perf(interfaces): list only the host's network devices

The interfaces page read the XML of every node device (183 on a test host)
to keep the network ones; libvirt now lists only those. Same names on both
test hosts, 756 -> 261 ms over TCP and 475 -> 173 ms over SSH.

vrtManager/connection.py

@@ -959,19 +959,11 @@ class wvmConnect(object):
         return instance
 
     def get_net_devices(self):
-        netdevice = []
-
-        def get_info(doc):
-            dev_type = util.get_xpath(doc, "/device/capability/@type")
-            interface = util.get_xpath(doc, "/device/capability/interface")
-            return dev_type, interface
-
-        for dev in self.wvm.listAllDevices(0):
-            xml = dev.XMLDesc(0)
-            (dev_type, interface) = util.get_xml_path(xml, func=get_info)
-            if dev_type == "net":
-                netdevice.append(interface)
-        return netdevice
+        """The host's network interface names; libvirt lists only the network devices"""
+        return [
+            util.get_xml_path(dev.XMLDesc(0), "/device/capability/interface")
+            for dev in self.wvm.listAllDevices(libvirt.VIR_CONNECT_LIST_NODE_DEVICES_CAP_NET)
+        ]
 
     def get_host_instances(self, raw_mem_size=False):
         vname = {}

vrtManager/test_host_reads.py

@@ -1,15 +1,18 @@
 """A wvmConnect reads the host's info and capabilities once, and its domain
 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."""
+info four times. An object lives for one request. The interfaces page lists
+only the host's network devices, not all of them."""
 
 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_CONNECT_LIST_NODE_DEVICES_CAP_NET
 from vrtManager.connection import wvmConnect
 from vrtManager.hostdetails import wvmHostDetails
 
@@ -97,3 +100,17 @@ class CapabilitiesOnceTestCase(unittest.TestCase):
         conn.wvm = FakeConn()
         conn.get_cap_xml()
         self.assertEqual(conn.wvm.reads, ["capabilities"])
+
+
+class NetDevicesTestCase(unittest.TestCase):
+    def test_only_the_network_devices_are_listed_and_read(self):
+        conn = connection()
+        devices = {
+            VIR_CONNECT_LIST_NODE_DEVICES_CAP_NET: [
+                MagicMock(**{"XMLDesc.return_value": f"<device><capability type='net'><interface>{name}</interface>"
+                                                     "<capability type='80203'/></capability></device>"})
+                for name in ("eth0", "br0")
+            ]
+        }
+        conn.wvm.listAllDevices = lambda flags: devices.get(flags, [])
+        self.assertEqual(conn.get_net_devices(), ["eth0", "br0"])