LightDM crashed when selecting "Try Ubuntu" on Ubuntu MATE

[Martin Wimpress ]

I am the maintainer of Ubuntu MATE Remix, we are currently working towards being an official Ubuntu flavour.

Since early September LightDM crashes when the option to "Try Ubuntu" is selected from the Welcome screen. LightDM was working fine upto the 28th August, if I track the timeline it appears to have started crashing since lightdm (1.11.8-0ubuntu1).

It is also worth noting that I include the following configurating in the live session `/etc/lightdm/lightdm.conf.d/90-ubuntu-mate.conf` which has the following content.

[SeatDefaults]
autologin-user=ubuntu
autologin-user-timeout=0
user-session=mate

You can download the current pre-release .iso images of Ubuntu MATE that exhibit this issue from https://ubuntu-mate.org/1369952/

ProblemType: Crash
DistroRelease: Ubuntu 14.10
Package: lightdm 1.11.9-0ubuntu1
ProcVersionSignature: Ubuntu 3.16.0-14.20-generic 3.16.2
Uname: Linux 3.16.0-14-generic x86_64
ApportVersion: 2.14.7-0ubuntu2
Architecture: amd64
CasperVersion: 1.344
Date: Tue Sep 16 09:11:18 2014
ExecutablePath: /usr/sbin/lightdm
LiveMediaBuild: Ubuntu MATE 14.10 "Utopic Unicorn" - pre-beta2.25818 amd64 (20140915)
ProcCmdline: lightdm
ProcEnviron:
 TERM=linux
 PATH=(custom, no user)
Signal: 6
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups:

[Apport retracing service (apport)]

StacktraceTop:
 __libc_message (do_abort=do_abort@entry=1, fmt=fmt@entry=0x7f0ab66fba40 "*** Error in `%s': %s: 0x%s ***\n") at ../sysdeps/posix/libc_fatal.c:175
 malloc_printerr (ptr=<optimized out>, str=0x7f0ab66fbc08 "double free or corruption (fasttop)", action=1) at malloc.c:4996
 _int_free (av=<optimized out>, p=<optimized out>, have_lock=0) at malloc.c:3840
 ?? ()
 ?? ()

[Martin Wimpress  (flexiondotorg)]

After some poking about I have discovered that LightDM no longer crashes if I change `/etc/lightdm/lightdm.conf.d/90-ubuntu-mate.conf` from:

[SeatDefaults]
autologin-user=ubuntu
autologin-user-timeout=0
user-session=mate

...to:

[SeatDefaults]
user-session=mate

That said, the original configuration was working prior to August 28th 2014 and continues to work in Ubuntu Trusty 14.04. I hope this additional information is useful.

[Robert Ancell (robert-ancell)]

Does this still occur since LightDM 1.12.1? This may be bug 1377373

[Martin Wimpress  (flexiondotorg)]

I re-tested this last night and can confirm it is now resolved in LightDM 1.12.1. Thank you!