ar the flags of a particular user.
	 *
	 * @access public
	 *
	 * @param int $user_id ID of the user.
	 */
	public function clear_flags( $user_id ) {
		if ( isset( $this->flags[ $user_id ] ) ) {
			unset( $this->flags[ $user_id ] );
		}
	}

	/**
	 * Add flags to a particular user.
	 *
	 * @access public
	 *
	 * @param int   $user_id ID of the user.
	 * @param array $flags   New flags to add for the user.
	 */
	public function add_flags( $user_id, $flags ) {
		$this->flags[ $user_id ] = wp_parse_args( $flags, $this->get_flags( $user_id ) );
	}

	/**
	 * Save the user meta, if we're interested in it.
	 * Also uses the time to add flags for the user.
	 *
	 * @access public
	 *
	 * @param int    $meta_id  ID of the meta object.
	 * @param int    $user_id  ID of the user.
	 * @param string $meta_key Meta key.
	 * @param mixed  $value    Meta value.
	 */
	public function maybe_save_user_meta( $meta_id, $user_id, $meta_key, $value ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
		if ( 'locale' === $meta_key ) {
			$this->add_flags( $user_id, array( 'locale_changed' => true ) );
		}

		$user = get_user_by( 'id', $user_id );
		if ( isset( $user->cap_key ) && $meta_key === $user->cap_key ) {
			$this->add_flags( $user_id, array( 'capabilities_changed' => true ) );
		}

		if ( $this->is_create_user() || $this->is_add_user_to_blog() || $this->is_delete_user() ) {
			return;
		}

		if ( isset( $this->flags[ $user_id ] ) ) {
			/**
			 * This action is documented already in this file
			 */
			do_action( 'jetpack_sync_save_user', $user_id, $this->get_flags( $user_id ) );
		}
	}

	/**
	 * Enqueue the users actions for full sync.
	 *
	 * @access public
	 *
	 * @param array   $config               Full sync configuration for this sync module.
	 * @param int     $max_items_to_enqueue Maximum number of items to enqueue.
	 * @param boolean $state                True if full sync has finished enqueueing this module, false otherwise.
	 * @return array Number of actions enqueued, and next module state.
	 */
	public function enqueue_full_sync_actions( $config, $max_items_to_enqueue, $state ) {
		global $wpdb;

		return $this->enqueue_all_ids_as_action( 'jetpack_full_sync_users', $wpdb->usermeta, 'user_id', $this->get_where_sql( $config ), $max_items_to_enqueue, $state );
	}

	/**
	 * Retrieve an estimated number of actions that will be enqueued.
	 *
	 * @access public
	 *
	 * @todo Refactor to prepare the SQL query before executing it.
	 *
	 * @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 ) {
		global $wpdb;

		$query = "SELECT count(*) FROM $wpdb->usermeta";

		$where_sql = $this->get_where_sql( $config );
		if ( $where_sql ) {
			$query .= ' WHERE ' . $where_sql;
		}

		// phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared
		$count = $wpdb->get_var( $query );

		return (int) ceil( $count / self::ARRAY_CHUNK_SIZE );
	}

	/**
	 * Retrieve the WHERE SQL clause based on the module config.
	 *
	 * @access public
	 *
	 * @param array $config Full sync configuration for this sync module.
	 * @return string WHERE SQL clause, or `null` if no comments are specified in the module config.
	 */
	public function get_where_sql( $config ) {
		global $wpdb;

		$query = "meta_key = '{$wpdb->prefix}user_level' AND meta_value > 0";

		// The $config variable is a list of user IDs to sync.
		if ( is_array( $config ) ) {
			$query .= ' AND user_id IN (' . implode( ',', array_map( 'intval', $config ) ) . ')';
		}

		return $query;
	}

	/**
	 * 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_users' );
	}

	/**
	 * Retrieve initial sync user config.
	 *
	 * @access public
	 *
	 * @todo Refactor the SQL query to call $wpdb->prepare() before execution.
	 *
	 * @return array|boolean IDs of users to initially sync, or false if tbe number of users exceed the maximum.
	 */
	public function get_initial_sync_user_config() {
		global $wpdb;

		// phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared
		$user_ids = $wpdb->get_col( "SELECT user_id FROM $wpdb->usermeta WHERE meta_key = '{$wpdb->prefix}user_level' AND meta_value > 0 LIMIT " . ( self::MAX_INITIAL_SYNC_USERS + 1 ) );
		$user_ids_count = is_countable( $user_ids ) ? count( $user_ids ) : 0;

		if ( $user_ids_count <= self::MAX_INITIAL_SYNC_USERS ) {
			return $user_ids;
		} else {
			return false;
		}
	}

