[ruby/singleton] Revert "Simplify the implementation

(https://github.com/ruby/singleton/pull/7)"

This reverts commit https://github.com/ruby/singleton/commit/545b6b61a40d.

  This change break Rails CI: https://bugs.ruby-lang.org/issues/19711

https://github.com/ruby/singleton/commit/911531d508

lib/singleton.rb

@@ -112,7 +112,7 @@ module Singleton
   module SingletonClassMethods # :nodoc:
 
     def clone # :nodoc:
-      super.include(Singleton)
+      Singleton.__init__(super)
     end
 
     # By default calls instance(). Override to retain singleton state.
@@ -121,18 +121,31 @@ module Singleton
     end
 
     def instance # :nodoc:
-      @singleton__instance__ || @singleton__mutex__.synchronize { @singleton__instance__ ||= new }
+      return @singleton__instance__ if @singleton__instance__
+      @singleton__mutex__.synchronize {
+        return @singleton__instance__ if @singleton__instance__
+        @singleton__instance__ = new()
+      }
+      @singleton__instance__
     end
 
     private
 
     def inherited(sub_klass)
       super
-      sub_klass.include(Singleton)
+      Singleton.__init__(sub_klass)
     end
   end
 
   class << Singleton # :nodoc:
+    def __init__(klass) # :nodoc:
+      klass.instance_eval {
+        @singleton__instance__ = nil
+        @singleton__mutex__ = Thread::Mutex.new
+      }
+      klass
+    end
+
     private
 
     # extending an object with Singleton is a bad idea
@@ -143,19 +156,14 @@ module Singleton
       unless mod.instance_of?(Class)
         raise TypeError, "Inclusion of the OO-Singleton module in module #{mod}"
       end
-
       super
     end
 
     def included(klass)
       super
-
       klass.private_class_method :new, :allocate
       klass.extend SingletonClassMethods
-      klass.instance_eval {
-        @singleton__instance__ = nil
-        @singleton__mutex__ = Thread::Mutex.new
-      }
+      Singleton.__init__(klass)
     end
   end
 

test/test_singleton.rb

@@ -94,13 +94,6 @@ class TestSingleton < Test::Unit::TestCase
     assert_same a, b
   end
 
-  def test_inheritance_creates_separate_singleton
-    a = SingletonTest.instance
-    b = Class.new(SingletonTest).instance
-
-    assert_not_same a, b
-  end
-
   def test_class_level_cloning_preserves_singleton_behavior
     klass = SingletonTest.clone
 
@@ -108,8 +101,4 @@ class TestSingleton < Test::Unit::TestCase
     b = klass.instance
     assert_same a, b
   end
-
-  def test_class_level_cloning_creates_separate_singleton
-    assert_not_same SingletonTest.instance, SingletonTest.clone.instance
-  end
 end