Add [] method for accessing ci entry dependencies

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

@@ -20,6 +20,10 @@ module Gitlab
             @validator.validate(:new)
           end
 
+          def [](key)
+            @entries[key] || Node::Undefined.new
+          end
+
           def compose!(deps = nil)
             return unless valid?
 

spec/lib/gitlab/ci/config/node/global_spec.rb

@@ -253,4 +253,27 @@ describe Gitlab::Ci::Config::Node::Global do
       expect(global.specified?).to be true
     end
   end
+
+  describe '#[]' do
+    before { global.compose! }
+
+    let(:hash) do
+      { cache: { key: 'a' }, rspec: { script: 'ls' } }
+    end
+
+    context 'when node exists' do
+      it 'returns correct entry' do
+        expect(global[:cache])
+          .to be_an_instance_of Gitlab::Ci::Config::Node::Cache
+        expect(global[:jobs][:rspec][:script].value).to eq ['ls']
+      end
+    end
+
+    context 'when node does not exist' do
+      it 'always return unspecified node' do
+        expect(global[:some][:unknown][:node])
+          .not_to be_specified
+      end
+    end
+  end
 end