Use Mutex from core, prevent it from being stubbed

Author: pirj

lib/rspec/support/mutex.rb

@@ -1,15 +1,19 @@
 module RSpec
   module Support
+    # This class protects us against Mutex.new stubbed out within tests.
+    # @private
+    class Mutex < ::Mutex
+      class << self
+        define_method(:new, &::Mutex.method(:new))
+      end
+    end
+
     # Allows a thread to lock out other threads from a critical section of code,
     # while allowing the thread with the lock to reenter that section.
     #
     # Based on Monitor as of 2.2 -
     # https://github.com/ruby/ruby/blob/eb7ddaa3a47bf48045d26c72eb0f263a53524ebc/lib/monitor.rb#L9
     #
-    # Depends on Mutex, but Mutex is only available as part of core since 1.9.1:
-    #   exists - http://ruby-doc.org/core-1.9.1/Mutex.html
-    #   dne    - http://ruby-doc.org/core-1.9.0/Mutex.html
-    #
     # @private
     class ReentrantMutex
       def initialize
@@ -40,22 +44,5 @@ def exit
         @mutex.unlock
       end
     end
-
-    if defined? ::Mutex
-      # On 1.9 and up, this is in core, so we just use the real one
-      class Mutex < ::Mutex
-        # If you mock Mutex.new you break our usage of Mutex, so
-        # instead we capture the original method to return Mutexs.
-        NEW_MUTEX_METHOD = Mutex.method(:new)
-
-        def self.new
-          NEW_MUTEX_METHOD.call
-        end
-      end
-    else # For 1.8.7
-      # :nocov:
-      RSpec::Support.require_rspec_support "mutex"
-      # :nocov:
-    end
   end
 end

lib/rspec/support/reentrant_mutex.rb

@@ -1,8 +1,15 @@
 require 'rspec/support/reentrant_mutex'
 require 'thread_order'
 
+RSpec.describe RSpec::Support::Mutex do
+  it "allows ::Mutex to be mocked" do
+    expect(Mutex).to receive(:new)
+    ::Mutex.new
+  end
+end
+
 # There are no assertions specifically
-# They are pass if they don't deadlock
+# They pass if they don't deadlock
 RSpec.describe RSpec::Support::ReentrantMutex do
   let!(:mutex) { described_class.new }
   let!(:order) { ThreadOrder.new }