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Dev Server 500S On Every Request: Concurrent GetStaticPaths Corrupts Prerender-Manifest.Json

Concurrent getStaticPaths executions under the same dynamic segment perform unsynchronized read-modify-write on prerender-manifest.json. Non-atomic writes leave the file invalid JSON, causing persistent 500s across all routes in dev server.

highConfidence 95%Next.js Affected V16.2.7 Affected V16.3.0-Canary.97

Origin Analysis

packages/next/src/server/dev/next-dev-server.ts performs unsynchronized read-modify-write on prerender-manifest.json. Two concurrent requests read the same manifest, mutate different routes, and write back with a non-atomic writeFile. Depending on interleaving, a shorter write truncates a longer one, leaving partial JSON or trailing bytes.

1. Clone https://github.com/LouisTsang-jk/nextjs-prerender-manifest-race 2. npm install 3. ./repro.sh (starts next dev, requests 10 pages under app/[locale]/ concurrently, then checks manifest and issues a request) 4. Observe 500 responses with SyntaxError: Unexpected end of JSON input or Unexpected non-whitespace character after 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

This lock is in-process only; if multiple Next.js dev server processes or workers share the same .next directory, lost updates may still occur. Rollback: remove .next/dev/prerender-manifest.json or restore from backup; the patch can be reverted by unapplying the diff. The atomic rename uses a unique temp file, but concurrent cross-process writes are not serialized; consider a filesystem lock if multi-process dev is used.