[libvirt]log XML if nova fails to parse it

In case nova fails to parse the XML from libvirt the lxml exception is
dumped to the log but it only states the location of the error in the
XML string like

  lxml.etree.XMLSyntaxError: StartTag: invalid element name, line 40, column 35

To be able to troubleshoot the actual XML error we need to see the
invalid XML as well. So this patch makes sure that if nova fails to
parse the XML then the XML itself is dumped to DEBUG log.

Change-Id: I14cce6db4c86f663e61d3668d081858741e88add

nova/tests/unit/virt/libvirt/test_config.py

@@ -13,6 +13,8 @@
 #    under the License.
 
 import ddt
+from unittest import mock
+
 from lxml import etree
 from oslo_utils.fixture import uuidsentinel as uuids
 from oslo_utils import units
@@ -80,6 +82,17 @@ class LibvirtConfigTest(LibvirtConfigBaseTest):
         obj = config.LibvirtConfigObject(root_name="demo")
         obj.parse_str(inxml)
 
+    @mock.patch.object(config.LOG, 'debug')
+    def test_config_parse_error_xml_logged(self, mock_debug):
+        inxml = "<demo><vendor_field index='Z'>6<1</vendor_field></demo>"
+        obj = config.LibvirtConfigObject(root_name="demo")
+
+        self.assertRaises(etree.XMLSyntaxError, obj.parse_str, inxml)
+
+        mock_debug.assert_called_once_with(
+            'Failed to parse the libvirt XML: %s',
+            "<demo><vendor_field index='Z'>6<1</vendor_field></demo>")
+
     def test_parse_on_off_str(self):
         obj = config.LibvirtConfigObject(root_name="demo")
         self.assertTrue(obj.parse_on_off_str('on'))

nova/virt/libvirt/config.py

@@ -28,6 +28,7 @@ import typing as ty
 
 from collections import OrderedDict
 from lxml import etree
+from oslo_log import log as logging
 from oslo_utils import strutils
 from oslo_utils import units
 
@@ -37,6 +38,7 @@ from nova.objects import fields
 from nova.pci import utils as pci_utils
 from nova.virt import hardware
 
+LOG = logging.getLogger(__name__)
 
 # Namespace to use for Nova specific metadata items in XML
 NOVA_NS = "http://openstack.org/xmlns/libvirt/nova/1.1"
@@ -83,7 +85,11 @@ class LibvirtConfigObject(object):
         return self._new_node(self.root_name)
 
     def parse_str(self, xmlstr):
-        self.parse_dom(etree.fromstring(xmlstr))
+        try:
+            self.parse_dom(etree.fromstring(xmlstr))
+        except etree.Error:
+            LOG.debug("Failed to parse the libvirt XML: %s", xmlstr)
+            raise
 
     def parse_dom(self, xmldoc):
         if self.root_name != xmldoc.tag: