devices: implement DeviceCollection.update_persistent()

Allow attached device to be converted from persistent to non-persistent
and the other way around.
This is to allow starting a VM with some device attached temporarily.
When VM is not running, it is possible to attach device only
persistently, so this change will allow to do that, then, after starting
the VM, change it to non-persistent - so it will not be attached again
at further startups.

QubesOS/qubes-issues#3055

qubes/devices.py

@@ -270,6 +270,27 @@ class DeviceCollection(object):
         device_assignment.bus = self._bus
         self._set.add(device_assignment)
 
+    def update_persistent(self, device: DeviceInfo, persistent: bool):
+        '''Update `persistent` flag of already attached device.
+        '''
+
+        if self._vm.is_halted():
+            raise qubes.exc.QubesVMNotStartedError(self._vm,
+                'VM must be running to modify device persistence flag')
+        assignments = [a for a in self.assignments() if a.device == device]
+        if not assignments:
+            raise qubes.exc.QubesValueError('Device not assigned')
+        assert len(assignments) == 1
+        assignment = assignments[0]
+
+        # be careful to use already present assignment, not the provided one
+        # - to not change options as a side effect
+        if persistent and device not in self._set:
+            assignment.persistent = True
+            self._set.add(assignment)
+        elif not persistent and device in self._set:
+            self._set.discard(assignment)
+
     @asyncio.coroutine
     def detach(self, device_assignment: DeviceAssignment):
         '''Detach (remove) device from domain.

qubes/tests/devices.py

@@ -71,6 +71,9 @@ class TestVM(qubes.tests.TestEmitter):
     def is_halted(self):
         return not self.running
 
+    def is_running(self):
+        return self.running
+
 
 
 class TC_00_DeviceCollection(qubes.tests.QubesTestCase):
@@ -130,8 +133,6 @@ class TC_00_DeviceCollection(qubes.tests.QubesTestCase):
         self.loop.run_until_complete(self.collection.attach(self.assignment))
         self.assertEventFired(self.emitter, 'device-list-attached:testclass')
         self.assertEqual({self.device}, set(self.collection.persistent()))
-        self.assertEqual({self.device},
-            set(self.collection.persistent()))
         self.assertEqual(set([]),
             set(self.collection.attached()))
 
@@ -153,6 +154,52 @@ class TC_00_DeviceCollection(qubes.tests.QubesTestCase):
         self.assertEqual({self.device}, set(self.collection))
         self.assertEventFired(self.emitter, 'device-list:testclass')
 
+    def test_020_update_persistent_to_false(self):
+        self.emitter.running = True
+        self.assertEqual(set([]), set(self.collection.persistent()))
+        self.loop.run_until_complete(self.collection.attach(self.assignment))
+        # device-attach event not implemented, so manipulate object manually
+        self.device.frontend_domain = self.emitter
+        self.assertEqual({self.device}, set(self.collection.persistent()))
+        self.assertEqual({self.device}, set(self.collection.attached()))
+        self.assertEqual({self.device}, set(self.collection.persistent()))
+        self.assertEqual({self.device}, set(self.collection.attached()))
+        self.collection.update_persistent(self.device, False)
+        self.assertEqual(set(), set(self.collection.persistent()))
+        self.assertEqual({self.device}, set(self.collection.attached()))
+
+    def test_021_update_persistent_to_true(self):
+        self.assignment.persistent = False
+        self.emitter.running = True
+        self.assertEqual(set([]), set(self.collection.persistent()))
+        self.loop.run_until_complete(self.collection.attach(self.assignment))
+        # device-attach event not implemented, so manipulate object manually
+        self.device.frontend_domain = self.emitter
+        self.assertEqual(set(), set(self.collection.persistent()))
+        self.assertEqual({self.device}, set(self.collection.attached()))
+        self.assertEqual(set(), set(self.collection.persistent()))
+        self.assertEqual({self.device}, set(self.collection.attached()))
+        self.collection.update_persistent(self.device, True)
+        self.assertEqual({self.device}, set(self.collection.persistent()))
+        self.assertEqual({self.device}, set(self.collection.attached()))
+
+    def test_022_update_persistent_reject_not_running(self):
+        self.assertEqual(set([]), set(self.collection.persistent()))
+        self.loop.run_until_complete(self.collection.attach(self.assignment))
+        self.assertEqual({self.device}, set(self.collection.persistent()))
+        self.assertEqual(set(), set(self.collection.attached()))
+        with self.assertRaises(qubes.exc.QubesVMNotStartedError):
+            self.collection.update_persistent(self.device, False)
+
+    def test_023_update_persistent_reject_not_attached(self):
+        self.assertEqual(set(), set(self.collection.persistent()))
+        self.assertEqual(set(), set(self.collection.attached()))
+        self.emitter.running = True
+        with self.assertRaises(qubes.exc.QubesValueError):
+            self.collection.update_persistent(self.device, True)
+        with self.assertRaises(qubes.exc.QubesValueError):
+            self.collection.update_persistent(self.device, False)
+
 
 class TC_01_DeviceManager(qubes.tests.QubesTestCase):
     def setUp(self):