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.

instances/models.py

@@ -275,6 +275,14 @@ class Instance(models.Model):
         """Whether the next start has hotpluggable vCPUs; the hot plug form shows it."""
         return any(vcpu["hotpluggable"] == "yes" for vcpu in self.proxy.get_vcpus(config=True).values())
 
+    @cached_property
+    def config_rng(self) -> str | None:
+        return self.proxy.get_rng()
+
+    @cached_property
+    def config_on_crash(self) -> str:
+        return self.proxy.get_on_crash()
+
     @cached_property
     def guest_agent_ready(self) -> bool:
         return self.proxy.is_agent_ready()

instances/templates/instances/settings_tab.html

@@ -870,6 +870,49 @@
                         </div>
                     </div>
                 </form>
+                <form class="list-group-item p-3 p-md-4" action="{% url 'instances:set_rng' instance.id %}" method="post">{% csrf_token %}
+                    <div class="row g-3 align-items-center">
+                        <div class="col-12 col-lg-4">
+                            <label class="form-label fw-semibold mb-0" for="rng">{% trans "Random number generator" %}</label>
+                            <p class="small text-body-secondary mb-0" id="rng-help">{% trans "A virtio device that feeds the guest's entropy from the host: a new VM does not wait for it at boot. From the next start." %}</p>
+                        </div>
+                        <div class="col-12 col-lg-8">
+                            <div class="form-check form-switch">
+                                <input class="form-check-input" type="checkbox" role="switch" name="rng" value="True" id="rng" aria-describedby="rng-help rng-state"
+                                       {% if instance.config_rng %}checked{% endif %}{% if instance.config_rng == "custom" %} disabled{% endif %} onchange="this.form.submit()">
+                                <label class="form-check-label" for="rng">{% trans "Virtio RNG" %}</label>
+                            </div>
+                            {% if instance.config_rng == "custom" %}
+                            <p class="small text-body-secondary mt-1 mb-0" id="rng-state">{% trans "The VM has its own RNG device: change it in the XML editor." %}</p>
+                            {% endif %}
+                        </div>
+                    </div>
+                </form>
+                <form class="list-group-item p-3 p-md-4" action="{% url 'instances:set_on_crash' instance.id %}" method="post">{% csrf_token %}
+                    <div class="row g-3 align-items-center">
+                        <div class="col-12 col-lg-4">
+                            <label class="form-label fw-semibold mb-0" for="on_crash">{% trans "When the guest crashes" %}</label>
+                            <p class="small text-body-secondary mb-0" id="on-crash-help">{% trans "For a crash the guest reports, not a hang. Stop keeps a VM that crashes again and again down for a look. From the next start." %}</p>
+                        </div>
+                        <div class="col-12 col-lg-8">
+                            {% if instance.config_on_crash == "restart" or instance.config_on_crash == "destroy" %}
+                            <div class="row g-2">
+                                <div class="col-12 col-sm">
+                                    <select id="on_crash" class="form-select" name="on_crash" aria-describedby="on-crash-help">
+                                        <option value="restart"{% if instance.config_on_crash == "restart" %} selected{% endif %}>{% trans "Restart" %}</option>
+                                        <option value="destroy"{% if instance.config_on_crash == "destroy" %} selected{% endif %}>{% trans "Stop" %}</option>
+                                    </select>
+                                </div>
+                                <div class="col-12 col-sm-auto d-grid">
+                                    <button class="btn btn-secondary" type="submit" name="set_on_crash">{% trans "Save" %}</button>
+                                </div>
+                            </div>
+                            {% else %}
+                            <p class="small mb-0" id="on_crash">{% blocktrans with action=instance.config_on_crash %}{{ action }}: change it in the XML editor.{% endblocktrans %}</p>
+                            {% endif %}
+                        </div>
+                    </div>
+                </form>
                 <form class="list-group-item p-3 p-md-4" action="{% url 'instances:set_guest_agent' instance.id %}" method="post">{% csrf_token %}
                     <div class="row g-3 align-items-center">
                         <div class="col-12 col-lg-4">

