Bug#1050981: xmonad: please set XDG_CURRENT_DESKTOP and use it to configure xdg-desktop-portal

[Simon McVittie]

Package: xmonad
Version: 0.17.1-1
Severity: normal
User: xdg-desktop-portal at packages.debian.org
Usertags: portals.conf

xdg-desktop-portal 1.17.x introduces a new way to select which portals will
be used for which desktop environments, modelled on mimeapps.list:

- each desktop environment should provide a file like
  /usr/share/xdg-desktop-portal/xmonad-portals.conf

- the filename is ${DESKTOP}-portals.conf where ${DESKTOP} is the desktop
  environment's entry in $XDG_CURRENT_DESKTOP (the same as the DesktopNames
  from /usr/share/{x,wayland-}sessions/*.desktop), folded to lower case

- sysadmins and users can override this via files named portals.conf or
  ${DESKTOP}-portals.conf in various locations like /etc/xdg-desktop-portal
  and ~/.config/xdg-desktop-portal

But as far as I can tell, XMonad doesn't set XDG_CURRENT_DESKTOP, so for
the purposes of this mechanism, it's not programmatically distinguishable
from any other desktop environment or window manager.

XDG_CURRENT_DESKTOP is also used in pre-existing freedesktop.org standards
like the OnlyShowIn/NotShowIn fields for .desktop files, and the ability to
provide a desktop-environment-specific mimeapps.list. Setting
XDG_CURRENT_DESKTOP would allow XMonad to participate in those
specifications.

The "GNOME Flashback (Xmonad)" session *does* set XDG_CURRENT_DESKTOP, but
to a value that is indistinguishable from the ordinary GNOME Flashback
session with the metacity window manager, and I don't know whether that's
the most appropriate thing for the purposes of desktop-specific
configuration or not.

To reproduce
============

* Start from a basic non-GUI virtual machine (I used autopkgtest-build-qemu)
* Ensure that a user account exists
* apt install lightdm xorg xmonad
* reboot
* Log in as the user account, selecting "XMonad" from the menu of
  possible X11 sessions
* In a ssh session as the same user, run:
  systemctl --user show-environment
* apt install gnome-session-flashback
* Repeat for the "GNOME Flashback (Xmonad)" session

(I couldn't work out how to open a shell in an otherwise unconfigured
XMonad session, but in any case it's the systemd activation environment
that matters here, more than `env`, because xdg-desktop-portal will
typically be run as a systemd user service.)

Expected result
===============

XDG_CURRENT_DESKTOP should be set to a colon-separated sequence of
desktop environment names, most specific first. XMonad seems to be its
own thing rather than being based on another desktop environment or
window manager, so

    XDG_CURRENT_DESKTOP=XMonad

would seem appropriate.

This would allow the XMonad session to have its own
desktop-environment-specific mimeapps.list or portals.conf(5), for
example /usr/share/xdg-desktop-portal/xmonad-portals.conf.

For XMonad's GNOME Flashback variant, I don't know whether
XDG_CURRENT_DESKTOP should be set to GNOME-Flashback:GNOME (exactly
the same as GNOME Flashback with metacity), or whether it would be better
to set it to something like XMonad-GNOME-Flashback:GNOME-Flashback:GNOME
so that XMonad's GNOME Flashback variant can have its own portal
configuration, mimeapps.list, OnlyShowIn/NotShowIn, etc. if it wants to
(in principle someone could develop a portal backend similar to
xdg-desktop-portal-gtk that works better with tiling window managers,
although I'm not aware of such a thing having been written).

Actual result
=============

In the XMonad session, XDG_CURRENT_DESKTOP is unset.

This means that xdg-desktop-portal configuration can only be done via a
non-desktop-specific portals.conf, but that's not really something that a
non-opinionated distribution like Debian can usefully ship in a centralized
way, so each user of XMonad who wants a working xdg-desktop-portal will
have to configure it themselves.

At the moment, this is mitigated by xdg-desktop-portal (>= 1.17) having
been patched to fall back to xdg-desktop-portal-gtk as a last-resort
desktop-environment-specific backend, but hard-coding that implementation
isn't really something we should be doing centrally (and the idea was
rejected upstream), so I intend to remove that patch before trixie
is released.

For XMonad's GNOME Flashback variant, XDG_CURRENT_DESKTOP is currently set
to GNOME-Flashback:GNOME (exactly the same as GNOME Flashback with metacity),
so it will always use the same portal configuration, etc. as
gnome-session-flashback (see #1050799).

Suggested fix
=============

Add a sequence of semicolon-separated desktop environment names to
/usr/share/xsessions/xmonad.desktop, most likely just "XMonad":

DesktopNames=XMonad;

(For example, icewm and windowmaker use "ICEWM" and "WindowMaker" in
their equivalent xsessions file.)

And then create a /usr/share/xdg-desktop-portal/xmonad-portals.conf
with whatever portal backends are desired for an XMonad session,
for example perhaps this:

[preferred]
default=gtk;

Please see portals.conf(5) or its source code
https://github.com/flatpak/xdg-desktop-portal/blob/main/doc/portals-conf.rst
for full details.

For XMonad's GNOME Flashback variant, there are two options. If it's
intentional that it is configured the same as the metacity-based GNOME
Flashback session in all respects (portals, mimeapps.list,
OnlyShowIn/NotShowIn, etc.) then it's OK to leave it as-is, with
"DesktopNames=GNOME-Flashback;GNOME;". Or, if it might be desirable to give
it its own desktop-specific configuration, then it should have something
like "DesktopNames=XMonad-GNOME-Flashback;GNOME-Flashback;GNOME;".

Thanks,
    smcv

-- 
This is part of a mass bug filing:
https://lists.debian.org/debian-devel/2023/08/msg00311.html