Adjust the libvirt config classes' API contract for parsing

Don't not automatically invoke the 'parse_dom' method from
the LibvirtConfigObject constructor, because this is too
early in construction - no child class constructors have
initialized yet.

Instead require the 'parse_dom' method to be invoked after
the bare object has been constructed. Also make the default
impl of 'parse_dom' take an etree.Element instance and
validate its root element name. Add a convenient 'parse_str'
method for turning a XML string into an etree.Element
instance & parsing it.

Change-Id: I7eda2b59869f3b4ef9dce1602cbb05f7662f4528
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

2 files changed, 14 insertions, 5 deletions

diff --git a/nova/tests/test_libvirt_config.py b/nova/tests/test_libvirt_config.py
index 1e0818bd7..4f9970e79 100644
--- a/nova/tests/test_libvirt_config.py
+++ b/nova/tests/test_libvirt_config.py
@@ -53,6 +53,11 @@ class LibvirtConfigTest(LibvirtConfigBaseTest):
         xml = etree.tostring(root)
         self.assertXmlEqual(xml, "<demo><foo>bar</foo></demo>")
 
+    def test_config_parse(self):
+        inxml = "<demo><foo/></demo>"
+        obj = config.LibvirtConfigObject(root_name="demo")
+        obj.parse_str(inxml)
+
 
 class LibvirtConfigGuestTimerTest(LibvirtConfigBaseTest):
     def test_config_platform(self):
diff --git a/nova/virt/libvirt/config.py b/nova/virt/libvirt/config.py
index 3ebdf21f0..c67f11550 100644
--- a/nova/virt/libvirt/config.py
+++ b/nova/virt/libvirt/config.py
@@ -21,6 +21,7 @@ Classes to represent the configuration of various libvirt objects
 and support conversion to/from XML
 """
 
+from nova import exception
 from nova import log as logging
 
 from lxml import etree
@@ -38,9 +39,6 @@ class LibvirtConfigObject(object):
         self.ns_prefix = kwargs.get('ns_prefix')
         self.ns_uri = kwargs.get('ns_uri')
 
-        if "xml_str" in kwargs:
-            self.parse_dom(kwargs.get("xml_str"))
-
     def _text_node(self, name, value):
         child = etree.Element(name)
         child.text = str(value)
@@ -53,8 +51,14 @@ class LibvirtConfigObject(object):
             return etree.Element("{" + self.ns_uri + "}" + self.root_name,
                                  nsmap={self.ns_prefix: self.ns_uri})
 
-    def parse_dom(xmldoc):
-        raise NotImplementedError()
+    def parse_str(self, xmlstr):
+        self.parse_dom(etree.fromstring(xmlstr))
+
+    def parse_dom(self, xmldoc):
+        if self.root_name != xmldoc.tag:
+            raise exception.InvalidInput(
+                "Root element name should be '%s' not '%s'"
+                % (self.root_name, xmldoc.tag))
 
     def to_xml(self, pretty_print=True):
         root = self.format_dom()