Allow successful mocking of Mutex#synchronize

lib/rspec/mocks/proxy.rb

@@ -1,3 +1,5 @@
+RSpec::Support.require_rspec_support 'mutex'
+
 module RSpec
   module Mocks
     # @private
@@ -9,11 +11,6 @@ def ==(expectation)
         end
       end
 
-      unless defined?(Mutex)
-        Support.require_rspec_support 'mutex'
-        Mutex = Support::Mutex
-      end
-
       # @private
       def ensure_implemented(*_args)
         # noop for basic proxies, see VerifyingProxy for behaviour.
@@ -27,7 +24,7 @@ def initialize(object, order_group, options={})
         @order_group = order_group
         @error_generator = ErrorGenerator.new(object)
         @messages_received = []
-        @messages_received_mutex = Mutex.new
+        @messages_received_mutex = Support::Mutex.new
         @options = options
         @null_object = false
         @method_doubles = Hash.new { |h, k| h[k] = MethodDouble.new(@object, k, self) }

spec/rspec/mocks/mutex_spec.rb

@@ -0,0 +1,18 @@
+module RSpec
+  module Mocks
+    RSpec.describe "Mocking Mutex" do
+      let(:mocked_mutex) { instance_double(Mutex) }
+      before do
+        allow(Mutex).to receive(:new).and_return(mocked_mutex)
+        allow(mocked_mutex).to receive(:synchronize).and_yield
+      end
+
+      it "successfully yields" do
+        called = false
+        mutex = Mutex.new
+        mutex.synchronize { called = true }
+        expect(called).to be_truthy
+      end
+    end
+  end
+end