Boilerplate for a grunt multi task that leverages YUI's Promises module to sanely handle asynchronous tasks that atomically process multiple files.

yui-async-multi-grunt.js

/*jshint node:true*/
module.exports = function(grunt) {
  var Y = require("yui/promise"),
    name = "...",
    desc = "...",
    defaultOptions = { /* */ };

  function executeAsyncTask(resolve, reject, file, options) {
    // this is where your business logic goes
    // call `resolve` when the task is finished
    // call `reject(error[, errorCode])` if something goes wrong.
  }

  // return a function that returns an array of promises, one for
  // each file passe to the multi task.
  function getPromises(options) {
    return function (file) {
      return new Y.Promise(function (resolve, reject) {
        executeAsyncTask(resolve, reject, file, options);
      });
    };
  }
  grunt.registerMultiTask(name, desc, function () {
    var options  = this.options(defaultOptions),
      iterator = getPromises(options),
      promises = this.files.map(iterator);
    Y.batch.apply(Y, promises).then(this.async(), grunt.warn);
  });
};