RUBY-32905 Add basic test for deducing source dir mechanism

Author: Dmitry Pogrebnoy

debase-ruby_core_source.gemspec

@@ -18,4 +18,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency "archive-tar-minitar", ">= 0.5.2"
   s.add_development_dependency 'rake', '>= 0.9.2'
   s.add_development_dependency 'minitar-cli'
+  s.add_development_dependency('test-unit', '~> 3.6.7')
 end

lib/debase/ruby_core_source.rb

@@ -3,6 +3,8 @@
 
 module Debase
   module RubyCoreSource
+    MOCK_RUBY_VERSION_KEY = "MOCK_RUBY_VERSION"
+
     REVISION_MAP = {
         # Add pre-release version here since they do not have patchlevel to refer to.
         # Revision can be found at `revision.h` of ruby sources.
@@ -54,7 +56,7 @@ def self.deduce_packaged_source_dir(ruby_dir)
         expected_directory
       else
         # Fallback to an older version.
-        ruby_version = Gem::Version.new(RUBY_VERSION)
+        ruby_version = Gem::Version.new(ENV[MOCK_RUBY_VERSION_KEY] || RUBY_VERSION)
         path, = Dir.glob(prefix + 'ruby-*').
           select { |d| File.directory?(d) }.
           map { |d| [d, ruby_source_dir_version(d)] }.

test/deduce_source_dir_test.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+DIR_NAME = File.dirname(__FILE__)
+$:.unshift File.join(DIR_NAME, "..", "lib")
+
+require 'test/unit'
+require 'debase/ruby_core_source'
+
+class DeduceSourceDirTest < Test::Unit::TestCase
+
+  def test_deduce_source_dir
+    version_to_answer = {
+      "3.4.1" => "ruby-3.4.0-p-1",
+      "3.4.0" => "ruby-3.4.0-p-1",
+      "3.3.6" => "ruby-3.3.5-p100",
+      "3.3.5" => "ruby-3.3.5-p100",
+      "3.3.4" => "ruby-3.3.0-p0",
+      "3.3.3" => "ruby-3.3.0-p0",
+      "3.2.6" => "ruby-3.2.0-p0",
+      "3.1.6" => "ruby-3.1.0-p0",
+      "3.0.7" => "ruby-3.0.0-p0",
+    }
+
+    version_to_answer.each do |version, answer|
+      ENV[Debase::RubyCoreSource::MOCK_RUBY_VERSION_KEY] = version
+      result = Debase::RubyCoreSource.deduce_packaged_source_dir(version)
+      deduced_source_dir_name = result.split(File::SEPARATOR).last
+      assert_equal(answer, deduced_source_dir_name)
+    end
+  end
+end