breaking: require nodejs_als compatibility flag

[Rich-Harris]

Supersedes #13605.

Honestly I don't know if this guidance is 100% correct — in particular I'm not sure if this contains the right advice around configuration Pages apps (the Workers documentation says 'Cloudflare Pages and Workers Assets is preferred over this approach', while the Pages documentation says 'We recommend using Cloudflare Workers for new projects', so who knows which approach you're meant to use) — but I eventually managed to deploy a test app using the wrangler CLI (though unfortunately not via the git integration/dashboard, which is just spinners for me) and confirmed that this flag does allow you to use getRequestEvent asynchronously.

Erroring on build might be a little aggressive given that most apps don't need getRequestEvent synchronously, but it feels better than failing at runtime for the minority of cases that do.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14189-svelte.vercel.app/

[benmccann]

this says you need to enable it via the dashboard or config file, but it looks like the code will error if it's not in the config file, so maybe we should drop the reference to the dashboard here?

[Rich-Harris]

oh right, I've remembered why I hesitated to put this PR up — it's entirely possible that you've set the flag via the dashboard and so don't need the config file at all. The way this should work is that the framework should be able to configure this stuff on the user's behalf but I don't think that exists on Cloudflare

So maybe it should only error for workers, and be a warning for pages? Ugh

[teemingc]

The way this should work is that the framework should be able to configure this stuff on the user's behalf but I don't think that exists on Cloudflare

I've talked to the Cloudflare folks and the recommended way to handle this is through a generated Wrangler config: https://developers.cloudflare.com/workers/wrangler/configuration/#generated-wrangler-configuration . I'm not a big fan of it though as it requires duplicating the user's config since it doesn't extend an existing one. Duplicating can also get tricky because any relative paths set need to be changed to resolve from where the generated config is instead of where the original config is.

[teemingc]

We should check that a main key is specified in the config too so that it's only needed if a user is deploying a Worker. I've met some folks who are only using the Cloudflare adapter for static sites / single-page apps. The docs should also mention something similar

[teemingc]

Suggested change

	SvelteKit uses the [`AsyncLocalStorage`](https://nodejs.org/api/async_context.html#class-asynclocalstorage) API internally. You need to enable the `nodejs_als` [compatibility flag](https://developers.cloudflare.com/workers/runtime-apis/nodejs/), either via the Cloudflare dashboard (if using Pages) or your Wrangler configuration, or your app may break in production.
	SvelteKit uses the [`AsyncLocalStorage`](https://nodejs.org/api/async_context.html#class-asynclocalstorage) API internally. You need to enable the `nodejs_als` [compatibility flag](https://developers.cloudflare.com/workers/runtime-apis/nodejs/) via your Wrangler configuration, or your app may break in production.

It may be better to force Cloudflare Pages users to have a Wrangler configuration since it's required when migrating to Cloudflare Workers anyway. Also makes it easier for us since we can't detect dashboard settings during a local build.

[elliott-with-the-longest-name-on-github]

Converted to draft due to inactivity

[teemingc]

I think we can close this since we'll be programatically adding these flags in Kit 3 anyway