Prevent worker stdout/stderr from ending when the first stream ends

packages/jest-worker/src/workers/ChildProcessWorker.ts

@@ -6,6 +6,7 @@
  */
 
 import childProcess, {ChildProcess} from 'child_process';
+import {PassThrough} from 'stream';
 import mergeStream from 'merge-stream';
 import supportsColor from 'supports-color';
 
@@ -44,13 +45,15 @@ export default class ChildProcessWorker implements WorkerInterface {
   private _child!: ChildProcess;
   private _options: WorkerOptions;
   private _onProcessEnd!: OnEnd;
+  private _fakeStream: PassThrough | null;
   private _request: ChildMessage | null;
   private _retries!: number;
   private _stderr: ReturnType<typeof mergeStream> | null;
   private _stdout: ReturnType<typeof mergeStream> | null;
 
   constructor(options: WorkerOptions) {
     this._options = options;
+    this._fakeStream = null;
     this._request = null;
     this._stderr = null;
     this._stdout = null;
@@ -75,15 +78,19 @@ export default class ChildProcessWorker implements WorkerInterface {
 
     if (child.stdout) {
       if (!this._stdout) {
-        this._stdout = mergeStream();
+        // We need to add a permanent stream to the merged stream to prevent it
+        // from ending when the subprocess stream ends
+        this._stdout = mergeStream(this._getFakeStream());
       }
 
       this._stdout.add(child.stdout);
     }
 
     if (child.stderr) {
       if (!this._stderr) {
-        this._stderr = mergeStream();
+        // We need to add a permanent stream to the merged stream to prevent it
+        // from ending when the subprocess stream ends
+        this._stderr = mergeStream(this._getFakeStream());
       }
 
       this._stderr.add(child.stderr);
@@ -119,6 +126,14 @@ export default class ChildProcessWorker implements WorkerInterface {
     }
   }
 
+  private _shutdown() {
+    // End the temporary streams so the merged streams end too
+    if (this._fakeStream) {
+      this._fakeStream.end();
+      this._fakeStream = null;
+    }
+  }
+
   onMessage(response: ParentMessage) {
     let error;
 
@@ -171,6 +186,8 @@ export default class ChildProcessWorker implements WorkerInterface {
       if (this._request) {
         this._child.send(this._request);
       }
+    } else {
+      this._shutdown();
     }
   }
 
@@ -199,4 +216,11 @@ export default class ChildProcessWorker implements WorkerInterface {
   getStderr(): NodeJS.ReadableStream | null {
     return this._stderr;
   }
+
+  private _getFakeStream() {
+    if (!this._fakeStream) {
+      this._fakeStream = new PassThrough();
+    }
+    return this._fakeStream;
+  }
 }

packages/jest-worker/src/workers/NodeThreadsWorker.ts

@@ -6,6 +6,7 @@
  */
 
 import path from 'path';
+import {PassThrough} from 'stream';
 // ESLint doesn't know about this experimental module
 // eslint-disable-next-line import/no-unresolved
 import {Worker} from 'worker_threads';
@@ -32,12 +33,14 @@ export default class ExperimentalWorker implements WorkerInterface {
   private _retries!: number;
   private _stderr: ReturnType<typeof mergeStream> | null;
   private _stdout: ReturnType<typeof mergeStream> | null;
+  private _fakeStream: PassThrough | null;
 
   constructor(options: WorkerOptions) {
     this._options = options;
     this._request = null;
     this._stderr = null;
     this._stdout = null;
+    this._fakeStream = null;
 
     this.initialize();
   }
@@ -62,15 +65,19 @@ export default class ExperimentalWorker implements WorkerInterface {
 
     if (this._worker.stdout) {
       if (!this._stdout) {
-        this._stdout = mergeStream();
+        // We need to add a permanent stream to the merged stream to prevent it
+        // from ending when the subprocess stream ends
+        this._stdout = mergeStream(this._getFakeStream());
       }
 
       this._stdout.add(this._worker.stdout);
     }
 
     if (this._worker.stderr) {
       if (!this._stderr) {
-        this._stderr = mergeStream();
+        // We need to add a permanent stream to the merged stream to prevent it
+        // from ending when the subprocess stream ends
+        this._stderr = mergeStream(this._getFakeStream());
       }
 
       this._stderr.add(this._worker.stderr);
@@ -104,6 +111,14 @@ export default class ExperimentalWorker implements WorkerInterface {
     }
   }
 
+  private _shutdown() {
+    // End the permanent stream so the merged stream end too
+    if (this._fakeStream) {
+      this._fakeStream.end();
+      this._fakeStream = null;
+    }
+  }
+
   onMessage(response: ParentMessage) {
     let error;
 
@@ -154,6 +169,8 @@ export default class ExperimentalWorker implements WorkerInterface {
       if (this._request) {
         this._worker.postMessage(this._request);
       }
+    } else {
+      this._shutdown();
     }
   }
 
@@ -183,4 +200,11 @@ export default class ExperimentalWorker implements WorkerInterface {
   getStderr(): NodeJS.ReadableStream | null {
     return this._stderr;
   }
+
+  private _getFakeStream() {
+    if (!this._fakeStream) {
+      this._fakeStream = new PassThrough();
+    }
+    return this._fakeStream;
+  }
 }

packages/jest-worker/src/workers/__tests__/ChildProcessWorker.test.js

@@ -141,6 +141,7 @@ it('provides stdout and stderr from the child processes', async () => {
   forkInterface.emit('exit');
   forkInterface.stdout.end('World!', {encoding: 'utf8'});
   forkInterface.stderr.end('Workers!', {encoding: 'utf8'});
+  forkInterface.emit('exit', 0);
 
   await expect(getStream(stdout)).resolves.toEqual('Hello World!');
   await expect(getStream(stderr)).resolves.toEqual('Jest Workers!');

packages/jest-worker/src/workers/__tests__/NodeThreadsWorker.test.js

@@ -153,6 +153,7 @@ it('provides stdout and stderr from the child processes', async () => {
   worker._worker.emit('exit');
   worker._worker.stdout.end('World!', {encoding: 'utf8'});
   worker._worker.stderr.end('Workers!', {encoding: 'utf8'});
+  worker._worker.emit('exit', 0);
 
   await expect(getStream(stdout)).resolves.toEqual('Hello World!');
   await expect(getStream(stderr)).resolves.toEqual('Jest Workers!');