Xorg guest session fails to start if the user has logged out and logged in again

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

If you search for "VT" in the Xorg.1.log you can see it attempts to start Xorg on VT 7... which is the VT where :0 is running. When everything works correctly, :1 should be started on VT 8, and the log confirms it.

This is visible on the lightdm log too. Here it is, trying to start :1 on VT7, which is already in use by :0

[+101.59s] DEBUG: Seat: Starting local X display
[+101.59s] DEBUG: Using VT 7
[+101.59s] DEBUG: DisplayServer x-1: Logging to /var/log/lightdm/x-1.log
[+101.59s] DEBUG: DisplayServer x-1: Writing X server authority to /var/run/lightdm/root/:1
[+101.59s] DEBUG: DisplayServer x-1: Launching X Server
[+101.59s] DEBUG: Launching process 2773: /usr/bin/X -core :1 -auth /var/run/lightdm/root/:1 -nolisten tcp vt7 -novtswitch
[+101.59s] DEBUG: DisplayServer x-1: Waiting for ready signal from X server :1
[+107.81s] DEBUG: Process 2773 terminated with signal 6
[+107.81s] DEBUG: DisplayServer x-1: X server stopped
[+107.81s] DEBUG: Releasing VT 7
[+107.81s] DEBUG: DisplayServer x-1: Removing X server authority /var/run/lightdm/root/:1

I now know exactly what causes this bug. The problem is that the VT gets unref'd once when the server shuts down, and then unref'd again much later when the object gets finalized. This might happen after the VT has already been allocated and ref'd to another server. This makes lightdm think the VT is free, when it isn't.

This is an annotated log, with extra debugging code added:

*** I've logged in (once) and now I log out for the first time.

[+34.12s] DEBUG: Session pid=13236: Exited with return value 0
[+34.12s] DEBUG: Seat: Session stopped
[+34.12s] DEBUG: Seat: Stopping display server, no sessions require it
[+34.12s] DEBUG: Sending signal 15 to process 13176
[+34.47s] DEBUG: Process 13176 exited with return value 0
[+34.47s] DEBUG: DisplayServer x-0: X server stopped :0, pid was 0
[+34.47s] DEBUG: Releasing VT 7

*** The display server shuts down and the VT is unref'd.

[+34.47s] DEBUG: DisplayServer x-0: Removing X server authority /var/run/lightdm/root/:0
[+34.47s] DEBUG: Seat: Display server stopped
[+34.47s] DEBUG: Seat: Active display server stopped, starting greeter
[+34.47s] DEBUG: Seat: Creating greeter session
[+34.47s] DEBUG: Seat: Setting XDG_SEAT=seat0
[+34.47s] DEBUG: Seat: Creating display server of type x
[+34.47s] DEBUG: Seat: Starting local X display

*** New server starts for the login screen

[+34.47s] DEBUG: DisplayServer x-0: Set VT of server:0 to 7

*** It uses VT7 because it's unref'd

[+34.47s] DEBUG: Using VT 7

*** This causes VT7 to get ref'd.

[+34.47s] DEBUG: DisplayServer x-0: Logging to /var/log/lightdm/x-0.log
[+34.47s] DEBUG: DisplayServer x-0: Writing X server authority to /var/run/lightdm/root/:0
[+34.47s] DEBUG: DisplayServer x-0: Launching X Server
[+34.47s] DEBUG: Launching process 13592: /usr/bin/X -core :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+34.47s] DEBUG: DisplayServer x-0: Waiting for ready signal from X server :0, pid=13592

*** New server launches.

[+34.47s] DEBUG: DisplayServer x-0: In finalize (null): vt=7
[+34.48s] DEBUG: Releasing VT 7

*** The old x-server-local object gets finalized, which causes VT7 to get unref'd for a second time. The next server that starts up will start on VT7, which is really currently already used.

The fix from the linked branch does work for me, no more screen corruption after relogging once and then locking the screen via lightdm (light-locker).

Fix committed into lp:lightdm at revision None, scheduled for release in lightdm, milestone Unknown

This bug was fixed in the package lightdm - 1.9.4-0ubuntu1

---------------
lightdm (1.9.4-0ubuntu1) trusty; urgency=low

  * New upstream release:
    - Fix issue where VTs are double used when switching sessions.
      (LP: #1256150)
    - Remove lightdm-set-defaults and gdmflexiserver.
    - Add new ability to specify a list of seat types to try, rather than just
      one.
    - Allow Mir sessions in the surfaceflinger seat.
    - Rename the guest session wrapper to have a simpler name.
    - Make sure sessions are associated with the display server before starting
      them.
    - Add a dm-tool man page.
 -- Robert Ancell <email address hidden> Tue, 03 Dec 2013 19:07:28 +1300

Hello Alistair, or anyone else affected,

Accepted lightdm into saucy-proposed. The package will build now and be available at http://launchpad.net/ubuntu/+source/lightdm/1.8.6-0ubuntu1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, and change the tag from verification-needed to verification-done. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed. In either case, details of your testing will help us make a better decision.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance!

Possible duplicate of 1248948.

saucy has seen the end of its life and is no longer receiving any updates. Marking the saucy task for this ticket as "Won't Fix".