Bug#802769: xinit: startx failed if started after su - <user> from a autologged root (getty@.service)

Package: xinit
Version: 1.3.4-3
Severity: normal

Dear Maintainer,

After the apt-get upgrade which changed the xinit from 1.3.4-1 to 1.3.4-3 the before functioning system stops to work with the following error in exec startx:

Fatal server error:
(EE) xf86OpenConsole: Cannot open virtual console 1 (Permission denied)

The invocation of the startx is:

1. The system autologs the root user using getty@.service ( via ExecStart=-/sbin/agetty -a root %I $TERM )
2. the root user starts the <user> via su - <user>
3. the user's .bash_profile contains the exec startx ...
    (but is the same if you start it manually)

The ls -al /dev/tty give:
crw------- 1 root tty 4, 1 Jan 1 01:11 /dev/tty1

if you chown <user> or chmod 666 /dev/tty1, the startx displays the graphical screen, but the mouse is not moving the cursor.

If you autostart the <user> directly from getty@.service, all is working as expected,

ls -al /dev/tty1
crw------- 1 <user> tty 4, 1 Jan 1 01:11 /dev/tty1

I filled this bug report because is the upgrade that broke the functioning system (twice, on two systems build on same image), nothing other changed in the meantime except upgrade.

-- System Information:
Debian Release: stretch/sid
APT prefers unstable
APT policy: (500, 'unstable')
Architecture: armhf (armv7l)

Kernel: Linux 4.1.7-v7+ (SMP w/4 CPU cores; PREEMPT)
Locale: LANG=en_GB, LC_CTYPE=en_GB (charmap=locale: Cannot set LC_ALL to default locale: No such file or directory ISO-8859-1)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages xinit depends on:
ii coreutils   8.23-4
ii libc6       2.19-22
ii libx11-6    2:1.6.3-1
ii x11-common 1:7.7+12
ii xauth       1:1.0.9-1

Versions of packages xinit recommends:
ii xfce4-session [x-session-manager]     4.12.1-3
ii xfce4-terminal [x-terminal-emulator] 0.6.3-2
ii xfwm4 [x-window-manager]              4.12.3-1
ii xserver-xephyr [xserver]              2:1.17.2-3
ii xserver-xorg [xserver]                1:7.7+12
ii xterm [x-terminal-emulator]           320-1

xinit suggests no packages.

-- debconf information:
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
    LANGUAGE = (unset),
    LC_ALL = (unset),
    LC_TIME = "en_US.UTF-8",
    LC_MONETARY = "hr_HR.UTF-8",
    LC_ADDRESS = "hr_HR.UTF-8",
    LC_TELEPHONE = "hr_HR.UTF-8",
    LC_NAME = "hr_HR.UTF-8",
    LC_MEASUREMENT = "hr_HR.UTF-8",
    LC_IDENTIFICATION = "hr_HR.UTF-8",
    LC_NUMERIC = "hr_HR.UTF-8",
    LC_PAPER = "hr_HR.UTF-8",
    LANG = "en_GB"
    are supported and installed on your system.
perl: warning: Falling back to a fallback locale ("en_GB").
locale: Cannot set LC_ALL to default locale: No such file or directory