fix(nix): don't report a broken install for a freshly mounted /nix volume (#2601)

[mikeland73]

Summary

Fixes #2601.

When running Devbox in a container (e.g. Docker or Kubernetes) with /nix mounted as a persistent volume, the volume starts out effectively empty. Devbox is supposed to install Nix into it, but instead it fails with:

Error: We found a /nix directory but nix binary is not in your PATH and we were not able to find it in the usual locations. Your nix installation might be broken. [...]

Root cause

EnsureNixInstalled decided whether /nix already contained an installation by checking whether the directory was non-empty:

if dirExistsAndIsNotEmpty("/nix") { ... return "broken installation" error ... }

A truly empty /nix already falls through to the installer, but a freshly mounted volume is usually not truly empty: ext4 (and other filesystems) create a lost+found directory at the root of the mount. So /nix looks non-empty, Devbox assumes a pre-existing (broken) install, and refuses to install Nix.

Fix

Detect an existing installation by ignoring known filesystem cruft (lost+found, .DS_Store) rather than treating any non-empty /nix as installed. A /nix that contains only those entries is now treated as a fresh install target, so Devbox proceeds to install Nix as expected. A /nix that contains real content (e.g. a store directory) is still treated as an existing installation, preserving the original "broken install" diagnostics.

The generic dirExistsAndIsNotEmpty helper was only used here, so it's replaced by a /nix-specific nixDirIsInstalled helper.

How was it tested?

• go test ./internal/nix/ -run TestNixDirIsInstalled -v — new table-driven test covering: empty dir, dir with files, dir with a nix store, dir with hidden files, dir with only lost+found (the regression case → treated as not installed), dir with lost+found + store (→ installed), and non-existent dir. All pass.
• go build ./... / go vet ./internal/nix/ / gofmt clean.

(Pre-existing TestConfigIsUserTrusted failures in the package are unrelated — they require a nix binary that isn't present in the CI sandbox and are not touched by this change.)

cc @ascknx (issue reporter)

---

Generated by Claude Code

[mikeland73]

@claude please review this PR for correctness, and flag anything that would block merging.

[claude]

Claude finished @mikeland73's task in 1m 34s —— View job

---

Review of PR #2939: fix(nix): don't report a broken install for a freshly mounted /nix volume

• Gather context (diff, changed files)
• Understand the request (correctness review, blockers for merge)
• Review internal/nix/install.go changes
• Review internal/nix/install_test.go changes
• Attempt to run build/vet/tests locally
• Post final review feedback

Note: go build/go vet/go test were blocked by sandbox approval in this environment (network/tool access), so I verified correctness by manual code reading rather than execution. If you'd like me to actually run the test suite, allow go build/go test/go vet in --allowedTools.

Summary

The fix is correct and well-targeted. nixDirIsInstalled (internal/nix/install.go:43-54) replaces the old dirExistsAndIsNotEmpty (which used fileutil.IsDirEmpty) and preserves the original error semantics: os.ReadDir failing (e.g. /nix doesn't exist) still returns false → falls through to the installer, same as before. The new logic just adds one refinement: entries matching nonNixDirEntries (lost+found, .DS_Store) are skipped when deciding "installed," so a freshly mounted volume containing only lost+found is correctly treated as uninstalled, while a store dir (or any other real content) still trips the "broken installation" error path — matching the PR description exactly.

Confirmed:

• dirExistsAndIsNotEmpty had no other call sites (internal/nix/install.go was the only user), so removing it is safe. fileutil.IsDirEmpty itself is now unused outside its own package/tests — not a blocker, just a minor future cleanup opportunity if you want to trim it (it's an exported helper, so may be intentionally kept as public API).
• The one caller of nixDirIsInstalled (EnsureNixInstalled, internal/nix/install.go:84) is only reached when BinaryInstalled() is already false, so the marginal cost of os.ReadDir reading/sorting all entries (vs. the old early-exit IsDirEmpty) is not a practical concern — this only runs once, off the hot path.
• Test coverage (internal/nix/install_test.go) is thorough: empty dir, populated dir, store dir, hidden file, lost+found-only (the regression case), lost+found + store, and non-existent dir. Table-driven and easy to extend.
• Comments on nonNixDirEntries and nixDirIsInstalled clearly explain the "why" (container/VM volume mounts, filesystem cruft) and link to the originating issue — good practice for a non-obvious fix like this.

No correctness issues, no regressions, no blockers for merging. Nice, minimal fix with good regression test coverage.

[mikeland73]

Reviewed for correctness and merge-readiness.

Correctness — looks good ✅

The change swaps the generic dirExistsAndIsNotEmpty("/nix") check for a /nix-specific nixDirIsInstalled that ignores known filesystem cruft (lost+found, .DS_Store). I checked behavior parity across every path in EnsureNixInstalled:

/nix contents	old behavior	new behavior
missing / empty	install	install (unchanged)
real content (store, var, …)	"broken install" error	"broken install" error (unchanged)
unreadable (permission error)	install¹	install¹ (unchanged)
only lost+found (mounted volume)	"broken install" error	install (fixed)

¹ os.ReadDir erroring returns false here, mirroring the old fileutil.IsDirEmpty error path — so no regression for root-owned /nix read by a non-root user.

The only intended behavior change is the last row, which is exactly #2601. go build ./..., go vet, gofmt, and the new table-driven TestNixDirIsInstalled all pass locally. The now-unused dirExistsAndIsNotEmpty helper and its fileutil import are cleanly removed (no other references).

Blocking merge 🚧

1. The PR is in draft (converted by @savil) — GitHub won't merge a draft, and the conversion dropped any auto-merge/merge-queue membership. It needs to be marked Ready for review by a maintainer. I'm intentionally not flipping it back myself.
2. CI + approval — status is currently blocked / checks pending. Needs CI green and a maintainer approval under branch protection.

Non-blocking notes

• This fixes Devbox's own premature guard. Whether the underlying Nix installer then installs cleanly into a mounted /nix that contains a lost+found is a separate concern outside this function — worth an end-to-end check in a real Docker volume before closing the issue, but it applied equally to the pre-existing empty-/nix install path, so it's not a regression from this PR.
• .DS_Store is included defensively (harmless on macOS); happy to drop it if you'd prefer to scope the ignore-list strictly to lost+found.

---

Generated by Claude Code