[Bug 1779667] Re: Unlocking existing session yields black screen. Closing session does not help as affected user cannot open a successful X session until next reboot. Other users are ok.

[Simon McVittie]

Please attach anything vaguely relevant-looking from the system log
(systemd Journal if you use systemd, or /var/log/syslog). The bug will
probably be somewhere in the vicinity of logind, PAM, dbus-daemon,
lightdm, X or XFCE, or possibly graphics drivers in the kernel (which
have been known to lock up during fast-user-switching), but without
looking at the log it's impossible to say which.

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https://bugs.launchpad.net/bugs/1779667

Title:
  Unlocking existing session yields black screen. Closing session does
  not help as affected user cannot open a successful X session until
  next reboot. Other users are ok.

Status in dbus package in Ubuntu:
  New

Bug description:
  # Context

  * Xubuntu 18.04 using lightdm, no specific customization, no xscreensaver/gnome-screensaver.
  * Problem observed two machines:
  ** one machine with several users often swapping sessions,
  ** one machine with just one regular user who just locks/unlocks session often

  
  # Action

  * Open a XFCE session.
  * Lock it.
  * On greeter, type password to unlock.

  
  # Expected

  * Get back to opened session.

  
  # Observed

  * After typing password, screen turns to black.  All pixels are fully black.
  * Not even a mouse pointer is visible.

  
  # Reproducible

  * Seems random.
  * Perhaps one time out of 20-50 unlock operations.
  * Never occurred in 16.04 14.04

  
  # Detailed information

  From start:

  * On unlocking an already opened session, after typing password screen
  turns to full black.

  
  ## Investigation: is machine still okay?

  * As a sanity check, switching to text VT works, login on text VT
  works.

  
  ## Investigation: problem does not affect all sessions at once

  * Bug observed both in one system with one user at a time, and on another system with two users alternating X sessions.
  * From tty1, using "dm-tool switch-to-greeter" with proper environment works, allows to select other user's session, type password and return to that other user's opened session. Any attempt to return to affected user session yields black screen.

  
  ## Investigation: problem affects user even after closing all their programs

  * Manually killing affected user processes allows to close faulty session and see lightdm greeter come back.
  * Trying to reopen a session (as usual with lightdm) as affected user shows an error dialog:

  "Unable to contact settings server
  Failed to connect to socket /run/user/1000/bus: Connection refused"
  and a "Close" button.

  Clicking on it yields a second dialog :

  "Unable to load failsafe session
  Unable to determine a failsafe session name. Possible causes:
  xfconfd isn't running (D-Bus setup problem);
  environment variable $XDG_CONFIG_DIRS is set incorrectly (must include "/etc"),
  or xfce4-session is installed incorrectly."
  and a "Quit" button.

  Clicking "quit" button goes back to lightdm greeter.

  
  ## Investigation: reboot solves the problem

  * Rebooting the machine solves the problem (but closes all user sessions and processes).
  * Problem reappears after a few days.

  
  ## Investigation: startx affected, too

  * From text VT, startx has same behavior as running from lightdm and
  thus also cannot reach a working XFCE desktop.

  
  ## Investigation: affected user has no dbus-daemon running.

  * No "/usr/bin/dbus-daemon" runs as affected user while it runs as
  other users.

  
  ## Additional information

  * It is believed that the multi-user system makes the bug more salient than usual workflow because users switch more often from one session to another.
  * It is believed that the single-user system makes the bug more salient than usual workflow because users often locks and unlocks their session.


  
  1) The release of Ubuntu you are using, via 'lsb_release -rd' or System -> About Ubuntu

  Description:	Ubuntu 18.04 LTS
  Release:	18.04

  Actually Xubuntu 18.04.

  2) The version of the package you are using, via 'apt-cache policy
  pkgname' or by checking in Software Center

  export LC_ALL=C ; apt-cache policy dbus

  dbus:
    Installed: 1.12.2-1ubuntu1
    Candidate: 1.12.2-1ubuntu1
    Version table:
   *** 1.12.2-1ubuntu1 500
          500 http://fr.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
          100 /var/lib/dpkg/status

  
  3) What you expected to happen
  4) What happened instead

  Described above.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: dbus 1.12.2-1ubuntu1
  ProcVersionSignature: Ubuntu 4.15.0-23.25-generic 4.15.18
  Uname: Linux 4.15.0-23-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.2
  Architecture: amd64
  Date: Mon Jul  2 13:43:08 2018
  InstallationDate: Installed on 2018-05-25 (38 days ago)
  InstallationMedia: Xubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   LANG=fr_FR.UTF-8
   SHELL=/bin/bash
  SourcePackage: dbus
  UpgradeStatus: No upgrade log present (probably fresh install)

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