untrusted_qdb invoked before qubesdb fully started

[qubest]

How to file a helpful issue

Qubes OS release

Qubes OS 4.2.0 (R4.2)

Brief summary

During vm start sys-gui-gpu failed to start. After checking journalctl the following error found:

vm.sys-gui-gpu: Failed to list usb devices
Traceback (most recent call last):
  File "/usr/lib/python3.11/site-packages/qubes/devices.py", line 349, in assignments
    devices = self._vm.fire_event('device-list-attached:' + self._bus,
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/qubes/events.py", line 195, in fire_event
    sync_effects, async_effects = self._fire_event(event, kwargs,
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/qubes/events.py", line 168, in _fire_event
    effects.extend(effect)
  File "/usr/lib/python3.11/site-packages/qubesusbproxy/core3ext.py", line 298, in on_device_list_attached
    for dev in self.get_all_devices(vm.app):
  File "/usr/lib/python3.11/site-packages/qubesusbproxy/core3ext.py", line 287, in get_all_devices
    for dev in vm.devices['usb']:
  File "/usr/lib/python3.11/site-packages/qubes/devices.py", line 377, in __iter__
    return iter(self.available())
                ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/qubes/devices.py", line 373, in available
    devices = self._vm.fire_event('device-list:' + self._bus)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/qubes/events.py", line 195, in fire_event
    sync_effects, async_effects = self._fire_event(event, kwargs,
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/qubes/events.py", line 168, in _fire_event
    effects.extend(effect)
  File "/usr/lib/python3.11/site-packages/qubesusbproxy/core3ext.py", line 252, in on_device_list_usb
    if not vm.is_running() or not hasattr(vm, 'untrusted_qdb'):
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/qubes/vm/qubesvm.py", line 851, in untrusted_qdb
    self._qdb_connection = qubesdb.QubesDB(self.name)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^
qubesdb.Error: (2, 'No such file or directory')

The error causes sys-gui-gpu to restart and get new xid. Some vms might start before sys-gui-gpu and uses the old qubes-gui-domain-xid. As a result, qubes-gui-agent run with unsynced qubes-gui-domain-xid.

After tracing the error untrusted_qdb is used at core3ext. which might be invoked before starting qubesdb

I tried to add a check at untrusted_qdb for the sock file /var/run/qubes/qubesdb.sys-gui-gpu.sock created by qubesdb to make sure it started before anyone tries to invoke it. After adding the check the error didn't appear again.

Steps to reproduce

Occurs randomly

Expected behavior

sys-gui-gpu starts normally and gui-agent in appvms run with correct xid

Actual behavior

sys-gui-gpu restart once which causes gui-agents in appvm to run with wrong gui-domain-xid