SSR: `onDestroy` callbacks are skipped when the render throws

[Nic-Polumeyv]

Describe the bug

Looking into lifecycle internals for #18583 led me here. If a server render throws partway through, onDestroy callbacks registered by components that already ran are never called: #close_render is the only caller of #collect_on_destroy and is unreachable on the error path. The abort-signal cleanup in the same finally blocks does run (renderer.js:643, renderer.js:669), so getAbortSignal().addEventListener('abort', ...) fires on failed renders while onDestroy doesn't.

#10296 added server onDestroy so per-request resources get released, and a failed render is where skipping that hurts most, since errors tend to repeat across requests. If success-only cleanup is intended, I couldn't find it documented.

Affects both the sync and async (top-level await) render paths.

Reproduction

<!-- Parent.svelte -->
<script>
	import { onDestroy, getAbortSignal } from 'svelte';
	import Child from './Child.svelte';
	let { fail } = $props();
	onDestroy(() => console.log('onDestroy fired'));
	getAbortSignal().addEventListener('abort', () => console.log('abort fired'));
</script>
{#if fail}<Child />{/if}

<!-- Child.svelte -->
<script>
	throw new Error('boom');
</script>

render(Parent, { props: { fail: false } }) logs both callbacks. With fail: true it logs only abort fired.

Logs

--- successful render ---
onDestroy fired
abort fired
--- failing render ---
abort fired
threw: boom

System Info

svelte 5.56.7, node 22.23.1

Severity

annoyance