Extract CI config YAML parser to a separate class

With this approach it would be easier to add different sources of
configuration, that we do not necessairly have to be in YAML format.

lib/gitlab/ci/config.rb

@@ -4,13 +4,13 @@ module Gitlab
       class ParserError < StandardError; end
 
       def initialize(config)
-        @config = YAML.safe_load(config, [Symbol], [], true)
+        parser = Parser.new(config)
 
-        unless @config.is_a?(Hash)
-          raise ParserError, 'YAML should be a hash'
+        unless parser.valid?
+          raise ParserError, 'Invalid configuration format!'
         end
 
-        @config = @config.deep_symbolize_keys
+        @config = parser.parse
       end
 
       def to_hash

lib/gitlab/ci/config/parser.rb

@@ -0,0 +1,25 @@
+module Gitlab
+  module Ci
+    class Config
+      class Parser
+        class FormatError < StandardError; end
+
+        def initialize(config)
+          @config = YAML.safe_load(config, [Symbol], [], true)
+        end
+
+        def valid?
+          @config.is_a?(Hash)
+        end
+
+        def parse
+          unless valid?
+            raise FormatError, 'Invalid configuration format'
+          end
+
+          @config.deep_symbolize_keys
+        end
+      end
+    end
+  end
+end

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

@@ -0,0 +1,54 @@
+require 'spec_helper'
+
+describe Gitlab::Ci::Config::Parser do
+  let(:parser) { described_class.new(yml) }
+
+  context 'when yaml syntax is correct' do
+    let(:yml) { 'image: ruby:2.2' }
+
+    describe '#valid?' do
+      it 'returns true' do
+        expect(parser.valid?).to be true
+      end
+    end
+
+    describe '#parse' do
+      it 'returns a hash' do
+        expect(parser.parse).to be_a Hash
+      end
+
+      it 'returns a valid hash' do
+        expect(parser.parse).to eq(image: 'ruby:2.2')
+      end
+    end
+  end
+
+  context 'when yaml syntax is incorrect' do
+    let(:yml) { '// incorrect' }
+
+    describe '#valid?' do
+      it 'returns false' do
+        expect(parser.valid?).to be false
+      end
+    end
+
+    describe '#parse' do
+      it 'raises error' do
+        expect { parser.parse }.to raise_error(
+          Gitlab::Ci::Config::Parser::FormatError,
+          'Invalid configuration format'
+        )
+      end
+    end
+  end
+
+  context 'when yaml config is empty' do
+    let(:yml) { '' }
+
+    describe '#valid?' do
+      it 'returns false' do
+        expect(parser.valid?).to be false
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/config_spec.rb

@@ -5,7 +5,7 @@ describe Gitlab::Ci::Config do
     described_class.new(yml)
   end
 
-  context 'when yml config is valid' do
+  context 'when config is valid' do
     let(:yml) do
       <<-EOS
         image: ruby:2.2
@@ -30,5 +30,17 @@ describe Gitlab::Ci::Config do
         expect(config.to_hash).to eq hash
       end
     end
+
+    context 'when config is invalid' do
+      let(:yml) { '// invalid' }
+
+      describe '.new' do
+        it 'raises error' do
+          expect { config }.to raise_error(
+            Gitlab::Ci::Config::ParserError, /Invalid configuration format/
+          )
+        end
+      end
+    end
   end
 end