New option --display-only-failed for junit format

Speeds up test report processing for large codebases and helps address
the sorts of concerns raised at
mikian/rubocop-junit-formatter#18.

Refactors file_finished method in junit_formatter.rb to reduce
complexity.

Fixes buggy error message when --disable-uncorrectable is run without
--auto-correct, which I noticed because I added similar error messaging
when you try to run --display-only-failed without --format junit

Tests, CHANGELOG.md, doc updates, and help message updates to accompany
new option.

CHANGELOG.md

@@ -9,6 +9,7 @@
 * [#7908](https://github.com/rubocop-hq/rubocop/pull/7908): Add new `Style/RedundantRegexpEscape` cop. ([@owst][])
 * [#7978](https://github.com/rubocop-hq/rubocop/pull/7978): Add new option `OnlyFor` to the `Bundler/GemComment` cop. ([@ric2b][])
 * [#8063](https://github.com/rubocop-hq/rubocop/issues/8063): Add new `AllowedNames` option for `Naming/ClassAndModuleCamelCase`. ([@tejasbubane][])
+* New option `--display-only-failed` that can be used with `--format junit`. Speeds up test report processing for large codebases and helps address the sorts of concerns raised at [mikian/rubocop-junit-formatter #18](https://github.com/mikian/rubocop-junit-formatter/issues/18). ([@burnettk][])
 
 ### Bug fixes
 
@@ -4558,3 +4559,4 @@
 [@CvX]: https://github.com/CvX
 [@jschneid]: https://github.com/jschneid
 [@ric2b]: https://github.com/ric2b
+[@burnettk]: https://github.com/burnettk

docs/modules/ROOT/pages/formatters.adoc

@@ -280,6 +280,19 @@ $ rubocop --format junit
 </testsuites>
 ----
 
+The `junit` style formatter is very useful for continuous integration systems
+such as Jenkins, most of which support junit formatting when parsing test
+results. A typical invocation in this type of scenario might look like:
+
+[source,sh]
+----
+$ rubocop --format junit --out test-reports/junit.xml
+----
+
+Since there is one XML node for each cop for each file, the size of the resulting
+XML can get quite large. If it is too large for you, you can restrict the output
+to just failures by adding the `--display-only-failed` option.
+
 == Offense Count Formatter
 
 Sometimes when first applying RuboCop to a codebase, it's nice to be able to

legacy-docs/formatters.md

@@ -242,7 +242,7 @@ The JSON structure is like the following example:
 **Machine-parsable**
 
 The `junit` style formatter provides the JUnit formatting.
-This formatter is based on [rubocop-junit-formatter gem](https://github.com/mikian/rubocop-junit-formatter).
+This formatter is based on the [rubocop-junit-formatter gem](https://github.com/mikian/rubocop-junit-formatter).
 
 ```sh
 $ rubocop --format junit
@@ -268,6 +268,18 @@ $ rubocop --format junit
 </testsuites>
 ```
 
+The `junit` style formatter is very useful for continuous integration systems
+such as Jenkins, most of which support junit formatting when parsing test
+results. A typical invocation in this type of scenario might look like:
+
+```sh
+$ rubocop --format junit --out test-reports/junit.xml
+```
+
+Since there is one XML node for each cop for each file, the size of the resulting
+XML can get quite large. If it is too large for you, you can restrict the output
+to just failures by adding the `--display-only-failed` option.
+
 ## Offense Count Formatter
 
 Sometimes when first applying RuboCop to a codebase, it's nice to be able to

lib/rubocop/formatter/junit_formatter.rb

@@ -35,19 +35,29 @@ def file_finished(file, offenses)
         #
         # In the future, it would be preferable to return only enabled cops.
         Cop::Cop.all.each do |cop|
+          target_offenses = offenses_for_cop(offenses, cop)
+
+          next unless relevant_for_output?(options, target_offenses)
+
           REXML::Element.new('testcase', @testsuite).tap do |testcase|
             testcase.attributes['classname'] = classname_attribute_value(file)
             testcase.attributes['name'] = cop.cop_name
 
-            target_offenses = offenses.select do |offense|
-              offense.cop_name == cop.cop_name
-            end
-
             add_failure_to(testcase, target_offenses, cop.cop_name)
           end
         end
       end
 
+      def relevant_for_output?(options, target_offenses)
+        !options[:display_only_failed] || target_offenses.any?
+      end
+
+      def offenses_for_cop(all_offenses, cop)
+        all_offenses.select do |offense|
+          offense.cop_name == cop.cop_name
+        end
+      end
+
       def classname_attribute_value(file)
         file.gsub(/\.rb\Z/, '').gsub("#{Dir.pwd}/", '').tr('/', '.')
       end

lib/rubocop/options.rb

@@ -143,6 +143,8 @@ def add_formatting_options(opts)
           @options[:output_path] = path
         end
       end
+
+      option(opts, '--display-only-failed')
     end
 
     def add_severity_option(opts)
@@ -286,6 +288,7 @@ def validate_compatibility # rubocop:disable Metrics/MethodLength
       end
       validate_auto_gen_config
       validate_auto_correct
+      validate_display_only_failed
       validate_parallel
 
       return if incompatible_options.size <= 1
@@ -309,13 +312,20 @@ def validate_auto_gen_config
       end
     end
 
+    def validate_display_only_failed
+      return unless @options.key?(:display_only_failed)
+      return if @options[:format] == 'junit'
+
+      raise OptionArgumentError,
+            format('--display-only-failed can only be used together with --format junit.')
+    end
+
     def validate_auto_correct
       return if @options.key?(:auto_correct)
       return unless @options.key?(:disable_uncorrectable)
 
       raise OptionArgumentError,
-            format('--%<flag>s can only be used together with --auto-correct.',
-                   flag: '--disable-uncorrectable')
+            format('--disable-uncorrectable can only be used together with --auto-correct.')
     end
 
     def validate_parallel
@@ -431,6 +441,8 @@ module OptionsHelp
                                          'if no format is specified.'],
       fail_level:                       ['Minimum severity (A/R/C/W/E/F) for exit',
                                          'with error code.'],
+      display_only_failed:              ['Only output offense messages. Omit passing',
+                                         'cops. Only valid for --format junit.'],
       display_only_fail_level_offenses:
                                         ['Only output offense messages at',
                                          'the specified --fail-level or above'],

spec/rubocop/options_spec.rb

@@ -92,6 +92,8 @@ def abs(path)
                                                This option applies to the previously
                                                specified --format, or the default format
                                                if no format is specified.
+                  --display-only-failed        Only output offense messages. Omit passing
+                                               cops. Only valid for --format junit.
               -r, --require FILE               Require Ruby file.
                   --fail-level SEVERITY        Minimum severity (A/R/C/W/E/F) for exit
                                                with error code.
@@ -224,6 +226,18 @@ def abs(path)
       end
     end
 
+    describe '--display-only-failed' do
+      it 'fails if given without --format junit' do
+        expect { options.parse %w[--display-only-failed] }
+          .to raise_error(RuboCop::OptionArgumentError)
+      end
+
+      it 'works if given with --format junit' do
+        expect { options.parse %w[--format junit --display-only-failed] }
+          .not_to raise_error(RuboCop::OptionArgumentError)
+      end
+    end
+
     describe '--fail-level' do
       it 'accepts full severity names' do
         %w[refactor convention warning error fatal].each do |severity|