fix: write the uniwind.css artifact atomically by florian-lefebvre · Pull Request #677 · uni-stack/uniwind

florian-lefebvre · 2026-09-16T13:15:25Z

For the past few weeks I've been getting this error unpredictably in CI with Expo:

SyntaxError: src/styles/global.css: Missing closing } at @theme
CssSyntaxError: Missing closing } at @theme

I pointed Claude at it and it found it's caused by concurrency.

Summary by CodeRabbit

Bug Fixes
- CSS and type declaration artifacts are now written atomically, preventing incomplete or partially updated files during builds.
- Existing shared files remain protected when artifacts are regenerated.
- Temporary files are cleaned up automatically if writing or replacement fails.
- Locked files are retried automatically during replacement.
Tests
- Added coverage for concurrent generation, artifact integrity, repeated builds, shared-file protection, retry behavior, and temporary-file cleanup.

@theme

`buildCSS` rewrote `uniwind.css` in place with `fs.writeFileSync`, which truncates the file to zero bytes before refilling it (~22 KB in a real project). Metro runs transforms in a worker pool and `expo export -p web` builds the client and SSR graphs concurrently, so one worker could sit inside that write while another worker's Tailwind pass read the same file through `@import "uniwind"`. The reader got a partial file, and Tailwind reported the failure against the consumer's entry file instead: SyntaxError: src/styles/global.css: Missing closing } at @theme `@theme {` sits near the end of the generated artifact, which is why a truncated read almost always landed there. The early return on unchanged content is what made this look flaky: the write only happens when the artifact is stale, which after a fresh install it always is, so it failed in CI and almost never on a developer machine where the file had been correct since the first build. Write to a unique temporary file beside the target and rename it into place. A rename within a filesystem is atomic, so a concurrent reader sees either the whole old file or the whole new one. The temporary name carries the pid and a random suffix, because the racing writers are separate Metro workers and a shared name would only move the race. The rename also fixes a second bug on the same line: package managers hardlink `uniwind.css` from a content-addressable store, so pnpm installs gave it a link count above 1 and writing in place mutated the store copy for every project on the machine. Replacing the directory entry breaks the link and leaves the store's inode alone. Tests spawn four writer/reader processes against one path and assert every read is byte-identical to one of the written contents, assert the hardlink is broken rather than followed, and assert a regeneration still produces the same bytes as before while a warm build leaves the inode untouched. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

coderabbitai · 2026-09-16T13:15:33Z

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📒 Files selected for processing (4)

CONTEXT.md
packages/uniwind/src/bundler/artifacts/dts.ts
packages/uniwind/src/bundler/artifacts/writeFileAtomic.ts
packages/uniwind/tests/web/bundler/write-file-atomic.test.ts

🚧 Files skipped from review as they are similar to previous changes (1)

CONTEXT.md

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📝 Walkthrough

Walkthrough

The bundler now writes generated CSS and declaration artifacts through an atomic temporary-file-and-rename helper. Tests cover concurrent reads, hardlink replacement, retry handling, cleanup, and rebuild behavior.

Changes

Atomic artifact writes

Layer / File(s)	Summary
Atomic write helper `packages/uniwind/src/bundler/artifacts/writeFileAtomic.ts`	Adds `writeFileAtomicSync` with unique temporary files, retries for transient rename errors, and cleanup after failures.
CSS and declaration build integration `packages/uniwind/src/bundler/artifacts/css/index.ts`, `packages/uniwind/src/bundler/artifacts/dts.ts`, `CONTEXT.md`	`buildCSS` and `buildDtsFile` use atomic writes after their existing content checks.
Atomic write and build validation `packages/uniwind/tests/web/bundler/write-file-atomic.test.ts`	Tests concurrent reads, hardlink replacement, rename retries, error handling, temporary-file cleanup, CSS output, and declaration inode and content changes.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: brentlok

Merge Risk: ⚪ Minimal · up to 9a63b

CSS and declaration artifacts are now replaced atomically with bounded retries, preventing readers from seeing partial files. The supplied coverage addresses concurrency, hardlinks, retries, cleanup, and rebuilds, so this change is merge-ready.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the primary change: writing the uniwind.css artifact atomically. It is concise and directly related to the pull request.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 4…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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greptile-apps · 2026-09-16T13:20:46Z

Confidence Score: 5/5

The PR appears safe to merge, with no actionable new defects or repository-rule violations identified.

