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.

vrtManager/connection.py

@@ -958,6 +958,17 @@ class wvmConnect(object):
                 instance.append(dom.name())
         return instance
 
+    def can_change_interfaces(self):
+        """False when the host's libvirt cannot define interfaces (the udev
+        backend only reads them). The definition sent is invalid, so nothing is
+        defined: a backend without define refuses it with VIR_ERR_NO_SUPPORT,
+        one with define with an XML error."""
+        try:
+            self.wvm.interfaceDefineXML("<interface/>", 0)
+        except libvirtError as err:
+            return err.get_error_code() != libvirt.VIR_ERR_NO_SUPPORT
+        return True
+
     def get_net_devices(self):
         """The host's network interface names; libvirt lists only the network devices"""
         return [

vrtManager/test_host_reads.py

@@ -2,7 +2,8 @@
 capabilities once per emulator, arch, machine and domain type: the create
 page's getters parse the capabilities some 60 times, the overview reads the
 info four times. An object lives for one request. The interfaces page lists
-only the host's network devices, not all of them."""
+only the host's network devices, not all of them, and knows whether the host
+can change interfaces at all."""
 
 import unittest
 from unittest.mock import MagicMock
@@ -12,7 +13,7 @@ from django.conf import settings
 if not settings.configured:
     settings.configure(MAC_OUI="52:54:10")
 
-from libvirt import VIR_CONNECT_LIST_NODE_DEVICES_CAP_NET
+from libvirt import VIR_CONNECT_LIST_NODE_DEVICES_CAP_NET, VIR_ERR_NO_SUPPORT, VIR_ERR_OPERATION_DENIED, VIR_ERR_XML_ERROR, libvirtError
 from vrtManager.connection import wvmConnect
 from vrtManager.hostdetails import wvmHostDetails
 
@@ -114,3 +115,20 @@ class NetDevicesTestCase(unittest.TestCase):
         }
         conn.wvm.listAllDevices = lambda flags: devices.get(flags, [])
         self.assertEqual(conn.get_net_devices(), ["eth0", "br0"])
+
+
+class ChangeInterfacesTestCase(unittest.TestCase):
+    def error(self, code):
+        err = libvirtError("refused")
+        err.get_error_code = lambda: code
+        return err
+
+    def test_a_backend_without_define_cannot_change_interfaces(self):
+        conn = connection()
+        # any other refusal (access denied, ...) leaves the page as it was
+        for code, changeable in ((VIR_ERR_NO_SUPPORT, False), (VIR_ERR_XML_ERROR, True), (VIR_ERR_OPERATION_DENIED, True)):
+            with self.subTest(code=code):
+                conn.wvm.interfaceDefineXML = MagicMock(side_effect=self.error(code))
+                self.assertIs(conn.can_change_interfaces(), changeable)
+                # an invalid definition: nothing can be defined by asking
+                conn.wvm.interfaceDefineXML.assert_called_once_with("<interface/>", 0)