Headers are not set when ssr = false

[ebeloded]

Describe the bug

In production build, the headers are not set if ssr = false in +layout.server.ts

Reproduction

1. Create SvelteKit skeleton project

2. Set headers in hooks.server.ts

export const handle: Handle = async ({ event, resolve }) => {
  event.setHeaders({
    'x-foo': 'bar',
    'cache-control': 'no-store',
  })
  return resolve(event)
}

3. Disable SSR in +layout.server.ts:

export const ssr = false

4. pnpm build
5. pnpm preview
6. Get headers (https -h http://127.0.0.1:4173/)
7. Note the missing headers

---

See reproduction repo: https://github.com/ebeloded/kit-headers-bug

Logs

❯ http -h http://127.0.0.1:4173/                      
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Connection: keep-alive
Date: Tue, 10 Jan 2023 01:58:08 GMT
Keep-Alive: timeout=5
Transfer-Encoding: chunked
content-type: text/html
etag: "1673315883560"

System Info

System:
    OS: macOS 13.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 370.16 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.17.1 - ~/.nvm/versions/node/v16.17.1/bin/node
    npm: 8.15.0 - ~/.nvm/versions/node/v16.17.1/bin/npm
  Browsers:
    Chrome: 108.0.5359.124
    Firefox: 108.0.1
    Safari: 16.2
  npmPackages:
    @sveltejs/adapter-auto: ^1.0.0 => 1.0.0 
    @sveltejs/adapter-node: ^1.1.0 => 1.1.0 
    @sveltejs/kit: ^1.0.0 => 1.0.10 
    svelte: ^3.54.0 => 3.55.0 
    vite: ^4.0.0 => 4.0.4

Additional Information

1. The headers are returned when in dev mode pnpm dev
2. In the reproduction, I use adapter-auto, but behavior is the same with adapter-node

[dummdidumm]

What do you mean by the step "get headers (https -h http://127.0.0.1:4173/)"? When I'm invoking the URL in the browser and look at the headers in the network tab, they are shown as expected.

[ebeloded]

@dummdidumm sorry about the confusion. The https comes from HTTPie.

I've double-checked and confirmed the issue.

Here is the screenshot of headers when the app running in dev mode (pnpm run dev). The headers are present:

And here is a screenshot of the app that runs in production mode (pnpm run preview). The headers are absent:

Please let me know if you get different results.

The problem I am trying to solve in my app is to set the cache-control header, but it proves to be impossible when ssr = false.

[Rich-Harris]

ohhh.... this must be because of #8131.

Two options:

1. we treat this as a bug and revert [breaking] prerender shells when ssr false and prerender not false #8131
2. add export const prerender = false alongside export const ssr = false

#8131 is a valuable optimisation so it would be a shame to lose it, but I wonder how we can make this behaviour clearer.

[dummdidumm]

Is there the possibility of a third option where we somehow check if setHeaders is invoked and opt out of prerendering in that case, too, or would that contain too many false positives?

[Rich-Harris]

No, that seems like a pretty good solution as long as it's documented (you could always opt back in with prerender = true). There's still the possibility that someone could do response.headers.set(...) after calling resolve in handle — don't know if we'd want to deoptimize in that case as well

[benmccann]

There's a feature request for saving the headers and serving them here: #9408