gracefully handle missing xml elements of relayables

parent-fetching failed, if the xml didn't contain the information needed
for it.

fixes NameError: uninitialized constant DiasporaFederation::Entities::Like::PARENT_TYPE

lib/diaspora_federation/entities/relayable.rb

@@ -203,8 +203,16 @@ module DiasporaFederation
         end
 
         def fetch_parent(data)
-          type = data[:parent_type] || self::PARENT_TYPE
-          guid = data[:parent_guid]
+          type = data.fetch(:parent_type) {
+            if const_defined?(:PARENT_TYPE)
+              self::PARENT_TYPE
+            else
+              raise DiasporaFederation::Entity::ValidationError, "invalid #{self}! missing 'parent_type'."
+            end
+          }
+          guid = data.fetch(:parent_guid) {
+            raise DiasporaFederation::Entity::ValidationError, "invalid #{self}! missing 'parent_guid'."
+          }
 
           data[:parent] = DiasporaFederation.callbacks.trigger(:fetch_related_entity, type, guid)
 

spec/lib/diaspora_federation/entities/like_spec.rb

@@ -32,5 +32,35 @@ XML
     it_behaves_like "an XML Entity"
 
     it_behaves_like "a relayable Entity"
+
+    context "invalid XML" do
+      it "raises a ValidationError if the parent_type is missing" do
+        broken_xml = <<-XML
+<like>
+  <parent_guid>#{parent.guid}</parent_guid>
+  <author_signature>#{data[:author_signature]}</author_signature>
+  <parent_author_signature>#{data[:parent_author_signature]}</parent_author_signature>
+</like>
+XML
+
+        expect {
+          DiasporaFederation::Entities::Like.from_xml(Nokogiri::XML::Document.parse(broken_xml).root)
+        }.to raise_error Entity::ValidationError, "invalid DiasporaFederation::Entities::Like! missing 'parent_type'."
+      end
+
+      it "raises a ValidationError if the parent_guid is missing" do
+        broken_xml = <<-XML
+<like>
+  <target_type>#{parent.entity_type}</target_type>
+  <author_signature>#{data[:author_signature]}</author_signature>
+  <parent_author_signature>#{data[:parent_author_signature]}</parent_author_signature>
+</like>
+        XML
+
+        expect {
+          DiasporaFederation::Entities::Like.from_xml(Nokogiri::XML::Document.parse(broken_xml).root)
+        }.to raise_error Entity::ValidationError, "invalid DiasporaFederation::Entities::Like! missing 'parent_guid'."
+      end
+    end
   end
 end

spec/lib/diaspora_federation/entities/relayable_spec.rb

@@ -295,6 +295,21 @@ XML
       end
     end
 
+    context "parse invalid XML" do
+      it "raises a ValidationError if the parent_guid is missing" do
+        broken_xml = <<-XML
+<some_relayable>
+  <author_signature/>
+  <parent_author_signature/>
+</some_relayable>
+        XML
+
+        expect {
+          SomeRelayable.from_xml(Nokogiri::XML::Document.parse(broken_xml).root)
+        }.to raise_error Entity::ValidationError, "invalid DiasporaFederation::SomeRelayable! missing 'parent_guid'."
+      end
+    end
+
     context "fetch parent" do
       before do
         expect_callback(:fetch_public_key, author).and_return(author_pkey.public_key)