greetd-moongreet/DECISIONS.md

Decisions

2026-04-08 – Show greeter UI on all monitors instead of just one

• Who: ClaudeCode, Dom
• Why: moonlock showed its UI on all monitors via ext-session-lock-v1, but moongreet only showed the login UI on one monitor (compositor-picked) with wallpaper-only windows on the rest. Inconsistent UX across the ecosystem.
• Tradeoffs: Each monitor gets its own full greeter widget tree (slightly more memory), but the UI is lightweight. Screen mirroring (e.g., wl-mirror/screencopy) was considered and rejected — it requires an external process, compositor screencopy support, adds latency, and fights Wayland's per-output model. One-window-per-monitor is the established Wayland pattern (swaylock, hyprlock, moonlock all do this).
• How: Create one create_greeter_window() per monitor with set_monitor(), only the first gets KeyboardMode::Exclusive. Removed create_wallpaper_window() (no longer needed). No layer shell fallback keeps single-window mode for development.

2026-04-06 – Restore explicit gtk-theme in moongreet config

• Who: ClaudeCode, Dom
• Why: GTK4 under greetd does not reliably read /etc/xdg/gtk-4.0/settings.ini — likely requires a settings daemon that doesn't run in the greeter session. moongreet fell back to Adwaita/Colloid-default (blue accent) instead of Colloid-Grey-Dark-Catppuccin.
• Tradeoffs: Reverts 094878f ("Remove gtk-theme from app config, use system-wide GTK settings instead"). Duplicates the theme name between settings.ini and moongreet.toml, but the explicit set via set_gtk_theme_name() is the only reliable path in a greetd context.
• How: Added gtk-theme = "Colloid-Grey-Dark-Catppuccin" to example config and deployed /etc/moongreet/moongreet.toml.

2026-04-02 – Replace hardcoded CSS colors with GTK theme variables

• Who: ClaudeCode, Dom
• Why: moongreet used hardcoded colors (#1a1a2e, white, #ff6b6b) while moonset already used @theme_bg_color, @theme_fg_color, @error_color etc. Inconsistent across the ecosystem and broke theme flexibility.
• Tradeoffs: Depends on the active GTK theme defining standard color variables. Catppuccin Colloid provides all needed vars (@theme_bg_color, @theme_fg_color, @error_color, @success_color, @theme_selected_bg_color). Fallback behavior if a theme lacks vars is GTK's default colors — acceptable.
• How: Replaced all hardcoded hex/named colors with GTK theme variables. Coordinated change across moongreet, moonlock, and moonset (all three now use identical pattern).

2026-03-31 – Fourth audit: power timeout, timing mitigation, release profile, GREETD_SOCK caching

• Who: ClaudeCode, Dom
• Why: Fourth triple audit found moongreet power.rs had no timeout on loginctl (greeter could freeze), username enumeration via timing differential, GREETD_SOCK re-read on every login, missing release profile, and missing GResource compression.
• Tradeoffs: 500ms minimum login response time adds slight delay on fast auth but prevents timing-based username enumeration. Power timeout (30s + SIGKILL) matches moonset pattern — aggressive but prevents greeter freeze.
• How: (1) power.rs adapted from moonset with 30s timeout + SIGKILL (nix dependency added). (2) 500ms min response floor in attempt_login via Instant + glib::timeout_future. (3) GREETD_SOCK cached in GreeterState at startup. (4) [profile.release] with LTO, codegen-units=1, strip. (5) compressed="true" on GResource entries. (6) SYNC comments on duplicated blur/background functions.

2026-03-30 – Full audit fix: security, quality, performance (v0.6.2)

• Who: ClaudeCode, Dom
• Why: Three parallel audits (security, code quality, performance) identified 10 actionable findings across the codebase — from world-readable cache dirs to a GPU blur geometry bug to a race condition in fingerprint probing.
• Tradeoffs: too_many_arguments Clippy warnings suppressed with #[allow] rather than introducing a UiWidgets struct — GTK's clone! macro with #[weak] refs requires individual widget parameters, a struct would fight the idiom. Async avatar loading skipped because Pixbuf is !Send; cache already prevents repeat loads. TOCTOU socket pre-check removed entirely — connect() in login_worker already handles errors, the metadata() check gave false security guarantees.
• How: Cache dirs use DirBuilder::mode(0o700) instead of create_dir_all. Blur config clamped to 0.0..=200.0 with is_finite() guard. Blur texture cached in Rc<RefCell<Option<gdk::Texture>>> across monitors. FingerprintProbe device proxy cached in GreeterState with generation counter to prevent stale async writes. GPU blur geometry fixed (-pad origin shift instead of texture stretching). is_valid_gtk_theme extracted as testable function. 9 new tests.

2026-03-29 – Fingerprint authentication via greetd multi-stage PAM

• Who: ClaudeCode, Dom
• Why: moonlock supports fprintd but moongreet rejected multi-stage auth. Users with enrolled fingerprints couldn't use them at the login screen.
• Tradeoffs: Direct fprintd D-Bus verification (like moonlock) can't start a greetd session — greetd controls session creation via PAM. Using greetd multi-stage means PAM decides the auth order (fingerprint first, then password fallback), not truly parallel. Acceptable — matches standard pam_fprintd behavior.
• How: Replace single-pass auth with a loop over auth_message rounds. Secret prompts get the password, non-secret prompts (fprintd) get None and block until PAM resolves. fprintd D-Bus probe (gio::DBusProxy) only for UI — detecting device availability and enrolled fingers. 60s socket timeout when fingerprint available. Config option fingerprint-enabled (default true).

2026-03-28 – Remove embedded wallpaper from binary

• Who: Selene, 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, greeter 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 wallpaper window creation and background picture when no path available.

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

• Who: ClaudeCode, Dom
• Why: CPU-side Gaussian blur (image crate) blocked the GTK main thread for 500ms–2s on 4K wallpapers at cold cache. Disk cache and async orchestration added significant complexity.
• Tradeoffs: GPU blur quality is slightly different (box-blur approximation vs true Gaussian), acceptable for wallpaper backgrounds. Removes image crate dependency entirely (~15 transitive crates eliminated). 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. Symmetric with moonlock and moonset.

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

• Who: Selene, Dom
• Why: Blurred wallpaper as greeter background is a common UX pattern for login screens
• 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 blurred gdk::Texture. Config option background-blur: Option<f32> in [appearance] TOML section.

2026-03-28 – Audit fixes for shared wallpaper texture (v0.4.1)

• Who: Selene, Dominik
• Why: Quality, performance, and security audits flagged issues in load_background_texture(), debug logging, and greetd error handling
• Tradeoffs: GResource path now requires UTF-8 (returns None for non-UTF-8 instead of aborting); 50 MB wallpaper limit is generous but prevents OOM; debug logging off by default trades observability for security
• How: GResource branch via resources_lookup_data() + from_bytes() (no abort), file size limit, error details only at debug level, MOONGREET_DEBUG env var for log level, greetd retry path truncation matching show_greetd_error()