fix: terser-webpack-plugin hanging on WSL (#6732)

* fix: terser-webpack-plugin hanging on WSL

* nits

* Merge branch 'master' into terser-parallel-bug-wsl

Co-authored-by: Jack Zhao <jzhao@fb.com>

packages/react-scripts/config/webpack.config.js

@@ -9,6 +9,7 @@
 'use strict';
 
 const fs = require('fs');
+const isWsl = require('is-wsl');
 const path = require('path');
 const webpack = require('webpack');
 const resolve = require('resolve');
@@ -222,7 +223,9 @@ module.exports = function(webpackEnv) {
           },
           // Use multi-process parallel running to improve the build speed
           // Default number of concurrent runs: os.cpus().length - 1
-          parallel: true,
+          // Disabled on WSL (Windows Subsystem for Linux) due to an issue with Terser
+          // https://github.com/webpack-contrib/terser-webpack-plugin/issues/21
+          parallel: !isWsl,
           // Enable file caching
           cache: true,
           sourceMap: shouldUseSourceMap,