Assert that expected attributes are present in XML

Raise a meaningful MissingAttributesError if not. The exception also
shows the raw XML node so the reason for the missing attribute can be
determined and addressed.

lib/cc/engine/csslint.rb

@@ -3,6 +3,8 @@
 
 module CC
   module Engine
+    MissingAttributesError = Class.new(StandardError)
+
     class CSSlint
       autoload :CheckDetails, "cc/engine/csslint/check_details"
 
@@ -18,32 +20,7 @@ def run
             path = file['name'].sub(/\A#{@directory}\//, '')
             file.children.each do |node|
               next unless node.name == "error"
-
-              lint = node.attributes
-              check_name = lint["identifier"].value
-              check_details = CheckDetails.fetch(check_name)
-
-              issue = {
-                type: "issue",
-                check_name: check_name,
-                description: lint["message"].value,
-                categories: check_details.categories,
-                remediation_points: check_details.remediation_points,
-                location: {
-                  path: path,
-                  positions: {
-                    begin: {
-                      line: lint["line"].value.to_i,
-                      column: lint["column"].value.to_i
-                    },
-                    end: {
-                      line: lint["line"].value.to_i,
-                      column: lint["column"].value.to_i
-                    }
-                  }
-                }
-              }
-
+              issue = create_issue(node, path)
               puts("#{issue.to_json}\0")
             end
           end
@@ -52,6 +29,34 @@ def run
 
       private
 
+      def create_issue(node, path)
+        check_name = node.attributes.fetch("identifier").value
+        check_details = CheckDetails.fetch(check_name)
+
+        {
+          type: "issue",
+          check_name: check_name,
+          description: node.attributes.fetch("message").value,
+          categories: check_details.categories,
+          remediation_points: check_details.remediation_points,
+          location: {
+            path: path,
+            positions: {
+              begin: {
+                line: node.attributes.fetch("line").value.to_i,
+                column: node.attributes.fetch("column").value.to_i
+              },
+              end: {
+                line: node.attributes.fetch("line").value.to_i,
+                column: node.attributes.fetch("column").value.to_i
+              }
+            }
+          }
+        }
+      rescue KeyError => ex
+        raise MissingAttributesError, "#{ex.message} on XML '#{node}' when analyzing file '#{path}'"
+      end
+
       def results
         @results ||= Nokogiri::XML(csslint_xml)
       end

spec/cc/engine/csslint_spec.rb

@@ -16,6 +16,11 @@ module Engine
           expect{ lint.run }.to output(/Don't use IDs in selectors./).to_stdout
         end
 
+        it 'fails on malformed file' do
+          create_source_file('foo.css', '�6�')
+          expect{ lint.run }.to raise_error(MissingAttributesError)
+        end
+
         it "doesn't analyze *.scss files" do
           create_source_file('foo.scss', id_selector_content)
           expect{ lint.run }.to_not output.to_stdout