ns that will be enqueued.
	 *
	 * @access public
	 *
	 * @param array $config Full sync configuration for this sync module.
	 * @return array Number of items yet to be enqueued.
	 */
	public function estimate_full_sync_actions( $config ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
		return 1;
	}

	/**
	 * Retrieve the actions that will be sent for this module during a full sync.
	 *
	 * @access public
	 *
	 * @return array Full sync actions of this module.
	 */
	public function get_full_sync_actions() {
		return array( 'jetpack_full_sync_callables' );
	}

	/**
	 * Unlock callables so they would be available for syncing again.
	 *
	 * @access public
	 */
	public function unlock_sync_callable() {
		delete_transient( self::CALLABLES_AWAIT_TRANSIENT_NAME );
	}

	/**
	 * Unlock callables on the next tick.
	 * Sometime the true callable values are only present on the next tick.
	 * When switching themes for example.
	 *
	 * @access public
	 */
	public function unlock_sync_callable_next_tick() {
		$this->force_send_callables_on_next_tick = true;
	}

	/**
	 * Unlock callables and plugin action links.
	 *
	 * @access public
	 */
	public function unlock_plugin_action_link_and_callables() {
		delete_transient( self::CALLABLES_AWAIT_TRANSIENT_NAME );
		delete_transient( 'jetpack_plugin_api_action_links_refresh' );
		add_filter( 'jetpack_check_and_send_callables', '__return_true' );
	}

	/**
	 * Parse and store the plugin action links if on the plugins page.
	 *
	 * @uses \DOMDocument
	 * @uses libxml_use_internal_errors
	 * @uses mb_convert_encoding
	 *
	 * @access public
	 */
	public function set_plugin_action_links() {
		if (
			! class_exists( '\DOMDocument' ) ||
			! function_exists( 'libxml_use_internal_errors' ) ||
			! function_exists( 'mb_convert_encoding' )
		) {
			return;
		}

		$current_screeen = get_current_screen();

		$plugins_action_links = array();
		// Is the transient lock in place?
		$plugins_lock = get_transient( 'jetpack_plugin_api_action_links_refresh', false );
		if ( ! empty( $plugins_lock ) && ( isset( $current_screeen->id ) && 'plugins' !== $current_screeen->id ) ) {
			return;
		}
		$plugins = array_keys( Functions::get_plugins() );
		foreach ( $plugins as $plugin_file ) {
			/**
			 *  Plugins often like to unset things but things break if they are not able to.
			 */
			$action_links = array(
				'deactivate' => '',
				'activate'   => '',
				'details'    => '',
				'delete'     => '',
				'edit'       => '',
			);
			/** This filter is documented in src/wp-admin/includes/class-wp-plugins-list-table.php */
			$action_links = apply_filters( 'plugin_action_links', $action_links, $plugin_file, null, 'all' );
			/** This filter is documented in src/wp-admin/includes/class-wp-plugins-list-table.php */
			$action_links = apply_filters( "plugin_action_links_{$plugin_file}", $action_links, $plugin_file, null, 'all' );
			// Verify $action_links is still an array to resolve warnings from filters not returning an array.
			if ( is_array( $action_links ) ) {
				$action_links = array_filter( $action_links );
			} else {
				$action_links = array();
			}
			$formatted_action_links = null;
			if ( ! empty( $action_links ) && count( $action_links ) > 0 ) {
				$dom_doc = new \DOMDocument();
				foreach ( $action_links as $action_link ) {
					// The @ is not enough to suppress errors when dealing with libxml,
					// we have to tell it directly how we want to handle errors.
					libxml_use_internal_errors( true );
					$dom_doc->loadHTML( mb_convert_encoding( $action_link, 'HTML-ENTITIES', 'UTF-8' ) );
					libxml_use_internal_errors( false );

					$link_elements = $dom_doc->getElementsByTagName( 'a' );
					if ( 0 === $link_elements->length ) {
						continue;
					}

					$link_element = $link_elements->item( 0 );
					// phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
					if ( $link_element->hasAttribute( 'href' ) && $link_element->nodeValue ) {
						$link_url = trim( $link_element->getAttribute( 'href' ) );

						// Add the full admin path to the url if the plugin did not provide it.
						$link_url_scheme = wp_parse_url( $link_url, PHP_URL_SCHEME );
						if ( empty( $link_url_scheme ) ) {
							$link_url = admin_url( $link_url );
						}

						// phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
						$formatted_action_links[ $link_element->nodeValue ] = $link_url;
					}
				}
			}
			if ( $formatted_action_links ) {
				$plugins_action_links[ $plugin_file ] = $formatted_action_links;
			}
		}
		// Cache things for a long time.
		set_transient( 'jetpack_plugin_api_action_links_refresh', time(), DAY_IN_SECONDS );
		update_option( 'jetpack_plugin_api_action_links', $plugins_action_links );
	}

