Bug#851974: choice of "gnome classic" session sticks, and make it impossible to go back to default "gnome" session

[Stefano Zacchiroli]

Package: gdm3
Version: 3.22.1-1
Severity: important

Heya,

yesterday evening my GNOME session (the default one) didn't want to start
properly, and just left me with the screen background after login - no Shell,
no anything else.

So I've tried logging in choosing the "GNOME Classic" session (not the metacity
one, the one based on GNOME Shell, but the panel-like look & feel). It worked
properly, and still does.  The problem is I can no longer go back to the
"GNOME" session: when I choose it upon login, I still get the "GNOME Classic"
session, and I've no idea about how to go back to the default GNOME session.

Any idea of how to fix and/or further debug this?

TIA,
Cheers

PS I'm not entirely sure it's GDM's fault. It might be gnome-session,
   gnome-shell, or something else. Feel free to reassign as needed.

-- System Information:
Debian Release: 9.0
  APT prefers testing
  APT policy: (500, 'testing'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.8.0-2-amd64 (SMP w/4 CPU cores)
Locale: LANG=it_IT.utf8, LC_CTYPE=it_IT.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages gdm3 depends on:
ii  accountsservice                              0.6.43-1
ii  adduser                                      3.115
ii  awesome [x-window-manager]                   3.5.9-1
ii  dconf-cli                                    0.26.0-2
ii  dconf-gsettings-backend                      0.26.0-2
ii  debconf [debconf-2.0]                        1.5.59
ii  gir1.2-gdm-1.0                               3.22.1-1
ii  gnome-session [x-session-manager]            3.22.2-1
ii  gnome-session-bin                            3.22.2-1
ii  gnome-session-flashback [x-session-manager]  3.22.0-3
ii  gnome-settings-daemon                        3.22.1-1
ii  gnome-shell                                  3.22.2-1
ii  gnome-terminal [x-terminal-emulator]         3.22.1-1
ii  gsettings-desktop-schemas                    3.22.0-1
ii  libaccountsservice0                          0.6.43-1
ii  libaudit1                                    1:2.6.7-1
ii  libc6                                        2.24-8
ii  libcanberra-gtk3-0                           0.30-3
ii  libcanberra0                                 0.30-3
ii  libgdk-pixbuf2.0-0                           2.36.3-1
ii  libgdm1                                      3.22.1-1
ii  libglib2.0-0                                 2.50.2-2
ii  libglib2.0-bin                               2.50.2-2
ii  libgtk-3-0                                   3.22.6-1
ii  libkeyutils1                                 1.5.9-9
ii  libpam-modules                               1.1.8-3.5
ii  libpam-runtime                               1.1.8-3.5
ii  libpam-systemd                               232-8
ii  libpam0g                                     1.1.8-3.5
ii  librsvg2-common                              2.40.16-1
ii  libselinux1                                  2.6-3
ii  libsystemd0                                  232-8
ii  libwrap0                                     7.6.q-26
ii  libx11-6                                     2:1.6.4-2
ii  libxau6                                      1:1.0.8-1
ii  libxdmcp6                                    1:1.1.2-1.1
ii  lsb-base                                     9.20161125
ii  metacity [x-window-manager]                  1:3.22.1-1
ii  mutter [x-window-manager]                    3.22.2-3
ii  policykit-1                                  0.105-17
ii  ucf                                          3.0036
ii  x11-common                                   1:7.7+18
ii  x11-xserver-utils                            7.7+7
ii  xterm [x-terminal-emulator]                  327-2

Versions of packages gdm3 recommends:
ii  at-spi2-core    2.22.0-5
ii  desktop-base    9.0.0
ii  x11-xkb-utils   7.7+3
ii  xserver-xephyr  2:1.19.0-3
ii  xserver-xorg    1:7.7+18
ii  zenity          3.22.0-1

Versions of packages gdm3 suggests:
ii  gnome-orca            3.22.2-1
ii  libpam-gnome-keyring  3.20.0-3

-- Configuration Files:
/etc/gdm3/daemon.conf changed:
[daemon]
WaylandEnable=false
[security]
[xdmcp]
[greeter]
[chooser]
[debug]


-- debconf information:
  gdm3/daemon_name: /usr/sbin/gdm3
* shared/default-x-display-manager: gdm3