Demo Site: add concept multiple middlewares that get chained together

[nsams]

This should avoid having one possibly large middleware file doing all sorts of different things. With this we can separete and structure better.

based on https://www.58bits.com/blog/chaining-or-combining-nextjs-middleware

TODO

• merge parent PR Demo Site: move admin redirect and dam rewrite into middleware #2727
• add in starter
• think about having middlewares in library (out of scope)

[johnnyomair]

Alternatively, we could use a behavior similar to the Next middleware where returning undefined is the same as NextResponse.next():

export async function middleware(request: NextRequest, response: NextResponse) {
    return await withPredefinedPagesMiddleware(request, response) ?? 
        await withAdminRedirectMiddleware(request, response) ??
        await withDamRewriteMiddleware(request, response)
}

The separate "middlewares" could then be simplified to:

async withAdminRedirectMiddleware(request: NextRequest, response: NextResponse) => {
    if (request.nextUrl.pathname === "/admin" && process.env.ADMIN_URL) {
        return NextResponse.redirect(new URL(process.env.ADMIN_URL));
    }
}

What do you think?

[fraxachun]

The default response should not be created here. For example in comet-starter we have to get the current scope here and create a rewrite. This logic does not belong to this helper function.

[fraxachun]

We're returning here a custom response without calling the "next" middleware. How would we solve this for the block-preview, which - if implemented as middleware - would also make this return but still needs to apply the CSP-Headers (which I guess is implemented as middleware).

[fraxachun]

In comet-starter getting the scope is more complicated and can possibly return a redirect: https://github.com/vivid-planet/comet-starter/blob/main/site/src/middleware.ts#L33
Should this be handled in every middleware which needs the scope?
E.g.:

const scope = getScopeFromHost(getHostByHeaders(request.headers);
if (scope instanceof Response) return response; // pseudocode
...