Experimentally implement shared workers

ava.config.js

@@ -1,6 +1,9 @@
 const skipTests = [];
-if (process.versions.node < '12.14.0') {
-	skipTests.push('!test/configurable-module-format/module.js');
+if (process.versions.node < '12.17.0') {
+	skipTests.push(
+		'!test/configurable-module-format/module.js',
+		'!test/shared-workers/!(requires-newish-node)/**'
+	);
 }
 
 export default {

lib/api.js

@@ -17,6 +17,7 @@ const RunStatus = require('./run-status');
 const fork = require('./fork');
 const serializeError = require('./serialize-error');
 const {getApplicableLineNumbers} = require('./line-numbers');
+const sharedWorkers = require('./plugin-support/shared-workers');
 
 function resolveModules(modules) {
 	return arrify(modules).map(name => {
@@ -206,6 +207,8 @@ class Api extends Emittery {
 				concurrency = 1;
 			}
 
+			const deregisteredSharedWorkers = [];
+
 			// Try and run each file, limited by `concurrency`.
 			await pMap(selectedFiles, async file => {
 				// No new files should be run once a test has timed out or failed,
@@ -231,15 +234,19 @@ class Api extends Emittery {
 
 				const worker = fork(file, options, apiOptions.nodeArguments);
 				runStatus.observeWorker(worker, file, {selectingLines: lineNumbers.length > 0});
+				deregisteredSharedWorkers.push(sharedWorkers.observeWorkerProcess(worker, runStatus));
 
 				pendingWorkers.add(worker);
 				worker.promise.then(() => {
 					pendingWorkers.delete(worker);
 				});
 				restartTimer();
 
-				return worker.promise;
+				await worker.promise;
 			}, {concurrency, stopOnError: false});
+
+			// Allow shared workers to clean up before the run ends.
+			await Promise.all(deregisteredSharedWorkers);
 		} catch (error) {
 			if (error && error.name === 'AggregateError') {
 				for (const err of error) {

lib/fork.js

@@ -14,7 +14,55 @@ if (fs.realpathSync(__filename) !== __filename) {
 const AVA_PATH = path.resolve(__dirname, '..');
 const WORKER_PATH = require.resolve('./worker/subprocess');
 
+class SharedWorkerChannel extends Emittery {
+	constructor({channelId, filename, initialData}, sendToFork) {
+		super();
+
+		this.id = channelId;
+		this.filename = filename;
+		this.initialData = initialData;
+		this.sendToFork = sendToFork;
+	}
+
+	signalReady() {
+		this.sendToFork({
+			type: 'shared-worker-ready',
+			channelId: this.id
+		});
+	}
+
+	signalError() {
+		this.sendToFork({
+			type: 'shared-worker-error',
+			channelId: this.id
+		});
+	}
+
+	emitMessage({messageId, replyTo, serializedData}) {
+		this.emit('message', {
+			messageId,
+			replyTo,
+			serializedData
+		});
+	}
+
+	forwardMessageToFork({messageId, replyTo, serializedData}) {
+		this.sendToFork({
+			type: 'shared-worker-message',
+			channelId: this.id,
+			messageId,
+			replyTo,
+			serializedData
+		});
+	}
+}
+
+let forkCounter = 0;
+
 module.exports = (file, options, execArgv = process.execArgv) => {
+	const forkId = `fork/${++forkCounter}`;
+	const sharedWorkerChannels = new Map();
+
 	let finished = false;
 
 	const emitter = new Emittery();
@@ -27,6 +75,7 @@ module.exports = (file, options, execArgv = process.execArgv) => {
 	options = {
 		baseDir: process.cwd(),
 		file,
+		forkId,
 		...options
 	};
 
@@ -69,6 +118,16 @@ module.exports = (file, options, execArgv = process.execArgv) => {
 				case 'ready-for-options':
 					send({type: 'options', options});
 					break;
+				case 'shared-worker-connect': {
+					const channel = new SharedWorkerChannel(message.ava, send);
+					sharedWorkerChannels.set(channel.id, channel);
+					emitter.emit('connectSharedWorker', channel);
+					break;
+				}
+
+				case 'shared-worker-message':
+					sharedWorkerChannels.get(message.ava.channelId).emitMessage(message.ava);
+					break;
 				case 'ping':
 					send({type: 'pong'});
 					break;
@@ -99,6 +158,7 @@ module.exports = (file, options, execArgv = process.execArgv) => {
 
 	return {
 		file,
+		forkId,
 		promise,
 
 		exit() {
@@ -110,6 +170,10 @@ module.exports = (file, options, execArgv = process.execArgv) => {
 			send({type: 'peer-failed'});
 		},
 
+		onConnectSharedWorker(listener) {
+			return emitter.on('connectSharedWorker', listener);
+		},
+
 		onStateChange(listener) {
 			return emitter.on('stateChange', listener);
 		}

lib/load-config.js

@@ -7,7 +7,13 @@ const pkgConf = require('pkg-conf');
 
 const NO_SUCH_FILE = Symbol('no ava.config.js file');
 const MISSING_DEFAULT_EXPORT = Symbol('missing default export');
-const EXPERIMENTS = new Set(['configurableModuleFormat', 'disableNullExpectations', 'disableSnapshotsInHooks', 'reverseTeardowns']);
+const EXPERIMENTS = new Set([
+	'configurableModuleFormat',
+	'disableNullExpectations',
+	'disableSnapshotsInHooks',
+	'reverseTeardowns',
+	'sharedWorkers'
+]);
 
 // *Very* rudimentary support for loading ava.config.js files containing an `export default` statement.
 const evaluateJsConfig = configFile => {