fix(instances): change the edited NIC instead of adding a new one

The edit form posts net-*-<n> fields for NIC n and carries only the
new MAC. The backend read index 0 and looked the NIC up by the new MAC,
so editing a MAC detached nothing and attached a second NIC, and edits
of any NIC but the first were applied with empty values.

The form now also posts the current MAC (net-old-mac-<n>). The view
reads the submitted NIC's fields, validates the new MAC and reports
errors. The backend replaces the NIC with the old MAC, refuses an
unknown one, and on a failed attach restores the old NIC and raises
instead of reporting success.

The view crashed with a TypeError without a Referer header; it now
returns to the instance page, and a Referer from another site is
ignored.

vrtManager/instance.py

@@ -1707,27 +1707,20 @@ class wvmInstance(wvmConnect):
                 return new_xml
         return None
 
-    def change_network(self, network_data):
-        net_mac = network_data.get("net-mac-0")
-        net_source = network_data.get("net-source-0")
-        net_source_type = network_data.get("net-source-0-type")
-        net_filter = network_data.get("net-nwfilter-0")
-        net_model = network_data.get("net-model-0")
-
-        # Remove interface first, but keep network interface XML definition
-        # If there is an error happened while adding changed one, then add removed one to back.
-        status = self.delete_network(net_mac)
+    def change_network(self, old_mac, mac, source, source_type, model, nwfilter):
+        """Replace the NIC with old_mac; put it back if the new one cannot be added."""
+        old_xml = self.delete_network(old_mac)
+        if old_xml is None:
+            raise libvirtError(f"No network interface with MAC {old_mac}")
         try:
-            self.add_network(
-                net_mac, net_source, net_source_type, net_model, net_filter
-            )
+            self.add_network(mac, source, source_type, model, nwfilter)
         except libvirtError:
-            if status is not None:
-                if self.get_status() == 1:
-                    self.instance.attachDeviceFlags(status, VIR_DOMAIN_AFFECT_LIVE)
-                    self.instance.attachDeviceFlags(status, VIR_DOMAIN_AFFECT_CONFIG)
-                if self.get_status() == 5:
-                    self.instance.attachDeviceFlags(status, VIR_DOMAIN_AFFECT_CONFIG)
+            if self.get_status() == 1:
+                self.instance.attachDeviceFlags(old_xml, VIR_DOMAIN_AFFECT_LIVE)
+                self.instance.attachDeviceFlags(old_xml, VIR_DOMAIN_AFFECT_CONFIG)
+            if self.get_status() == 5:
+                self.instance.attachDeviceFlags(old_xml, VIR_DOMAIN_AFFECT_CONFIG)
+            raise
 
     def change_network_oldway(self, network_data):
         """

vrtManager/test_change_network.py

@@ -0,0 +1,56 @@
+import unittest
+from unittest.mock import MagicMock
+
+from django.conf import settings
+
+if not settings.configured:
+    settings.configure(MAC_OUI="52:54:10")
+
+from libvirt import VIR_DOMAIN_AFFECT_CONFIG, libvirtError
+from vrtManager.instance import wvmInstance
+
+DOMAIN_XML = """<domain><devices>
+<interface type='network'><mac address='52:54:00:00:00:01'/><source network='default'/><model type='virtio'/></interface>
+<interface type='network'><mac address='52:54:00:00:00:02'/><source network='default'/><model type='virtio'/></interface>
+</devices></domain>"""
+
+
+def stopped_instance():
+    inst = wvmInstance.__new__(wvmInstance)
+    inst.instance = MagicMock()
+    inst._XMLDesc = MagicMock(return_value=DOMAIN_XML)
+    inst.get_status = MagicMock(return_value=5)
+    return inst
+
+
+class TestChangeNetwork(unittest.TestCase):
+    def test_replaces_the_nic_with_the_old_mac(self):
+        inst = stopped_instance()
+        inst.change_network("52:54:00:00:00:02", "52:54:00:00:00:03", "default", "net", "e1000", "")
+
+        detached = inst.instance.detachDeviceFlags.call_args[0][0]
+        self.assertIn("52:54:00:00:00:02", detached)
+        attached = inst.instance.attachDeviceFlags.call_args[0][0]
+        self.assertIn("52:54:00:00:00:03", attached)
+        self.assertIn("e1000", attached)
+
+    def test_unknown_old_mac_adds_nothing(self):
+        inst = stopped_instance()
+        with self.assertRaises(libvirtError):
+            inst.change_network("52:54:00:00:00:09", "52:54:00:00:00:03", "default", "net", "virtio", "")
+        inst.instance.detachDeviceFlags.assert_not_called()
+        inst.instance.attachDeviceFlags.assert_not_called()
+
+    def test_failed_add_restores_the_old_nic(self):
+        inst = stopped_instance()
+        inst.instance.attachDeviceFlags.side_effect = [libvirtError("bad source"), 0]
+        with self.assertRaises(libvirtError):
+            inst.change_network("52:54:00:00:00:01", "52:54:00:00:00:03", "missing", "net", "virtio", "")
+
+        restored, flags = inst.instance.attachDeviceFlags.call_args[0]
+        self.assertIn("52:54:00:00:00:01", restored)
+        self.assertEqual(flags, VIR_DOMAIN_AFFECT_CONFIG)
+
+
+if __name__ == "__main__":
+    unittest.main()