Add support for LazyOutputFormat in the DSL

The `output` DSL method now takes a `:lazy` option, which configures LazyOutputFormat.

lib/rubydoop/dsl.rb

@@ -139,6 +139,9 @@ def output(dir=nil, options={})
         end
         format.set_output_path(@job, Hadoop::Fs::Path.new(@output_dir))
         @job.set_output_format_class(format)
+        if options[:lazy]
+          Hadoop::Mapreduce::Lib::Output::LazyOutputFormat.set_output_format_class(@job, format)
+        end
       end
       @output_dir
     end

spec/integration/hadoop_system_spec.rb

@@ -155,5 +155,12 @@ def isolated_run(dir, cmd)
         expect(uniques['e']).to eq 128
       end
     end
+
+    context 'the lazy output job' do
+      it 'produces no output files' do
+        expect(File.exist?('data/output/lazy_output/_SUCCESS')).to be_truthy
+        expect(Dir['data/output/lazy_output/part-r-*']).to be_empty
+      end
+    end
   end
 end

spec/resources/test_project/lib/lazy_output.rb

@@ -0,0 +1,8 @@
+# encoding: utf-8
+
+module LazyOutput
+  class Reducer
+    def reduce(key, values, context)
+    end
+  end
+end

spec/resources/test_project/lib/test_project.rb

@@ -6,6 +6,7 @@
 
 require 'word_count'
 require 'uniques'
+require 'lazy_output'
 
 
 Rubydoop.configure do |input_path, output_path|
@@ -46,6 +47,17 @@
     output_key Hadoop::Io::Text
     output_value Hadoop::Io::Text
   end
+
+  job 'lazy_output' do
+    input input_path
+    output "#{output_path}/lazy_output", lazy: true
+
+    mapper WordCount::Mapper
+    reducer LazyOutput::Reducer
+
+    output_key Hadoop::Io::Text
+    output_value Hadoop::Io::IntWritable
+  end
 end
 
 cc = Rubydoop::ConfigurationDefinition.new