Support websocket connection to existing app/pages paths

[ddadaal]

Goals

Support websocket connection to existing app/pages paths.

Non-Goals

No response

Background

We are developing an app that allows to proxy user requests (both HTTP and websocket) from next.js to an background web server (using http-proxy package). We are blocked by #55299 for months, and extremely glad to hear that #57245 has finally fixed it by adding back support for registering custom upgrade handler for server.

But currently it is disallowed to upgrade to existing app/pages paths.

next.js/packages/next/src/server/lib/router-server.ts

Lines 596 to 600 in 2238696

	// TODO: allow upgrade requests to pages/app paths?
	// this was not previously supported
	if (matchedOutput) {
	return socket.end()
	}

Why adding this limitation? It makes the fix useless if there is an optional catch-all route, adds another source of errors that users using existing paths as websocket endpoints may encounter and spend hours to debug (the connection will be closed with code 1006 with no reason), but has no clear advantages IMO.

Proposal

Just remove the code.