generations: fix current generation flag for specialisations

[cloud-yu]

Hi,
this pr is for #508

Sanity Checking

• I have updated the changelog as per my changes
• I have tested, and self-reviewed my code
• Style and consistency
  • I ran nix fmt to format my Nix code
  • I ran cargo fmt to format my Rust code
  • I have added appropriate documentation to new code
  • My changes are consistent with the rest of the codebase
• Correctness
  • I ran cargo clippy and fixed any new linter warnings.
• If new changes are particularly complex:
  • My code includes comments in particularly complex areas to explain the
    logic
  • I have documented the motive for those changes in the PR body or commit
    description.
• Tested on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin

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Summary by CodeRabbit

• New Features

  • nh os info now shows the current-generation flag when run with a specialization.

• Bug Fixes

  • Active specialization is now marked with an asterisk (*) while other specializations are shown without per-item prefixes for clearer output.
  • Improved detection of the active generation so the info output reliably reflects which generation is current.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Walkthrough

Switches current-generation detection from /run/current-system to /nix/var/nix/profiles/system, changes active specialisation identification to compare canonical paths so only the active one is prefixed with *, and updates CLI specialisation output formatting and changelog entry.

Changes

Cohort / File(s)	Change Summary
Documentation CHANGELOG.md	Adds release notes: nh os info shows current-generation flag when run with a specialisation; specialisations are prefixed with * only when active.
Generation / Specialisation Logic src/generations.rs	Replace /run/current-system lookup with canonical comparison against /nix/var/nix/profiles/system; require canonical existence+match for current-generation detection; mark only the active specialisation with * by comparing canonical targets; change print_info() formatting to join specialisation names without per-item prefixes.

Sequence Diagram(s)

(omitted)

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• generations: cleanup; hide empty columns #435 — overlaps changes to specialisations detection/formatting and print_info output.
• Always print fields when they are explicitly requested via --fields #455 — related adjustments to src/generations.rs and output/field handling in print_info.

Suggested reviewers

• NotAShelf

Pre-merge checks and finishing touches

✅ 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 clearly and specifically describes the main fix: correcting the current generation flag behavior for specialisations, which aligns directly with the primary changes in both CHANGELOG.md and src/generations.rs.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

✨ Finishing touches

• 📝 Generate docstrings

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[NotAShelf]

Two tiny nits, code LGTM otherwise.

[NotAShelf]

Suggested change

	// Only prefixed specialisation name by '*' when it's currently actived
	// Only prefix specialisation name with '*' when it's currently active

[NotAShelf]

Suggested change

	- `nh os info` now shows the current generation flag,
	when activated with a specialisation [#508](https://github.com/nix-community/nh/issues/508).
	`nh os info` now only prefixes the specialisation name with '\*' when the specialisation is activated.
	- `nh os info` now shows the current generation flag,
	when activated with a specialisation [#508](https://github.com/nix-community/nh/issues/508).
	- `nh os info` now only prefixes the specialisation name with '\*' when the specialisation is activated.

I don't quite understand the first sentence. What's the "current generation flag" ?

[cloud-yu]

Currently, a prefix '' is used to every specialisation name. This is confusing, I can't recognize which specialisation i'm using. whether i'm using a specialisation or not ?
So, I changed this, only prefix '' to the specialisation name currently activate.

[coderabbitai]

Actionable comments posted: 0

♻️ Duplicate comments (1)

CHANGELOG.md (1)

55-57: Changelog entry looks good after typo fixes.

The previous review flagged "actived" typos which have been corrected to "activated". The wording now accurately describes the fix for issue #508.

🧹 Nitpick comments (1)

src/generations.rs (1)

260-277: Hoist fs::canonicalize("/run/current-system") outside the loop.

fs::canonicalize("/run/current-system") is called for every entry in the iterator. This filesystem operation should be computed once before the loop.

🔎 Proposed fix

     let specialisation_path = generation_dir.join("specialisation");
     if specialisation_path.exists() {
       // Only prefix specialisation name with '*' when it's currently active
+      let current_system = fs::canonicalize("/run/current-system").ok();
       let specs = fs::read_dir(specialisation_path)
         .map(|entries| {
           let mut specs_vec = entries
             .filter_map(Result::ok)
             .filter_map(|e| {
-              if fs::canonicalize(e.path()).ok()
-                == fs::canonicalize("/run/current-system").ok()
-              {
+              if fs::canonicalize(e.path()).ok() == current_system {
                 Some(format!("*{}", e.file_name().into_string().ok()?))
               } else {
                 e.file_name().into_string().ok()
               }
             })
             .collect::<Vec<String>>();
           specs_vec.sort();
           specs_vec
         })
         .unwrap_or_default();

📜 Review details

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 0d961f9 and 7321d83.

📒 Files selected for processing (2)

• CHANGELOG.md
• src/generations.rs

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🔇 Additional comments (2)

src/generations.rs (2)

286-292: Current generation detection logic looks correct.

Using /nix/var/nix/profiles/system instead of /run/current-system correctly identifies the current generation even when a specialisation is active. The guard run_current_target.is_some() prevents false positives when both paths fail to canonicalize.

---

423-426: LGTM!

Clean formatting change that joins specialisations with spaces, correctly displaying the * prefix for active ones.