Hiya,

I was looking through the available greetd greeters and it occurred to
me that virtually all of them are doing the legwork to find the list of
valid X11/Wayland sessions:

* https://gitlab.com/marcusbritanicus/QtGreet/-/blob/main/src/widgets/session/session.cpp#L60
* https://github.com/deathowl/ddlm/blob/master/src/main.rs#L437
* https://github.com/rharish101/ReGreet/blob/main/src/sysutil.rs#L157
* https://github.com/Decodetalkers/greetd-qmlgreet/blob/603cad4dcb57be129d25df3fbc373809d021fce0/src/DesktopListModel.cpp#L10
* https://github.com/Decodetalkers/marine_greetdm/blob/master/desktopparse/src/lib.rs#L61

Outside of greetd, it's more of the same:

 * https://github.com/GNOME/gdm/blob/c732f814ff6fc1cde53fdc30496bfe62b11bd081/libgdm/gdm-sessions.c#L246
 * https://github.com/Obsidian-StudiosInc/entrance/blob/master/src/daemon/entrance_session.c#L473
 * https://github.com/canonical/lightdm/blob/8c22a5100f04c30be034cc12d9058ee88c6329d5/liblightdm-gobject/session.c#L116
 * https://github.com/GalliumOS/lxdm/blob/7aeab6a9dc589f71999eb8de05033733b5f2dd9e/src/lxdm.c#L587
 * https://github.com/sddm/sddm/blob/develop/src/common/Session.cpp#L40

I was curious if it might make sense for greetd to do this work once,
and then offer the list to the greeters via some sort of IPC mechanism.

-Sam

My original stance was that I did not like the over-complicated use of 
.desktop files and their built-in assumptions about the type of session 
being started. greetd was designed to make no assumptions about what is 
being run, which is why it just handles plan command-lines. For an X11 
session file, it assumes you're going to pick a DISPLAY value, deal with 
xauth, start an X11 server and wait for it to be ready, and only then 
run the specified command.

On the contrib branch, I made some example shell scripts that could be 
used to interact with such session definitions without any code in greetd.