worker: flush stdout and stderr on exit

lib/internal/worker/io.js

@@ -292,6 +292,9 @@
       chunks: ArrayPrototypeMap(chunks,
                                 ({ chunk, encoding }) => ({ chunk, encoding })),
     });
+    if (process._exiting) {
+      cb()
+    }
     ArrayPrototypePush(this[kWritableCallbacks], cb);
     if (this[kPort][kWaitingStreams]++ === 0)
       this[kPort].ref();

test/parallel/test-worker-stdio-flush.js

@@ -0,0 +1,28 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const util = require('util');
+const { Writable } = require('stream');
+const { Worker, isMainThread } = require('worker_threads');
+
+if (isMainThread) {
+  const w = new Worker(__filename, { stdout: true });
+  const expected = 'hello world'
+
+  let data = ''
+  w.stdout.setEncoding('utf8')
+  w.stdout.on('data', (chunk) => {
+    data += chunk
+  })
+
+  w.on('exit', common.mustCall(() => {
+    assert.strictEqual(data, expected)
+  }));
+} else {
+  process.on('exit', () => {
+    process.stdout.write(' ')
+    process.stdout.write('world')
+  })
+  process.stdout.write('hello')
+}