fixup! Combine Mutex with ReentrantMutex

lib/rspec/support/reentrant_mutex.rb

@@ -1,7 +1,13 @@
-RSpec::Support.require_rspec_support "mutex"
-
 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.
     #

spec/rspec/support/reentrant_mutex_spec.rb

@@ -1,6 +1,13 @@
 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 pass if they don't deadlock
 RSpec.describe RSpec::Support::ReentrantMutex do