PR #89040: fix: ensure LRU cache items have minimum size of 1 to prevent unbounded growth

• Repository: vercel/next.js
• Author: timneutkens
• State: closed | merged: True
• Link: https://github.com/vercel/next.js/pull/89040

Description (problem / solution / changelog)

What?

Prevent LRU cache unbounded growth by requiring calculateSize to return values > 0.

Why?

When calculateSize returned 0 (e.g., for null values in the filesystem route cache), items were never evicted because they didn't contribute to totalSize. This caused unbounded memory growth when handling dynamic routes like /api/logs/[id] where each unique ID created a new cache entry with size 0.

How?

1. LRUCache now throws an error if size ≤ 0 - This catches bugs at development time rather than silently masking them:

   if (size <= 0) {
     throw new Error(
       \`LRUCache: calculateSize returned ${size}, but size must be > 0. \` +
       \`Items with size 0 would never be evicted, causing unbounded cache growth.\`
     )
   }

2. Fixed filesystem.ts - Null entries (negative cache) now return size 1 instead of 0:

   if (!value) {
     // Null entries (negative cache) still need a non-zero size for LRU eviction
     return 1
   }

3. Fixed next-dev-server.ts - Static paths cache now uses || 1 instead of ?? 0:

   return JSON.stringify(value.staticPaths)?.length || 1

Fixes #89033

Changed files

• packages/next/errors.json (modified, +2/-1)
• packages/next/src/server/dev/next-dev-server.ts (modified, +2/-1)
• packages/next/src/server/lib/lru-cache.ts (modified, +6/-0)
• packages/next/src/server/lib/router-utils/filesystem.ts (modified, +4/-1)