From 542c70aab748cf17960cc2a0959ba490983ceae6 Mon Sep 17 00:00:00 2001
From: Daniel Pepper <pepper.daniel@gmail.com>
Date: Sun, 4 Jun 2023 21:29:32 -0700
Subject: [PATCH] [ruby/singleton] Simplify implementation of
 `Singleton#instance`. (https://github.com/ruby/singleton/pull/9)

- Add more tests to cover rails' usage.
---
 lib/singleton.rb       |  7 +------
 test/test_singleton.rb | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/lib/singleton.rb b/lib/singleton.rb
index 07420d2ea2..7ed17a769c 100644
--- a/lib/singleton.rb
+++ b/lib/singleton.rb
@@ -121,12 +121,7 @@ module Singleton
     end
 
     def instance # :nodoc:
-      return @singleton__instance__ if @singleton__instance__
-      @singleton__mutex__.synchronize {
-        return @singleton__instance__ if @singleton__instance__
-        @singleton__instance__ = new()
-      }
-      @singleton__instance__
+      @singleton__instance__ || @singleton__mutex__.synchronize { @singleton__instance__ ||= new }
     end
 
     private
diff --git a/test/test_singleton.rb b/test/test_singleton.rb
index b3c48bb5f5..e474a0ccc5 100644
--- a/test/test_singleton.rb
+++ b/test/test_singleton.rb
@@ -94,6 +94,23 @@ 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_inheritance_instantiation
+    klass = Class.new do
+      include Singleton
+
+      public_class_method :new
+    end
+
+    assert Class.new(klass).new
+  end
+
   def test_class_level_cloning_preserves_singleton_behavior
     klass = SingletonTest.clone
 
@@ -101,4 +118,8 @@ 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