Cannot autologin when X crashes

hi all,

I find that LightDM cannot autologin my user when X crashes or when I kill X process, even if I configure /etc/lightdm/lightdm.conf with autologin options.

My /etc/lightdm/lightdm.conf is as follows:
[SeatDefaults]
allow-guest=false
autologin-guest=false
autologin-user=mytestuser
autologin-user-timeout=0
autologin-session=lightdm-autologin

There are only 2 users in my system: root, mytestuser.

When I "sudo kill xxxx" to kill X process, LightDM just shows a dialog to ask for username and password, while I expect an autologin.

/var/log/lightdm/lightdm.log says:
[+0.03s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.04s] DEBUG: Starting Light Display Manager 1.10.0, UID=0 PID=3505
[+0.04s] DEBUG: Loading configuration dirs from /usr/share/lightdm/lightdm.conf.d
[+0.04s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
[+0.04s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
[+0.04s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
[+0.04s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf
[+0.04s] DEBUG: Loading configuration dirs from /usr/local/share/lightdm/lightdm.conf.d
[+0.04s] DEBUG: Loading configuration dirs from /etc/xdg/lightdm/lightdm.conf.d
[+0.04s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf.d/10-xfce.conf
[+0.04s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf
[+0.04s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.04s] DEBUG: Registered seat module xlocal
[+0.04s] DEBUG: Registered seat module xremote
[+0.04s] DEBUG: Registered seat module unity
[+0.04s] DEBUG: Registered seat module surfaceflinger
[+0.34s] DEBUG: Adding default seat
[+0.34s] DEBUG: Seat: Starting
[+0.34s] DEBUG: Seat: Creating user session
[+0.99s] DEBUG: Loading users from org.freedesktop.Accounts
[+1.08s] DEBUG: Seat: Creating display server of type x
[+1.08s] DEBUG: Deactivating Plymouth
[+1.11s] DEBUG: Using VT 7
[+1.11s] DEBUG: Seat: Starting local X display on VT 7
[+1.11s] DEBUG: DisplayServer x-0: Logging to /var/log/lightdm/x-0.log
[+1.11s] DEBUG: DisplayServer x-0: Writing X server authority to /var/run/lightdm/root/:0
[+1.11s] DEBUG: DisplayServer x-0: Launching X Server
[+1.11s] DEBUG: Launching process 3713: /usr/bin/X -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+1.11s] DEBUG: DisplayServer x-0: Waiting for ready signal from X server :0
[+1.11s] DEBUG: User /org/freedesktop/Accounts/User999 added
[+1.23s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+1.23s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+6.88s] DEBUG: Got signal 10 from process 3713
[+6.88s] DEBUG: DisplayServer x-0: Got signal from X server :0
[+6.88s] DEBUG: DisplayServer x-0: Connecting to XServer :0
[+6.88s] DEBUG: Quitting Plymouth; retaining splash
[+6.91s] DEBUG: Seat: Display server ready, starting session authentication
[+6.91s] DEBUG: Session pid=4246: Started with service 'lightdm-autologin', username 'mytestuser'
[+6.93s] DEBUG: Session pid=4246: Authentication complete with return value 0: Success
[+6.93s] DEBUG: Seat: Session authenticated, running command
[+6.93s] DEBUG: Registering session with bus path /org/freedesktop/DisplayManager/Session0
[+6.94s] DEBUG: Session pid=4246: Running command /usr/sbin/lightdm-session startxfce4
[+6.94s] DEBUG: Creating shared data directory /var/lib/lightdm-data/mytestuser
[+6.94s] DEBUG: Session pid=4246: Logging to .xsession-errors
[+6.94s] DEBUG: Activating VT 7
[+6.94s] DEBUG: Activating login1 session /org/freedesktop/login1/session/c6
[+191.16s] DEBUG: Session pid=4246: Exited with return value 0
[+191.16s] DEBUG: Seat: Session stopped
[+191.16s] DEBUG: Seat: Stopping display server, no sessions require it
[+191.16s] DEBUG: Sending signal 15 to process 3713
[+191.36s] DEBUG: Process 3713 exited with return value 0
[+191.36s] DEBUG: DisplayServer x-0: X server stopped
[+191.36s] DEBUG: Releasing VT 7
[+191.36s] DEBUG: DisplayServer x-0: Removing X server authority /var/run/lightdm/root/:0
[+191.36s] DEBUG: Seat: Display server stopped
[+191.36s] DEBUG: Seat: Active display server stopped, starting greeter
[+191.36s] DEBUG: Seat: Creating greeter session
[+191.37s] DEBUG: Seat: Creating display server of type x
[+191.37s] DEBUG: Using VT 7
[+191.37s] DEBUG: Seat: Starting local X display on VT 7
