xhost access control fails to add other users

[asgard2]

Since 15.04 it is not possible to allow access control to a specific user.

Tested with the current xubuntu 15.04 live iso file, it is the same problem.
$ md5sum xubuntu-15.04-desktop-amd64.iso
586604cdb814f906f7c18345138f503b xubuntu-15.04-desktop-amd64.iso

It can easily reproduced by adding another user:

sudo adduser user2

xhost +SI:localuser:user2
localuser:user2 being added to access control list

su user2

But it is sill not possible to start gui applications.

user2@xubuntu:~$ xclock
No protocol specified
Error: Can't open display: :0.0

user2@xubuntu:~$ /usr/bin/firefox

(process:3130): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
No protocol specified
No protocol specified
Error: cannot open display: :0.0

a workaround would be:
xhost +local:
non-network local connections being added to access control list

added apport-collect 1448431
---
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CasperVersion: 1.360
CurrentDesktop: XFCE
DistroRelease: Ubuntu 15.04
LiveMediaBuild: Xubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422.1)
Package: x11-xserver-utils 7.7+2ubuntu2
PackageArchitecture: amd64
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 3.19.0-15.15-generic 3.19.3
Tags: vivid
Uname: Linux 3.19.0-15-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo
_MarkForUpload: True

[asgard2 (kamp000x)]

apport information

[asgard2 (kamp000x)]

apport information

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[ntfwc (ntfwc)]

I have used this functionality extensively, and wrote a few scripts to make it easier to run programs as other users. So I ran into this rather quickly on 15.04. As a workaround, you can use xauth instead. For example, as a script:

#!/bin/sh
OTHER_USER=user2

echo "Giving $OTHER_USER xauth key"
XAUTH_KEY=$(xauth list | grep $(uname -n) | sed -n '1,1p' | cut -d ' ' -f 5)

#If XAUTHORITY is set, it probably won't be a file writable by the other user
unset XAUTHORITY

sudo -Hu $OTHER_USER xauth add $DISPLAY . $XAUTH_KEY

sudo -Hu $OTHER_USER xclock

echo "Taking $OTHER_USER xauth key"
sudo -Hu $OTHER_USER xauth remove $DISPLAY