Unable to render _error page, renders 404 page instead

[stefee]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

3.25.0

Plugin version

7.0.0

Node.js version

16.13.0

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

11.6.1

Description

The plugin uses app.render under the hood, which explicitly disallows rendering _app, _document and _error pages:

https://github.com/vercel/next.js/blob/e75361fd03872b097e817634c049b3185f24cf56/packages/next/server/base-server.ts#L1704

Attempting to render the _error path (as described in the readme) results in a 404 Not Found response.

If you need to render with Next.js from within a custom handler (such as an error handler), use reply.nextRender

app.setErrorHandler((err, req, reply) => {
  reply.status(err.statusCode || 500)
  return reply.nextRender('/_error')
})

Steps to Reproduce

See description.

Expected Behavior

The plugin should either:

1. Decorate the request with the app instance, so that app.renderError can be called manually
2. Decorate fastify with a reply.nextRenderError which proxies the app.renderError similar to how reply.nextRender works

[mcollina]

Would you like to send a Pull Request to address this issue? Remember to add unit tests.

I'd go with both 1 and 2.

[stefee]

Cheers - I think option 2 makes most sense specifically for this problem. Option 1 is a more general feature suggestion.

👉🏻 #436