Keep test worker alive until explicitly freed

This change ensures that all messages from the worker are received by
the main process before the worker exits.

It solves issues on Windows that are occurring with Node.js v20 and later
when `workerThreads: false` is set in the AVA configuration.

Fixes: #3390

* Update comment and rename message

Whether the worker exits depends on whether user code is keeping the event loop busy. 'Freed' is a better term.

---------

Co-authored-by: Mark Wubben <mark@novemberborn.net>

Author: matz3

lib/fork.js

@@ -104,6 +104,11 @@ export default function loadFork(file, options, execArgv = process.execArgv) {
 			}
 
 			switch (message.ava.type) {
+				case 'worker-finished': {
+					send({type: 'free-worker'});
+					break;
+				}
+
 				case 'ready-for-options': {
 					send({type: 'options', options});
 					break;

lib/worker/base.js

@@ -120,6 +120,15 @@ const run = async options => {
 			return;
 		}
 
+		channel.send({type: 'worker-finished'});
+
+		// Reference the channel until the worker is freed. This should prevent Node.js from terminating the child process
+		// prematurely, which has been witnessed on Windows. See discussion at
+		// <https://github.com/avajs/ava/issues/3390#issuecomment-3056119361>.
+		channel.ref();
+		await channel.workerFreed;
+		channel.unref();
+
 		nowAndTimers.setImmediate(() => {
 			const unhandled = currentlyUnhandled();
 			if (unhandled.length === 0) {

lib/worker/channel.cjs

@@ -107,7 +107,9 @@ handle.ref();
 
 exports.options = selectAvaMessage(handle.channel, 'options').then(message => message.ava.options);
 exports.peerFailed = selectAvaMessage(handle.channel, 'peer-failed');
+exports.workerFreed = selectAvaMessage(handle.channel, 'free-worker');
 exports.send = handle.send.bind(handle);
+exports.ref = handle.ref.bind(handle);
 exports.unref = handle.unref.bind(handle);
 
 let channelCounter = 0;

test/child-process/fixtures/package.json

@@ -0,0 +1,3 @@
+{
+	"type": "module"
+}

test/child-process/fixtures/test.js

@@ -0,0 +1,7 @@
+import test from 'ava';
+
+for (let i = 0; i < 50; i++) {
+	test('Test ' + (i + 1), t => {
+		t.true(true);
+	});
+}

test/child-process/test.js

@@ -0,0 +1,9 @@
+import test from '@ava/test';
+
+import {fixture} from '../helpers/exec.js';
+
+// Regression test for https://github.com/avajs/ava/issues/3390
+test('running 50 tests in a child process works as expected', async t => {
+	const result = await fixture(['--no-worker-threads']);
+	t.is(result.stats.passed.length, 50);
+});