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Detached-Flush Never Exits, Leaking Reparented Process Per Next Dev Shutdown

Regression in Next.js 16.0.3: detached-flush child spawned on dev server shutdown hangs after submitting telemetry when next.config leaves a live handle, causing orphaned processes to accumulate and consume large memory/CPU.

highConfidence 93%Next.js Affected V16.0.3 Affected V16.1.x Affected V16.2.x Affected V16.3.1 Affected V16.4.0-Canary.0

Origin Analysis

Commit ad93e452 removed process.exit(0) after unlinkSync in packages/next/src/telemetry/detached-flush.ts. Because loadConfig(PHASE_DEVELOPMENT_SERVER, dir) evaluates the entire user next.config and can leave a ref'd libuv handle (e.g., setInterval), the detached child never exits after completing telemetry. Spawned with detached:true, it is reparented to init and never supervised.

1. Scaffold a Next.js app with next@16.0.3 or later. 2. Add setInterval(() => {}, 1000); to next.config.ts/module.exports (or any config plugin that keeps a live handle). 3. Run next dev, issue at least one request to queue telemetry. 4. Send SIGINT/Ctrl+C to the dev server. 5. Observe a detached-flush.js child remains alive after parent exit (ps -o pid,ppid,rss,command). It never exits. Or use the provided repro: git clone https://github.com/gesposito/nextjs-detached-flush-orphan && cd nextjs-detached-flush-orphan && npm install && npm run repro.

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 hotfix force-exits the detached process. However, process.exit does not automatically kill child processes spawned during config evaluation; those grandchildren may be reparented and continue running. A more complete fix would avoid evaluating next.config in the flush process or use detached:true with process group termination, but that is a larger change. Rollback: if this patch causes NEW issues with telemetry debug output or other side effects, revert by pinning next@16.0.2 or manually pkill -f 'telemetry/detached-flush.js'. Add test coverage for both normal exit and lingering-handle scenarios.