fix(terser): always make at least 1 worker thread

package.json

@@ -20,7 +20,7 @@
     "@ava/babel": "2.0.0",
     "@rollup/plugin-typescript": "^9.0.1",
     "@types/conventional-commits-parser": "^3.0.2",
-    "@types/node": "14.18.30",
+    "@types/node": "20.8.4",
     "@types/semver": "^7.3.7",
     "@types/source-map-support": "^0.5.4",
     "@types/yargs-parser": "^20.2.1",

packages/terser/src/worker-pool.ts

@@ -1,6 +1,6 @@
 import { AsyncResource } from 'async_hooks';
 import { Worker } from 'worker_threads';
-import { cpus } from 'os';
+import os from 'os';
 import { EventEmitter } from 'events';
 
 import serializeJavascript from 'serialize-javascript';
@@ -40,7 +40,14 @@ export class WorkerPool extends EventEmitter {
   constructor(options: WorkerPoolOptions) {
     super();
 
-    this.maxInstances = options.maxWorkers || cpus().length;
+    if (options.maxWorkers) {
+      this.maxInstances = options.maxWorkers;
+    } else if (typeof os.availableParallelism === 'function') {
+      this.maxInstances = os.availableParallelism();
+    } else {
+      // cpus() can return [] on some platforms, and we always need at least 1 worker
+      this.maxInstances = os.cpus().length || 1;
+    }
     this.filePath = options.filePath;
 
     this.on(freeWorker, () => {

packages/terser/test/test.js

@@ -1,3 +1,5 @@
+const os = require('os');
+
 const test = require('ava');
 const { rollup } = require('rollup');
 
@@ -356,3 +358,23 @@ test.serial('terser preserve vars in nameCache when provided', async (t) => {
     }
   });
 });
+
+test.serial('minify when os.cpus().length === 0', async (t) => {
+  const originalAvailableParallelism = os.availableParallelism;
+  const originalCpus = os.cpus;
+  os.cpus = () => [];
+  try {
+    const bundle = await rollup({
+      input: 'test/fixtures/unminified.js',
+      plugins: [terser()]
+    });
+    const result = await bundle.generate({ format: 'cjs' });
+    t.is(result.output.length, 1);
+    const [output] = result.output;
+    t.is(output.code, '"use strict";window.a=5,window.a<3&&console.log(4);\n');
+    t.falsy(output.map);
+  } finally {
+    os.availableParallelism = originalAvailableParallelism;
+    os.cpus = originalCpus;
+  }
+});