Eliminate Farm work results leakage by clearing callback reference

packages/jest-worker/src/Farm.ts

@@ -127,9 +127,12 @@ export default class Farm {
     // Reference the task object outside so it won't be retained by onEnd,
     // and other properties of the task object, such as task.request can be
     // garbage collected.
-    const taskOnEnd = task.onEnd;
+    let taskOnEnd: OnEnd | null = task.onEnd;
     const onEnd: OnEnd = (error, result) => {
-      taskOnEnd(error, result);
+      if (taskOnEnd) {
+        taskOnEnd(error, result);
+      }
+      taskOnEnd = null;
 
       this._unlock(workerId);
       this._process(workerId);

packages/jest-worker/src/__tests__/leak-integration.test.ts

@@ -11,30 +11,71 @@ import {writeFileSync} from 'graceful-fs';
 import LeakDetector from 'jest-leak-detector';
 import {Worker} from '../../build/index';
 
-let workerFile!: string;
-beforeAll(() => {
-  workerFile = join(tmpdir(), 'baz.js');
-  writeFileSync(workerFile, 'module.exports.fn = () => {};');
-});
+describe('WorkerThreads leaks', () => {
+  let workerFile!: string;
+  beforeAll(() => {
+    workerFile = join(tmpdir(), 'baz.js');
+    writeFileSync(workerFile, 'module.exports.fn = () => {};');
+  });
 
-let worker!: Worker;
-beforeEach(() => {
-  worker = new Worker(workerFile, {
-    enableWorkerThreads: true,
-    exposedMethods: ['fn'],
+  let worker!: Worker;
+  beforeEach(() => {
+    worker = new Worker(workerFile, {
+      enableWorkerThreads: true,
+      exposedMethods: ['fn'],
+    });
+  });
+  afterEach(async () => {
+    await worker.end();
+  });
+
+  it('does not retain arguments after a task finished', async () => {
+    let leakDetector!: LeakDetector;
+    await new Promise((resolve, reject) => {
+      const obj = {};
+      leakDetector = new LeakDetector(obj);
+      (worker as any).fn(obj).then(resolve, reject);
+    });
+
+    expect(await leakDetector.isLeaking()).toBe(false);
   });
 });
-afterEach(async () => {
-  await worker.end();
-});
 
-it('does not retain arguments after a task finished', async () => {
-  let leakDetector!: LeakDetector;
-  await new Promise((resolve, reject) => {
-    const obj = {};
-    leakDetector = new LeakDetector(obj);
-    (worker as any).fn(obj).then(resolve, reject);
+describe('Worker leaks', () => {
+  let workerFile!: string;
+  beforeAll(() => {
+    workerFile = join(tmpdir(), 'baz.js');
+    writeFileSync(workerFile, 'module.exports.fn = (obj) => [obj];');
+  });
+
+  let worker!: Worker;
+  beforeEach(() => {
+    worker = new Worker(workerFile, {
+      enableWorkerThreads: false,
+      exposedMethods: ['fn'],
+    });
   });
+  afterEach(async () => {
+    await worker.end();
+  });
+
+  it('does not retain result after next task call', async () => {
+    let leakDetector!: LeakDetector;
+    await new Promise((resolve, reject) => {
+      const obj = {};
+      (worker as any)
+        .fn(obj)
+        .then((result: unknown) => {
+          leakDetector = new LeakDetector(result);
+          return result;
+        })
+        .then(resolve, reject);
+    });
+    await new Promise((resolve, reject) => {
+      const obj = {};
+      (worker as any).fn(obj).then(resolve, reject);
+    });
 
-  expect(await leakDetector.isLeaking()).toBe(false);
+    expect(await leakDetector.isLeaking()).toBe(false);
+  });
 });