getStaticPaths fallback fails only in production

[brandonpittman]

I’ve got a fallback page that reads the file system for Markdown in getStaticProps. The fallback page works in dev, but in production, it shows the loading state while router.isFallback is true and then the server errors out. Here’s a typical error.

[GET] /_next/data/ZrNsHZ4IschKCZ7AEV5cr/tags/mac-os.json
22:45:31:14
2020-05-17T13:45:31.218Z	3f2bd3f5-e11d-4373-81b3-bf8779acb3c9	ERROR	Error: ENOENT: no such file or directory, scandir ‘/var/task/_posts’
    at Object.readdirSync (fs.js:872:3)
    at getPostSlugs (/var/task/.next/serverless/pages/tags/[slug].js:3590:52)
    at getAllPosts (/var/task/.next/serverless/pages/tags/[slug].js:3632:17)
    at getStaticProps (/var/task/.next/serverless/pages/tags/[slug].js:129:85)
    at renderToHTML (/var/task/node_modules/next/dist/next-server/server/render.js:277:30)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async renderReqToHTML (/var/task/.next/serverless/pages/tags/[slug].js:2919:22)
    at async Server.render (/var/task/.next/serverless/pages/tags/[slug].js:2966:22) {
  errno: -2,
  syscall: ‘scandir’,
  path: ‘/var/task/_posts’
}

Am I using this feature wrong?

[dotlouis]

Does your getStaticProps use an environment variable?
I stumbled a similar misunderstanding. It may be unrelated, but posting here just in case.

What's not immediately clear about this feature is that getStaticProps() runs multiple times.
Once during the build context (when you run next build), but also every time you request a new page that matches the route (SSG), to potentially build a page or fall back to whatever you chose to display as a fallback.

The thing is that, during the normal build phase. It can read environment variables from the build context. But in the SSG phase, it can't and needs to read them from the runtime context.
If you set these variables via the Vercel dashboard, it should work, as there are made available in both contexts.
However, if like me, you have to set them via the deployment API, or vercel.json, you have to set them in both env AND build.env for getStaticProps() to work in SSG.

I do think that with the lines blurring between build-time and run-time, the Vercel team is trying to simplify this whole environment variables thing. They just need to update the API subsequently ;)

Hope it helps