fix: prevent duplicate /nix entries in fstab

[cole-h]

Filter out all existing /nix mount point entries and prelude comments, then always append exactly one new fstab entry to ensure at most one /nix entry in the output.

Forward-ported from NixOS/nix-installer#55

Description

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

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Summary by CodeRabbit

• Bug Fixes
  • Always ensures exactly one /nix filesystem entry in system config, removing duplicate or outdated entries.
  • Improves cleanup of old /nix lines and treats a missing config file as empty.
  • Writes updates atomically and preserves trailing newline to avoid partial or malformed configs.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This change updates macOS fstab entry creation to always remove nix-installer prelude comments and any existing /nix mount lines, then unconditionally append a single new /nix fstab entry; revert logic similarly detects and drops /nix lines using ASCII-whitespace splitting.

Changes

Cohort / File(s)	Change Summary
macOS fstab entry management src/action/macos/create_fstab_entry.rs	Rewrote execute to filter out prelude comments and any existing /nix mount lines, always push a single new fstab entry, ensure trailing newline before atomic write; updated revert cleanup to detect /nix via split_ascii_whitespace.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested labels

upload to s3

Suggested reviewers

• grahamc
• colemickens

Poem

🐇 I hop through fstab, nibbling old lines away,
Toss out the clutter, then plant one new stay.
No more checking twice, a fresh /nix in sight—
A tidy mount for dawn, soft and bright!

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix: prevent duplicate /nix entries in fstab' directly and concisely describes the main change in the pull request: filtering out existing /nix mount points and ensuring exactly one /nix entry is present in fstab.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

---

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@src/action/macos/create_fstab_entry.rs`:
- Around line 85-92: The filter closure in create_fstab_entry.rs currently
checks for the second field using line.split(&[' ', '\t']).nth(1), which fails
on lines with multiple or leading separators; update the condition in the
closure (the filter handling "Remove any existing /nix mount point entries") to
use line.split_whitespace().nth(1) == Some("/nix") so the second fstab field is
parsed robustly and existing /nix entries are reliably detected and removed.