Add basic support for CI/CD config extension

lib/gitlab/ci/config/extendable.rb

+module Gitlab
+  module Ci
+    class Config
+      class Extendable
+        include Enumerable
+
+        ExtensionError = Class.new(StandardError)
+
+        def initialize(hash)
+          @hash = hash
+        end
+
+        def each
+          @hash.each_pair do |key, value|
+            next unless value.key?(:extends)
+
+            yield key, value.fetch(:extends).to_sym, value
+          end
+        end
+
+        def extend!
+          @hash.tap do
+            each do |key, extends, value|
+              @hash[key] = @hash.fetch(extends).deep_merge(value)
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/config/extendable_spec.rb

+require 'fast_spec_helper'
+
+describe Gitlab::Ci::Config::Extendable do
+  subject { described_class.new(hash) }
+
+  describe '#each' do
+    context 'when there is extendable entry in the hash' do
+      let(:test) do
+        { extends: 'something', only: %w[master] }
+      end
+
+      let(:hash) do
+        { something: { script: 'ls' }, test: test }
+      end
+
+      it 'yields the test hash' do
+        expect { |b| subject.each(&b) }
+          .to yield_with_args(:test, :something, test)
+      end
+    end
+
+    pending 'when not extending using a hash'
+  end
+
+  describe '#extend!' do
+    context 'when a hash has a single simple extension' do
+      let(:hash) do
+        {
+          something: {
+            script: 'deploy',
+            only: { variables: %w[$SOMETHING] }
+          },
+          test: {
+            extends: 'something',
+            script: 'ls',
+            only: { refs: %w[master] }
+          }
+        }
+      end
+
+      it 'extends a hash with reverse merge' do
+        expect(subject.extend!).to eq(
+          something: {
+            script: 'deploy',
+            only: { variables: %w[$SOMETHING] }
+          },
+          test: {
+            extends: 'something',
+            script: 'ls',
+            only: {
+              refs: %w[master],
+              variables: %w[$SOMETHING]
+            }
+          }
+        )
+      end
+    end
+
+    pending 'when a hash recursive extensions'
+
+    pending 'when invalid `extends` is specified'
+  end
+end