Introduce Ruby linting (#1699)

* Add standardrb gem for Ruby linting

* Add standardrb to CI lint job

* Auto-fix standardrb violations

* Add .standard.yml to exclude test fixture repos from linting

The repos/ directory contains external Rails application fixtures used for
integration testing. These should not be linted as they represent external
code, not the buildpack's own code.

* Fix Style/StringLiterals in Gemfile

Lint failure: Style/StringLiterals - Prefer double-quoted strings unless you
need single quotes to avoid extra backslashes for escaping.

The fix uses double quotes consistently for all gem declarations, matching
the style used by the rest of the Gemfile.

* Disable lint rules for intentional patterns in bin/support scripts

Lint failures disabled:
- Lint/RescueException: Rescuing Exception is intentional in buildpack entry
  points to ensure all errors (including SignalException, SystemExit) are
  properly reported to users rather than causing cryptic failures.
- Style/MixinUsage: Including modules at top level is intentional for these
  standalone scripts, which are not library code and benefit from direct
  access to shell helper methods.

* Fix lint violations in heroku_build_report.rb

Lint failures:
- Lint/NonLocalExitFromIterator: Changed 'return' to 'next' in the each block.
  Using 'return' would exit the entire capture method, while 'next' correctly
  skips to the next iteration.
- Style/RedundantInterpolation: Changed "\#{key}" to key.to_s since the key
  is already a string after strip. This is more explicit and avoids
  unnecessary string interpolation.

* Fix lint violations in language_pack.rb

Lint failures:
- Lint/AssignmentInCondition: Wrapped 'pack = LanguagePack.detect(...)' in
  parentheses to clarify that the assignment is intentional.
- Style/SafeNavigation: Changed 'if pack_klass; pack_klass.new(...); end'
  to 'pack_klass&.new(...)' using safe navigation operator. This is more
  idiomatic Ruby and clearly expresses the intent to call new only if
  pack_klass is not nil.

* Fix Performance/UnfreezeString violations

Lint failure: Performance/UnfreezeString - Use unary plus to get an unfrozen
string literal.

Changed String.new("") and String.new("...") to +"" and +"..."
respectively. The unary plus operator on a frozen string literal returns
an unfrozen copy, which is more idiomatic and performant than String.new.
These mutable strings are necessary because we append to them with <<.

* Fix Lint/AssignmentInCondition and Lint/MixedRegexpCaptureTypes

Lint failures fixed:
- Lint/AssignmentInCondition: Wrapped 'if var = expr' in parentheses to
  clarify that the assignment is intentional, not a typo for '=='.
- Lint/MixedRegexpCaptureTypes: Changed numbered capture groups (\r?\n)
  to non-capturing groups (?:\r?\n) in regex patterns. Mixing named and
  numbered captures causes confusion because numbered captures are
  renumbered when named captures are present, making the regex behavior
  unpredictable.

* Fix Style/RedundantSort in outdated_ruby_version.rb

Lint failure: Style/RedundantSort - Use max_by instead of sort_by...last.

Changed 'sort_by { |v| Gem::Version.new(v) }.last' to
'max_by { |v| Gem::Version.new(v) }'. Using max_by is more efficient as it
finds the maximum in O(n) time without sorting the entire array, and clearly
expresses the intent to find the maximum value.

* Fix Lint/DuplicateMethods in rake_runner.rb

Lint failure: Lint/DuplicateMethods - Method RakeTask#status is defined at
both line 11 (via attr_accessor) and line 37 (explicit def).

The explicit status method includes validation logic that ensures the status
is set to an allowed value before returning it. Changed 'attr_accessor :status'
to 'attr_writer :status' so we only generate the setter, keeping the custom
getter with its validation logic.

* Fix lint violations in ruby.rb

Lint failures fixed:
- Performance/UnfreezeString: Changed String.new("") to +"" for creating
  a mutable string to append to.
- Lint/BinaryOperatorWithIdenticalOperands: Fixed 'version != version' which
  always evaluates to false. Changed to 'old_version != version' to correctly
  detect when the default Node.js/Yarn version has changed. This was a bug
  where the version change warning would never be displayed.
- Lint/IneffectiveAccessModifier: Added file-level ignore in .standard.yml.
  The class has public class methods interspersed after a 'private' declaration
  that only affects instance methods. The class methods are intentionally
  public and called from lib/language_pack.rb.

