how do i have my route /auth/callback handle setting a cookie AND returning a response ("logged in!" or "something happened!")

I'm not sure what you're asking? You can just set the cookie then send the response

how

that is indeed what i'm trying to achieve, however i can't navigate the maze of docs to figure out how

i can only use that from a server action or route handler

which one of those are you implying i use?

also, i saw this

what is startTransition, and where are the docs for it?

You most likely don't need this, this is part of react. Are you just trying to set a cookie on certain/all page loads?
https://react.dev/reference/react/startTransition

i'm trying to set a cookie on a certain route, /auth/callback and have that route return a page with my layout

You can set it in middleware
https://nextjs.org/docs/app/building-your-application/routing/middleware#using-cookies

in pseudocode:

function Page() {
setCookie(...)
return #Unknown ChannelAuthenticated!</>
}

so i'd set it in the middleware then read whether the cookie got set to determine whether to return authenticated or not authenticated

Anything auth related should happen in middleware

so i create a middleware.ts file in app/auth/callback/ directory?

in a nutshell, yes. You do your token check in middleware, which runs before anything is sent to the page

looks like there's a single middleware file

I recommend next-auth if you're not familiar
https://next-auth.js.org/

thanks, i might

tbh I recommend it anyway, it's lightweight and easier than configuring it manually

i'm not trying to protect any routes or anything like that

i really think i just want to implement it myself

i'm not using any existing identity provider

it looks like i could set cookies in getServerSideProps with the pages router

is there no equivalent for app router?

ah, i see how next auth does it, a route handler with a 302

ok then redirect to an error page with a query param with the error string

honestly setting a cookie before rendering a page shouldn't be this difficult

More important to ask why you're setting the cookie. You wouldn't typically need to just from a page.

is there a way to render a page from a route handler

no, that's not something you would ever do

well, suppose i don't want another round trip via sending a 302

how would i handle an oauth callback and return a page

That depends entirely on your authentication solution

but that would happen somewhere between a server action for the login and the redirect to the next page

It's not 1:1 returning a page with a response, you just direct to the next page

ok let me try to rephrase the issue

how would you set a cookie that increments a number when a page has been accessed

every time i visit /foo/bar/baz i want to set a cookie that has a number that increments

You could do that in middleware, but you shouldn't use cookies for data storage because they get sent with every single subsequent request. Should generally just keep it to tokens or flags

can you please stop telling me to not use cookies or use a certain library etc

it's very frustrating

if the answer is "you can only do it from middleware"

then that's fine

and, i suppose, more generally, does that mean you can only set headers for a page response from middleware?