EROFS on every Vercel publish — production fs is read-only

Symptom

First real publish attempt from the admin on the live site:

EROFS: read-only file system,
open '/var/task/content/posts/en/qqf.mdx'

The GitHub commit never ran. The admin UI showed an error and the post vanished.

Cause

lib/storage.ts:savePost was written as:

await fs.writeFile(abs, content, "utf-8");   // ← crashed here
if (githubConfigured()) {
  await commitFile({ path, content, message });
}

This works in dev. It also works in production until the function tries to write — at which point Vercel's serverless filesystem (/var/task/) refuses, because that path is read-only by design. The function bundle is sealed at build time.

Same bug existed in saveBinaryAsset, deletePost, and saveSiteConfig. All four wrote to disk first.

Fix

Skip the local fs write entirely when GitHub is configured:

if (githubConfigured()) {
  const result = await commitFile({ path, content, message });
  return { path, via: "github", commitUrl: result.commitUrl };
}
// Dev-only fallback below
await fs.mkdir(path.dirname(abs), { recursive: true });
await fs.writeFile(abs, content, "utf-8");
return { path, via: "local" };

In dev (no GITHUB_TOKEN), local fs still works because dev fs is writable. In production, the local fs branch is unreachable.

Commit: 559da07.

Why I didn't catch this in local dev

Local next dev runs on a writable filesystem. The first branch (commit) was never exercised in local testing because I didn't set GITHUB_TOKEN locally — the path that hits GitHub in production was hidden behind a config flag.

Pattern lesson: code paths gated by env-set-only-in-prod are effectively untested. Either set the env locally and exercise the prod path, or write a unit test that mocks the commit.

Reading after this

docs/architecture.md now explains the build-snapshot fs model in depth. The next troubleshoot entry (fs-vs-github-raw) is about the other half of the same bug — even after publishing worked, the post didn't appear on the live site for 60s, for a related but different reason.