From 1516361e1f021a8bfcfbf2a3fa158b9d0038b682 Mon Sep 17 00:00:00 2001
From: Christoph Tavan <dev@tavan.de>
Date: Fri, 25 Oct 2019 15:06:41 +0200
Subject: [PATCH] Only enable worker threads if there is more than 1 worker

Multiple worker threads can cause problems when certain dependencies are
being used, see e.g. https://github.com/zeit/next.js/issues/7894

This patch allows disabling of worker threads by setting
`config.experimental.cpus = 1`.

The benefit of spawning 1 worker thread, if there is any at all, should
very limited anyways, so the workload can just as well be processed in
the main thread.
---
 packages/next/build/index.ts                                    | 2 +-
 .../webpack/plugins/terser-webpack-plugin/src/TaskRunner.js     | 2 +-
 packages/next/export/index.ts                                   | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/packages/next/build/index.ts b/packages/next/build/index.ts
index 3feb83a48b348..2a2cd8d1b0845 100644
--- a/packages/next/build/index.ts
+++ b/packages/next/build/index.ts
@@ -337,7 +337,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
 
   const staticCheckWorkers = new Worker(staticCheckWorker, {
     numWorkers: config.experimental.cpus,
-    enableWorkerThreads: true,
+    enableWorkerThreads: config.experimental.cpus > 1,
   })
 
   const analysisBegin = process.hrtime()
diff --git a/packages/next/build/webpack/plugins/terser-webpack-plugin/src/TaskRunner.js b/packages/next/build/webpack/plugins/terser-webpack-plugin/src/TaskRunner.js
index b306966292ede..cb905786e3ea2 100644
--- a/packages/next/build/webpack/plugins/terser-webpack-plugin/src/TaskRunner.js
+++ b/packages/next/build/webpack/plugins/terser-webpack-plugin/src/TaskRunner.js
@@ -28,8 +28,8 @@ export default class TaskRunner {
 
     if (this.maxConcurrentWorkers > 1) {
       this.workers = new Worker(worker, {
-        enableWorkerThreads: true,
         numWorkers: this.maxConcurrentWorkers,
+        enableWorkerThreads: this.maxConcurrentWorkers > 1,
       })
       this.boundWorkers = options => this.workers.default(options)
     } else {
diff --git a/packages/next/export/index.ts b/packages/next/export/index.ts
index dba1116a3f3d1..0792904475bf4 100644
--- a/packages/next/export/index.ts
+++ b/packages/next/export/index.ts
@@ -267,7 +267,7 @@ export default async function(
     {
       maxRetries: 0,
       numWorkers: threads,
-      enableWorkerThreads: true,
+      enableWorkerThreads: threads > 1,
       exposedMethods: ['default'],
     }
   ) as any