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greetd-moongreet/DECISIONS.md
nevaforget a2dc89854d
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Details
fix: security hardening, blur geometry, and performance audit fixes (v0.6.2) 
Security: cache dirs now 0o700 via DirBuilder::mode(), blur config
validated (finite + clamp 0–200), TOCTOU socket pre-check removed.

Quality: GPU blur geometry fixed (texture shifted instead of stretched),
is_valid_username hardened, is_valid_gtk_theme extracted as testable fn,
save_last_session error handling consistent with save_last_user.

Performance: blurred texture cached across monitors (1x GPU renderpass
instead of N), FingerprintProbe device proxy cached in GreeterState with
generation counter to prevent race condition on fast user-switch.

Clippy: all 7 warnings resolved (collapsible if-let, redundant closure,
manual_range_contains, too_many_arguments suppressed for GTK widget fns).

Tests: 109 → 118 (GTK theme validation, Unicode usernames, cache dir
permissions, unwritable dir handling, blur config edge cases).
2026-03-30 14:31:28 +02:00

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Decisions

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

  • Who: Ragnar, 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: Ragnar, 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: Ragnar, 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()