instances/tests/test_live_xml_effects.py

@@ -125,6 +125,16 @@ EDITS = {
         lambda t: first(t, "devices/video/model/@type") == "virtio",
         ["devices/video"], True, False,
     ),
+    "set_rng": (
+        lambda vm: vm.set_rng(False),
+        lambda t: not t.xpath("devices/rng"),
+        ["devices/rng"], True, False,
+    ),
+    "set_on_crash": (
+        lambda vm: vm.set_on_crash("restart"),
+        lambda t: t.findtext("on_crash") == "restart",
+        ["on_crash"], True, False,
+    ),
     "resize_cpu": (
         lambda vm: vm.resize_cpu("1", "2"),
         lambda t: t.findtext("vcpu") == "2" and first(t, "vcpu/@current") == "1",

instances/tests/test_pending_edits.py

@@ -141,6 +141,15 @@ class PendingEditsTestCase(TestCase):
         self.assertEqual(doc.xpath("//select[@name='vcpu_hotplug']/option[@selected]/@value"), ["True"])
         self.assertIn("No channel", doc.xpath("string(//form[.//select[@name='guest_agent']])"))
 
+    def test_a_custom_rng_or_crash_action_is_left_to_the_xml_editor(self):
+        doc = self.page(get_rng=("custom", "custom"), get_on_crash=("preserve", "preserve"))
+        self.assertIsNotNone(doc.xpath("//input[@name='rng']")[0].get("disabled"))
+        self.assertEqual(doc.xpath("//select[@name='on_crash']"), [])
+        self.assertIn("preserve: change it in the XML editor.", doc.xpath("string(//p[@id='on_crash'])"))
+        doc = self.page(get_rng=("virtio", "virtio"), get_on_crash=("restart", "restart"))  # read from the persistent definition
+        self.assertEqual(doc.xpath("//input[@name='rng']/@checked"), ["checked"])
+        self.assertEqual(doc.xpath("//select[@name='on_crash']/option[@selected]/@value"), ["restart"])
+
     def test_buttons_shown_as_disabled_cannot_submit(self):
         # running: the CPU resize, vCPU hot plug, template and disk removal forms wait for a shutdown;
         # the title, description and video model are of the next start, saved at once

instances/urls.py

@@ -54,6 +54,8 @@ urlpatterns = [
     path("<int:pk>/change_xml/", views.change_xml, name="change_xml"),
     path("<int:pk>/set_guest_agent/", views.set_guest_agent, name="set_guest_agent"),
     path("<int:pk>/set_video_model/", views.set_video_model, name="set_video_model"),
+    path("<int:pk>/set_rng/", views.set_rng, name="set_rng"),
+    path("<int:pk>/set_on_crash/", views.set_on_crash, name="set_on_crash"),
     path("<int:pk>/change_network/", views.change_network, name="change_network"),
     path("<int:pk>/add_network/", views.add_network, name="add_network"),
     path("<int:pk>/delete_network/", views.delete_network, name="delete_network"),

instances/views.py

@@ -1749,6 +1749,34 @@ def set_guest_agent(request, pk):
     return _back(request, pk, "options")
 
 
+@require_POST
+@superuser_only
+@serialize_instance_mutation
+def set_rng(request, pk):
+    instance = get_instance(request.user, pk)
+    enabled = bool(request.POST.get("rng"))
+    try:
+        with audit_vm(request, instance, "add RNG device" if enabled else "remove RNG device"):
+            instance.proxy.set_rng(enabled)
+    except util.OperationError as err:
+        messages.error(request, err)
+    return _back(request, pk, "options")
+
+
+@require_POST
+@superuser_only
+@serialize_instance_mutation
+def set_on_crash(request, pk):
+    instance = get_instance(request.user, pk)
+    action = request.POST.get("on_crash", "")
+    try:
+        with audit_vm(request, instance, f"set crash action {action}"):
+            instance.proxy.set_on_crash(action)
+    except util.OperationError as err:
+        messages.error(request, err)
+    return _back(request, pk, "options")
+
+
 @require_POST
 @superuser_only
 @serialize_instance_mutation

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, "", ""))