LightDM leaves a systemd session after login

[Andrey Vihrov]

LightDM leaves a running systemd session after the user logs in. The session is permanently in the "closing" state. Typical loginctl output:

> loginctl session-status c7
c7 - lightdm (620)
           Since: Tue 2015-04-07 23:01:15 EEST; 11min ago
          Leader: 13751
            Seat: seat0; vc1
         Display: :0
         Service: lightdm-greeter; type x11; class greeter
           State: closing
            Unit: session-c7.scope
                  |-13766 dbus-launch --autolaunch=6f5e72077d76e68732c97a0900000a8c --binary-syntax --close-stderr
                  `-13767 /usr/bin/dbus-daemon --fork --print-pid 5 --print-address 7 --session

The manual fix is to run "sudo loginctl terminate-session <session>" each time.

My component versions are:

lightdm 1.14.0
lightdm-gtk-greeter 2.0.0
systemd 218
dbus 1.8.16

[Andrew P. (kalgasnik)]

Results of small investigation.
This bug caused by deadlock deep inside X/XCB library.

1. Lightdm wants to close a greeter and sends SIGTERM to it.
2. Greeter catches that signal and calls gtk_main_quit() to stop main gtk loop.
3. There is a gdk_flush() call inside gtk_main → it calls gdk_display_sync() → XSync() → deadlock.
4. Greeter hangs.

0x00007f62c335f4ed in poll () from /usr/lib/libc.so.6
(gdb) bt
#0 0x00007f62c335f4ed in poll () from /usr/lib/libc.so.6
#1 0x00007f62c30689f2 in ?? () from /usr/lib/libxcb.so.1
#2 0x00007f62c306a2ff in ?? () from /usr/lib/libxcb.so.1
#3 0x00007f62c306a411 in xcb_wait_for_reply () from /usr/lib/libxcb.so.1
#4 0x00007f62c59bf727 in _XReply () from /usr/lib/libX11.so.6
#5 0x00007f62c59bb11d in XSync () from /usr/lib/libX11.so.6
#6 0x00007f62c4bbd13c in gdk_flush () from /usr/lib/libgdk-3.so.0
#7 0x00007f62c507069f in gtk_main () from /usr/lib/libgtk-3.so.0
#8 0x000000000040cdcc in main ()

Any call to XSync() after SIGTERM leads to deadlock.

This issue appeared after upgrading Gtk from 3.15.1 to 3.15.2.
I'm not familiar with all this X stuff, so I don't have real solution.
The simplest workaround - exit() in SIGTERM handler.

[Andrey Vihrov (andrey.vihrov)]

I think that #1445461 is not a duplicate bug. There seem to be two separate problems:

* The problem described in #1445461 happens on lock/unlock, leaves a running greeter and started happening with GTK 3.15+.
* The problem described here happens on login only, leaves only two dbus processes in the session and is reproducible with GTK 3.14 too.

[Andrew P. (kalgasnik)]

Yep, just checked it on another machine - #1441356 still present even if greeter terminates correctly.