Next-generation configuration loading

* When worker threads are available, support asynchronous configuration loading in the ESLint plugin helper

* Experimental implementation of next-generation configuration loading. This adds support for `.mjs` files, fixing #2346. I've removed the special handling of `ava.config.js` files, relying on Node.js to follow the package type instead. We now also support asynchronous factories.

Author: novemberborn

ava.config.js

@@ -1,6 +1,7 @@
 const skipTests = [];
 if (process.versions.node < '12.17.0') {
 	skipTests.push(
+		'!test/config/next-gen.js',
 		'!test/configurable-module-format/module.js',
 		'!test/shared-workers/!(requires-newish-node)/**'
 	);

docs/06-configuration.md

@@ -73,11 +73,13 @@ To use these files:
 2. Your `package.json` must not contain an `ava` property (or, if it does, it must be an empty object)
 3. You must not both have an `ava.config.js` *and* an `ava.config.cjs` file
 
-AVA recognizes `ava.config.mjs` files but refuses to load them.
+AVA 3 recognizes `ava.config.mjs` files but refuses to load them. This is changing in AVA 4, [see below](#next-generation-configuration).
 
 ### `ava.config.js`
 
-For `ava.config.js` files you must use `export default`. You cannot use ["module scope"](https://nodejs.org/docs/latest-v12.x/api/modules.html#modules_the_module_scope). You cannot import dependencies.
+In AVA 3, for `ava.config.js` files you must use `export default`. You cannot use ["module scope"](https://nodejs.org/docs/latest-v12.x/api/modules.html#modules_the_module_scope). You cannot import dependencies.
+
+This is changing in AVA 4, [see below](#next-generation-configuration).
 
 The default export can either be a plain object or a factory function which returns a plain object:
 
@@ -111,7 +113,7 @@ export default ({projectDir}) => {
 };
 ```
 
-Note that the final configuration must not be a promise.
+Note that the final configuration must not be a promise. This is changing in AVA 4, [see below](#next-generation-configuration).
 
 ### `ava.config.cjs`
 
@@ -149,12 +151,14 @@ module.exports = ({projectDir}) => {
 };
 ```
 
-Note that the final configuration must not be a promise.
+Note that the final configuration must not be a promise. This is changing in AVA 4, [see below](#next-generation-configuration).
 
 ## Alternative configuration files
 
 The [CLI] lets you specify a specific configuration file, using the `--config` flag. This file must have either a `.js` or `.cjs` extension and is processed like an `ava.config.js` or `ava.config.cjs` file would be.
 
+AVA 4 also supports `.mjs` extensions, [see below](#next-generation-configuration).
+
 When the `--config` flag is set, the provided file will override all configuration from the `package.json` and `ava.config.js` or `ava.config.cjs` files. The configuration is not merged.
 
 The configuration file *must* be in the same directory as the `package.json` file.
@@ -182,6 +186,25 @@ module.exports = {
 
 You can now run your unit tests through `npx ava` and the integration tests through `npx ava --config integration-tests.config.cjs`.
 
+## Next generation configuration
+
+AVA 4 will add full support for ESM configuration files as well as allowing you to have asynchronous factory functions. If you're using Node.js 12 or later you can opt-in to these features in AVA 3 by enabling the `nextGenConfig` experiment. Say in an `ava.config.mjs` file:
+
+```js
+export default {
+	nonSemVerExperiments: {
+		nextGenConfig: true
+	},
+	files: ['unit-tests/**/*]
+};
+```
+
+This also allows you to pass an `.mjs` file using the `--config` argument.
+
+With this experiment enabled, AVA will no longer have special treatment for `ava.config.js` files. Instead AVA follows Node.js' behavior, so if you've set [`"type": "module"`](https://nodejs.org/docs/latest/api/packages.html#packages_type) you must use ESM, and otherwise you must use CommonJS.
+
+You mustn't have an `ava.config.mjs` file next to an `ava.config.js` or `ava.config.cjs` file.
+
 ## Object printing depth
 
