Avoid domain undefine on configuration update (#1496)

Calling undefine on a domain and recreating it can result in some edge
case errors where if the current capabilities of libvirt have been
reduced, it may not be possible to restore the old definition.

Instead switch to calling `domain_define` with the new definition and
check that the resulting libvirt domain definition has been updated in
the expected manner, otherwise report an error to the user.

Fixes: #949
Relates-to: #1329
Relates-to: #1027
Relates-to: #1371

lib/vagrant-libvirt/action/start_domain.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require 'diffy'
 require 'log4r'
 require 'rexml/document'
 
@@ -14,8 +15,6 @@ module VagrantPlugins
         end
 
         def call(env)
-          env[:ui].info(I18n.t('vagrant_libvirt.starting_domain'))
-
           domain = env[:machine].provider.driver.connection.servers.get(env[:machine].id.to_s)
           raise Errors::NoDomainError if domain.nil?
           config = env[:machine].provider_config
@@ -33,6 +32,7 @@ module VagrantPlugins
                 libvirt_domain.memory = libvirt_domain.max_memory
               end
             end
+
             begin
               # XML definition manipulation
               descr = libvirt_domain.xml_desc(1)
@@ -248,7 +248,7 @@ module VagrantPlugins
               # TPM
               if [config.tpm_path, config.tpm_version].any?
                 if config.tpm_path
-                  raise Errors::FogCreateServerError, 'The TPM Path must be fully qualified' unless config.tpm_path[0].chr == '/'
+                  raise Errors::UpdateServerError, 'The TPM Path must be fully qualified' unless config.tpm_path[0].chr == '/'
                 end
 
                 # just build the tpm element every time
@@ -394,7 +394,6 @@ module VagrantPlugins
                 loader.parent.delete_element(loader)
               end
 
-              undefine_flags = 0
               nvram = REXML::XPath.first(xml_descr, '/domain/os/nvram')
               if config.nvram
                 if nvram.nil?
@@ -407,28 +406,57 @@ module VagrantPlugins
                     descr_changed = true
                     nvram.text = config.nvram
                   end
-                  undefine_flags |= ProviderLibvirt::Util::DomainFlags::VIR_DOMAIN_UNDEFINE_KEEP_NVRAM
                 end
               elsif !nvram.nil?
                 descr_changed = true
-                undefine_flags |= ProviderLibvirt::Util::DomainFlags::VIR_DOMAIN_UNDEFINE_NVRAM
                 nvram.parent.delete_element(nvram)
               end
 
               # Apply
               if descr_changed
+                env[:ui].info(I18n.t('vagrant_libvirt.updating_domain'))
+                new_xml = String.new
+                xml_descr.write(new_xml)
                 begin
-                  libvirt_domain.undefine(undefine_flags)
-                  new_descr = String.new
-                  xml_descr.write new_descr
-                  env[:machine].provider.driver.connection.servers.create(xml: new_descr)
-                rescue Fog::Errors::Error => e
-                  env[:machine].provider.driver.connection.servers.create(xml: descr)
-                  raise Errors::FogCreateServerError, error_message: e.message
+                  # providing XML for the same name and UUID will update the existing domain
+                  libvirt_domain = env[:machine].provider.driver.connection.define_domain(new_xml)
+                rescue ::Libvirt::Error => e
+                  raise Errors::UpdateServerError, error_message: e.message
+                end
+
+                begin
+                  proposed = Nokogiri::XML(new_xml, &:noblanks)
+
+                  # This normalizes the attribute order to be consistent across both XML docs to eliminate differences
+                  # for subsequent comparison by diffy
+                  updated_xml_descr = REXML::Document.new(libvirt_domain.xml_desc(1))
+                  updated_xml = String.new
+                  updated_xml_descr.write(updated_xml)
+
+                  updated = Nokogiri::XML(updated_xml, &:noblanks)
+
+                  pretty_proposed = StringIO.new
+                  pretty_updated = StringIO.new
+                  proposed.write_xml_to(pretty_proposed, indent: 2)
+                  updated.write_xml_to(pretty_updated, indent: 2)
+
+                  diff = Diffy::Diff.new(pretty_proposed.string, pretty_updated.string, :context => 3).to_s(:text)
+
+                  unless diff.empty?
+                    error_msg = "Libvirt failed to fully update the domain with the specified XML. Result differs from requested:\n" +
+                      "--- requested\n+++ result\n#{diff}\n" +
+                      "Typically this means there is a bug in the XML being sent, please log an issue"
+
+                    raise Errors::UpdateServerError, error_message: error_msg
+                  end
+                rescue Exception => e
+                  env[:machine].provider.driver.connection.define_domain(descr)
+                  raise
                 end
               end
             rescue Errors::VagrantLibvirtError => e
               env[:ui].error("Error when updating domain settings: #{e.message}")
+              raise
             end
             # Autostart with host if enabled in Vagrantfile
             libvirt_domain.autostart = config.autostart
@@ -436,6 +464,7 @@ module VagrantPlugins
               "Starting Domain with XML:\n#{libvirt_domain.xml_desc}"
             }
             # Actually start the domain
+            env[:ui].info(I18n.t('vagrant_libvirt.starting_domain'))
             domain.start
           rescue Fog::Errors::Error, Errors::VagrantLibvirtError => e
             raise Errors::FogError, message: e.message

lib/vagrant-libvirt/errors.rb

@@ -109,7 +109,7 @@ module VagrantPlugins
       end
 
       class FogCreateServerError < VagrantLibvirtError
-        error_key(:fog_create_server_error)
+        error_key(:create_server_error)
       end
 
       # Network exceptions
@@ -154,6 +154,10 @@ module VagrantPlugins
       end
 
       # Other exceptions
+      class UpdateServerError < VagrantLibvirtError
+        error_key(:create_server_error)
+      end
+
       class InterfaceSlotNotAvailable < VagrantLibvirtError
         error_key(:interface_slot_not_available)
       end