Summary

Writes generated CSS and TypeScript declaration artifacts through unique adjacent temporary files, then atomically renames them into place.

Prevents concurrent readers from observing partially written artifacts and avoids modifying shared hardlinked files.
Adds bounded retries for transient rename failures and removes temporary files when writing or replacement fails.
Adds coverage for concurrent readers and writers, hardlink protection, retry behavior, cleanup, and unchanged-content checks.
No actionable new issues were identified. Tests were inspected but not executed.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Generate CSS or declarations] --> B{Content changed?}
    B -->|No| C[Leave artifact untouched]
    B -->|Yes| D[Write unique adjacent temporary file]
    D --> E[Rename over destination]
    E -->|Success| F[Complete artifact available to readers]
    E -->|Transient failure, retries remain| G[Bounded synchronous delay]
    G --> E
    D -->|Write failure| H[Remove temporary file and throw]
    E -->|Permanent failure or retries exhausted| H

_{Reviews (2) · Last reviewed commit: "Merge branch 'main' into florian-lefebvr..."}

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Actionable comments posted: 1

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Inline comments:
In `@packages/uniwind/src/bundler/artifacts/writeFileAtomic.ts`:
- Around line 20-37: Update writeFileAtomicSync to retry transient Windows
EPERM/EACCES failures from fs.renameSync, with a bounded retry strategy and
brief delays, while preserving cleanup of tmpPath on final failure and retaining
atomic replacement behavior.

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Reviewing files that changed from the base of the PR and between 44388b4 and edd8b6e.

📒 Files selected for processing (4)

CONTEXT.md
packages/uniwind/src/bundler/artifacts/css/index.ts
packages/uniwind/src/bundler/artifacts/writeFileAtomic.ts
packages/uniwind/tests/web/bundler/write-file-atomic.test.ts

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Brentlok

Please also verify coderabbitai finding

Addresses review on uni-stack#677. `buildDtsFile` has the same shape as `buildCSS` - read, compare, early return, write - and runs in the same `generateArtifacts` call from the same Metro worker pool, so it carried the same torn-read and pnpm hardlink exposure. It writes through `writeFileAtomicSync` now too. Windows uses mandatory file locking: a rename over the target needs delete access on it, so an antivirus scanner, the search indexer or another worker holding a handle makes `renameSync` fail with EPERM, EACCES or EBUSY. The error would propagate out of the Metro transformer and fail the build. This is the bug graceful-fs patches `rename` for on Windows, which is not a dependency here, so retry those codes five times with exponential backoff (~620 ms in total) before giving up, still cleaning up the temporary file. The retry is not gated on win32: EBUSY also shows up on network filesystems, and gating it would make the path untestable on CI. A permanently failing rename, a read-only node_modules for instance, now takes ~620 ms longer to report the same error. Also trims the helper's doc comment down to why the rename is there.

Brentlok

Thanks for another useful contribution 🚀

florian-lefebvre marked this pull request as ready for review September 16, 2026 13:18

coderabbitai Bot reviewed Sep 16, 2026

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Comment thread packages/uniwind/src/bundler/artifacts/writeFileAtomic.ts

Brentlok reviewed Sep 20, 2026

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Comment thread packages/uniwind/src/bundler/artifacts/writeFileAtomic.ts

Comment thread packages/uniwind/src/bundler/artifacts/css/index.ts

florian-lefebvre added 2 commits September 22, 2026 10:11

Merge branch 'main' into florian-lefebvre/fix-atomic-css-write

9a63b5b

Brentlok approved these changes Sep 22, 2026

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Brentlok merged commit ba7ac68 into uni-stack:main Sep 22, 2026
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fix: write the uniwind.css artifact atomically - #677

fix: write the uniwind.css artifact atomically#677
Brentlok merged 3 commits into
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florian-lefebvre:florian-lefebvre/fix-atomic-css-write

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florian-lefebvre commented Sep 16, 2026 •

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