home_dir() Uses Only HOME Env Var (Not getpwuid_r Fallback)

Problem Statement

refs.rs uses std::env::var("HOME") to locate the RefMap directory. If the tool is invoked in an environment where HOME is unset (sandboxed environment, su/sudo context, CI runner, systemd service), it will fail with a confusing error instead of falling back to the system password database (getpwuid_r).

Findings

File: crates/core/src/refs.rs:112-114

let home = std::env::var("HOME")
    .map_err(|_| AppError::internal("HOME env var not set"))?;

HOME can be unset in sandboxed macOS apps, in CI via sudo, or in daemon contexts. The POSIX-correct approach is to try HOME first, then fall back to getpwuid_r(getuid()).

Proposed Solutions

Option A: Use dirs::home_dir() crate (Recommended)

The dirs crate handles HOME + getpwuid_r fallback correctly on all platforms.

Effort: Tiny (add one dependency)
Risk: Low

Option B: Manual getpwuid_r fallback

Call libc::getpwuid_r(libc::getuid(), ...) when HOME is unset. Platform-correct but verbose.

Effort: Small
Risk: Low

Option C: Make RefMap path configurable via env var

Introduce AGENT_DESKTOP_DATA_DIR env var. Default to $HOME/.agent-desktop when set. Useful for testing and CI regardless.

Effort: Small
Risk: Low — good flexibility, independent of the HOME fallback fix

Recommended Action

Option A (dirs crate) + Option C (configurable env var).

Technical Details

File: crates/core/src/refs.rs
Lines: 112–114
Component: RefMap path resolution

Acceptance Criteria

Works correctly when HOME is unset
Falls back to getpwuid_r or equivalent
Optional: accepts AGENT_DESKTOP_DATA_DIR env override

Work Log

2026-02-19: Finding identified by security-sentinel review agent

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