Who reads and who writes: CinnamonBg Reference Manual

Who reads and who writes

Three processes build a CinnamonBgList: the background daemon, the settings panel, and the slideshow daemon.

cinnamon-background-daemon (the renderer) never writes the configuration. It asks the model what each monitor should show, paints the result, and pushes the primary monitor's picture uri out to AccountsService so the greeter matches. It also publishes its own RenderedLayout property on its dbus interface (org.Cinnamon.Background), useful for reporting what images are currently being rendered to an applet.
The settings panel owns the mode. It writes the background-mode key directly and calls cinnamon_bg_list_save() once the list reports the new mode; cinnamon_bg_list_set_mode() is the equivalent for a C consumer. It makes configuration edits by setting properties on model items and calling cinnamon_bg_list_save().
The slideshow daemon only advances picture-uri on model items as it rotates, persisting with cinnamon_bg_list_save_pictures(), which moves pictures without changing which entries the key holds. A rotation is not a configuration edit, so it must not prune the entry of a monitor that happens to be unplugged. It never writes background-mode.
Cinnamon's slideshowManager only ever reads GSettings, and builds no BgList. It calls cinnamon_bg_list_has_slideshow(), which reads the keys directly.
Cinnamon's backgroundManager builds no list either. It is the only thing that reacts to a flat picture-uri, which is how an external "set as wallpaper" arrives: either on our own key (what xdg-desktop-portal-xapp writes) or on org.gnome.desktop.background's (which some applications still set directly). It turns either into cinnamon_bg_list_set_single_uri() — deliberately destructive, since asking for one wallpaper means one image on every monitor.

That both of Cinnamon's own consumers reach the library only through cinnamon_bg_list_has_slideshow() and cinnamon_bg_list_set_single_uri() is deliberate. Both are static and touch nothing but GSettings, so the shell never constructs a list and never acquires the window manager's DisplayConfig proxy that one would bring with it.

cinnamon_bg_list_save() writes picture-uri-list alone, so a component that only meant to change a picture cannot write back whatever mode it happens to be holding. Changing the mode is a separate, deliberate call. cinnamon_bg_list_set_single_uri() is the one exception, and an explicit one: it sets the mode to mirror along with the picture, because one wallpaper on every monitor is what it means.