Node Memory Leaks: Detached Cache Graphs That Heap Snapshots Reveal

// scripts/leak-repro.ts
import { writeHeapSnapshot } from "node:v8";
import { app } from "../src/app.js";

async function burst() {
 for (let i = 0; i < 5_000; i++) {
 await app.inject({ method: "GET", url: `/items/${i % 200}` });
 }
 global.gc?.();
}

await burst();
writeHeapSnapshot("/tmp/before.heapsnapshot");
await burst();
await burst();
global.gc?.();
writeHeapSnapshot("/tmp/after.heapsnapshot");
// Diff in Chrome DevTools → Comparison → Rank by Retained Size delta

// ❌ Debugging only with process.memoryUsage().rss in Grafana
// RSS moves for many reasons — allocator caches, fragmentation — not just leaks

// src/bad-cache.ts
const cache = new Map<string, object>(); // ❌ unbounded

export function getUser(id: string) {
 if (!cache.has(id)) cache.set(id, loadUser(id));
 return cache.get(id);
}

// src/good-cache.ts
import { LRUCache } from "lru-cache";

const cache = new LRUCache<string, object>({
 max: 5_000,
 ttl: 60_000,
 // ✅ updateAgeOnGet only if product wants sliding TTL
});

// EventEmitter / ALS traps
const sockets = new Set<Socket>();
server.on("connection", (s) => {
 sockets.add(s);
 // ❌ missing sockets.delete on close → classic growth
 s.on("close", () => sockets.delete(s)); // ✅
});

// Use WeakRef only when values may disappear and you can recompute
const weakUsers = new Map<string, WeakRef<object>>();
const registry = new FinalizationRegistry((id: string) => weakUsers.delete(id));

export function remember(id: string, obj: object) {
 weakUsers.set(id, new WeakRef(obj));
 registry.register(obj, id);
}
// ✅ Still bound the Map of WeakRefs by key count — WeakRef doesn't shrink the key set