Bug#724731: XDG_RUNTIME_DIR has incorrect value

[Gabriel Mainberger]

Same issue for me after upgrading to gdm 3.8.4-3. A black screen after 
starting gdm, except if I chmod 777 /run/user/0.

/var/log/gdm3/:0-slave.log:

gdm-simple-slave[5325]: Failed to give slave programs access to the 
display. Trying to proceed.
gdm-launch-environment][5358]: pam_env(gdm-launch-environment:session): 
Unable to open env file: /etc/environment: Datei oder Verzeichnis nicht 
gefunden
gdm-launch-environment][5358]: pam_unix(gdm-launch-environment:session): 
session opened for user Debian-gdm by (uid=0)
gdm-launch-environment][5358]: 
pam_systemd(gdm-launch-environment:session): Asking logind to create 
session: uid=115 pid=5358 service=gdm-launch-environment type=x11 
class=greeter seat=seat0 vtnr=7 tty= display=:0 remote=no remote_user= 
remote_host=
gdm-launch-environment][5358]: 
pam_systemd(gdm-launch-environment:session): Reply from logind: id=c1 
object_path=/org/freedesktop/login1/session/c1 runtime_path=/run/user/0 
session_fd=13 seat=seat0 vtnr=1
gdm-launch-environment][5358]: 
pam_ck_connector(gdm-launch-environment:session): nox11 mode, ignoring 
PAM_TTY :0
gdm3][5591]: pam_env(gdm3:session): Unable to open env file: 
/etc/environment: Datei oder Verzeichnis nicht gefunden
gdm3][5591]: pam_unix(gdm3:session): session opened for user gabriel by 
(uid=0)
gdm3][5591]: pam_systemd(gdm3:session): Asking logind to create session: 
uid=1000 pid=5591 service=gdm3 type=x11 class=user seat=seat0 vtnr=7 
tty= display=:0 remote=no remote_user= remote_host=
gdm3][5591]: pam_systemd(gdm3:session): Reply from logind: id=c1 
object_path=/org/freedesktop/login1/session/c1 runtime_path=/run/user/0 
session_fd=15 seat=seat0 vtnr=1
gdm3][5591]: pam_ck_connector(gdm3:session): nox11 mode, ignoring PAM_TTY :0
gdm-launch-environment][5358]: pam_unix(gdm-launch-environment:session): 
session closed for user Debian-gdm
gdm-simple-slave[5325]: Failed to remove slave program access to the 
display. Trying to proceed.
gdm-simple-slave[5325]: Child process -5369 was already dead.
gdm3][5591]: pam_unix(gdm3:session): session closed for user gabriel
gdm-simple-slave[5325]: GLib-GObject: g_object_ref: assertion 
`object->ref_count > 0' failed
gdm-simple-slave[5325]: GLib-GObject: g_object_unref: assertion 
`object->ref_count > 0' failed

If I login with my user account, I still get the wrong runtime_path.