# Decisions

Architectural and design decisions for Moonlock, in reverse chronological order.

## 2026-03-30 – Second audit: zeroize CString, FP account check, PAM timeout, blur downscale

- **Who**: Nyx, Dom
- **Why**: Second triple audit (quality, performance, security) found: CString password copy not zeroized (HIGH), fingerprint unlock bypassing pam_acct_mgmt (MEDIUM), no PAM timeout leaving user locked out on hanging modules (MEDIUM), GPU blur on full wallpaper resolution (MEDIUM), no-monitor edge case doing `return` instead of `exit(1)` (MEDIUM).
- **Tradeoffs**: PAM timeout (30s) uses a generation counter to avoid stale result interference — adds complexity but prevents parallel PAM sessions. FP restart after failed account check re-claims the device, adding a D-Bus round-trip, but prevents permanent FP death on transient failures. Blur downscale to 1920px cap trades negligible quality for ~4x less GPU work on 4K wallpapers.
- **How**: (1) `Zeroizing<CString>` wraps password in auth.rs, `zeroize/std` feature enabled. (2) `check_account()` calls pam_acct_mgmt after FP match; `resume_async()` restarts FP on transient failure. (3) `auth_generation` counter invalidates stale PAM results; 30s timeout re-enables UI. (4) `MAX_BLUR_DIMENSION` caps blur input at 1920px, sigma scaled proportionally. (5) `exit(1)` on no-monitor after `lock.lock()`.

## 2026-03-28 – Remove embedded wallpaper from binary

- **Who**: Nyx, Dom
- **Why**: Wallpaper is installed by moonarch to /usr/share/moonarch/wallpaper.jpg. Embedding a 374K JPEG in the binary is redundant. GTK background color (Catppuccin Mocha base) is a clean fallback.
- **Tradeoffs**: Without moonarch installed AND without config, lockscreen shows plain dark background instead of wallpaper. Acceptable — that's the expected minimal state.
- **How**: Remove wallpaper.jpg from GResources, return None from resolve_background_path when no file found, skip background picture creation when no texture available.

## 2026-03-28 – Audit-driven security and lifecycle fixes (v0.6.0)

- **Who**: Nyx, Dom
- **Why**: Triple audit (quality, performance, security) revealed a critical D-Bus signal spoofing vector, fingerprint lifecycle bugs, and multi-monitor performance issues.
- **Tradeoffs**: `cleanup_dbus()` extraction adds a method but clarifies the stop/match ownership; `running_flag: Rc<Cell<bool>>` adds a field but prevents race between async restart and stop; sender validation adds a check per signal but closes the only known auth bypass.
- **How**: (1) Validate D-Bus VerifyStatus sender against fprintd's unique bus name. (2) Extract `cleanup_dbus()` from `stop()`, call it on verify-match. (3) `Rc<Cell<bool>>` running flag checked after await in `restart_verify_async`. (4) Consistent 3s D-Bus timeouts. (5) Panic hook before logging. (6) Blur and avatar caches shared across monitors. (7) Peek icon disabled. (8) Symlink rejection for background_path. (9) TOML parse errors logged.

## 2026-03-28 – GPU blur via GskBlurNode replaces CPU blur

- **Who**: Nyx, Dom
- **Why**: CPU-side Gaussian blur (`image` crate) blocked the GTK main thread for 500ms–2s on 4K wallpapers at cold cache. Disk cache mitigated repeat starts but added ~100 lines of complexity.
- **Tradeoffs**: GPU blur quality is slightly different (box-blur approximation vs true Gaussian), acceptable for wallpaper. Removes `image` and `dirs` dependencies entirely. No disk cache needed.
- **How**: `Snapshot::push_blur()` + `GskRenderer::render_texture()` on `connect_realize`. Blur happens once on the GPU when the widget gets its renderer, producing a concrete `gdk::Texture`. Zero startup latency.

## 2026-03-28 – Optional background blur via `image` crate (superseded)

- **Who**: Nyx, Dom
- **Why**: Consistent with moonset/moongreet — blurred wallpaper as lockscreen background is a common UX pattern
- **Tradeoffs**: Adds `image` crate dependency (~15 transitive crates); CPU-side Gaussian blur at load time adds startup latency proportional to image size and sigma. Acceptable because blur runs once and the texture is shared across monitors.
- **How**: `load_background_texture(bg_path, blur_radius)` loads texture, optionally applies `imageops::blur()`, returns `gdk::Texture`. Config option `background_blur: Option<f32>` in TOML.

## 2026-03-28 – Shared wallpaper texture pattern (aligned with moonset/moongreet)

- **Who**: Nyx, Dom
- **Why**: Previously loaded wallpaper per-window via `Picture::for_filename()`. Multi-monitor setups decoded the JPEG redundantly. Blur feature requires texture pixel access anyway.
- **Tradeoffs**: Slightly more code in main.rs (texture loaded before window creation), but avoids redundant decoding and enables the blur feature.
- **How**: `load_background_texture()` in lockscreen.rs decodes once, `create_background_picture()` wraps shared `gdk::Texture` in `gtk::Picture`. Same pattern as moonset/moongreet.