fix(instances): a network device keeps its source unless another is chosen

The Edit dialog of a VM's network device selected the listed network
or interface whose name matched the device's source, and the first one
when none did: saving only a new model moved a macvtap device or a
bridge the host's lists did not show onto the first network, a bridge
named like a network became that network, and a macvtap device chosen
again lost its mode.

The dialog now offers the device's own source first, "<name>, as it
is", and posts nothing for it; the view passes no source and the
device's source element is left as it is. Choosing another source
changes it as before.

vrtManager/instance.py

@@ -2706,7 +2706,8 @@ class wvmInstance(wvmConnect):
         """Edit the NIC with old_mac in the persistent definition, in place.
 
         Settings this form does not edit (QoS, link state, driver, PCI address)
-        stay. A running VM gets the change at its next start.
+        stay, and so does the source when source is None. A running VM gets
+        the change at its next start.
         """
         tree = etree.fromstring(self._XMLDesc(PERSISTENT_XML))
         found = tree.xpath("./devices/interface[mac/@address=$mac]", mac=old_mac)
@@ -2716,14 +2717,15 @@ class wvmInstance(wvmConnect):
         if iface.get("type") == "hostdev":
             raise util.OperationError("A network interface that is a host device is changed in the XML editor")
 
-        iface_type = {"net": "network", "bridge": "bridge"}.get(source_type, "direct")
-        iface.set("type", iface_type)
-        for old_source in iface.findall("source"):
-            iface.remove(old_source)
-        source_attrs = {"network": {"network": source}, "bridge": {"bridge": source}}.get(
-            iface_type, {"dev": source, "mode": "bridge"}
-        )
-        iface.insert(1, etree.Element("source", **source_attrs))
+        if source is not None:
+            iface_type = {"net": "network", "bridge": "bridge"}.get(source_type, "direct")
+            iface.set("type", iface_type)
+            for old_source in iface.findall("source"):
+                iface.remove(old_source)
+            source_attrs = {"network": {"network": source}, "bridge": {"bridge": source}}.get(
+                iface_type, {"dev": source, "mode": "bridge"}
+            )
+            iface.insert(1, etree.Element("source", **source_attrs))
 
         iface.find("mac").set("address", mac)
         for old_model in iface.findall("model"):

vrtManager/tests/test_change_network.py

@@ -14,6 +14,7 @@ DOMAIN_XML = """<domain><devices>
 <interface type='network'><mac address='52:54:00:00:00:02'/><source network='default'/><model type='virtio'/>
 <bandwidth><outbound average='1000'/></bandwidth><link state='down'/>
 <address type='pci' domain='0x0000' bus='0x02' slot='0x00' function='0x0'/></interface>
+<interface type='direct'><mac address='52:54:00:00:00:04'/><source dev='eth0' mode='vepa'/><model type='virtio'/></interface>
 </devices></domain>"""
 
 
@@ -34,7 +35,7 @@ class TestChangeNetwork(unittest.TestCase):
         inst.change_network("52:54:00:00:00:02", "52:54:00:00:00:03", "br0", "bridge", "e1000", "clean-traffic")
 
         defined = etree.fromstring(inst.wvm.defineXML.call_args[0][0])
-        first, second = defined.findall("devices/interface")
+        first, second, _direct = defined.findall("devices/interface")
         self.assertEqual(first.find("mac").get("address"), "52:54:00:00:00:01")
         self.assertEqual(second.get("type"), "bridge")
         self.assertEqual(second.find("mac").get("address"), "52:54:00:00:00:03")
@@ -46,6 +47,18 @@ class TestChangeNetwork(unittest.TestCase):
         self.assertEqual(second.find("link").get("state"), "down")
         self.assertEqual(second.find("address").get("bus"), "0x02")
 
+    def defined_nic(self, *args):
+        from lxml import etree
+
+        inst = stopped_instance()
+        inst.change_network(*args)
+        return etree.fromstring(inst.wvm.defineXML.call_args[0][0]).findall("devices/interface")[2]
+
+    def test_no_source_keeps_the_one_the_nic_has(self):
+        nic = self.defined_nic("52:54:00:00:00:04", "52:54:00:00:00:04", None, None, "e1000", "")
+        self.assertEqual((nic.get("type"), nic.find("source").attrib), ("direct", {"dev": "eth0", "mode": "vepa"}))
+        self.assertEqual(nic.find("model").get("type"), "e1000")
+
     def test_unknown_old_mac_changes_nothing(self):
         inst = stopped_instance()
         with self.assertRaises(libvirtError):