 By default, AVA prints nested objects to a depth of `3`. However, when debugging tests with deeply nested objects, it can be useful to print with more detail. This can be done by setting [`util.inspect.defaultOptions.depth`](https://nodejs.org/api/util.html#util_util_inspect_defaultoptions) to the desired depth, before the test is executed:

eslint-plugin-helper.js

@@ -1,26 +1,25 @@
 'use strict';
-const normalizeExtensions = require('./lib/extensions');
+let isMainThread = true;
+let supportsWorkers = false;
+try {
+	({isMainThread} = require('worker_threads'));
+	supportsWorkers = true;
+} catch {}
+
 const {classify, hasExtension, isHelperish, matches, normalizeFileForMatching, normalizeGlobs, normalizePatterns} = require('./lib/globs');
-const loadConfig = require('./lib/load-config');
-const providerManager = require('./lib/provider-manager');
 
-const configCache = new Map();
-const helperCache = new Map();
+let resolveGlobs;
+let resolveGlobsSync;
 
-function load(projectDir, overrides) {
-	const cacheKey = `${JSON.stringify(overrides)}\n${projectDir}`;
-	if (helperCache.has(cacheKey)) {
-		return helperCache.get(cacheKey);
-	}
+if (!supportsWorkers || !isMainThread) {
+	const normalizeExtensions = require('./lib/extensions');
+	const {loadConfig, loadConfigSync} = require('./lib/load-config');
+	const providerManager = require('./lib/provider-manager');
 
-	let conf;
-	let providers;
-	if (configCache.has(projectDir)) {
-		({conf, providers} = configCache.get(projectDir));
-	} else {
-		conf = loadConfig({resolveFrom: projectDir});
+	const configCache = new Map();
 
-		providers = [];
+	const collectProviders = ({conf, projectDir}) => {
+		const providers = [];
 		if (Reflect.has(conf, 'babel')) {
 			const {level, main} = providerManager.babel(projectDir);
 			providers.push({
@@ -39,12 +38,125 @@ function load(projectDir, overrides) {
 			});
 		}
 
