Check for undefined worker.stdio

src/WorkerPool.js

@@ -21,6 +21,15 @@ class PoolWorker {
     this.worker = childProcess.spawn(process.execPath, [].concat(options.nodeArgs || []).concat(workerPath, options.parallelJobs), {
       stdio: ['ignore', 1, 2, 'pipe', 'pipe'],
     });
+
+    // This prevents a problem where the worker stdio can be undefined
+    // when the kernel hits the limit of open files.
+    // More info can be found on: https://github.com/webpack-contrib/thread-loader/issues/2
+    if (!this.worker.stdio) {
+      throw new Error(`Failed to create the worker pool with workerId: ${workerId} and ${''
+      }configuration: ${JSON.stringify(options)}. Please verify if you hit the OS open files limit.`);
+    }
+
     const [, , , readPipe, writePipe] = this.worker.stdio;
     this.readPipe = readPipe;
     this.writePipe = writePipe;

test/__snapshots__/workerPool.test.js.snap

@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`workerPool should throw an error when worker.stdio is undefined 1`] = `"Failed to create the worker pool with workerId: 1 and configuration: {}. Please verify if you hit the OS open files limit."`;

test/workerPool.test.js

@@ -0,0 +1,32 @@
+import childProcess from 'child_process';
+import stream from 'stream';
+import WorkerPool from '../src/WorkerPool';
+
+jest.mock('child_process', () => {
+  return {
+    spawn: jest.fn(() => {
+      return {};
+    }),
+  };
+});
+
+describe('workerPool', () => {
+  it('should throw an error when worker.stdio is undefined', () => {
+    childProcess.spawn.mockImplementationOnce(() => { return {}; });
+
+    const workerPool = new WorkerPool({});
+    expect(() => workerPool.createWorker()).toThrowErrorMatchingSnapshot();
+    expect(() => workerPool.createWorker()).toThrowError('Please verify if you hit the OS open files limit');
+  });
+
+  it('should not throw an error when worker.stdio is defined', () => {
+    childProcess.spawn.mockImplementationOnce(() => {
+      return {
+        stdio: new Array(5).fill(new stream.PassThrough()),
+      };
+    });
+
+    const workerPool = new WorkerPool({});
+    expect(() => workerPool.createWorker()).not.toThrow();
+  });
+});