util: Blacklist some session-specific variables

[leigh123linux]

Based on: GNOME/gnome-session@646b9bc
GNOME/gnome-session@9d8b0709

Fixes #140

[smcv]

Thanks, this looks good to me. While you're here you might also want to sync up with later changes to the corresponding file in gnome-session, which fixes various bugs related to this code:

• 9d8b0709 adds NOTIFY_SOCKET to this list (that variable should basically never propagate between processes)
• b7b24627 uses systemd's UnsetAndSetEnvironment to actively unset some variables (instead of just not setting them, as done here)
• 60e619b8 removes these variables from the environment of /etc/xdg/autostart apps, too
• fe22c4ee fixes handling of rare whitespace characters like \r
• 87d92fec avoids warnings when systemd is not managing the session
• 3b57d117 fixes a stack overflow (session manager crash) if users have long environment variables
• d44888fb is the opposite side of this: the change requested in Cinnamon session shouldn’t upload XDG_SESSION_ID to systemd —user #140 stops inappropriate variables from "leaking" out of a Cinnamon session into a later login session, and d44888fb stops those same variables from "leaking" into the later login session, on the principle that where there's a bad interaction between two components, the most robust thing to do is to fix it on both sides

Those all seem like good fixes to have.

[ItzSwirlz]

Yeah, to be fair it's been a while since we've checked upstream for anything that may be useful for -session. Thanks @smcv :)

[ItzSwirlz]

I'd love to implement GNOME/gnome-session@b7b24627 but we aren't sure what the idea with Wayland is.. I was asked if we would follow the proper XDG_DECORATION in muffin and stuff. Does any of the devs know what we want to do with Wayland or are we sticking with Xorg?

[smcv]

GNOME can be run in either X11-only or Wayland mode (they appear as separate session types in gdm), and its handling of those environment variables is the same for both modes - so I expect that Cinnamon should probably do the same, regardless of whether Cinnamon is permanently X11-only, currently X11-only with plans to adopt Wayland in future (similar to old GNOME versions), or already supports Wayland (similar to current GNOME).

In an X11-only session, if a previous session (maybe GNOME or another desktop) has pushed WAYLAND_DISPLAY and WAYLAND_SOCKET into the systemd environment, you'll want to remove them from the environment so that they don't confuse apps into thinking they can still use Wayland. In a Wayland session, you'd want to remove those variables from the environment and replace them with your own values for them (if any). I think the gnome-session code does the right thing for both of those cases.

[smcv]

If Cinnamon/muffin becomes a Wayland compositor in future, then it's up to the compositor developers whether it prefers client-side decoration (like Weston and mutter), server-side decoration (like traditional X11), or some combination of the two.

I don't think getting the environment variable handling right has any impact on that decision.

[ItzSwirlz]

Implemented the commits above in leigh123linux#1 except for GNOME/gnome-session@60e619b8 because we need to add a few more commits to catch up to it.

[mtwebster]

@ItzSwirlz please open a new pr for that other stuff. Also please be sure all of that even applies to us.
For instance, CINNAMON_SHELL_SESSION_MODE isn't a thing - I believe that had to do with gnome's short-lived 'classic mode'.