* Fix lint violations in ruby_version.rb and shell_helpers.rb

Lint failures fixed:
- Style/RedundantInterpolation: Changed "\#{engine_version}" to
  engine_version.to_s and "\#{path}" to path.to_s. String interpolation
  on a single variable is redundant when the variable is already a string
  or should be explicitly converted.
- Removed unused RUBY_VERSION_REGEX constant (dead code).

* Fix lint violations in spec files

Lint failures fixed:
- Lint/RescueException (ruby_spec.rb): Added inline disable. The test
  rescues Exception intentionally to provide debugging output for any failure.
- Style/RedundantInterpolation (outdated_ruby_version_spec.rb): Removed
  redundant string interpolation.
- Lint/ShadowedArgument (ruby_version_spec.rb): Renamed block arg to _dir
  since it's immediately shadowed by a local variable assignment.
- Lint/ConstantDefinitionInBlock (shell_spec.rb): Added inline disable.
  Defining test helper classes inside describe blocks is a common RSpec pattern.
- Lint/DuplicateRequire (spec_helper.rb): Removed duplicate require 'hatchet'.
- Lint/MixedRegexpCaptureTypes (spec_helper.rb): Changed numbered capture
  group to non-capturing group.
- Lint/DuplicateMethods (spec_helper.rb): Removed duplicate hatchet_path
  method definition.

* Fix Layout/IndentationConsistency in ruby.rb

Lint failure: Layout/IndentationConsistency - Inconsistent indentation detected.

Fixed extra indentation on the bundle_command assignment line.

* Changelog

* Remove unused include

* Apply linting fixes

Author: schneems

.github/workflows/ci.yml

@@ -16,6 +16,13 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v6
+      - name: Install Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+          ruby-version: "3.3.9"
+      - name: Run StandardRB
+        run: bundle exec standardrb
       - name: Run ShellCheck bin top level
         run: |
             # All bash files that don't end in '.rb'

.standard.yml

@@ -0,0 +1,7 @@
+# StandardRB configuration
+# https://github.com/standardrb/standard
+
+ignore:
+  - "repos/**/*"  # Test fixture repos (external Rails apps for integration tests)
+  - "lib/language_pack/ruby.rb":
+    - Lint/IneffectiveAccessModifier  # Class methods after `private` are intentionally public; private only affects instance methods

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - Set `WEB_CONCURRENCY_SET_BY=heroku/ruby` if `WEB_CONCURRENCY` gets set in (deprecated) `SENSIBLE_DEFAULTS` mode (https://github.com/heroku/heroku-buildpack-ruby/pull/1700)
 - Default Node.js version now 24.13.0 (https://github.com/heroku/heroku-buildpack-ruby/pull/1704)
+- Fix yarn and nodejs version change warnings (https://github.com/heroku/heroku-buildpack-ruby/pull/1699)
 
 ## [v344] - 2026-01-14
 
@@ -14,7 +15,6 @@
 
 - Ruby 4.0.1 is now available
 
-
 ## [v342] - 2026-01-09
 
 - Update bundler version warning output (https://github.com/heroku/heroku-buildpack-ruby/pull/1697)

Gemfile

@@ -11,7 +11,8 @@ group :development, :test do
   gem "excon"
   gem "rake"
   gem "parallel_split_test"
-  gem 'rspec-retry'
-  gem 'json'
-  gem 'redis'
+  gem "rspec-retry"
+  gem "json"
+  gem "redis"
+  gem "standard"
 end

