feat(instances): a VM's RNG device and crash action in its options

Two settings the owner chose to add to Settings > Options after the
page's review with Codex. Both change the persistent definition, so
they are saved while the VM runs and used from its next start.

- Random number generator: a switch for a virtio RNG device fed from
  the host's /dev/urandom, so a new VM does not wait for entropy at
  boot. Another RNG device, or the panel's with a rate or driver, is
  shown as the VM's own and left to the XML editor.
- When the guest crashes: Restart or Stop (on_crash restart or
  destroy); libvirt's default, Stop, when the definition has none.
  Stop keeps a VM that crashes again and again down for a look. Any
  other action is shown and left to the XML editor.

vrtManager/instance.py

@@ -83,6 +83,7 @@ PERSISTENT_XML = VIR_DOMAIN_XML_INACTIVE | VIR_DOMAIN_XML_SECURE
 USB_HOSTDEVS = "devices/hostdev[@mode='subsystem'][@type='usb']"
 PCI_HOSTDEVS = "devices/hostdev[@mode='subsystem'][@type='pci']"
 # what of the host a VM holds: host devices, and NICs that are host devices
+RNG_DEVICE = "<rng model='virtio'><backend model='random'>/dev/urandom</backend></rng>"
 GUEST_AGENT_CHANNEL = "/domain/devices/channel[@type='unix']/target[@name='org.qemu.guest_agent.0']"
 HOST_DEVICES = "devices/hostdev | devices/interface[@type='hostdev'] | devices/interface/actual[@type='hostdev']"
 
@@ -2866,6 +2867,46 @@ class wvmInstance(wvmConnect):
                     option = etree.SubElement(tree, o)
                 option.text = option_value
 
+    def get_rng(self):
+        """The persistent definition's RNG device: "virtio" for the panel's
+        own (virtio, from the host's /dev/urandom), "custom" for another one,
+        None without one."""
+        rngs = etree.fromstring(self._XMLDesc(PERSISTENT_XML)).xpath("devices/rng")
+        if not rngs:
+            return None
+        # libvirt adds only its address and alias to the panel's device; a rate or driver makes it another one
+        own = "@model='virtio' and backend[@model='random']='/dev/urandom' and not(*[not(self::backend or self::address or self::alias)])"
+        if len(rngs) == 1 and rngs[0].xpath(own):
+            return "virtio"
+        return "custom"
+
+    def set_rng(self, enabled):
+        """Adds the panel's virtio RNG device to the persistent definition, or
+        removes it; another RNG device is left to the XML editor."""
+        if self.get_rng() == "custom":
+            raise util.OperationError("The VM has its own RNG device: change it in the XML editor")
+        tree = etree.fromstring(self._XMLDesc(PERSISTENT_XML))
+        for rng in tree.xpath("devices/rng"):
+            rng.getparent().remove(rng)
+        if enabled:
+            tree.find("devices").append(etree.fromstring(RNG_DEVICE))
+        self._defineXML(etree.tostring(tree).decode())
+
+    def get_on_crash(self):
+        """What the next start does when the guest crashes; libvirt's default is destroy."""
+        return etree.fromstring(self._XMLDesc(PERSISTENT_XML)).findtext("on_crash") or "destroy"
+
+    def set_on_crash(self, action):
+        """restart or destroy (stop) the VM when the guest crashes, from the next start."""
+        if action not in ("restart", "destroy"):
+            raise util.OperationError(f"Unknown crash action {action}")
+        tree = etree.fromstring(self._XMLDesc(PERSISTENT_XML))
+        on_crash = tree.find("on_crash")
+        if on_crash is None:
+            on_crash = etree.SubElement(tree, "on_crash")
+        on_crash.text = action
+        self._defineXML(etree.tostring(tree).decode())
+
     def set_options(self, options):
         """
         Function change description, title

vrtManager/tests/test_device_edits.py

@@ -134,6 +134,42 @@ class DeviceEditsTestCase(unittest.TestCase):
         self.assertIsNotNone(p.get_guest_agent(config=True))
         self.assertEqual([v["hotpluggable"] for v in p.get_vcpus(config=True).values()], ["no", "yes"])
 
+    def test_the_rng_device_is_the_panel_s_own_or_left_to_the_xml_editor(self):
+        p = proxy()
+        self.assertIsNone(p.get_rng())
+        p.set_rng(True)
+        self.assertEqual(etree.tostring(defined(p).find("devices/rng")).decode(),
+                         "<rng model=\"virtio\"><backend model=\"random\">/dev/urandom</backend></rng>")
+        added = etree.tostring(defined(p)).decode()
+        p._XMLDesc = lambda flags: added
+        self.assertEqual(p.get_rng(), "virtio")
+        p.set_rng(False)
+        self.assertIsNone(defined(p).find("devices/rng"))
+        # the panel's device with the address libvirt gives it; another RNG device, or the panel's with
+        # a rate or driver, is shown as custom and changed only in the XML editor
+        backend = "<backend model='random'>{}</backend>"
+        for inner, found in ((backend.format("/dev/urandom") + "<address type='pci' bus='0x08' slot='0x00'/>", "virtio"),
+                             (backend.format("/dev/hwrng"), "custom"),
+                             (backend.format("/dev/urandom") + "<rate bytes='1024' period='1000'/>", "custom"),
+                             (backend.format("/dev/urandom") + "<driver iommu='on'/>", "custom")):
+            with self.subTest(inner):
+                p = proxy()
+                p._XMLDesc = lambda flags, inner=inner: PERSISTENT.replace("<video>", f"<rng model='virtio'>{inner}</rng><video>")
+                self.assertEqual(p.get_rng(), found)
+                if found == "custom":
+                    with self.assertRaises(util.OperationError):
+                        p.set_rng(False)
+                    p._defineXML.assert_not_called()
+
+    def test_the_crash_action_is_restart_or_stop(self):
+        p = proxy()
+        self.assertEqual(p.get_on_crash(), "destroy")  # libvirt's default
+        p.set_on_crash("restart")
+        self.assertEqual(defined(p).findtext("on_crash"), "restart")
+        with self.assertRaises(util.OperationError):
+            p.set_on_crash("preserve")
+        p._defineXML.assert_called_once()
+
     def test_cpu_title_and_description_are_read_from_either_definition(self):
         p = proxy()
         self.assertEqual((p.get_vcpu(), p.get_cur_vcpu(), p.get_title(), p.get_description()), (1, 1, "", ""))