-		configCache.set(projectDir, {conf, providers});
+		return providers;
+	};
+
+	const buildGlobs = ({conf, providers, projectDir, overrideExtensions, overrideFiles}) => {
+		const extensions = overrideExtensions ?
+			normalizeExtensions(overrideExtensions) :
+			normalizeExtensions(conf.extensions, providers);
+
+		return {
+			cwd: projectDir,
+			...normalizeGlobs({
+				extensions,
+				files: overrideFiles ? overrideFiles : conf.files,
+				providers
+			})
+		};
+	};
+
+	resolveGlobsSync = (projectDir, overrideExtensions, overrideFiles) => {
+		if (!configCache.has(projectDir)) {
+			const conf = loadConfigSync({resolveFrom: projectDir});
+			const providers = collectProviders({conf, projectDir});
+			configCache.set(projectDir, {conf, providers});
+		}
+
+		const {conf, providers} = configCache.get(projectDir);
+		return buildGlobs({conf, providers, projectDir, overrideExtensions, overrideFiles});
+	};
+
+	resolveGlobs = async (projectDir, overrideExtensions, overrideFiles) => {
+		if (!configCache.has(projectDir)) {
+			configCache.set(projectDir, loadConfig({resolveFrom: projectDir}).then(conf => { // eslint-disable-line promise/prefer-await-to-then
+				const providers = collectProviders({conf, projectDir});
+				return {conf, providers};
+			}));
+		}
+
+		const {conf, providers} = await configCache.get(projectDir);
+		return buildGlobs({conf, providers, projectDir, overrideExtensions, overrideFiles});
+	};
+}
+
+if (supportsWorkers) {
+	const v8 = require('v8');
+
+	const MAX_DATA_LENGTH_EXCLUSIVE = 100 * 1024; // Allocate 100 KiB to exchange globs.
+
+	if (isMainThread) {
+		const {Worker} = require('worker_threads');
+		let data;
+		let sync;
+		let worker;
+
+		resolveGlobsSync = (projectDir, overrideExtensions, overrideFiles) => {
+			if (worker === undefined) {
+				const dataBuffer = new SharedArrayBuffer(MAX_DATA_LENGTH_EXCLUSIVE);
+				data = new Uint8Array(dataBuffer);
+
+				const syncBuffer = new SharedArrayBuffer(4);
+				sync = new Int32Array(syncBuffer);
+
+				worker = new Worker(__filename, {
+					workerData: {
+						dataBuffer,
+						syncBuffer,
+						firstMessage: {projectDir, overrideExtensions, overrideFiles}
+					}
+				});
+				worker.unref();
+			} else {
+				worker.postMessage({projectDir, overrideExtensions, overrideFiles});
+			}
+
+			Atomics.wait(sync, 0, 0);
+
+			const byteLength = Atomics.exchange(sync, 0, 0);
+			if (byteLength === MAX_DATA_LENGTH_EXCLUSIVE) {
+				throw new Error('Globs are over 100 KiB and cannot be resolved');
+			}
+
+			const globsOrError = v8.deserialize(data.slice(0, byteLength));
+			if (globsOrError instanceof Error) {
+				throw globsOrError;
+			}
+
+			return globsOrError;
+		};
+	} else {
+		const {parentPort, workerData} = require('worker_threads');
+		const data = new Uint8Array(workerData.dataBuffer);
+		const sync = new Int32Array(workerData.syncBuffer);
+
+		const handleMessage = async ({projectDir, overrideExtensions, overrideFiles}) => {
+			let encoded;
+			try {
+				const globs = await resolveGlobs(projectDir, overrideExtensions, overrideFiles);
+				encoded = v8.serialize(globs);
+			} catch (error) {
+				encoded = v8.serialize(error);
+			}
+
+			const byteLength = encoded.length < MAX_DATA_LENGTH_EXCLUSIVE ? encoded.copy(data) : MAX_DATA_LENGTH_EXCLUSIVE;
+			Atomics.store(sync, 0, byteLength);
+			Atomics.notify(sync, 0);
+		};
+
+		parentPort.on('message', handleMessage);
+		handleMessage(workerData.firstMessage);
+		delete workerData.firstMessage;
 	}
+}
+
+const helperCache = new Map();
 
-	const extensions = overrides && overrides.extensions ?
-		normalizeExtensions(overrides.extensions) :
-		normalizeExtensions(conf.extensions, providers);
+function load(projectDir, overrides) {
+	const cacheKey = `${JSON.stringify(overrides)}\n${projectDir}`;
+	if (helperCache.has(cacheKey)) {
+		return helperCache.get(cacheKey);
+	}
 
 	let helperPatterns = [];
 	if (overrides && overrides.helpers !== undefined) {
@@ -55,14 +167,7 @@ function load(projectDir, overrides) {
 		helperPatterns = normalizePatterns(overrides.helpers);
 	}
 
-	const globs = {
-		cwd: projectDir,
-		...normalizeGlobs({
-			extensions,
-			files: overrides && overrides.files ? overrides.files : conf.files,
-			providers
-		})
-	};
+	const globs = resolveGlobsSync(projectDir, overrides && overrides.extensions, overrides && overrides.files);
 
 	const classifyForESLint = file => {
 		const {isTest} = classify(file, globs);

lib/cli.js

@@ -7,7 +7,7 @@ const arrify = require('arrify');
 const yargs = require('yargs');
 const readPkg = require('read-pkg');
 const isCi = require('./is-ci');
-const loadConfig = require('./load-config');
+const {loadConfig} = require('./load-config');
 
 function exit(message) {
 	console.error(`\n  ${require('./chalk').get().red(figures.cross)} ${message}`);
@@ -83,7 +83,7 @@ exports.run = async () => { // eslint-disable-line complexity
 	let confError = null;
 	try {
 		const {argv: {config: configFile}} = yargs.help(false);
-		conf = loadConfig({configFile});
+		conf = await loadConfig({configFile});
 	} catch (error) {
 		confError = error;
 	}