worker: make terminate() resolve for unref’ed Workers

Once `worker.terminate()` is called, the Worker instance will be
destroyed as soon as possible anyway, so in order to make
the Promise returned by `worker.terminate()` resolve always,
it should be okay to just call `.ref()` on it and keep the main
event loop alive temporarily.

PR-URL: #29484
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/internal/worker.js

@@ -226,6 +226,8 @@ class Worker extends EventEmitter {
   terminate(callback) {
     debug(`[${threadId}] terminates Worker with ID ${this.threadId}`);
 
+    this.ref();
+
     if (typeof callback === 'function') {
       process.emitWarning(
         'Passing a callback to worker.terminate() is deprecated. ' +

test/parallel/test-worker-terminate-unrefed.js

@@ -0,0 +1,16 @@
+'use strict';
+const common = require('../common');
+const { once } = require('events');
+const { Worker } = require('worker_threads');
+
+// Test that calling worker.terminate() on an unref()’ed Worker instance
+// still resolves the returned Promise.
+
+async function test() {
+  const worker = new Worker('setTimeout(() => {}, 1000000);', { eval: true });
+  await once(worker, 'online');
+  worker.unref();
+  await worker.terminate();
+}
+
+test().then(common.mustCall());