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Concurrent FileSystemCache Writes Can Corrupt Fetch-Cache JSON On A Shared Filesystem

Non-atomic writes in MultiFileWriter.append() can leave partial or invalid JSON in .next/cache/fetch-cache under concurrent writes; fix by writing to a temporary file and atomically renaming it into place.

highConfidence 92%Next.js Affected V15.5.15 Affected V16.4.0-Canary.9

Origin Analysis

MultiFileWriter.append() calls this.fs.writeFile(filePath, data) directly on the final cache path. Without a temporary file + rename, concurrent writers can truncate and overwrite the same destination, allowing readers to observe partial or mixed JSON documents. Next.js already has a write-atomic utility, but it is not used in this runtime FileSystemCache path.

1. Clone https://github.com/Little-Working/nextjs-shared-cache-race-repro\n2. Run corepack enable && pnpm install\n3. Run pnpm repro\n4. The script runs two concurrent MultiFileWriter instances writing to the same cache path through a slow filesystem adapter while another task reads and parses that path. Observe parseErrors > 0 and possible invalid JSON.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Atomic rename is not guaranteed on all network filesystems (e.g., some Windows SMB configurations) and may require FILE_RENAME_REPLACE semantics. Concurrent updates remain last-writer-wins, but no corruption occurs. If rename fails after the temporary write, the temp file may leak; a production fix should include a cleanup step. Rollback: revert to the original writeFile call, but re-introduces the race.