[Bug]: NEXTAUTH_URL is set as prod URL on deploy preview

[colbyfayock]

Summary

While trying to create a PR with Deploy Preview that includes Next.js, I noticed the NEXTAUTH_URL value was being overridden.

https://app.netlify.com/sites/50reactprojects/deploys/631aa4b96d37524db46b4ee1

10:28:50 PM: NextAuth package detected, setting NEXTAUTH_URL environment variable to https://50reactprojects.com/

I think I see 2 problems here:

• The value used is the prod domain, in this instance https://50reactprojects.com/ where the deploy is for a preview with a domain of https://deploy-preview-22--50reactprojects.netlify.app/
• I have NEXTAUTH_URL configured as a build environment variable and from what I understand, that should not be overridden based on what I'm seeing in the code

Steps to reproduce

1. Create a deploy preview where the production site does not have NextAuth.js installed, but the preview does
2. When the build/deploy runs, it sets NEXTAUTH_URL as the production site

A link to a reproduction repository

colbyfayock/50reactprojects.com#22

Next Runtime version

4.21.3

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]
  publish = ".next"
  command = "yarn build"

Your public/_redirects file

`_redirects`

# Paste content of your `_redirects` file here

Your next.config.js file

`next.config.js`

/** @type {import('next').NextConfig} */
const nextConfig = {
  reactStrictMode: false, // to prevent double entries
};

module.exports = nextConfig;

Builds logs (or link to your logs)

Build logs

https://app.netlify.com/sites/50reactprojects/deploys/631aa4b96d37524db46b4ee1

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.

[nickytonline]

Thanks for reporting this @colbyfayock! It should be all sorted now.

[colbyfayock]

awesome thank you all!

[kachar]

@nickytonline @colbyfayock

Since the #1613 PR got merged we've got a regression on our Netlify account regarding NextAuth deployments.
(Version 4.23.1 (2022-09-09))

We didn't set the environment var NEXTAUTH_URL manually (as the docs advise), so we ended up having the build redirecting users after login to https://develop--project-xxx-staging.netlify.app/login instead of our main custom domain URL (ex. https://app.project-xxx.com/login.

The custom domain is linked to our develop branch for automated deployments.

We've resolved the situation by manually setting the env var to our custom domain, however, we lose the ability to see and login to the preview builds if we want to use custom domains.

I guess we can enhance the functionality even further to use custom domains when they are set and if not to fall back to the deploy preview URLs.