_TEMPORARY_DISABLE_FLAG );

		// Queue locks.
		// Note that we are just unlocking the queues here, not reseting them.
		if ( $unlock_queues ) {
			$sync_queue = new Queue( 'sync' );
			$sync_queue->unlock();

			$full_sync_queue = new Queue( 'full_sync' );
			$full_sync_queue->unlock();
		}
	}
}