fix: Externalize app initialization to adapters

[GrygrFlzr]

Fixes #1784. Closes #1786 as its alternative.

This PR contains a breaking change to the adapter API.

Current Behavior

kit/packages/kit/src/core/build/index.js

Line 383 in 559aad5

init({ paths: ${s(config.kit.paths)} });

The built app.js file contains a side effect of calling the init function, which ends up running before polyfills like @sveltejs/kit/install-fetch. Although ESM (supposedly) guarantees a top-down order for import side effects, this is currently not necessarily true with esbuild nor rollup (see evanw/esbuild#399 (comment)).

Proposed Behavior

kit/packages/kit/src/core/build/index.js

Lines 288 to 292 in e2f1b3f

	const default_settings = { paths: ${s(config.kit.paths)} };
	// allow paths to be overridden in svelte-kit preview
	// and in prerendering
	export function init(settings = default_settings) {

The init call is removed from the built app.js, and instead it becomes the responsibility of the adapters to call it at least once. This lets it control when the initialization happens, allowing us to guarantee that it happens after any polyfills:

kit/packages/adapter-node/src/index.js

Lines 4 to 8 in e2f1b3f

	import { init, render } from '../output/server/app.js'; // eslint-disable-line import/no-unresolved
	import { host, port } from './env.js'; // eslint-disable-line import/no-unresolved
	init();
	const instance = createServer({ render }).listen(port, host, () => {

This may also be useful in the future should the init function need to perform other side effects.

Non-breaking alternate solutions

1. Use require() and the require shim to import the app to force it to run last:

   const { render } = require('../output/server/app.js');

   This is not a great long-term solution considering we're forging ahead with ESM anyway.
2. Call init from render the first time it's called:

   let initialized = false;
   export function render(/* ... */) {
     if (!initialized) {
       initialized = true;
       init(/* ... */);
     }
     // ...
   }

   In environments such as adapter-node it won't be evaluated until the server handles the quest, which may delay the initial response. May also affect serverless environments where lambdas may live longer than a single request.

[changeset-bot]

🦋 Changeset detected

Latest commit: 8346eb1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@sveltejs/adapter-cloudflare-workers	Patch
@sveltejs/adapter-netlify	Patch
@sveltejs/adapter-node	Patch
@sveltejs/adapter-vercel	Patch
@sveltejs/kit	Patch

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[GrygrFlzr]

defineProperties seems like a more robust solution, and also avoids having to slap ts-nocheck directives.

[GrygrFlzr]

Consistency with the rest of the codebase.

[benmccann]

Thank you!