Can't cache a page load (setting "cache-control" with `setHeaders` doesn't seem to work)

Describe the bug

For example, setting the cache here has no effect.

export const load: PageLoad = async ({ fetch, setHeaders }) => {
    const res = await fetch(`/api/users`);

    if (!res.ok) throw error(res.status);

    const users: UserV2[] = await res.json();

    setHeaders({
        'cache-control': 'max-age=86400',
    });

    return {
        users,
    };
};

Maybe it’s because the initial request to the page contains “Cache-Control: max-age=0”?

So the current behavior is that even if I set the “cache-control” header, the request is always processed by the server (instead of being served by disk/memory cache).

The expected behaviour is that the request is served from cache after being cached (obviously)

Reproduction

Unfortunately, I can’t really debug “cache-control” using node.new 🤔 The network panel doesn’t give me access to the “sandboxed” requests.

Logs

Raw response headers

HTTP/1.1 304 Not Modified
Access-Control-Allow-Origin: *
cache-control: max-age=86400
etag: "1ons99h"
Date: Sun, 02 Oct 2022 23:00:59 GMT
Connection: keep-alive
Keep-Alive: timeout=5

Raw request headers

GET /top-secret HTTP/1.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Cache-Control: max-age=0
Connection: keep-alive
Cookie: ...
Host: localhost:3000
If-None-Match: "1ons99h"
Referer: http://localhost:3000/top-secret
Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: same-origin
Sec-GPC: 1
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Safari/537.36

System Info

System:
    OS: macOS 12.6
    CPU: (8) arm64 Apple M2
    Memory: 2.30 GB / 24.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 18.7.0 - ~/.nvm/versions/node/v18.7.0/bin/node
    npm: 8.16.0 - ~/.nvm/versions/node/v18.7.0/bin/npm
  Browsers:
    Brave Browser: 106.1.44.105
    Safari: 16.0
  npmPackages:
    @sveltejs/adapter-node: ^1.0.0-next.96 => 1.0.0-next.96 
    @sveltejs/kit: ^1.0.0-next.504 => 1.0.0-next.504 
    svelte: ^3.50.1 => 3.50.1 `

Severity

serious, but I can work around it

Additional Information

I guess a temporary workaround is to do in-memory caching?

[Rich-Harris]

Yes, it’s all explained in https://github.com/sveltejs/kit/issues/6477#issuecomment-1255168104.

I’ve just added stringify and parse functions to devalue: https://github.com/Rich-Harris/devalue#stringify-and-parse. This would allow us to use a more conventional approach to serializing non-JSON-stringifiable values if we adopted it in SvelteKit. Payloads would get slightly larger, and we’d have to add 0.5kb of parsing logic, but the benefits probably outweigh the costs.

[Rich-Harris]

Yeah, I think we fixed it a while back but didn’t close this issue. Closing now