worker: perform initial port.unref() before preload modules

The refcount of the internal communication port is relevant for
stdio, but the `port.unref()` call effectively resets any `.ref()`
calls happening during stdio operations happening before it.

Therefore, do the `.unref()` call before loading preload modules,
which may cause stdio operations.

Fixes: #31777

PR-URL: #33455
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>

lib/internal/main/worker_thread.js

@@ -92,6 +92,7 @@ if (process.env.NODE_CHANNEL_FD) {
 
 port.on('message', (message) => {
   if (message.type === LOAD_SCRIPT) {
+    port.unref();
     const {
       argv,
       cwdCounter,
@@ -140,7 +141,6 @@ port.on('message', (message) => {
 
     debug(`[${threadId}] starts worker script ${filename} ` +
           `(eval = ${eval}) at cwd = ${process.cwd()}`);
-    port.unref();
     port.postMessage({ type: UP_AND_RUNNING });
     if (doEval) {
       const { evalScript } = require('internal/process/execution');

test/parallel/test-worker-stdio-from-preload-module.js

@@ -0,0 +1,20 @@
+'use strict';
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const { Worker } = require('worker_threads');
+const assert = require('assert');
+
+// Regression test for https://github.com/nodejs/node/issues/31777:
+// stdio operations coming from preload modules should count towards the
+// ref count of the internal communication port on the Worker side.
+
+for (let i = 0; i < 10; i++) {
+  const w = new Worker('console.log("B");', {
+    execArgv: ['--require', fixtures.path('printA.js')],
+    eval: true,
+    stdout: true
+  });
+  w.on('exit', common.mustCall(() => {
+    assert.strictEqual(w.stdout.read().toString(), 'A\nB\n');
+  }));
+}