fix: merge batches by Rich-Harris · Pull Request #16866 · sveltejs/svelte

Rich-Harris · 2025-09-29T19:06:22Z

This is part of a project to update some of the async batching logic to handle a few edge cases that have cropped up.

The observable change here is that if you have an await expression that runs twice, and the second one somehow resolves before the first, they are no longer forced to resolve in linear order. The easiest way to see it is in this example — click a++ then b++ in quick succession:

main — the fast b++ update is blocked on the slow a++ update
this PR — the b++ update occurs, followed by the a++ update

To make this work, whenever a batch is committed, we essentially 'rebase' other pending batches on top of the newly applied state. In the example above, when we commit the fast b++ update, we re-run any async effects that depend on b in the context of the pending a++ batch.

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changeset-bot · 2025-09-29T19:06:27Z

🦋 Changeset detected

Latest commit: a50d0ec

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

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svelte-docs-bot · 2025-09-29T19:06:58Z

Preview: https://svelte-dev-git-preview-svelte-16866-svelte.vercel.app/

github-actions · 2025-09-29T19:07:25Z

Playground

pnpm add https://pkg.pr.new/svelte@16866

Rich-Harris · 2025-09-29T20:44:59Z

moving back to draft, some more improvements incoming

Rich-Harris · 2025-09-29T21:15:10Z

In addition to the tests that are updated in the PR, this fixes the async-redirect test which, for reasons I don't begin to understand, passes on main despite being completely broken. Press 'b', then 'ok', then 'b' again and observe the difference in behaviour:

elliott-with-the-longest-name-on-github

This looks awesome. I'll wait for Simon to review as well before approving; I think everything is good but given he's more familiar with the clientside stuff he should definitely take a look

packages/svelte/src/internal/client/reactivity/batch.js

elliott-with-the-longest-name-on-github · 2025-09-29T21:49:26Z

packages/svelte/tests/runtime-runes/samples/async-derived-module/_config.js

-		// TODO why is this necessary? why isn't `await tick()` enough?
-		await Promise.resolve();
-		await Promise.resolve();
-		await Promise.resolve();
-		await Promise.resolve();
-		await Promise.resolve();
-		await Promise.resolve();
-		await Promise.resolve();
-		await Promise.resolve();
-		flushSync();


😆 I saw this while I was working on async SSR and I was really hopeful we could murder it at some point

packages/svelte/src/internal/client/reactivity/batch.js

…to merge-batches

dummdidumm

nice one!

Since #16866, when an async effect runs multiple times, we rebase older batches and rerun those effects. This can have unintended consequences: In a case where an async effect only depends on a single source, and that single source was updated in a later batch, we know that we don't need to / should not rerun the older batch. This PR makes it so: We collect all the sources of older batches that are not part of the current batch that just committed, and then only mark those async effects as dirty which depend on one of those other sources. Fixes the bug I noticed while working on #16935

* runtime-first approach * revert these * type safety, lint * fix: better input cursor restoration for `bind:value` (#16925) If cursor was at end and new input is longer, move cursor to new end No test because not possible to reproduce using our test setup. Follow-up to #14649, helps with #16577 * Version Packages (#16920) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> * docs: await no longer need pending (#16900) * docs: link to custom renderer issue in Svelte Native discussion (#16896) * fix code block (#16937) Updated code block syntax from Svelte to JavaScript for clarity. * fix: unset context on stale promises (#16935) * fix: unset context on stale promises When a stale promise is rejected in `async_derived`, and the promise eventually resolves, `d.resolve` will be noop and `d.promise.then(handler, ...)` will never run. That in turns means any restored context (via `(await save(..))()`) will never be unset. We have to handle this case and unset the context to prevent errors such as false-positive state mutation errors * fix: unset context on stale promises (slightly different approach) (#16936) * slightly different approach to #16935 * move unset_context call * get rid of logs --------- Co-authored-by: Rich Harris <rich.harris@vercel.com> * fix: svg `radialGradient` `fr` attribute missing in types (#16943) * fix(svg radialGradient): fr attribute missing in types * chore: add changeset * Version Packages (#16940) * Version Packages * Update packages/svelte/CHANGELOG.md --------- Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by: Rich Harris <rich.harris@vercel.com> * chore: simplify `batch.apply()` (#16945) * chore: simplify `batch.apply()` * belt and braces * note to self * unused * fix: don't rerun async effects unnecessarily (#16944) Since #16866, when an async effect runs multiple times, we rebase older batches and rerun those effects. This can have unintended consequences: In a case where an async effect only depends on a single source, and that single source was updated in a later batch, we know that we don't need to / should not rerun the older batch. This PR makes it so: We collect all the sources of older batches that are not part of the current batch that just committed, and then only mark those async effects as dirty which depend on one of those other sources. Fixes the bug I noticed while working on #16935 * fix: ensure map iteration order is correct (#16947) quick follow-up to #16944 Resetting a map entry does not change its position in the map when iterating. We need to make sure that reset makes that batch jump "to the front" for the "reject all stale batches" logic below. Edge case for which I can't come up with a test case but it _is_ a possibility. * feat: add `createContext` utility for type-safe context (#16948) * feat: add `createContext` utility for type-safe context * regenerate * Version Packages (#16946) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> * chore: Remove annoying sync-async warning (#16949) * fix * use `$state.eager(value)` instead of `$effect.pending(value)` --------- Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by: Hyunbin Seo <47051820+hyunbinseo@users.noreply.github.com> Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com> Co-authored-by: Rich Harris <rich.harris@vercel.com> Co-authored-by: Hannes Rüger <hannes@hannesrueger.de> Co-authored-by: Elliott Johnson <sejohnson@torchcloudconsulting.com>

