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Cannot find module 'webpack' in CI environment

See original GitHub issue

Bug report

Describe the bug

In CI environments, when initially trying to start the server after build, it errors out with the following error:

ERR Error: Cannot find module 'webpack'
ERR Require stack:
ERR - /home/vcap/deps/0/node_modules/mini-css-extract-plugin/dist/index.js
ERR - /home/vcap/deps/0/node_modules/mini-css-extract-plugin/dist/cjs.js
ERR - /home/vcap/deps/0/node_modules/@zeit/next-css/css-loader-config.js
ERR - /home/vcap/deps/0/node_modules/@zeit/next-sass/index.js
ERR - /home/vcap/app/next.config.js
ERR - /home/vcap/deps/0/node_modules/next/dist/next-server/server/config.js
ERR - /home/vcap/deps/0/node_modules/next/dist/next-server/server/next-server.js
ERR - /home/vcap/deps/0/node_modules/next/dist/server/next.js
ERR - /home/vcap/app/routes/next.js
ERR - /home/vcap/app/helpers/startServer.js
ERR - /home/vcap/app/server.js
ERR     at Function.Module._resolveFilename (internal/modules/cjs/loader.js:625:15)
ERR     at Function.Module._load (internal/modules/cjs/loader.js:527:27)
ERR     at Module.require (internal/modules/cjs/loader.js:683:19)
ERR     at require (internal/modules/cjs/helpers.js:16:16)
ERR     at Object.<anonymous> (/home/vcap/deps/0/node_modules/mini-css-extract-plugin/dist/index.js:7:16)
ERR     at Module._compile (internal/modules/cjs/loader.js:776:30)
ERR     at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)
ERR     at Module.load (internal/modules/cjs/loader.js:643:32)
ERR     at Function.Module._load (internal/modules/cjs/loader.js:556:12)
ERR     at Module.require (internal/modules/cjs/loader.js:683:19) {
ERR   code: 'MODULE_NOT_FOUND',
ERR   requireStack: [
ERR     '/home/vcap/deps/0/node_modules/mini-css-extract-plugin/dist/index.js',
ERR     '/home/vcap/deps/0/node_modules/mini-css-extract-plugin/dist/cjs.js',
ERR     '/home/vcap/deps/0/node_modules/@zeit/next-css/css-loader-config.js',
ERR     '/home/vcap/deps/0/node_modules/@zeit/next-sass/index.js',
ERR     '/home/vcap/app/next.config.js',
ERR     '/home/vcap/deps/0/node_modules/next/dist/next-server/server/config.js',
ERR     '/home/vcap/deps/0/node_modules/next/dist/next-server/server/next-server.js',
ERR     '/home/vcap/deps/0/node_modules/next/dist/server/next.js',
ERR     '/home/vcap/app/routes/next.js',
ERR     '/home/vcap/app/helpers/startServer.js',
ERR     '/home/vcap/app/server.js'
ERR   ]
ERR }

It seems there is an issue with the mini-css-extract-plugin as part of @zeit/next-sass. We’ve been successfully using this plugin for over a year, with different versions of next.js from 6.0 up to 9.2, and a seemingly unrelated commit is now introducing this error that prevents the site from starting.

This was reported previously in the mini-css-extract-plugin repo: https://github.com/webpack-contrib/mini-css-extract-plugin/issues/198