[kbn-build] Extract and test parallelizeBatches (#16596)

* [kbn-build] Extract and test parallelizeBatches

* Add test for rejects

* Simplify test

packages/kbn-build/src/commands/bootstrap.js

@@ -2,6 +2,7 @@ import chalk from 'chalk';
 
 import { topologicallyBatchProjects } from '../utils/projects';
 import { linkProjectExecutables } from '../utils/link_project_executables';
+import { parallelizeBatches } from '../utils/parallelize';
 
 export const name = 'bootstrap';
 export const description = 'Install dependencies and crosslink projects';
@@ -46,13 +47,3 @@ export async function run(projects, projectGraph, { options }) {
 
   console.log(chalk.green.bold('\nBootstrapping completed!\n'));
 }
-
-async function parallelizeBatches(batchedProjects, fn) {
-  for (const batch of batchedProjects) {
-    const running = batch.map(pkg => fn(pkg));
-
-    // We need to make sure the entire batch has completed before we can move on
-    // to the next batch
-    await Promise.all(running);
-  }
-}

packages/kbn-build/src/commands/run.js

@@ -1,6 +1,7 @@
 import chalk from 'chalk';
 
 import { topologicallyBatchProjects } from '../utils/projects';
+import { parallelizeBatches } from '../utils/parallelize';
 
 export const name = 'run';
 export const description =
@@ -31,13 +32,3 @@ export async function run(projects, projectGraph, { extraArgs }) {
     }
   });
 }
-
-async function parallelizeBatches(batchedProjects, fn) {
-  for (const batch of batchedProjects) {
-    const running = batch.map(pkg => fn(pkg));
-
-    // We need to make sure the entire batch has completed before we can move on
-    // to the next batch
-    await Promise.all(running);
-  }
-}

packages/kbn-build/src/utils/parallelize.js

@@ -0,0 +1,9 @@
+export async function parallelizeBatches(batches, fn) {
+  for (const batch of batches) {
+    const running = batch.map(obj => fn(obj));
+
+    // We need to make sure the entire batch has completed before we can move on
+    // to the next batch
+    await Promise.all(running);
+  }
+}

packages/kbn-build/src/utils/parallelize.test.js

@@ -0,0 +1,79 @@
+import { parallelizeBatches } from './parallelize';
+
+// As promises resolve async, we use this helper to wait for all promises that
+// have been resolved to complete (aka call `then`).
+const tick = () =>
+  new Promise(resolve => {
+    setTimeout(resolve, 0);
+  });
+
+test('parallelizes batches', async () => {
+  const foo = createPromiseWithResolve();
+  const bar = createPromiseWithResolve();
+  const baz = createPromiseWithResolve();
+
+  const batches = [[foo, bar], [baz]];
+  const parallelize = parallelizeBatches(batches, obj => {
+    obj.called = true;
+    return obj.promise;
+  });
+
+  const completed = [];
+  parallelize.then(() => {
+    completed.push('parallelizeBatches');
+  });
+  foo.promise.then(() => {
+    completed.push('foo');
+  });
+  bar.promise.then(() => {
+    completed.push('bar');
+  });
+  baz.promise.then(() => {
+    completed.push('baz');
+  });
+
+  expect(foo.called).toBe(true);
+  expect(bar.called).toBe(true);
+  expect(baz.called).toBe(false);
+  expect(completed).toEqual([]);
+
+  bar.resolve();
+  await tick();
+
+  expect(completed).toEqual(['bar']);
+  expect(baz.called).toBe(false);
+
+  foo.resolve();
+  await tick();
+
+  expect(completed).toEqual(['bar', 'foo']);
+  expect(baz.called).toBe(true);
+
+  baz.resolve();
+  await tick();
+
+  expect(completed).toEqual(['bar', 'foo', 'baz', 'parallelizeBatches']);
+});
+
+test('rejects if any promise rejects', async () => {
+  const foo = createPromiseWithResolve();
+  const bar = createPromiseWithResolve();
+  const baz = createPromiseWithResolve();
+
+  const batches = [[foo, bar], [baz]];
+  const parallelize = parallelizeBatches(batches, obj => obj.promise);
+
+  foo.reject(new Error('foo failed'));
+
+  await expect(parallelize).rejects.toThrow('foo failed');
+});
+
+function createPromiseWithResolve() {
+  let resolve;
+  let reject;
+  const promise = new Promise((_resolve, _reject) => {
+    resolve = _resolve;
+    reject = _reject;
+  });
+  return { promise, resolve, reject, called: false };
+}