🐛 BUG: SvelteKit Still Had Null Body Response Error

[DevOfManyThings]

What version of Wrangler are you using?

0.0.15

What operating system are you using?

Windows

Describe the Bug

SvelteKit still gets null body response error which was fixed in https://github.com/cloudflare/miniflare/releases/tag/v2.0.0-rc.3, not sure how Wrangler2 incorporates miniflare, is there a pending version update or is this a separate issue?

SETUP

npm init svelte@next my-app
npm i --save-dev @sveltejs/adapter-cloudflare@next
Update svelte.config.js to use adapter.
npm install

LAUNCH
npx wrangler pages dev --proxy 3000 -- npx svelte-kit dev --host
ERROR
When viewing project on http://localhost:8788/ page will alternate between rendering correctly and showing this:

BEHAVIOR
When viewing same page with network tab open and disable cache enabled, error does not occur

Output:

[pages:err] GET /: TypeError: Response with null body status cannot have body
    at new Response (E:\Programming\Projects\ArtPageSvelteWrangler\node_modules\undici\lib\fetch\response.js:160:15)
    at new Response (E:\Programming\Projects\ArtPageSvelteWrangler\node_modules\@miniflare\core\src\standards\http.ts:556:13)
    at Object.construct (E:\Programming\Projects\ArtPageSvelteWrangler\node_modules\@miniflare\core\src\plugins\core.ts:69:29)
    at Object.fetch (<script>:5:24)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at EventTarget.[kDispatchFetch] (E:\Programming\Projects\ArtPageSvelteWrangler\node_modules\@miniflare\core\src\standards\event.ts:359:13)
    at Server.<anonymous> (E:\Programming\Projects\ArtPageSvelteWrangler\node_modules\@miniflare\http-server\src\index.ts:167:20)
GET / 500 Internal Server Error (22.61ms)

[threepointone]

Thank you for reporting this, and sorry for the trouble! Could you try this with wrangler@alpha? There's a small chance we've already fixed this, but I'm not sure atm.

[DevOfManyThings]

No trouble at all! All comes with the fun of using new things eh?

Wrangler was updated to 0.0.0-536c7e5 and issue still occurs with same behaviour as above.

[threepointone]

Well damn. Alright, we'll figure this out. Thanks again for the report!

[GregBrimble]

Don't know where it is, but this is because somewhere (either in wrangler or in SvelteKit), a Response is being constructed incorrectly.

I've made the mistake myself several times before (usually when cloning an existing Response). This is what you need to do.

[GregBrimble]

Completely untested, but looks like it might be this line in SvelteKit: https://github.com/sveltejs/kit/blob/master/packages/adapter-cloudflare/files/worker.js#L17

[petebacondarwin]

Had a quick look at this. It seems that the Worker is returning a 304 - Unmodified response that also has a body, which undici is then complaining about. It might have been that this was always the case (?) but we were using a http client that didn't complain about this?

[petebacondarwin]

@GregBrimble - I tried changing that line but still got the error.

From what I can make out it is miniflare that is dispatching the response, but I guess that is just being proxied from svelte?

[petebacondarwin]

We've found the issue. The problem is in the Miniflare proxy. @GregBrimble is working on a fix. In the meantime, you can (sort of) workaround this by building the Svelte app and then running the build output via pages directly...

npm run build
npx wrangler pages dev .svelte-kit/cloudflare

[petebacondarwin]

Fix here: cloudflare/miniflare#172