Inconsistent behaviour when changing headers of generated response

Start with packages/kit/src/runtime/server/respond.js at the cited lines, then run the linked reproduction repository. Inspect src/routes/a/b/+page.server.ts and src/routes/a/+layout.server.ts to compare event.setHeaders and event.cookies.set after the redirect. Done means their post-response behavior is consistent with the intended contract.

Describe the bug

When setting headers of an already generated response through event.setHeaders and event.cookies.set the behaviour is inconsistent.

event.cookies.set throws an error, while event.setHeaders does not. Based on the code at https://github.com/sveltejs/kit/blob/c749e85d2502d5eac03f739c8f18a2f3a4dcb13e/packages/kit/src/runtime/server/respond.js#L541-L547 I would assume they are both supposed to fail.

Server logs when going to http://localhost:5173/a/b in the reproduction repo I've attached shows the issue:

starting /a/b
redirect
resolved /a/b
would error layout event.setHeaders: false
would error layout event.cookies.set: true

Since there is no indirection in front of these functions destructuring like the following, would prevent event.cookies.set from erroring as well:

export const load: LayoutServerLoad = async ({ setHeaders, cookies: { set }}) => {

Additionally, I would suggest adding some mechanism for checking this state, besides checking the toString of event.cookies.set. Something like event.responseGenerated() or event.headersSent(), since in many cases, unnecessary work can be completely skipped if the event has already been handled / responded to. Let me know if you want me to submit that as a separate feature request.

Reproduction

Reproduction repository:
https://github.com/evestera/sveltekit-headers-sent-repro

Commit of changes from "skeleton" project:
https://github.com/evestera/sveltekit-headers-sent-repro/commit/a37b3b1c6b4997b08575028499da587da6121d5b

src/routes/a/b/+page.server.ts redirects, while src/routes/a/+layout.server.ts inspects the two functions after a short delay.

Logs

No response

System Info

System:
    OS: macOS 14.2.1
    CPU: (8) arm64 Apple M1 Pro
    Memory: 73.73 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 21.6.1 - ~/Library/Caches/fnm_multishells/76568_1707148995605/bin/node
    Yarn: 1.22.19 - /opt/homebrew/bin/yarn
    npm: 10.2.4 - ~/Library/Caches/fnm_multishells/76568_1707148995605/bin/npm
    pnpm: 7.30.3 - /opt/homebrew/bin/pnpm
    bun: 1.0.20 - ~/.bun/bin/bun
  Browsers:
    Brave Browser: 116.1.57.47
    Chrome: 121.0.6167.139
    Safari: 17.2.1
  npmPackages:
    @sveltejs/adapter-auto: ^3.0.0 => 3.1.1
    @sveltejs/kit: ^2.0.0 => 2.5.0
    @sveltejs/vite-plugin-svelte: ^3.0.0 => 3.0.2
    svelte: ^4.2.7 => 4.2.10
    vite: ^5.0.3 => 5.0.12

Severity

annoyance

Additional Information

No response