fix(interfaces): no create, start, stop or delete where libvirt cannot change interfaces

libvirt's udev interface backend (RHEL 9 and later, Debian 12) only reads
interfaces, so the create form and the start, stop and delete buttons
only failed there. The pages ask the host with an invalid definition,
which defines nothing: a backend without define refuses it as not
supported. There they show the interface state and a note instead, and
posts change nothing; the host's devices, needed only by the create
form, are not read. Any other answer keeps the pages as they were.

interfaces/test_forms.py

@@ -8,7 +8,7 @@ from computes.models import Compute
 from django.contrib.auth import get_user_model
 from django.test import SimpleTestCase, TestCase
 from django.urls import reverse
-from lxml import etree
+from lxml import etree, html
 from vrtManager.interface import wvmInterfaces
 
 from interfaces.forms import AddInterface
@@ -295,6 +295,7 @@ class CreateViewTestCase(TestCase):
             with self.subTest(netdev=netdev), patch("interfaces.views.wvmInterfaces") as conn_cls:
                 conn = conn_cls.return_value
                 conn.get_ifaces.return_value = []
+                conn.can_change_interfaces.return_value = True
                 conn.get_net_devices.return_value = ["eth0", "eth1"]
                 self.client.post(
                     reverse("interfaces", args=[compute.id]),
@@ -309,6 +310,7 @@ class CreateViewTestCase(TestCase):
         with patch("interfaces.views.wvmInterfaces") as conn_cls:
             conn = conn_cls.return_value
             conn.get_ifaces.return_value = []
+            conn.can_change_interfaces.return_value = True
             conn.get_net_devices.side_effect = RuntimeError("nodedev driver not running")
             response = self.client.post(
                 reverse("interfaces", args=[compute.id]),
@@ -320,6 +322,78 @@ class CreateViewTestCase(TestCase):
         self.assertFalse(any("not on this host" in m for m in messages), messages)
 
 
+class UnchangeableHostTestCase(TestCase):
+    """A host whose libvirt cannot change interfaces (udev backend) gets no
+    create form and no start/stop/delete, only a note; posts change nothing."""
+
+    NOTE = "does not support changing network interfaces"
+
+    def setUp(self):
+        self.client.force_login(get_user_model().objects.create_superuser("iface_ro", "ro@example.com", "x"))
+        self.compute = Compute.objects.create(name="iface-ro", hostname="127.0.0.1:1", login="root", password="", type=1)
+
+    def interfaces_page(self, changeable, post=None):
+        with patch("interfaces.views.wvmInterfaces") as conn_cls:
+            conn = conn_cls.return_value
+            conn.get_ifaces.return_value = []
+            conn.can_change_interfaces.return_value = changeable
+            conn.get_net_devices.return_value = ["eth0", "eth1"]
+            url = reverse("interfaces", args=[self.compute.id])
+            response = self.client.post(url, post) if post else self.client.get(url)
+        return conn, response.content.decode()
+
+    def test_the_interfaces_page_offers_create_only_where_it_works(self):
+        for changeable in (True, False):
+            with self.subTest(changeable=changeable):
+                conn, page = self.interfaces_page(changeable)
+                self.assertEqual("#AddInterface" in page, changeable)
+                self.assertEqual(self.NOTE in page, not changeable)
+                # the devices are only the form's choices
+                self.assertEqual(conn.get_net_devices.called, changeable)
+
+    def test_a_create_post_creates_nothing(self):
+        conn, page = self.interfaces_page(False, {**BASE, "netdev": "eth1", "ipv6_type": "none", "create": ""})
+        conn.create_iface.assert_not_called()
+        self.assertIn(self.NOTE, page)
+
+    def interface_page(self, changeable, state, post=None):
+        with patch("interfaces.views.wvmInterface") as conn_cls:
+            conn = conn_cls.return_value
+            conn.can_change_interfaces.return_value = changeable
+            conn.is_active.return_value = state
+            conn.get_bridge_slave_ifaces.return_value = []
+            url = reverse("interface", args=[self.compute.id, "br0"])
+            response = self.client.post(url, post) if post else self.client.get(url)
+        return conn, response
+
+    def test_the_interface_page_shows_the_state_without_actions(self):
+        for state, shown in ((1, "Active"), (0, "Inactive")):
+            with self.subTest(state=state):
+                _conn, response = self.interface_page(False, state)
+                page = response.content.decode()
+                self.assertNotIn('name="start"', page)
+                self.assertNotIn('name="stop"', page)
+                state_cell = html.fromstring(page).xpath("//dt[normalize-space()='State']/following-sibling::dd[1]")[0]
+                self.assertEqual(state_cell.text_content().strip(), shown)
+                self.assertIn(self.NOTE, page)
+        _conn, response = self.interface_page(True, 0)
+        self.assertIn('name="start"', response.content.decode())
+
+    def test_a_host_that_could_not_be_read_keeps_the_page_as_it_was(self):
+        with patch("interfaces.views.wvmInterface", side_effect=libvirt.libvirtError("unreachable")):
+            page = self.client.get(reverse("interface", args=[self.compute.id, "br0"])).content.decode()
+        self.assertNotIn(self.NOTE, page)
+        self.assertIn("Interface start/stop/delete form", page)
+
+    def test_start_stop_and_delete_posts_change_nothing(self):
+        for action in ("start", "stop", "delete"):
+            with self.subTest(action=action):
+                conn, response = self.interface_page(False, 1, {action: ""})
+                self.assertEqual(response.status_code, 200)
+                for method in (conn.start_iface, conn.stop_iface, conn.delete_iface):
+                    method.assert_not_called()
+
+
 class EthernetDeviceTestCase(SimpleTestCase):
     def test_an_ethernet_interface_needs_no_bridge_member(self):
         self.assertTrue(form(itype="ethernet", name="eth0", netdev="", ipv6_type="none").is_valid())