adapter-node can't find build assets when paths.base is set to non-empty string

[DrDanRyan]

Describe the bug

Using a non-empty string value for paths.base in svelte.config.js with adapter-node will result in a site where all .js and .css files requests return 404.

There is a warning message in npm run build that says:
(!) "base" option should end with a slash.

However, if you try to put a value that ends in slash then an error is thrown that paths.base cannot end in a slash.

Reproduction

1. start a skeleton app
2. replace adapter-auto with adapter-node
3. set a value for paths.base in svelte.config.js like "/foo"
4. npm run build
5. node build/index.js
6. navigate to the app with devtools open and observe that the asset chunks are 404

Logs

Example request for an app with `paths.base == "/foo"`
GET
	
scheme
	http
host
	127.0.0.1:3000
filename
	/foo/_app/immutable/start-e87edd60.js
Address
	127.0.0.1:3000
Status
404
Not Found
Version HTTP/1.1
Transferred 592 B (0 B size)
Referrer Policy strict-origin-when-cross-origin

System Info

"node_modules/@sveltejs/adapter-node": {
			"version": "1.0.0-next.86",

"node_modules/@sveltejs/kit": {
			"version": "1.0.0-next.428",

Severity

blocking all usage of SvelteKit

Additional Information

I can't use Svelte kit for my project at this point because it must be served at a non-root endpoint.

[babichjacob]

Is #3726 the same as your issue?

[DrDanRyan]

Yes, it is, didn't find that issue in my first search. Closing this one.