Note that the description below is not fully correct:

 /suse/werner> sudo printenv | grep XDG
 XDG_CONFIG_DIRS=/etc/xdg:/usr/local/etc/xdg:/usr/etc/xdg
 XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session11
 XDG_MENU_PREFIX=fvwm-
 XDG_CONFIG_HOME=/suse/werner/.config
 XDG_SEAT=seat0
 XDG_SESSION_DESKTOP=
 XDG_SESSION_TYPE=x11
 XDG_CURRENT_DESKTOP=FVWM
 XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
 XDG_CACHE_HOME=/suse/werner/.cache/noether:0
 XDG_SESSION_CLASS=user
 XDG_DESKTOP_DIR=/suse/werner/Desktop
 XDG_SESSION_COOKIE=noether
 XDG_VTNR=7
 XDG_SESSION_ID=733
 XDG_RUNTIME_DIR=/run/user/223
 XDG_DATA_DIRS=/usr/local/share:/usr/share:/usr/share/gdm:/var/lib/menu-xdg

that is sudo does *not* remove the XDG variables from the ennvirnment nor does
the pam_systemd module set the correct XDG variables.


+++ This bug was initially created as a clone of Bug #1205109 +++

# Description

Emacs checks whether environment variable `XDG_RUNTIME_DIR` exists to determine
where a certain socket file should be placed. If `XDG_RUNTIME_DIR` doesn't
exist, emacs will fallback to some other location. See
<https://git.savannah.gnu.org/cgit/emacs.git/tree/lib-src/emacsclient.c?h=emacs-27.2#n1393>

`sudo` defaults to unset all environment variables and set some certain ones.
So any program run by `sudo` should miss `XDG_RUNTIME_DIR` in its environment.

If I run `sudo emacs-nox`, everything goes expectedly. But if I run `sudo
emacs`, emacs prompts `Unable to set up transient service directory:
XDG_RUNTIME_DIR "/run/user/0" not available: No such file or directory`.

# Possible reason

The binary `/usr/bin/emacs` is not built from emacs source, but made by the
SUSE package maintainer. The binary checks whether it's run in terminal or X to
determine whether it should exec `emacs-nox` or `emacs-x11`.

The `/usr/bin/emacs` binary defines an environment variable `XDG_RUNTIME_DIR`
as `/run/user/<USER_ID>` arbitrarily. But `/run/user/0` doesn't exist because
it  should be created by `/etc/profile` in a login shell. And `sudo` doesn't
spawn a new login shell.

# Reproduction

1. Install `emacs` and `emcas-nox` from zypper, and don't install `emacs-x11`.
1. Restart your computer and don't login as root in any tty, but login as a
regular user.
1. `sudo emacs`
1. Quit emacs.
1. You will see the prompt.

# Versions

openSUSE Leap 15.4
Emacs 27.2
Sudo 1.9.9