fix(child_process): pipe stdout and stderr to main thread

src/runtime/process-worker.ts

@@ -26,12 +26,16 @@ export default class ProcessWorker implements TinypoolWorker {
       options.argv,
       {
         ...options,
+        stdio: 'pipe',
         env: {
           ...options.env,
           TINYPOOL_WORKER_ID: options.workerData[0].workerId.toString(),
         },
       }
     )
+    this.process.stdout!.pipe(process.stdout)
+    this.process.stderr!.pipe(process.stderr)
+
     this.threadId = this.process.pid!
 
     this.process.on('exit', this.onUnexpectedExit)
@@ -136,6 +140,21 @@ export default class ProcessWorker implements TinypoolWorker {
     // This requires manual unreffing of its channel.
     this.process.channel?.unref()
 
+    if (hasUnref(this.process.stdout)) {
+      this.process.stdout.unref()
+    }
+
+    if (hasUnref(this.process.stderr)) {
+      this.process.stderr.unref()
+    }
+
     return this.process.unref()
   }
 }
+
+// unref is untyped for some reason
+function hasUnref(stream: null | object): stream is { unref: () => void } {
+  return (
+    stream != null && 'unref' in stream && typeof stream.unref === 'function'
+  )
+}

test/fixtures/stdio.mjs

@@ -0,0 +1,4 @@
+export default function run() {
+  process.stdout.write('Worker message')
+  process.stderr.write('Worker error')
+}

test/worker-stdio.test.ts

@@ -0,0 +1,56 @@
+import * as path from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { stripVTControlCharacters } from 'node:util'
+import { Tinypool } from 'tinypool'
+
+const runtimes = ['worker_threads', 'child_process'] as const
+const __dirname = path.dirname(fileURLToPath(import.meta.url))
+
+test.each(runtimes)(
+  "worker's stdout and stderr are piped to main thread when { runtime: '%s' }",
+  async (runtime) => {
+    const pool = createPool({
+      runtime,
+      minThreads: 1,
+      maxThreads: 1,
+    })
+
+    const getStdout = captureStandardStream('stdout')
+    const getStderr = captureStandardStream('stderr')
+
+    await pool.run({})
+
+    const stdout = getStdout()
+    const stderr = getStderr()
+
+    expect(stdout).toMatch('Worker message')
+
+    expect(stderr).toMatch('Worker error')
+  }
+)
+
+function createPool(options: Partial<Tinypool['options']>) {
+  const pool = new Tinypool({
+    filename: path.resolve(__dirname, 'fixtures/stdio.mjs'),
+    minThreads: 1,
+    maxThreads: 1,
+    ...options,
+  })
+
+  return pool
+}
+
+function captureStandardStream(type: 'stdout' | 'stderr') {
+  const spy = vi.fn()
+
+  // eslint-disable-next-line @typescript-eslint/unbound-method
+  const original = process[type].write
+  process[type].write = spy
+
+  return function collect() {
+    process[type].write = original
+    return stripVTControlCharacters(
+      spy.mock.calls.map((call) => call[0]).join('')
+    )
+  }
+}