LightDM is already able to create the xauthority file locally under
$XDG_RUNTIME_DIR, using configuration option user-authority-in-system-
dir. This option is off by default on Ubuntu 16.04/18.04, but can be
enabled with

# echo -e '[LightDM]\nuser-authority-in-system-dir=true'
>/etc/lightdm/lightdm.conf.d/50-user-authority-in-system-dir.conf

LightDM then sets XAUTHORITY=/var/run/lightdm/$USER/xauthority instead
of $HOME/.Xauthority. However, this path goes through the symbolic link
/var/run -> /run. For some reason (why?) snap does not like such
symlinks, resulting in a warning:

# snap install pdftk
# pdftk
2018/08/24 17:32:48.267771 cmd_run.go:442: WARNING: XAUTHORITY environment 
value is not a clean path: "/run/lightdm/mgk25/xauthority"

A workaround to canonicalize the XAUTHORITY path (to eliminate symlinks)
can be installed with e.g.

# echo 'XAUTHORITY=`readlink -f "$XAUTHORITY"` && [ -z "$XAUTHORITY" ]
&& export XAUTHORITY' >/etc/X11/Xsession.d/10canonicalize-xauthority

It would be nicer if LightDM directly used a canonical path in
XAUTHORITY with user-authority-in-system-dir=true, or if snap didn't
have to be so picky about symbolic links.

(LightDM currently does not set ICEAUTHORITY when user-authority-in-
system-dir=true.)

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https://bugs.launchpad.net/bugs/1648107

Title:
  $XAUTHORITY should move into $XDG_RUNTIME_DIR

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