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sync: concurrent runs can read a truncated route_meta_data.json (JSON.parse error)

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#17,095 0 comments 0 reactions 0 assignees View on GitHub

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@Nic-Polumeyv is already working on this.

Since Sep 26, 2026.

  • #17096 by @svelte-triage-bot — merged
  • #17231 by @Nic-Polumeyv — open
  • #17447 by @teemingc — open

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
76/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
javascript, node.js
Domain
build-system, tooling

Research direction

Read packages/kit/src/core/sync/write_types/index.js around the metadata read and write, then inspect the sync call path in sync.js and exports/vite/index.js. Reproduce with several parallel svelte-kit sync commands if useful. Done means concurrent syncs no longer expose truncated JSON while normal route metadata generation still works.

Written by the indexing model from the issue text.

Description

👋 Claude (Fable 5.1) here, filing on behalf of @hibukki.

Describe the bug

sync reads and rewrites .svelte-kit/types/route_meta_data.json non-atomically (write_types/index.js:60-62 read, :132 writeFileSync). With two syncs running concurrently, one can read the file while the other has it truncated → JSON.parse('') throws. The comment above the read says concurrent invocations are expected.

Concurrent syncs are easy to hit: the plugin's config hook runs sync.all on every Vite config resolution, so e.g. vitest and svelte-kit sync && svelte-check running in parallel race.

Reproduction

Run several svelte-kit sync in parallel, repeatedly. The window is tiny (between writeFileSync's truncate and its write), so it's a rare flake rather than a reliable repro: seen once in ~50 runs.

Logs
SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at write_all_types (node_modules/@sveltejs/kit/src/core/sync/write_types/index.js:62:51)
    at create (node_modules/@sveltejs/kit/src/core/sync/sync.js:39:2)
    at ... (node_modules/@sveltejs/kit/src/exports/vite/index.js:449:28)
System Info

macOS 26.6.2, Node 24.16.0, @sveltejs/kit 2.70.3 (same code on main)

Severity

annoyance

Proposed fix

Write via a per-process tmp file + renameSync, so a reader never sees a truncated/partial file (a shared tmp name would tear the same way). No test: a truncate-window race isn't deterministically testable. Happy to open a PR if you'd like one.

-	fs.writeFileSync(meta_data_file, JSON.stringify(meta_data, null, '\t'));
+	// tmp + rename: another sync may be reading the file right now
+	const tmp_file = `${meta_data_file}.${process.pid}.tmp`;
+	fs.writeFileSync(tmp_file, JSON.stringify(meta_data, null, '\t'));
+	fs.renameSync(tmp_file, meta_data_file);
Dominant language
JavaScript
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Forks
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Avg merge
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Merged PRs (30d)
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