	/**
	 * Whether a certain callable should be sent.
	 *
	 * @access public
	 *
	 * @param array  $callable_checksums Callable checksums.
	 * @param string $name               Name of the callable.
	 * @param string $checksum           A checksum of the callable.
	 * @return boolean Whether to send the callable.
	 */
	public function should_send_callable( $callable_checksums, $name, $checksum ) {
		$idc_override_callables = array(
			'main_network_site',
			'home_url',
			'site_url',
		);
		if ( in_array( $name, $idc_override_callables, true ) && \Jetpack_Options::get_option( 'migrate_for_idc' ) ) {
			return true;
		}

		return ! $this->still_valid_checksum( $callable_checksums, $name, $checksum );
	}

	/**
	 * Sync the callables if we're supposed to.
	 *
	 * @access public
	 */
	public function maybe_sync_callables() {
		$callables = $this->get_all_callables();
		if ( ! apply_filters( 'jetpack_check_and_send_callables', false ) ) {
			if ( ! is_admin() ) {
				// If we're not an admin and we're not doing cron and this isn't WP_CLI, don't sync anything.
				if ( ! Settings::is_doing_cron() && ! Jetpack_Constants::get_constant( 'WP_CLI' ) ) {
					return;
				}
				// If we're not an admin and we are doing cron, sync the Callables that are always supposed to sync ( See https://github.com/Automattic/jetpack/issues/12924 ).
				$callables = $this->get_always_sent_callables();
			}
			if ( get_transient( self::CALLABLES_AWAIT_TRANSIENT_NAME ) ) {
				if ( $this->force_send_callables_on_next_tick ) {
					$this->unlock_sync_callable();
				}
				return;
			}
		}

		if ( empty( $callables ) ) {
			return;
		}
		// No need to set the transiant we are trying to remove it anyways.
		if ( ! $this->force_send_callables_on_next_tick ) {
			set_transient( self::CALLABLES_AWAIT_TRANSIENT_NAME, microtime( true ), Defaults::$default_sync_callables_wait_time );
		}

		$callable_checksums = (array) \Jetpack_Options::get_raw_option( self::CALLABLES_CHECKSUM_OPTION_NAME, array() );
		$has_changed        = false;
		// Only send the callables that have changed.
		foreach ( $callables as $name => $value ) {
			$checksum = $this->get_check_sum( $value );

			// Explicitly not using Identical comparison as get_option returns a string.
			if ( ! is_null( $value ) && $this->should_send_callable( $callable_checksums, $name, $checksum ) ) {

				// Only send callable if the non sorted checksum also does not match.
				if ( $this->should_send_callable( $callable_checksums, $name, $this->get_check_sum( $value, false ) ) ) {

					/**
					 * Tells the client to sync a callable (aka function) to the server
					 *
					 * @param string The name of the callable
					 * @param mixed The value of the callable
					 *
					 * @since 4.2.0
					 */
					do_action( 'jetpack_sync_callable', $name, $value );
				}

				$callable_checksums[ $name ] = $checksum;
				$has_changed                 = true;
			} else {
				$callable_checksums[ $name ] = $checksum;
			}
		}
		if ( $has_changed ) {
			\Jetpack_Options::update_raw_option( self::CALLABLES_CHECKSUM_OPTION_NAME, $callable_checksums );
		}

		if ( $this->force_send_callables_on_next_tick ) {
			$this->unlock_sync_callable();
		}
	}

	/**
	 * Get the callables that should always be sent, e.g. on cron.
	 *
	 * @return array Callables that should always be sent
	 */
	protected function get_always_sent_callables() {
		$callables      = $this->get_all_callables();
		$cron_callables = array();
		foreach ( self::ALWAYS_SEND_UPDATES_TO_THESE_OPTIONS as $option_name ) {
			if ( array_key_exists( $option_name, $callables ) ) {
				$cron_callables[ $option_name ] = $callables[ $option_name ];
				continue;
			}

			// Check for the Callable name/key for the option, if different from option name.
			if ( array_key_exists( $option_name, self::OPTION_NAMES_TO_CALLABLE_NAMES ) ) {
				$callable_name = self::OPTION_NAMES_TO_CALLABLE_NAMES[ $option_name ];
				if ( array_key_exists( $callable_name, $callables ) ) {
					$cron_callables[ $callable_name ] = $callables[ $callable_name ];
				}
			}
		}
		return $cron_callables;
	}

	/**
	 * Expand the callables within a hook before they are serialized and sent to the server.
	 *
	 * @access public
	 *
	 * @param array $args The hook parameters.
	 * @return array $args The hook parameters.
	 */
	public function expand_callables( $args ) {
		if ( $args[0] ) {
			$callables           = $this->get_all_callables();
			$callables_checksums = array();
			foreach ( $callables as $name => $value ) {
				$callables_checksums[ $name ] = $this->get_check_sum( $value );
			}
			\Jetpack_Options::update_raw_option( self::CALLABLES_CHECKSUM_OPTION_NAME, $callables_checksums );
			return $callables;
		}

		return $args;
	}

	/**
	 * Return Total number of objects.
	 *
	 * @param array $config Full Sync config.
	 *
	 * @return int total
	 */
	public function total( $config ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
		return count( $this->get_callable_whitelist() );
	}

}