root@ubuntu:~# echo $GDK_SYNCHRONIZE
1
root@ubuntu:~# gdb
GNU gdb (Ubuntu 8.1-0ubuntu3) 8.1.0.20180409-git
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(gdb) break gdk_x_error
No symbol table is loaded.  Use the "file" command.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (gdk_x_error) pending.
(gdb) attach 4423
Attaching to process 4423
[New LWP 4492]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007fdde30c7bf9 in __GI___poll (fds=0x336f0f0, nfds=5, timeout=-1)
    at ../sysdeps/unix/sysv/linux/poll.c:29
29	../sysdeps/unix/sysv/linux/poll.c: No such file or directory.
(gdb) continue
Continuing.
[New Thread 0x7fddc53be700 (LWP 16740)]
[New Thread 0x7fddc49aa700 (LWP 16741)]
[New Thread 0x7fddcc95c700 (LWP 16742)]
[New Thread 0x7fdd70549700 (LWP 16743)]
[New Thread 0x7fdd6fd48700 (LWP 16744)]
[New Thread 0x7fdd6f1ff700 (LWP 16749)]
[New Thread 0x7fdd6e9fe700 (LWP 16750)]
[New Thread 0x7fdd6e1fd700 (LWP 16751)]

Thread 1 "ubiquity" received signal SIGTRAP, Trace/breakpoint trap.
0x00007fdde0fa0c41 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
(gdb) backtrace
#0  0x00007fdde0fa0c41 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#1  0x00007fdde0fa3571 in g_log_writer_default ()
   from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007fdde0fa17de in g_log_structured_array ()
   from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007fdde0fa222e in g_log_structured_standard ()
   from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#4  0x00007fddddcacc31 in ?? () from /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#5  0x00007fddddcb9ab3 in ?? () from /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#6  0x00007fdddba47c8d in _XError () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#7  0x00007fdddba44bb7 in ?? () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#8  0x00007fdddba44c75 in ?? () from /usr/lib/x86_64-linux-gnu/libX11.so.6
#9  0x00007fdddba45575 in _XEventsQueued ()
   from /usr/lib/x86_64-linux-gnu/libX11.so.6
#10 0x00007fdddba37237 in XPending ()
   from /usr/lib/x86_64-linux-gnu/libX11.so.6
#11 0x00007fddddcb408e in ?? () from /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#12 0x00007fdde0f9a998 in g_main_context_prepare ()
   from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#13 0x00007fdde0f9b36b in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#14 0x00007fdde0f9b7d2 in g_main_loop_run ()
   from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#15 0x00007fddd8d62385 in gtk_main ()
---Type <return> to continue, or q <return> to quit---
  .0
#16 0x00007fdde0d4cdae in ffi_call_unix64 ()
   from /usr/lib/x86_64-linux-gnu/libffi.so.6
#17 0x00007fdde0d4c71f in ffi_call ()
   from /usr/lib/x86_64-linux-gnu/libffi.so.6
#18 0x00007fdde1718dda in ?? ()
   from /usr/lib/python3/dist-packages/gi/_gi.cpython-36m-x86_64-linux-gnu.so
#19 0x00007fdde171a9b8 in ?? ()
   from /usr/lib/python3/dist-packages/gi/_gi.cpython-36m-x86_64-linux-gnu.so
#20 0x000000000045a0e3 in _PyObject_FastCallDict ()
#21 0x000000000054fd37 in ?? ()
#22 0x00000000005546cf in _PyEval_EvalFrameDefault ()
#23 0x000000000054fbe1 in ?? ()
#24 0x000000000054fe6d in ?? ()
#25 0x00000000005546cf in _PyEval_EvalFrameDefault ()
#26 0x000000000054fbe1 in ?? ()
#27 0x000000000054fe6d in ?? ()
#28 0x0000000000552b00 in _PyEval_EvalFrameDefault ()
#29 0x000000000054f0e8 in ?? ()
#30 0x0000000000550116 in ?? ()
#31 0x00000000005546cf in _PyEval_EvalFrameDefault ()
#32 0x000000000054fbe1 in ?? ()
#33 0x0000000000550b93 in PyEval_EvalCode ()
#34 0x000000000042b519 in PyRun_FileExFlags ()
#35 0x000000000042b705 in PyRun_SimpleFileExFlags ()
#36 0x0000000000441fcb in Py_Main ()
#37 0x0000000000421ff4 in main ()
(gdb)