[Bug]: Next.js middlewares are currently broken on Netlify

[ivorpad]

Summary

Next.js throws an error when hostname and port are not provided in the custom server since v12.0.5-canary.18 — as a result it breaks middleware in Netlify:

next.js@base-server.ts#L725-L729

// For middleware to "fetch" we must always provide an absolute URL
const url = getRequestMeta(params.request, '__NEXT_INIT_URL')!
 if (!url.startsWith('http')) {
   throw new Error(
     'To use middleware you must provide a `hostname` and `port` to the Next.js Server'
   )
}

Ref: https://github.com/netlify/netlify-plugin-nextjs/blob/main/src/templates/getHandler.ts#L46-L50

Steps to reproduce

1. Fork this simple Next.js app (npx create-next-app with one middleware) https://github.com/ivorpad/repro-nextjs-middleware
2. Deploy to Netlify
3. Go to /api/users
4. Watch the ___netlify-handler function logs

A link to a reproduction repository

https://github.com/ivorpad/repro-nextjs-middleware

Plugin version

4.2.0

More information about your build

• I am building using the CLI
• I am building using file-based configuration (netlify.toml)

What OS are you using?

No response

Your netlify.toml file

`netlify.toml`

[build]
command = "npm run build"
functions = "out_functions"
publish = ".next"

[[plugins]]
package = "@netlify/plugin-nextjs"

Your public/_redirects file

`_redirects`

# Paste content of your `_redirects` file here

Your next.config.js file

`next.config.js`

# Paste content of your `next.config.js` file here. Check there is no private info in there.

Builds logs (or link to your logs)

Build logs

# Paste logs here

Function logs

Function logs

# Paste logs here

.next JSON files

generated .next JSON files

# Paste file contents here. Please check there isn't any private info in them
# You can either build locally, or download the deploy from Netlify by clicking the arrow next to the deploy time.

[guscsales]

I have the same issue here but running the server locally. I did an update to version 12.0.8 yesterday and this error started happening. I rollback to 12.0.7 and worked. Maybe there is some bug in the latest version.

[ascorbic]

Thank you both for the detailed report and information. I have a fix ready and should be able to put out a release on Monday.