Return UnknownDevice for assignments where device is unknown

qubes/device_protocol.py

@@ -1369,7 +1369,9 @@ def devices(self) -> List[DeviceInfo]:
     @property
     def device(self) -> DeviceInfo:
         """
-        Get single DeviceInfo object or raise an error.
+        Get single DeviceInfo object or UnknownDevice, if the device has
+        not been found.
+        If there are more devices than one matching, raise ProtocolError.
 
         If port id is set we have exactly one device
         since we can attach ony one device to one port.
@@ -1380,7 +1382,7 @@ def device(self) -> DeviceInfo:
             return devices[0]
         if len(devices) > 1:
             raise ProtocolError("Too many devices matches to assignment")
-        raise ProtocolError("No devices matches to assignment")
+        return UnknownDevice(port=self.port, device_id=self.device_id)
 
     @property
     def port(self) -> Port:

qubes/devices.py

@@ -197,8 +197,13 @@ async def attach(self, assignment: DeviceAssignment):
 
         try:
             device = assignment.device
+            if isinstance(device, UnknownDevice):
+                # assignment matches no devices
+                raise ProtocolError(
+                    f"Cannot attach unknown {assignment.devclass} device."
+                )
         except ProtocolError:
-            # assignment matches no or top many devices
+            # assignment matches too many devices
             raise ProtocolError(
                 f"Cannot attach ambiguous {assignment.devclass} device."
             )