xdg-desktop-portal
A portal frontend service for Flatpak and possibly other desktop containment frameworks.
xdg-desktop-portal works by exposing a series of D-Bus interfaces known as portals under a well-known name (org.freedesktop.portal.Desktop) and object path (/org/freedesktop/portal/desktop).
The portal interfaces include APIs for file access, opening URIs, printing and others.
Documentation for the available D-Bus interfaces can be found here.
Version numbering
xdg-desktop-portal uses even minor vesion numbers for stable versions, and odd minor version numbers for unstable versions. During an unstable version cycle, portal APIs can make backward incompatible changes, meaning that applications should only depend on APIs defined in stable xdg-desktop-portal versions in production.
Building xdg-desktop-portal
xdg-desktop-portal depends on GLib and Flatpak. To build the documentation, you will need xmlto and the docbook stylesheets. For more instructions, please read CONTRIBUTING.md.
Using portals
Flatpak grants sandboxed applications talk access to names in the
org.freedesktop.portal.* prefix. One possible way to use the portal APIs
is thus just to make D-Bus calls. For many of the portals, toolkits (e.g.
GTK+) are expected to support portals transparently if you use suitable
high-level APIs.
To implement most portals, xdg-desktop-portal relies on a backend
that provides implementations of the org.freedesktop.impl.portal.* interfaces.
Here are some examples of available backends:
- GTK xdg-desktop-portal-gtk
- GNOME xdg-desktop-portal-gnome
- KDE xdg-desktop-portal-kde
- LXQt xdg-desktop-portal-lxqt
- Pantheon (elementary OS) xdg-desktop-portal-pantheon
- wlroots xdg-desktop-portal-wlr
- Deepin xdg-desktop-portal-dde
- Xapp (Cinnamon, MATE, Xfce) xdg-desktop-portal-xapp
- GNOME Keyring gnome-keyring
Portals available on your system
Each portal drops a file in /usr/share/xdg-desktop-portal/portals/ with the
portal name and the .portal suffix. it provides its DBus name and the list of
protocol interfaces it supports. It can provide a hist of the desktop
environment they are used in but this is deprecated
and desktop environment are asked to provide a list of portals they should use
(see below).
Portals in use in your desktop environment
xdg-desktop-portal makes use of the XDG_CURRENT_DESKTOP environment variable
to know your current desktop environment, then it will make take a configuration
file named CURRENTDESKTOP-portals.conf specific to your environment to know
which portal implementations to use.
System-wide configuration is in /usr/share/xdg-desktop-portal/ and it is
possible to override this configuration in ~/.config/xdg-desktop-portal.
You can check which portals are implemented in your environment using DoorKnocker and you can try out the portals with ASHPD Demo.
Troubleshooting
Make sure the XDG_CURRENT_DESKTOP environment variable is correctly set and
inherited by the xdg-desktop-portal process. You can check with
< "/proc/$(pidof xdg-desktop-portal)/environ" tr '\0' '\n' | grep '^XDG_CURRENT_DESKTOP='
that the variable is there.
Design considerations
There are several reasons for the frontend/backend separation of the portal code:
- We want to have native portal dialogs that match the session desktop (i.e. GTK+ dialogs for GNOME, Qt dialogs for KDE)
- One of the limitations of the D-Bus proxying in flatpak is that allowing a sandboxed app to talk to a name implicitly also allows it to talk to any other name owned by the same unique name. Therefore, sandbox-facing D-Bus apis should generally be hosted on a dedicated bus connection. For portals, the frontend takes care of this for us.
- The frontend can handle all the interaction with portal infrastructure, such as the permission store and the document store, freeing the backends to focus on just providing a user interface.
- The frontend can also handle argument validation, and be strict about only letting valid requests through to the backend.
The portal apis are all following the pattern of an initial method call, whose response returns an object handle for an org.freedesktop.portal.Request object that represents the portal interaction. The end of the interaction is done via a Response signal that gets emitted on that object. This pattern was chosen over a simple method call with return, since portal apis are expected to show dialogs and interact with the user, which may well take longer than the maximum method call timeout of D-Bus. Another advantage is that the caller can cancel an ongoing interaction by calling the Cancel method on the request object.
One consideration for deciding the shape of portal APIs is that we want them to 'hide' behind existing library APIs where possible, to make it as easy as possible to have apps use them transparently. For example, the OpenFile portal is working well as a backend for the GtkFileChooserNative API.
When it comes to files, we need to be careful to not let portal apis subvert the limited filesystem view that apps have in their sandbox. Therefore, files should only be passed into portal APIs in one of two forms:
- As a document ID referring to a file that has been exported in the document portal
- As an open fd. The portal can work its way back to a file path from the fd, and passing an fd proves that the app inside the sandbox has access to the file to open it.
When it comes to processes, passing pids around is not useful in a sandboxed world where apps are likely in their own pid namespace. And passing pids from inside the sandbox is problematic, since the app can just lie.