fix(server): prevent RequestEvent retention in AsyncLocalStorage async resources after SSR

[Zelys-DFKH]

closes #15764

Hey! I tracked down what's causing the heap to grow under SSR load with Svelte 4 apps.

What's going on

with_request_store wraps the SSR render in als.run(store, renderFn). Node.js has a quirk in async context propagation (nodejs/node#53408): every async resource created inside renderFn (Promise continuations, callbacks, the works) gets kResourceStore = store stamped onto it internally. In Svelte 4 apps, component_subscribe creates subscription callbacks inside this context that can outlive the render. Those callbacks hold a strong reference to store, so the RequestStore and its RequestEvent never get garbage-collected. Under any real load, this accumulates fast.

Svelte 5 isn't affected because subscriptions are released per-component immediately. Svelte 4 batches them until the full component tree renders.

The fix

Instead of passing store directly to als.run(), I wrap it in a single-property container ({ current: store }) and pass that instead. Async resources created during rendering hold a reference to the lightweight container, not the store itself.

with_request_store gets a gc_barrier parameter (default false). Only the SSR render call in render.js passes gc_barrier = true; after the render promise settles, container.current is nulled. That severs the only path from lingering Svelte 4 subscription callbacks back to the RequestStore, and normal GC takes over.

Other callers (command handlers, query batches, form handlers) keep gc_barrier = false. Their async continuations (the await 0 in get_response, the setTimeout(0) in batch processing) need store access for the duration of the request, so nulling would break them. Scoping the barrier to the render path leaves those flows untouched.

// in with_request_store:
const container = { current: store };
const result = als.run(container, fn);
// gc_barrier = true (SSR render only): container.current = null after render settles

// try_get_request_store dereferences through .current:
return sync_store ?? als?.getStore()?.current ?? null;

Changes

• packages/kit/src/exports/internal/event.js — wrap ALS value in a container; add gc_barrier parameter (default false) to null the container only when the caller opts in
• packages/kit/src/runtime/server/page/render.js — pass gc_barrier = true to the with_request_store call that wraps the component render (the only source of this leak)
• packages/kit/src/exports/internal/event.spec.js — new unit tests including a regression test that spawns a long-lived async resource inside als.run() with gc_barrier = true and asserts it cannot reach the store after render completes (fails on pre-fix code)

All existing tests pass. Happy to adjust anything — thanks for all the work you put into this.

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Preview: https://svelte-dev-git-preview-kit-15770-svelte.vercel.app/