# 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