Add specs for attributable aspect of ci config entry

lib/gitlab/ci/config/entry/attributable.rb

@@ -8,7 +8,9 @@ module Gitlab
           class_methods do
             def attributes(*attributes)
               attributes.flatten.each do |attribute|
-                raise ArgumentError if method_defined?(attribute)
+                if method_defined?(attribute)
+                  raise ArgumentError, 'Method already defined!'
+                end
 
                 define_method(attribute) do
                   return unless config.is_a?(Hash)

spec/lib/gitlab/ci/config/entry/attributable_spec.rb

 require 'spec_helper'
 
 describe Gitlab::Ci::Config::Entry::Attributable do
-  let(:node) { Class.new }
+  let(:node) do
+    Class.new do
+      include Gitlab::Ci::Config::Entry::Attributable
+    end
+  end
+
   let(:instance) { node.new }
 
   before do
-    node.include(described_class)
-
     node.class_eval do
       attributes :name, :test
     end
   end
 
-  context 'config is a hash' do
+  context 'when config is a hash' do
     before do
       allow(instance)
         .to receive(:config)
@@ -29,7 +32,7 @@ describe Gitlab::Ci::Config::Entry::Attributable do
     end
   end
 
-  context 'config is not a hash' do
+  context 'when config is not a hash' do
     before do
       allow(instance)
         .to receive(:config)
@@ -40,4 +43,18 @@ describe Gitlab::Ci::Config::Entry::Attributable do
       expect(instance.test).to be_nil
     end
   end
+
+  context 'when method is already defined in a superclass' do
+    it 'raises an error' do
+      expectation = expect do
+        Class.new(String) do
+          include Gitlab::Ci::Config::Entry::Attributable
+
+          attributes :length
+        end
+      end
+
+      expectation.to raise_error(ArgumentError, 'Method already defined!')
+    end
+  end
 end