Fallback engine

The four tiers

Tier 1  XDG Desktop Portal
Tier 2  Native DE D-Bus / IPC
Tier 3  CLI tools (probe PATH)
Tier 4  UdaError::NotSupported (typed error)

When all three service tiers fail there is no panic and no bare io::Error — the caller receives a UdaError::NotSupported carrying a diagnosis. This is the mechanical form of AGENTS.md Principle 1: never assume a capability, always degrade.

Tier 1 — XDG Desktop Portal

Probe: does org.freedesktop.portal.Desktop exist on the session bus, and is the target interface exported?

Covers: appearance (org.freedesktop.portal.Settings), wallpaper (org.freedesktop.portal.Wallpaper), file selection, screen capture, global shortcuts.

Why first: the portal is the freedesktop-standard answer, implemented by the desktop itself, consistent across environments, and already handles consent.

The catch: not every desktop implements every interface. GNOME 42+ is complete, KDE is partial, Wayland tiling compositors usually have none. Hence probe, never assume.

Every portal call runs inside a timeout, because the portal may present a consent dialog and block indefinitely. A timeout becomes a typed error rather than a hung caller.

Tier 2 — Native DE D-Bus / IPC

Probe: read $XDG_CURRENT_DESKTOP, map it to a concrete D-Bus service, or check the compositor socket paths ($HYPRLAND_INSTANCE_SIGNATURE, $SWAYSOCK).

Desktop	Service / mechanism
GNOME	`org.gnome.desktop.background`, `org.gnome.desktop.interface`
KDE	`org.kde.plasmashell` → `/PlasmaShell` → `evaluateScript`
Hyprland	`$HYPRLAND_INSTANCE_SIGNATURE` socket → `hyprpaper` / `swww`
Sway	`$SWAYSOCK` → IPC
Any (session)	`org.freedesktop.ScreenSaver`, `org.freedesktop.Notifications`

Why it is needed: native paths cover what the portal does not, and are usually richer — GNOME’s dark/light wallpaper pairing, for example, has no portal equivalent.

Tier 3 — CLI tools

Probe: look for an executable on PATH in a fixed order.

Feature	Tool order
Wallpaper (X11)	`feh` → `nitrogen`
Wallpaper (Wayland)	`swww` → `hyprpaper`
Theme (XFCE)	`xfconf-query`
Logout (generic)	`loginctl`

Why last: a CLI tool is a process — expensive to start, coarse to interpret (exit code and stderr only), and not necessarily installed. But on a bare X11 environment with no IPC channel it is the only option, which is why the tier exists at all.

Tier 4 — typed error

The final tier is not a workaround, it is a contract: UdaError::NotSupported("...") with a message naming the tier chain that was tried. A host can render it verbatim, or match on the type to hide the feature entirely.

What the caller sees

The tiers are internal. A host observes only:

Observation	Meaning
`Ok(...)`	some tier answered
`SupportLevel::Full`	Tier 1 or 2 answered, with no known limitation
`SupportLevel::Partial(reason)`	available, but in a degraded form, with the reason attached
`UdaError::NotSupported`	all four tiers are exhausted