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fix: race condition when importing AsyncLocalStorage #17350

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Rich-Harris merged 8 commits into from
Dec 12, 2025
Merged

fix: race condition when importing AsyncLocalStorage #17350

Rich-Harris merged 8 commits into from
Dec 12, 2025

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@elliott-with-the-longest-name-on-github
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@elliott-with-the-longest-name-on-github elliott-with-the-longest-name-on-github commented Dec 11, 2025

Closes #17345 -- basically if you get the timings actually perfectly right it appears you get two copies of ALS, one which gets used for the first half of the first render and another that gets used for every subsequent render (including the second half of the first render, which causes the error). I have no idea how to test this other than running the reproduction in the original issue, which this fixes. (@florian-lefebvre, can you verify as well? Thanks!)

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changeset-bot bot commented Dec 11, 2025 •
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Latest commit: 237fde0

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github-actions bot commented Dec 11, 2025

Playground

pnpm add https://pkg.pr.new/svelte@17350

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matthewp commented Dec 11, 2025

This still looks racey to me. Maybe it's not actually possible to have a race due to how this code is used that I don't have context for, but in isolation if init_render_context where called twice sequentially there would be 2 instances of als created. Maybe that doesn't matter because by the time its used there's only 1. I think what this code is really attempting to do is memoize the result of the full init_render_context() call.

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Rich-Harris commented Dec 11, 2025

Yeah, maybe this?

/** @type {AsyncLocalStorage<RenderContext> | null} */
let als = null;

/** @type {Promise<void> | null} */
let als_promise = null;

export function init_render_context() {
	als_promise ??= import('node:async_hooks')
		.then((hooks) => {
			als = new hooks.AsyncLocalStorage();
		})
		.catch(noop);

	return als_promise;
}

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elliott-with-the-longest-name-on-github commented Dec 11, 2025

lol, one step ahead of you

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matthewp commented Dec 11, 2025

Can confirm this fixes the Astro test that was breaking. Thanks a lot!

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elliott-with-the-longest-name-on-github commented Dec 11, 2025

that somehow broke everything, brb

@Rich-Harris Rich-Harris merged commit 00459a1 into main Dec 12, 2025
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@Rich-Harris Rich-Harris deleted the elliott/als-race-condition branch December 12, 2025 03:23
@github-actions github-actions bot mentioned this pull request Dec 12, 2025
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@Rich-Harris Rich-Harris mentioned this pull request Dec 12, 2025
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5.44 breaks async rendering in Astro

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