	/**
	 * Expand the users within a hook before they are serialized and sent to the server.
	 *
	 * @access public
	 *
	 * @param array $args The hook arguments.
	 * @return array $args The hook arguments.
	 */
	public function expand_users( $args ) {
		list( $user_ids, $previous_end ) = $args;

		return array(
			'users'        => array_map(
				array( $this, 'sanitize_user_and_expand' ),
				get_users(
					array(
						'include' => $user_ids,
						'orderby' => 'ID',
						'order'   => 'DESC',
					)
				)
			),
			'previous_end' => $previous_end,
		);
	}

	/**
	 * Handler for user removal from a particular blog.
	 *
	 * @access public
	 *
	 * @param int $user_id ID of the user.
	 * @param int $blog_id ID of the blog.
	 */
	public function remove_user_from_blog_handler( $user_id, $blog_id ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
		// User is removed on add, see https://github.com/WordPress/WordPress/blob/0401cee8b36df3def8e807dd766adc02b359dfaf/wp-includes/ms-functions.php#L2114.
		if ( $this->is_add_new_user_to_blog() ) {
			return;
		}

		$reassigned_user_id = $this->get_reassigned_network_user_id();

		// Note that we are in the context of the blog the user is removed from, see https://github.com/WordPress/WordPress/blob/473e1ba73bc5c18c72d7f288447503713d518790/wp-includes/ms-functions.php#L233.
		/**
		 * Fires when a user is removed from a blog on a multisite installation
		 *
		 * @since 1.6.3
		 * @since-jetpack 5.4.0
		 *
		 * @param int $user_id - ID of the removed user
		 * @param int $reassigned_user_id - ID of the user the removed user's posts are reassigned to (if any).
		 */
		do_action( 'jetpack_removed_user_from_blog', $user_id, $reassigned_user_id );
	}

	/**
	 * Whether we're adding a new user to a blog in this request.
	 *
	 * @access protected
	 *
	 * @return boolean
	 */
	protected function is_add_new_user_to_blog() {
		return $this->is_function_in_backtrace( 'add_new_user_to_blog' );
	}

	/**
	 * Whether we're adding an existing user to a blog in this request.
	 *
	 * @access protected
	 *
	 * @return boolean
	 */
	protected function is_add_user_to_blog() {
		return $this->is_function_in_backtrace( 'add_user_to_blog' );
	}

	/**
	 * Whether we're removing a user from a blog in this request.
	 *
	 * @access protected
	 *
	 * @return boolean
	 */
	protected function is_delete_user() {
		return $this->is_function_in_backtrace( array( 'wp_delete_user', 'remove_user_from_blog' ) );
	}

	/**
	 * Whether we're creating a user or adding a new user to a blog.
	 *
	 * @access protected
	 *
	 * @return boolean
	 */
	protected function is_create_user() {
		$functions = array(
			'add_new_user_to_blog', // Used to suppress jetpack_sync_save_user in save_user_cap_handler when user registered on multi site.
			'wp_create_user', // Used to suppress jetpack_sync_save_user in save_user_role_handler when user registered on multi site.
			'wp_insert_user', // Used to suppress jetpack_sync_save_user in save_user_cap_handler and save_user_role_handler when user registered on single site.
		);

		return $this->is_function_in_backtrace( $functions );
	}

	/**
	 * Retrieve the ID of the user the removed user's posts are reassigned to (if any).
	 *
	 * @return int ID of the user that got reassigned as the author of the posts.
	 */
	protected function get_reassigned_network_user_id() {
		$backtrace = debug_backtrace( false ); // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_debug_backtrace
		foreach ( $backtrace as $call ) {
			if (
				'remove_user_from_blog' === $call['function'] &&
				3 === count( $call['args'] )
			) {
				return $call['args'][2];
			}
		}

		return false;
	}

	/**
	 * Checks if one or more function names is in debug_backtrace.
	 *
	 * @access protected
	 *
	 * @param array|string $names Mixed string name of function or array of string names of functions.
	 * @return bool
	 */
	protected function is_function_in_backtrace( $names ) {
		$backtrace = debug_backtrace( false ); // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_debug_backtrace
		if ( ! is_array( $names ) ) {
			$names = array( $names );
		}
		$names_as_keys = array_flip( $names );

		$backtrace_functions         = array_column( $backtrace, 'function' );
		$backtrace_functions_as_keys = array_flip( $backtrace_functions );
		$intersection                = array_intersect_key( $backtrace_functions_as_keys, $names_as_keys );
		return ! empty( $intersection );
	}
}