fix: detect systemd unit directory at runtime for cross-distro compatibility

[Benoît Cortier (CBenoit)]

Fixes installation failure on Rocky Linux 8.4 and other RHEL-based distributions where systemd units are located in /usr/lib/systemd/system instead of /lib/systemd/system.

• Add detect_systemd_unit_dir() with three-tier detection:
  1. CEVICHE_SYSTEMD_UNITDIR environment variable (takes precedence)
  2. pkg-config --variable=systemdsystemunitdir systemd
  3. Fallback probing: /lib/systemd/system, then /usr/lib/systemd/system
• Update get_service_unit_path() and get_service_dropin_dir() to use detection
• Add comprehensive documentation in README explaining the approach and caveats
• Include rationale comments about when to use distro-specific tooling

This pragmatic runtime detection works across distributions while still recommending proper packaging tools (dh_installsystemd, %{_unitdir}) for production packages.

Prompt

I used Claude Code to generate this fix, and audited it.

Here is the prompt I used:

Implement build/packaging-time detection of the systemd *system* unit directory by invoking:

  pkg-config --variable=systemdsystemunitdir systemd

Requirements:
1) - Resolve UNITDIR via pkg-config as mentioned above.
   - Allow an env override: CEVICHE_SYSTEMD_UNITDIR (takes precedence).
   - Fail clearly if UNITDIR is empty; otherwise install the ".service" file.

2) Fallback behavior if pkg-config is unavailable:
   - If SYSTEMD_UNITDIR is unset and pkg-config not found, probe common vendor dirs in order:
     /lib/systemd/system, then /usr/lib/systemd/system
   - Emit a warning comment/message explaining it’s a heuristic and may be distro-specific (suggest using CEVICHE_SYSTEMD_UNITDIR env variable).

3) Add a short “Rationale” comment that explains:
   - This isn’t the best approach for Linux — packagers should normally choose the destination and rely on distro tooling (e.g., Debian’s dh_installsystemd or RPM’s %{_unitdir} macros).
   - Using pkg-config at build/packaging time is a pragmatic, good-enough approach in many situations to discover the vendor unit dir without hardcoding paths.
   - Caveat: we can’t automatically determine whether it should go into user/ or system/, and we default to system/. Use CEVICHE_SYSTEMD_UNITDIR if you need to choose.

4) Quality:
   - Keep snippets minimal and self-contained.

Deliverables:
- Update the library.
- README section titled “Where we install systemd units and why”

[Copilot]

Pull Request Overview

This PR adds runtime detection of systemd unit directories to make the library more portable across different Linux distributions, replacing the hardcoded /lib/systemd/system/ path. The version is bumped from 0.6.1 to 0.7.0.

• Implements a three-tier detection strategy: environment variable override, pkg-config query, and fallback probing
• Updates all methods that reference systemd paths to use the new detection function
• Adds comprehensive documentation explaining the approach and its caveats

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/controller/linux.rs	Adds detect_systemd_unit_dir() function with runtime detection logic, updates get_service_unit_path() and get_service_dropin_dir() to return Result<PathBuf, Error>, adjusts callers to handle new error cases
README.md	Documents the systemd unit detection strategy, caveats for packagers, and usage instructions for the CEVICHE_SYSTEMD_UNITDIR environment variable
Cargo.toml	Bumps version from 0.6.1 to 0.7.0

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[Copilot]

The detect_systemd_unit_dir() function is called separately in both get_service_unit_path() and get_service_dropin_dir(). When both methods are called (e.g., in write_service_config() and delete()), this results in redundant detection work including potential Command execution and filesystem probing. Consider caching the detected directory in the LinuxController struct or calling the detection once at a higher level.

[Copilot]

The error message references path.display() but the variable being created is dropin_dir. This will show the wrong path (the config file path instead of the directory path) if directory creation fails. Change to dropin_dir.display().

Suggested change

	.map_err(|e| Error::new(&format!("Failed to create {}: {}", path.display(), e)))?;
	.map_err(|e| Error::new(&format!("Failed to create {}: {}", dropin_dir.display(), e)))?;