libvirt: Optimize test_connection and capabilities

The getCapabilities call can be very slow so it is not a good choice
for testing the libvirt connection. This patch switches to
getLibVersion and also caches the result of getCapabilities so it
doesn't need to be requested every time. Note that this means that
nova-compute will need to be restarted if the capaabilities of the
host changes. This is an acceptable risk because capabilities
changes should be very rare and nova-compute should be restarted
if libvirt is restarted or reinstalled.

This simple change lowers boot time in my devstack install from
22 seconds down to 8 seconds!

Fixes bug 1100446

Change-Id: I1b5072a906b19c6130957cf255e8d35b20990828

nova/tests/test_libvirt.py

@@ -2854,11 +2854,11 @@ def test_broken_connection(self):
             conn = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
 
             self.mox.StubOutWithMock(conn, "_wrapped_conn")
-            self.mox.StubOutWithMock(conn._wrapped_conn, "getCapabilities")
+            self.mox.StubOutWithMock(conn._wrapped_conn, "getLibVersion")
             self.mox.StubOutWithMock(libvirt.libvirtError, "get_error_code")
             self.mox.StubOutWithMock(libvirt.libvirtError, "get_error_domain")
 
-            conn._wrapped_conn.getCapabilities().AndRaise(
+            conn._wrapped_conn.getLibVersion().AndRaise(
                     libvirt.libvirtError("fake failure"))
 
             libvirt.libvirtError.get_error_code().AndReturn(error)

nova/virt/libvirt/driver.py

@@ -277,6 +277,7 @@ def __init__(self, virtapi, read_only=False):
         self._host_state = None
         self._initiator = None
         self._wrapped_conn = None
+        self._caps = None
         self.read_only = read_only
         self.firewall_driver = firewall.load_driver(
             DEFAULT_FIREWALL_DRIVER,
@@ -362,7 +363,7 @@ def _get_connection(self):
 
     def _test_connection(self):
         try:
-            self._wrapped_conn.getCapabilities()
+            self._wrapped_conn.getLibVersion()
             return True
         except libvirt.libvirtError as e:
             if (e.get_error_code() == libvirt.VIR_ERR_SYSTEM_ERROR and
@@ -1422,11 +1423,11 @@ def _volume_in_mapping(mount_device, block_device_info):
     def get_host_capabilities(self):
         """Returns an instance of config.LibvirtConfigCaps representing
            the capabilities of the host"""
-        xmlstr = self._conn.getCapabilities()
-
-        caps = vconfig.LibvirtConfigCaps()
-        caps.parse_str(xmlstr)
-        return caps
+        if not self._caps:
+            xmlstr = self._conn.getCapabilities()
+            self._caps = vconfig.LibvirtConfigCaps()
+            self._caps.parse_str(xmlstr)
+        return self._caps
 
     def get_host_uuid(self):
         """Returns a UUID representing the host."""