Gemfile.lock

@@ -1,6 +1,7 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    ast (2.4.3)
     base64 (0.3.0)
     citrus (3.0.2)
     connection_pool (2.5.3)
@@ -22,24 +23,32 @@ GEM
       thor (~> 1)
       threaded (~> 0)
     json (2.18.0)
+    language_server-protocol (3.17.0.5)
+    lint_roller (1.1.0)
     logger (1.7.0)
     moneta (1.0.0)
     multi_json (1.17.0)
     parallel (1.27.0)
     parallel_split_test (0.10.0)
       parallel (>= 0.5.13)
       rspec-core (>= 3.9.0)
+    parser (3.3.10.0)
+      ast (~> 2.4.1)
+      racc
     platform-api (3.8.0)
       heroics (~> 0.1.1)
       moneta (~> 1.0.0)
       rate_throttle_client (~> 0.1.0)
+    prism (1.7.0)
     racc (1.8.1)
+    rainbow (3.1.1)
     rake (13.3.1)
     rate_throttle_client (0.1.2)
     redis (5.4.1)
       redis-client (>= 0.22.0)
     redis-client (0.25.1)
       connection_pool
+    regexp_parser (2.11.3)
     rrrretry (1.0.0)
     rspec-core (3.13.6)
       rspec-support (~> 3.13.0)
@@ -49,11 +58,45 @@ GEM
     rspec-retry (0.6.2)
       rspec-core (> 3.3)
     rspec-support (3.13.6)
+    rubocop (1.81.7)
+      json (~> 2.3)
+      language_server-protocol (~> 3.17.0.2)
+      lint_roller (~> 1.1.0)
+      parallel (~> 1.10)
+      parser (>= 3.3.0.2)
+      rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 2.9.3, < 3.0)
+      rubocop-ast (>= 1.47.1, < 2.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (>= 2.4.0, < 4.0)
+    rubocop-ast (1.49.0)
+      parser (>= 3.3.7.2)
+      prism (~> 1.7)
+    rubocop-performance (1.26.1)
+      lint_roller (~> 1.1)
+      rubocop (>= 1.75.0, < 2.0)
+      rubocop-ast (>= 1.47.1, < 2.0)
+    ruby-progressbar (1.13.0)
+    standard (1.52.0)
+      language_server-protocol (~> 3.17.0.2)
+      lint_roller (~> 1.0)
+      rubocop (~> 1.81.7)
+      standard-custom (~> 1.0.0)
+      standard-performance (~> 1.8)
+    standard-custom (1.0.2)
+      lint_roller (~> 1.0)
+      rubocop (~> 1.50)
+    standard-performance (1.9.0)
+      lint_roller (~> 1.1)
+      rubocop-performance (~> 1.26.0)
     thor (1.4.0)
     threaded (0.0.4)
     toml-rb (4.1.0)
       citrus (~> 3.0, > 3.0)
       racc (~> 1.7)
+    unicode-display_width (3.2.0)
+      unicode-emoji (~> 4.1)
+    unicode-emoji (4.2.0)
     webrick (1.9.1)
 
 PLATFORMS
@@ -69,6 +112,7 @@ DEPENDENCIES
   rspec-core
   rspec-expectations
   rspec-retry
+  standard
   toml-rb
 
 RUBY VERSION

Rakefile

@@ -5,8 +5,8 @@
 #
 require "fileutils"
 require "tmpdir"
-require 'hatchet/tasks'
-require_relative 'lib/rake/deploy_check'
+require "hatchet/tasks"
+require_relative "lib/rake/deploy_check"
 
 namespace :buildpack do
   desc "prepares the next version of the buildpack for release"
@@ -44,13 +44,13 @@ namespace :buildpack do
 end
 
 begin
-  require 'rspec/core/rake_task'
+  require "rspec/core/rake_task"
 
   desc "Run specs"
   RSpec::Core::RakeTask.new(:spec) do |t|
-    t.rspec_opts = %w(-fd --color)
-    #t.ruby_opts  = %w(-w)
+    t.rspec_opts = %w[-fd --color]
+    # t.ruby_opts  = %w(-w)
   end
-  task :default => :spec
-rescue LoadError => e
+  task default: :spec
+rescue LoadError
 end