* WIP implement `$effect.pending(...)` * feat: `$state.eager(value)` (#16926) * runtime-first approach * revert these * type safety, lint * fix: better input cursor restoration for `bind:value` (#16925) If cursor was at end and new input is longer, move cursor to new end No test because not possible to reproduce using our test setup. Follow-up to #14649, helps with #16577 * Version Packages (#16920) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> * docs: await no longer need pending (#16900) * docs: link to custom renderer issue in Svelte Native discussion (#16896) * fix code block (#16937) Updated code block syntax from Svelte to JavaScript for clarity. * fix: unset context on stale promises (#16935) * fix: unset context on stale promises When a stale promise is rejected in `async_derived`, and the promise eventually resolves, `d.resolve` will be noop and `d.promise.then(handler, ...)` will never run. That in turns means any restored context (via `(await save(..))()`) will never be unset. We have to handle this case and unset the context to prevent errors such as false-positive state mutation errors * fix: unset context on stale promises (slightly different approach) (#16936) * slightly different approach to #16935 * move unset_context call * get rid of logs --------- Co-authored-by: Rich Harris <rich.harris@vercel.com> * fix: svg `radialGradient` `fr` attribute missing in types (#16943) * fix(svg radialGradient): fr attribute missing in types * chore: add changeset * Version Packages (#16940) * Version Packages * Update packages/svelte/CHANGELOG.md --------- Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by: Rich Harris <rich.harris@vercel.com> * chore: simplify `batch.apply()` (#16945) * chore: simplify `batch.apply()` * belt and braces * note to self * unused * fix: don't rerun async effects unnecessarily (#16944) Since #16866, when an async effect runs multiple times, we rebase older batches and rerun those effects. This can have unintended consequences: In a case where an async effect only depends on a single source, and that single source was updated in a later batch, we know that we don't need to / should not rerun the older batch. This PR makes it so: We collect all the sources of older batches that are not part of the current batch that just committed, and then only mark those async effects as dirty which depend on one of those other sources. Fixes the bug I noticed while working on #16935 * fix: ensure map iteration order is correct (#16947) quick follow-up to #16944 Resetting a map entry does not change its position in the map when iterating. We need to make sure that reset makes that batch jump "to the front" for the "reject all stale batches" logic below. Edge case for which I can't come up with a test case but it _is_ a possibility. * feat: add `createContext` utility for type-safe context (#16948) * feat: add `createContext` utility for type-safe context * regenerate * Version Packages (#16946) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> * chore: Remove annoying sync-async warning (#16949) * fix * use `$state.eager(value)` instead of `$effect.pending(value)` --------- Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by: Hyunbin Seo <47051820+hyunbinseo@users.noreply.github.com> Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com> Co-authored-by: Rich Harris <rich.harris@vercel.com> Co-authored-by: Hannes Rüger <hannes@hannesrueger.de> Co-authored-by: Elliott Johnson <sejohnson@torchcloudconsulting.com> * decouple from boundaries * use queue_micro_task * add test * fix * changeset * revert * tidy up * update docs * Update packages/svelte/src/internal/client/reactivity/batch.js Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com> * minor tweak --------- Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com> Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by: Hyunbin Seo <47051820+hyunbinseo@users.noreply.github.com> Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com> Co-authored-by: Hannes Rüger <hannes@hannesrueger.de> Co-authored-by: Elliott Johnson <sejohnson@torchcloudconsulting.com>

Rich-Harris added 9 commits September 25, 2025 21:52

WIP

e286d30

update test

fe693be

tweak

cb6e824

fix

ecdfd5b

fix

8f85d7d

tweak

8cc385a

simplify

bb1c91f

tidy up

93a5f54

changeset

dfe85ba

Rich-Harris added 2 commits September 29, 2025 16:31

abort stale promises on a given batch

357c077

tidy up

cbb4b94

Rich-Harris marked this pull request as draft September 29, 2025 20:45

add test

f8eddac

Rich-Harris marked this pull request as ready for review September 29, 2025 20:50

Rich-Harris added 3 commits September 29, 2025 17:00

lint

a43dc29

add explanatory comment

e6b5adc

Merge branch 'main' into merge-batches

9e8fcbf

Rich-Harris added 2 commits September 29, 2025 17:24

unused

afdad8b

doesn't look like we need the queueMicrotask here

99d31b4

elliott-with-the-longest-name-on-github reviewed Sep 29, 2025

View reviewed changes

Rich-Harris and others added 2 commits September 29, 2025 21:07

unused

49e1ab5

lint / reinstate comment

3bcbc60

dummdidumm reviewed Sep 30, 2025

View reviewed changes

packages/svelte/src/internal/client/reactivity/batch.js Outdated Show resolved Hide resolved

Rich-Harris added 2 commits September 30, 2025 08:45

fix

1bdc518

Merge branch 'merge-batches' of https://github.com/sveltejs/svelte in…

a50d0ec

…to merge-batches

dummdidumm approved these changes Sep 30, 2025

View reviewed changes

dummdidumm merged commit 25cbdc8 into main Sep 30, 2025
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dummdidumm deleted the merge-batches branch September 30, 2025 13:57

github-actions bot mentioned this pull request Sep 29, 2025

Version Packages #16848

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dummdidumm mentioned this pull request Oct 14, 2025

fix: don't rerun async effects unnecessarily #16944

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fix: merge batches#16866

fix: merge batches#16866
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merge-batches

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