fix: prevent infinite loop when HMRing a component with an await

[Rich-Harris]

Alternative to #17374. I think this is a more correct fix — it makes the $.hmr block effect a no-op when it isn't running as a result of a hot.accept callback, rather than excluding all block effects from re-running after the first await is encountered during traversal.

It also makes the HMR implementation simpler, both in terms of the generated code...

if (import.meta.hot) {
	A = $.hmr(A, () => A[$.HMR].source);

	import.meta.hot.accept((module) => {
		module.default[$.HMR].source = A[$.HMR].source;
		$.set(A[$.HMR].source, module.default[$.HMR].original);
	});
}

if (import.meta.hot) {
	A = $.hmr(A);

	import.meta.hot.accept((module) => {
		A[$.HMR](module.default);
	});
}

...and the dance that needs to happen inside $.hmr.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 8ae4726

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

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17380

[dominikg]

/ecosystem-ci run vite-plugin-svelte

[svelte-ecoystem-ci]

📝 Ran ecosystem CI on 92fcc14: Open

✅ vite-plugin-svelte

[dominikg]

vite-plugin-svelte contains an e2e test for hmr modificactions but the components in there are not using await so this bug wasn't caught there.

Updating the test components there would help if ecosystem-ci was triggered in PRs, otherwise the svelte repo would need hmr tests itself where a devserver is started, files edited and result of hmr is asserted.

v-p-s tests can only be updated after a svelte release so its not ideal for testing changes to the generated hmr code. For this case i think it would be best if the svelte repo itself had an e2e test that edits files in a "kitchensink" app and tests all behaviors for hmr.

[dummdidumm]

This wasn't quite the same as before: module.default in A[$.HMR](module.default); is already the HMR-wrapped version, so subsequent updates have one needless block in-between. Furthermore this created one extra block effect each update. I therefore reverted this to the old logic (but using the update function) and adding a bunch of comments so we don't regress again in the future.

Might need another review now but good to go from my side