bin/support/ruby_compile.rb

@@ -25,8 +25,8 @@
     app_path: app_path,
     cache_path: cache_path,
     gemfile_lock: gemfile_lock,
-    bundle_default_without: "development:test",
+    bundle_default_without: "development:test"
   )
-rescue Exception => e
+rescue Exception => e # standard:disable Lint/RescueException
   LanguagePack::ShellHelpers.display_error_and_exit(e)
 end

bin/support/ruby_test-compile.rb

@@ -16,7 +16,6 @@
 require "language_pack"
 require "language_pack/shell_helpers"
 require "language_pack/test"
-include LanguagePack::ShellHelpers
 
 begin
   app_path = Pathname(ARGV[0])
@@ -30,8 +29,8 @@
     cache_path: cache_path,
     gemfile_lock: gemfile_lock,
     bundle_default_without: "development",
-    environment_name: "test",
+    environment_name: "test"
   )
-rescue Exception => e
+rescue Exception => e # standard:disable Lint/RescueException
   LanguagePack::ShellHelpers.display_error_and_exit(e)
 end

bin/support/ruby_test.rb

@@ -12,7 +12,7 @@
 require "language_pack/test"
 require "language_pack/ruby"
 
-include LanguagePack::ShellHelpers
+include LanguagePack::ShellHelpers # standard:disable Style/MixinUsage
 
 def execute_test(command)
   topic("Running test: #{command}")
@@ -26,7 +26,7 @@ def execute_command(command)
   # having no whitespace before output. To avoid adding whitespace
   # for the original Kernel.puts to be used by passing in the
   # Kernel object.
-  pipe(command, :user_env => true, :output_object => Kernel)
+  pipe(command, user_env: true, output_object: Kernel)
 end
 
 # $ bin/test app_path ENV_DIR ARTIFACT_DIR
@@ -35,7 +35,7 @@ def execute_command(command)
 Dir.chdir(app_path)
 
 gems_list = LanguagePack::Helpers::BundleList::HumanCommand.new(
-  stream_to_user: false,
+  stream_to_user: false
 ).call
 
 execute_test(
@@ -53,4 +53,3 @@ def execute_command(command)
     "rake test"
   end
 )
-

lib/heroku_build_report.rb

@@ -1,6 +1,6 @@
-require 'yaml'
-require 'json'
-require 'pathname'
+require "yaml"
+require "json"
+require "pathname"
 
 # Observability reporting for builds
 #
@@ -17,7 +17,7 @@ class JsonReport
     MALFORMED_JSON_KEY = "build_report_file_malformed"
     attr_reader :data
 
-    def initialize(path: , io: $stdout)
+    def initialize(path:, io: $stdout)
       @io = io
       @path = Pathname(path).expand_path
       @path.dirname.mkpath
@@ -35,9 +35,9 @@ def safe_load(contents)
       else
         JSON.parse(contents)
       end
-    rescue => e
+    rescue
       @io.puts "Internal Warning: Expected build report to be JSON, but it is malformed: #{contents.inspect}"
-      { MALFORMED_JSON_KEY => true }
+      {MALFORMED_JSON_KEY => true}
     end
 
     def complex_object?(value)
@@ -46,7 +46,7 @@ def complex_object?(value)
 
     def capture(metrics = {})
       metrics.each do |(key, value)|
-        return if key.nil? || key.to_s.strip.empty?
+        next if key.nil? || key.to_s.strip.empty?
 
         key = key&.strip
         raise "Key cannot be empty (#{key.inspect} => #{value})" if key.nil? || key.empty?
@@ -56,15 +56,15 @@ def capture(metrics = {})
           value = value.to_s
         end
 
-        @data["#{key}"] = value
+        @data[key.to_s] = value
       end
 
       @path.write(@data.to_json)
     end
   end
 
   # Current load order of the various "language packs"
-  def self.set_global(path: )
+  def self.set_global(path:)
     JsonReport.new(path: path).tap { |report|
       # Silence warning about setting a constant
       begin
@@ -82,5 +82,5 @@ def self.dev_null
     JsonReport.new(path: "/dev/null")
   end
 
-  GLOBAL = self.dev_null # Changed via `set_global`
+  GLOBAL = dev_null # Changed via `set_global`
 end