MDL-73915 js: Switch amd minification to terser

Unfortunately the babel minify-mangle plugin seems to be abandoned and
in certain circumstances can be very buggy. The only safe options are to
disable it, or to switch to a different minification library.

Not minifying our javascript is not ideal, so this commit updates the
javascript tasks to use a rollup, combined with babel, and terser.

Babel still converts code from ES/UMD/AMD to AMD modules with the
relevant browser support, whilst terser minifies the code.

The rollup bundler handles tracking and creation of sourcemaps, and
supports better parallelisation of the tasks.

Since the upgrade to Node LTS/Gallium requires an upgrade to @babel/core
and eslint, which change the built files anyway, this seems like the
ideal time to make this change.

.grunt/tasks/javascript.js

@@ -30,6 +30,7 @@
  * @param {String} srcPath the  matched src path
  * @return {String} The rewritten destination path.
  */
+
 const babelRename = function(destPath, srcPath) {
     destPath = srcPath.replace('src', 'build');
     destPath = destPath.replace('.js', '.min.js');
@@ -53,65 +54,139 @@ module.exports = grunt => {
 
     // Register JS tasks.
     grunt.registerTask('yui', ['eslint:yui', 'shifter']);
-    grunt.registerTask('amd', ['ignorefiles', 'eslint:amd', 'babel']);
+    grunt.registerTask('amd', ['ignorefiles', 'eslint:amd', 'rollup']);
     grunt.registerTask('js', ['amd', 'yui']);
 
     // Register NPM tasks.
     grunt.loadNpmTasks('grunt-contrib-uglify');
     grunt.loadNpmTasks('grunt-contrib-watch');
+    grunt.loadNpmTasks('grunt-rollup');
+
+    const babelTransform = require('@babel/core').transform;
+    const babel = (options = {}) => {
+        return {
+            name: 'babel',
+
+            transform: (code, id) => {
+                grunt.log.debug(`Transforming ${id}`);
+                options.filename = id;
+                const transformed = babelTransform(code, options);
+
+                return {
+                    code: transformed.code,
+                    map: transformed.map
+                };
+            }
+        };
+    };
+
+    // Note: We have to use a rate limit plugin here because rollup runs all tasks asynchronously and in parallel.
+    // When we kick off a full run, if we kick off a rollup of every file this will fork-bomb the machine.
+    // To work around this we use a concurrent Promise queue based on the number of available processors.
+    const rateLimit = () => {
+        const queue = [];
+        let queueRunner;
+
+        const startQueue = () => {
+            if (queueRunner) {
+                return;
+            }
+
+            queueRunner = setTimeout(() => {
+                const limit = Math.max(1, require('os').cpus().length / 2);
+                grunt.log.debug(`Starting rollup with queue size of ${limit}`);
+                runQueue(limit);
+            }, 100);
+        };
 
-    // Load the Babel tasks and config.
-    grunt.loadNpmTasks('grunt-babel');
+        // The queue runner will run the next `size` items in the queue.
+        const runQueue = (size = 1) => {
+            queue.splice(0, size).forEach(resolve => {
+                resolve();
+            });
+        };
+
+        return {
+            name: 'ratelimit',
+
+            // The options hook is run in parallel.
+            // We can return an unresolved Promise which is queued for later resolution.
+            options: async() => {
+                return new Promise(resolve => {
+                    queue.push(resolve);
+                    startQueue();
+                });
+            },
+
+            // When an item in the queue completes, start the next item in the queue.
+            buildEnd: () => {
+                runQueue();
+            },
+        };
+    };
+
+    const terser = require('rollup-plugin-terser').terser;
     grunt.config.merge({
-        babel: {
+        rollup: {
             options: {
-                sourceMaps: true,
-                comments: false,
+                format: 'esm',
+                dir: 'output',
+                sourcemap: true,
+                treeshake: false,
+                context: 'window',
                 plugins: [
-                    'transform-es2015-modules-amd-lazy',
-                    'system-import-transformer',
-                    // This plugin modifies the Babel transpiling for "export default"
-                    // so that if it's used then only the exported value is returned
-                    // by the generated AMD module.
-                    //
-                    // It also adds the Moodle plugin name to the AMD module definition
-                    // so that it can be imported as expected in other modules.
-                    path.resolve('.grunt/babel-plugin-add-module-to-define.js'),
-                    '@babel/plugin-syntax-dynamic-import',
-                    '@babel/plugin-syntax-import-meta',
-                    ['@babel/plugin-proposal-class-properties', {'loose': false}],
-                    '@babel/plugin-proposal-json-strings'
+                    rateLimit(),
+                    babel({
+                        sourceMaps: true,
+                        comments: false,
+                        compact: false,
+                        plugins: [
+                            'transform-es2015-modules-amd-lazy',
+                            'system-import-transformer',
+                            // This plugin modifies the Babel transpiling for "export default"
+                            // so that if it's used then only the exported value is returned
+                            // by the generated AMD module.
+                            //
+                            // It also adds the Moodle plugin name to the AMD module definition
+                            // so that it can be imported as expected in other modules.
+                            path.resolve('.grunt/babel-plugin-add-module-to-define.js'),
+                            '@babel/plugin-syntax-dynamic-import',
+                            '@babel/plugin-syntax-import-meta',
+                            ['@babel/plugin-proposal-class-properties', {'loose': false}],
+                            '@babel/plugin-proposal-json-strings'
+                        ],
+                        presets: [
+                            ['@babel/preset-env', {
+                                targets: {
+                                    browsers: [
+                                        ">0.25%",
+                                        "last 2 versions",
+                                        "not ie <= 10",
+                                        "not op_mini all",
+                                        "not Opera > 0",
+                                        "not dead"
+                                    ]
+                                },
+                                modules: false,
+                                useBuiltIns: false
+                            }]
+                        ]
+                    }),
+
+                    terser({
+                        // Do not mangle variables.
+                        // Makes debugging easier.
+                        mangle: false,
+                    }),
                 ],
-                presets: [
-                    ['minify', {
-                        // This minification plugin needs to be disabled because it breaks the
-                        // source map generation and causes invalid source maps to be output.
-                        simplify: false,
-                        builtIns: false
-                    }],
-                    ['@babel/preset-env', {
-                        targets: {
-                            browsers: [
-                                ">0.25%",
-                                "last 2 versions",
-                                "not ie <= 10",
-                                "not op_mini all",
-                                "not Opera > 0",
-                                "not dead"
-                            ]
-                        },
-                        modules: false,
-                        useBuiltIns: false
-                    }]
-                ]
             },
             dist: {
                 files: [{
                     expand: true,
                     src: grunt.moodleEnv.files ? grunt.moodleEnv.files : grunt.moodleEnv.amdSrc,
                     rename: babelRename
-                }]
-            }
+                }],
+            },
         },
     });
 
@@ -135,7 +210,7 @@ module.exports = grunt => {
     let changedFiles = Object.create(null);
     const onChange = grunt.util._.debounce(function() {
         const files = Object.keys(changedFiles);
-        grunt.config('babel.dist.files', [{expand: true, src: files, rename: babelRename}]);
+        grunt.config('rollup.dist.files', [{expand: true, src: files, rename: babelRename}]);
         changedFiles = Object.create(null);
     }, 200);