Rotate workers when they are all idling

packages/jest-worker/src/__tests__/index.test.js

@@ -291,9 +291,9 @@ it('checks that once a sticked task finishes, next time is sent to that worker',
   await promise;
 
   // Note that the stickiness is not created by the method name or the arguments
-  // it is solely controlled by the provided "computeWorkerKey" method, which in the
-  // test example always returns the same key, so all calls should be redirected
-  // to worker 1 (which is the one that resolved the first call).
+  // it is solely controlled by the provided "computeWorkerKey" method, which in
+  // the test example always returns the same key, so all calls should be
+  // redirected to worker 1 (which is the one that resolved the first call).
   farm.bar();
 
   // The first time, a call with a "1234567890abcdef" hash had never been done
@@ -325,3 +325,42 @@ it('checks that once a non-sticked task finishes, next time is sent to all worke
   expect(mockWorkers[1].send).toHaveBeenCalledTimes(2);
   expect(mockWorkers[2].send).toHaveBeenCalledTimes(2);
 });
+
+it('rotates workers when they are idling', async () => {
+  let order;
+  let promise;
+
+  // Note there is no "computeWorkerKey".
+  const farm = new Farm('/tmp/baz.js', {
+    exposedMethods: ['foo', 'bar'],
+    numWorkers: 3,
+  });
+
+  [0, 1, 2].forEach(i => {
+    mockWorkers[i].send.mockReset();
+    mockWorkers[i].send.mockImplementation(() => order.push(i));
+  });
+
+  // First time, the order is 0, 1, 2.
+  order = [];
+  promise = farm.foo('car', 'plane');
+  expect(order).toEqual([0, 1, 2]);
+
+  // Worker 1 successfully replies with "17" as a result.
+  workerReply(1, null, 17);
+  await promise;
+
+  [0, 1, 2].forEach(i => {
+    mockWorkers[i].send.mockReset();
+    mockWorkers[i].send.mockImplementation(() => order.push(i));
+  });
+
+  // Now, the order is 1, 2, 0 (shifted one).
+  order = [];
+  promise = farm.foo('car', 'plane');
+  expect(order).toEqual([1, 2, 0]);
+
+  // Worker 1 successfully replies again.
+  workerReply(1, null, 17);
+  await promise;
+});

packages/jest-worker/src/index.js

@@ -54,6 +54,7 @@ export default class {
   _cacheKeys: {[string]: Worker, __proto__: null};
   _options: FarmOptions;
   _workers: Array<Worker>;
+  _offset: number;
 
   constructor(workerPath: string, options?: FarmOptions = {}) {
     const numWorkers = options.numWorkers || os.cpus().length - 1;
@@ -116,6 +117,7 @@ export default class {
     this._cacheKeys = Object.create(null);
     this._options = options;
     this._workers = workers;
+    this._offset = 0;
   }
 
   getStdout(): Readable {
@@ -151,6 +153,7 @@ export default class {
     return new Promise((resolve, reject) => {
       const {computeWorkerKey} = this._options;
       const workers = this._workers;
+      const length = workers.length;
       const cacheKeys = this._cacheKeys;
       const request = [CHILD_MESSAGE_CALL, false, method, args];
 
@@ -183,9 +186,11 @@ export default class {
       }
 
       // ... otherwise use all workers, so the first one available will pick it.
-      for (let i = 0; i < workers.length; i++) {
-        workers[i].send(request, callback);
+      for (let i = 0; i < length; i++) {
+        workers[(i + this._offset) % length].send(request, callback);
       }
+
+      this._offset++;
     });
   }
 }