[feat] pass data from `locals` to `load`

Describe the problem

My site queries an external API with the following pattern:

• handle extracts user ID from cookie
• +layout.svelte fetches user by user ID
• +page.svelte fetches profile by user ID

These fetches previously happened in parallel. With $session removed I have no way to write this code without introducing a waterfall

Describe the proposed solution

I’m happy with any solution folks can think of that solves this problem

I think we could potentially solve this as part of https://github.com/sveltejs/kit/issues/7105

Alternatives considered

No response

Importance

i cannot use SvelteKit in a performant manner without it

Additional Information

No response

[dummdidumm]

Copying my Discord-thoughts into here:

What became clear to me that getSession was used for multiple things at once.

1. pass data from server to client load functions
2. make cross-cutting data easily available across all client load functions
3. trigger a load function to rerun when data changes (e.g. $session.user = null)
4. a convenient writable store that is somewhat global

Because it had so many use cases in once, it became somewhat confusing what runs when, because people may only think about 4 and then wonder about 3. So I think removing it in its form was correct, but we need some features that bring back the mentioned points in a easy-to-use way. Right now we have this situation:

1. pass data -> can do this now by having a +layout.server.js and passing whatever you want to the client
2. cross-cutting data -> use await parent() -> this introduces a waterfall because you can’t tell “hey I’m only interested in the data from parent X, don’t wait on the 3 other layout loads after that”. Something like locals for the client side would help with this.
3. trigger rerun -> there’s nothing built-in for this right now. Rich shared a pattern somewhere about creating a custom state which triggers a unique invalidate function under the hood. Maybe it’s enough to make this a blessed pattern and/or tidy it up a little and provide a wrapper around it.
4. convenient writable store -> again, nothing built-in, this would be #7105 3+4. It’s not clear to me how to achieve a combination of these two in a nice API. session made it possible because it was on both sides.

–> I feel like we need to add recipes and/or APIs to ease this

[dummdidumm]

The idea of more fine-grained await parent() is a interesting idea. Another way could be to optionally pass parent a function to tell it what to wait for, or a list of top level keys.

export function load({ parent }) {
  const user = await parent(['user']); // resolves as soon as `user` resolves, only reruns when `user` changes
}

Edit: They key-version is probably not possible. How would the function know that the user key that is returned by some upper load function is actually the last with that key? If there are two load functions returning user, we need to wait for the later load function in the chain, but we don’t know that there’s something coming later on.