[+191.37s] DEBUG: DisplayServer x-0: Logging to /var/log/lightdm/x-0.log
[+191.37s] DEBUG: DisplayServer x-0: Writing X server authority to /var/run/lightdm/root/:0
[+191.37s] DEBUG: DisplayServer x-0: Launching X Server
[+191.37s] DEBUG: Launching process 9629: /usr/bin/X -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+191.37s] DEBUG: DisplayServer x-0: Waiting for ready signal from X server :0
[+191.43s] DEBUG: Got signal 10 from process 9629
[+191.43s] DEBUG: DisplayServer x-0: Got signal from X server :0
[+191.43s] DEBUG: DisplayServer x-0: Connecting to XServer :0
[+191.43s] DEBUG: Seat: Display server ready, starting session authentication
[+191.43s] DEBUG: Session pid=9646: Started with service 'lightdm-greeter', username 'lightdm'
[+191.45s] DEBUG: Session pid=9646: Authentication complete with return value 0: Success
[+191.45s] DEBUG: Seat: Session authenticated, running command
[+191.45s] DEBUG: Session pid=9646: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/lightdm-gtk-greeter
[+191.45s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+191.45s] DEBUG: Session pid=9646: Logging to /var/log/lightdm/x-0-greeter.log
[+191.53s] DEBUG: Activating VT 7
[+191.53s] DEBUG: Activating login1 session /org/freedesktop/login1/session/c8
[+191.69s] DEBUG: Session pid=9646: Greeter connected version=1.10.0
[+193.10s] DEBUG: Session pid=9646: Greeter start authentication
[+193.10s] DEBUG: Session pid=9701: Started with service 'lightdm', username '(null)'
[+193.11s] DEBUG: Session pid=9701: Got 1 message(s) from PAM
[+193.11s] DEBUG: Session pid=9646: Prompt greeter with 1 message(s)
[+195.42s] DEBUG: Session pid=9646: Greeter start authentication for mytestuser
[+195.42s] DEBUG: Session pid=9701: Sending SIGTERM
[+195.42s] DEBUG: Session pid=9716: Started with service 'lightdm', username 'mytestuser'
[+195.42s] DEBUG: Session pid=9701: Terminated with signal 15
[+195.42s] DEBUG: Session: Failed during authentication
[+195.42s] DEBUG: Seat: Session stopped
[+195.43s] DEBUG: Session pid=9716: Got 1 message(s) from PAM
[+195.43s] DEBUG: Session pid=9646: Prompt greeter with 1 message(s)
[+298.05s] DEBUG: Session pid=9646: Continue authentication
[+298.06s] DEBUG: Session pid=9716: Authentication complete with return value 0: Success
[+298.06s] DEBUG: Session pid=9646: Authenticate result for user mytestuser: Success
[+298.06s] DEBUG: Session pid=9646: User mytestuser authorized
[+298.06s] DEBUG: Session pid=9646: Greeter sets language zh_CN
[+298.07s] CRITICAL: g_dbus_connection_call_sync_internal: assertion 'object_path != NULL && g_variant_is_object_path (object_path)' failed
[+298.07s] DEBUG: Writing /home/mytestuser/.dmrc
[+298.12s] DEBUG: Session pid=9646: Greeter requests session xfce
[+298.12s] CRITICAL: g_dbus_connection_call_sync_internal: assertion 'object_path != NULL && g_variant_is_object_path (object_path)' failed
[+298.12s] DEBUG: Writing /home/mytestuser/.dmrc
[+298.27s] DEBUG: Seat: Stopping greeter; display server will be re-used for user session
[+298.27s] DEBUG: Session pid=9646: Sending SIGTERM
[+298.32s] DEBUG: Session pid=9646: Greeter closed communication channel
[+298.32s] DEBUG: Session pid=9646: Exited with return value 0
[+298.32s] DEBUG: Seat: Session stopped
[+298.32s] DEBUG: Seat: Greeter stopped, running session
[+298.32s] DEBUG: Registering session with bus path /org/freedesktop/DisplayManager/Session1
[+298.33s] DEBUG: Session pid=9716: Running command /usr/sbin/lightdm-session startxfce4
[+298.33s] DEBUG: Creating shared data directory /var/lib/lightdm-data/mytestuser
[+298.33s] DEBUG: Session pid=9716: Logging to .xsession-errors
[+298.38s] DEBUG: Activating VT 7
[+298.38s] DEBUG: Activating login1 session /org/freedesktop/login1/session/c9

Let me explain this log.
1. [+0.03s]: On system boot, the first LightDM login: it's an autologin action as configured by lightdm.conf.
2. [+6.94s]: End of the first LightDM login, starts a session on VT 7.
3. [+191.16s]: I killed X process to emulate X crashing. LightDM detected this, and restarted a new greeter. Now a dialog showed and asked for username and password. I did not input username nor password now.
4. [+298.05s]: I input username and password, and hitted Login button.

So my question is: how to do autologin when X crashes?

Thx!