fix: handle rejected streamed data race condition

[scarab-systems]

Related to #9785, but this PR targets a distinct streamed-data rejection race condition demonstrated by the added regression test.

Summary

Thanks for SvelteKit. I really appreciate the care that goes into this project.

This PR fixes a timing issue in streamed server data handling. Previously, the JSON data response waited for all server load promises to settle before serializing any node. If one load returned a rejected streamed promise while another load was still pending, the rejection could occur before SvelteKit had attached the stream rejection handler.

The change creates the JSON data serializer earlier and adds each node as its load result resolves, while still waiting for all nodes before emitting the final data response. That preserves the response shape/order but attaches stream handlers earlier.

I also added a regression route/test where one server load delays serialization and another returns a rejected streamed promise.

Validation

Passed:

• pnpm exec prettier --check ... on touched files
• pnpm --dir packages/kit check
• pnpm --dir packages/kit/test/apps/basics check
• pnpm run lint
• pnpm run check
• Focused Playwright regression in dev mode
• Focused Playwright regression in build/preview mode

I also ran pnpm test:kit locally with reduced workers/retries. The new regression passed inside that run, but the full command failed on an existing unrelated no-SSR dev test (SPA mode / no SSR › cannot use browser-only global on page because of ssr config in +page.js). I did not change that area.

Disclosure: I used AI-assisted coding tools while preparing this PR. I reviewed the changes myself, tested them, and take responsibility for the implementation and any follow-up revisions needed.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 456e378:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/456e37877955e14003d12a30473b398375c67a98

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16268

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 456e378

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

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[teemingc]

See #9785 (comment)

[teemingc]

Actually, this PR targets a different problem than the aforementioned issue (at least the failing test is failing)

[scarab-systems]

Thanks for taking another look, and for reopening/renaming this.

I updated the PR description so it no longer says Fixes #9785. It now frames this as related to #9785, but targeting the distinct streamed-data rejection race demonstrated by the regression test.

Happy to adjust the scope or wording further if you would prefer a different framing.

[teemingc]

Do you mind rebasing this on the version-3 branch? We're not actively developing against the main branch currently.

[scarab-systems]

Thanks — rebased onto version-3 and updated the PR base.

I resolved the rebase conflict by keeping the current version-3 data-response error/redirect status behavior while preserving the earlier streamed-data serializer attachment from this PR.

Local validation after the rebase:

• pnpm install --frozen-lockfile
• pnpm exec prettier --check ... on touched files
• pnpm --dir packages/kit check
• pnpm --dir packages/kit/test/apps/basics check
• Focused regression in dev mode
• Focused regression in build/preview mode
• pnpm run lint
• pnpm run check
• pnpm exec changeset status --since upstream/version-3 shows @sveltejs/kit patch

CI is rerunning on the rebased branch now.

[Rich-Harris]

thank you!

[scarab-systems]

thank you!

You’re very welcome, and thank you for the review and merge. We’re big fans of SvelteKit and were happy to help.