Non-writable pages/_app breaks build

[elado]

What version of Next.js are you using?

10.2.0

What version of Node.js are you using?

14.15

What browser are you using?

Chrome

What operating system are you using?

macOS

How are you deploying your application?

Other

Describe the Bug

In environments that make files non-writable like Bazel, builds break with

Failed to compile.

./src/<file>.css
Global CSS cannot be imported from files other than your Custom <App>. Please move all global CSS imports to pages/_app.js. Or convert the import to Component-Level CSS (CSS Modules).
Read more: https://nextjs.org/docs/messages/css-global
Location: src/pages/_app.tsx

This is due to checking if pages/_app isWritable:

next.js/packages/next/server/lib/find-page-file.ts

Line 34 in 0af3b52

if (await isWriteable(pagePath)) {

which ends up making customAppFile = null even when the file exists.

Is there a reason to check if file is writable and not just if it exists?

Expected Behavior

Expected build not to fail even if pages/_app is not writable

To Reproduce

Remove write permission on pages/_app:

chmod -w pages/_app.js

[evanxd]

Not sure why to check if a file is writable in the findPageFile function, but it looks like we should check if it exists instead of checking if it is writable there.

[balazsorban44]

This issue has been automatically locked due to no recent activity. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.