When NM quits it destroys all singletons including NMOvsdb, which
invokes callbacks for every pending method call. In the shutdown,
extra care must be taken to not access objects that are already in a
inconsistent state; for example here, the callback changes the device
state, and this causes an access to data that has already been
cleared:

 #0  _g_log_abort (breakpoint=breakpoint@entry=1) at gmessages.c:554
 #1  g_logv (log_domain=0x5635653b6817 "NetworkManager", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=args@entry=0x7fffb4b2c1e0) at gmessages.c:1362
 #2  g_log (log_domain=log_domain@entry=0x5635653b6817 "NetworkManager", log_level=log_level@entry=G_LOG_LEVEL_CRITICAL, format=format@entry=0x7fbb3f58fa4a "%s: assertion '%s' failed") at gmessages.c:1403
 #3  g_return_if_fail_warning (log_domain=log_domain@entry=0x5635653b6817 "NetworkManager", pretty_function=pretty_function@entry=0x5635653b6b00 <__func__.34463> "nm_device_factory_manager_find_factory_for_connection", expression=expression@entry=0x5635653b6719 "factories_by_setting") at gmessages.c:2702
 #4  nm_device_factory_manager_find_factory_for_connection (connection=connection@entry=0x56356627e0e0) at src/devices/nm-device-factory.c:243
 #5  nm_manager_get_connection_iface (self=0x563566241080 [NMManager], connection=connection@entry=0x56356627e0e0, out_parent=out_parent@entry=0x0, error=error@entry=0x0) at src/nm-manager.c:1458
 #6  check_connection_compatible (self=<optimized out>, connection=0x56356627e0e0) at src/devices/nm-device.c:4679
 #7  check_connection_compatible (device=0x56356647b1b0 [NMDeviceOvsInterface], connection=0x56356627e0e0) at src/devices/ovs/nm-device-ovs-interface.c:95
 #8  _nm_device_check_connection_available (self=0x56356647b1b0 [NMDeviceOvsInterface], connection=0x56356627e0e0, flags=NM_DEVICE_CHECK_CON_AVAILABLE_NONE, specific_object=0x0) at src/devices/nm-device.c:12102
 #9  nm_device_check_connection_available (self=self@entry=0x56356647b1b0 [NMDeviceOvsInterface], connection=0x56356627e0e0, flags=flags@entry=NM_DEVICE_CHECK_CON_AVAILABLE_NONE, specific_object=specific_object@entry=0x0) at src/devices/nm-device.c:12131
 #10 nm_device_recheck_available_connections (self=self@entry=0x56356647b1b0 [NMDeviceOvsInterface]) at src/devices/nm-device.c:12238
 #11 _set_state_full (self=self@entry=0x56356647b1b0 [NMDeviceOvsInterface], state=state@entry=NM_DEVICE_STATE_FAILED, reason=reason@entry=NM_DEVICE_STATE_REASON_OVSDB_FAILED, quitting=quitting@entry=0) at src/devices/nm-device.c:13065
 #12 nm_device_state_changed (self=self@entry=0x56356647b1b0 [NMDeviceOvsInterface], state=state@entry=NM_DEVICE_STATE_FAILED, reason=reason@entry=NM_DEVICE_STATE_REASON_OVSDB_FAILED) at src/devices/nm-device.c:13328
 #13 del_iface_cb (error=<optimized out>, user_data=0x56356647b1b0) at src/devices/ovs/nm-device-ovs-port.c:160
 #14 _transact_cb (self=self@entry=0x5635662b9ba0 [NMOvsdb], result=result@entry=0x0, error=0x563566259a10, user_data=user_data@entry=0x5635662ff320) at src/devices/ovs/nm-ovsdb.c:1449
 #15 ovsdb_disconnect (self=self@entry=0x5635662b9ba0 [NMOvsdb]) at src/devices/ovs/nm-ovsdb.c:1331
 #16 dispose (object=0x5635662b9ba0 [NMOvsdb]) at src/devices/ovs/nm-ovsdb.c:1558
 #17 g_object_unref (_object=0x5635662b9ba0) at gobject.c:3293
 #18 _nm_singleton_instance_destroy () at src/nm-core-utils.c:138
 #19 _dl_fini () at dl-fini.c:253
 #20 __run_exit_handlers (status=status@entry=0, listp=0x7fbb3e1ad6c8 <__exit_funcs>, run_list_atexit=run_list_atexit@entry=true) at exit.c:77
 #21 __GI_exit (status=status@entry=0) at exit.c:99
 #22 main (argc=1, argv=0x7fffb4b2cc38) at src/main.c:468

Add a new error code to indicate to callbacks that we are quitting and
no further action must be taken. This is preferable to having
additional references because it allows us to free the resources owned
by callbacks immediately, while references can easily create loops.

https://bugzilla.redhat.com/show_bug.cgi?id=1543871