fsCache default: node_modules/.cache/jiti never resolves for file-path instance ids (silent TMP_DIR fallback) #459

Describe the bug

When fsCache: true (default) and the jiti instance id is a file path (the documented usage createJiti(import.meta.url)), the default cache dir almost never resolves to node_modules/.cache/jiti — it silently falls back to ${TMP_DIR}/jiti.

To reproduce

import { createJiti } from "jiti";
// id = a file nested below the package root, e.g. <pkg>/src/loader.ts
const jiti = createJiti("file:///home/user/my-pkg/src/loader.ts", { fsCache: true });
console.log(jiti.options.fsCache); // → /tmp/jiti  (not <pkg>/node_modules/.cache/jiti)

Even with <pkg>/node_modules present and writable.

Cause

getCacheDir() in src/cache.ts computes:

const nmDir = ctx.filename && resolve(ctx.filename, "../node_modules");

pathe.resolve treats the id (a FILE path, e.g. <pkg>/src/loader.ts) as a directory path, so "../node_modules" resolves relative to the file's parent directory: <pkg>/src/loader.ts/../node_modules → <pkg>/src/node_modules — which never exists. Same for ids inside node_modules (node_modules/foo/index.js → node_modules/foo/node_modules). The documented node_modules/.cache/jiti only materializes when the id sits directly at the project root (<proj>/index.js → <proj>/node_modules).

Impact

• Transform cache lands in ${TMP_DIR}/jiti: volatile (wiped on reboot), shared machine-wide, unbounded growth (stale content-hash variants accumulate forever).
• Every consumer whose entry module isn't at the project root silently re-pays cold babel transpiles after every reboot (measured: ~1.2s cold vs ~4ms warm for a small config graph).
• On some systems TMPDIR handling has extra workarounds (pnpm #6140) that a proper node_modules cache dir avoids entirely.

Suggested fix

Use the file's directory:

const nmDir = ctx.filename && resolve(dirname(ctx.filename), "../node_modules");

(dirname is already imported in src/cache.ts.) This makes node_modules/.cache/jiti work for file ids at any depth; the tmpdir fallback remains for the no-node_modules case. Note: our consumer (pi-steering, cad0p/pi-steering#39) worked around it by passing an explicit fsCache string — an upstream fix would make the default correct for everyone.

Environment

• jiti 2.7.0 (verified against current main, src/cache.ts getCacheDir), Node 22/24, Linux.