Switch to xml-simple for document comparison (#1567)

The switch to compare-xml exhibits non intuitive behaviour in that
comparing without use of verbose mode does not detect documents that are
identical. The if statement allowed the tests to pass but reads that if
the documents compare as true then emit an error message and raise an
exception. However this is because it returns false even with the docs
are the same content. Unfortunately there was no pre-existing test
case added when

Since there were concerns about equivalent-xml not being active upstream
raised by a distribution maintainer, switching back should be avoided.
Attempted use of nokogiri-diff indicated that whitespace changes would
be treated as differences with it being unclear how to easily exclude.

Moving to xml-simple, which although will treat whitespace as
significant and even with NormaliseSpace set to disregard any whitespace
around content elements, it requires walking the returned structure to
remove the empty content attribute that is left behind to allow a direct
comparison between the data structures.

To ensure the XML comparison is validated add a test where libvirt
returns XML that is different to what was requested, and assert that
the expected error is raised, an error message emitted and that the
domain define would be reverted to the previous state if possible.

Relates-To: #1565
Fixes: #1556

lib/vagrant-libvirt/action/start_domain.rb

@@ -2,14 +2,16 @@
 
 require 'log4r'
 
-require 'compare-xml'
 require 'rexml/document'
 
+require 'vagrant-libvirt/util/xml'
+
 module VagrantPlugins
   module ProviderLibvirt
     module Action
       # Just start the domain.
       class StartDomain
+
         def initialize(app, _env)
           @logger = Log4r::Logger.new('vagrant_libvirt::action::start_domain')
           @app = app
@@ -427,27 +429,13 @@ module VagrantPlugins
 
                 begin
                   # need to check whether the updated XML contains all the changes requested
+                  proposed = VagrantPlugins::ProviderLibvirt::Util::Xml.new(new_xml)
+                  applied = VagrantPlugins::ProviderLibvirt::Util::Xml.new(libvirt_domain.xml_desc(1))
 
-                  # This normalizes the attribute order to be consistent across both XML docs to
-                  # eliminate differences for subsequent comparison by diffy
-                  applied_xml_descr = REXML::Document.new(libvirt_domain.xml_desc(1))
-                  applied_xml = String.new
-                  applied_xml_descr.write(applied_xml)
-
-                  proposed = Nokogiri::XML(new_xml, &:noblanks)
-                  applied = Nokogiri::XML(applied_xml, &:noblanks)
-
-                  if CompareXML.equivalent?(proposed, applied, { force_children: true })
+                  if proposed != applied
                     require 'diffy'
 
-                    # pretty print the XML as even though there can be additional changes,
-                    # the output with diffy appears to be clearer
-                    pretty_proposed = StringIO.new
-                    pretty_applied = StringIO.new
-                    proposed.write_xml_to(pretty_proposed, indent: 2)
-                    applied.write_xml_to(pretty_applied, indent: 2)
-
-                    diff = Diffy::Diff.new(pretty_proposed.string, pretty_applied.string, :context => 3).to_s(:text)
+                    diff = Diffy::Diff.new(proposed, applied, :context => 3).to_s(:text)
 
                     error_msg = "Libvirt failed to fully update the domain with the specified XML. Result differs from requested:\n" +
                       "--- requested\n+++ result\n#{diff}\n" +

lib/vagrant-libvirt/util/xml.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+require 'xmlsimple'
+
+module VagrantPlugins
+  module ProviderLibvirt
+    module Util
+      class Xml
+        attr_reader :xml
+
+        def initialize(xmlstr)
+          @xml = compact_content(XmlSimple.xml_in(xmlstr, {'NormaliseSpace' => 2}))
+        end
+
+        def to_str
+          XmlSimple.xml_out(@xml)
+        end
+
+        def ==(other)
+          @xml == other.xml
+        end
+
+        private
+
+        # content elements that are empty are preserved by xml-simple and will result
+        # in the structures being considered different even if functionality the same
+        # strip any empty ones to avoid.
+        def compact_content(node)
+          if node.is_a?(Array)
+            node.map! do |element|
+              compact_content(element)
+            end
+          elsif node.is_a?(Hash)
+            if node['content'] and node['content'].empty?
+              node.delete('content')
+            end
+            node.each do |k, v|
+              node[k] = compact_content(v)
+            end
+          else
+            return node
+          end
+        end
+      end
+    end
+  end
+end

spec/unit/action/start_domain_spec.rb

@@ -72,7 +72,30 @@ describe VagrantPlugins::ProviderLibvirt::Action::StartDomain do
 
         expect(subject.call(env)).to be_nil
       end
+    end
 
+    context 'when xml not applied' do
+      let(:test_file) { 'default_with_different_formatting.xml' }
+      let(:updated_domain_xml) {
+        new_xml = domain_xml.dup
+        new_xml.gsub!(/<cpu .*<\/cpu>/m, '<cpu mode="host-passthrough"/>')
+        new_xml
+      }
+      let(:vagrantfile_providerconfig) do
+        <<-EOF
+        libvirt.cpu_mode = "host-passthrough"
+        EOF
+      end
+
+      it 'should error and revert the update' do
+        expect(ui).to receive(:error)
+        expect(connection).to receive(:define_domain).and_return(libvirt_domain)
+        expect(connection).to receive(:define_domain).with(domain_xml) # undo
+        expect(libvirt_domain).to receive(:xml_desc).and_return(domain_xml, updated_domain_xml)
+        expect(domain).to_not receive(:start)
+
+        expect { subject.call(env) }.to raise_error(VagrantPlugins::ProviderLibvirt::Errors::FogError)
+      end
     end
 
     context 'when any setting changed' do

vagrant-libvirt.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'fog-libvirt', '>= 0.6.0'
   s.add_runtime_dependency 'fog-core', '~> 2'
   s.add_runtime_dependency 'rexml'
-  s.add_runtime_dependency 'compare-xml'
+  s.add_runtime_dependency 'xml-simple'
   s.add_runtime_dependency 'diffy'
 
   # Make sure to allow use of the same version as Vagrant by being less specific