Comment 121 for bug 1197395

(In reply to Clement Lefebvre from comment #53)
> Is the solution either for systemd to set
> XDG_RUNTIME_DIR differently when the UID is 0, or for techs like glib and
> apps using this variable, to only rely on it when the UID isn't 0?
[...]
> - Should we patch glib and individual apps to use a different runtime
> directory than XDG_RUNTIME_DIR when UID == 0?

The problem is the exact opposite of this. When someone is root
and 'su -' to a user, XDG_RUNTIME_DIR still points to the root-owned
directory and so is unusable by applications.

I am recommending that no one uses XDG_RUNTIME_DIR at all since you
cannot be sure it exists and has usable permissions.