revert r65822

Because mon_initialized may be used to re-initialize copied objects intentionally.

mon_initialize_spec.rb fails:

1)
MonitorMixin#mon_initialize can be called in initialize_copy to get a new Mutex and used with synchronize ERROR
ThreadError: already initialized
/home/shugo/src/ruby/lib/monitor.rb:255:in `mon_initialize'
/home/shugo/src/ruby/spec/ruby/library/monitor/mon_initialize_spec.rb:19:in `initialize_copy'
/home/shugo/src/ruby/spec/ruby/library/monitor/mon_initialize_spec.rb:28:in `initialize_dup'
/home/shugo/src/ruby/spec/ruby/library/monitor/mon_initialize_spec.rb:28:in `dup'
/home/shugo/src/ruby/spec/ruby/library/monitor/mon_initialize_spec.rb:28:in `block (2 levels) in <top (required)>'
/home/shugo/src/ruby/spec/ruby/library/monitor/mon_initialize_spec.rb:4:in `<top (required)>'

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65827 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/monitor.rb

@@ -251,12 +251,9 @@ module MonitorMixin
   # Initializes the MonitorMixin after being included in a class or when an
   # object has been extended with the MonitorMixin
   def mon_initialize
-    if defined?(@mon_mutex)
-      raise ThreadError, "already initialized"
-    end
-    @mon_mutex = Thread::Mutex.new
     @mon_owner = nil
     @mon_count = 0
+    @mon_mutex = Thread::Mutex.new
   end
 
   def mon_check_owner

test/monitor/test_monitor_mixin.rb

@@ -1,37 +0,0 @@
-# frozen_string_literal: false
-require 'test/unit'
-require 'monitor'
-
-class TestMonitorMixin < Test::Unit::TestCase
-  def test_cond
-    a = "foo"
-    a.extend(MonitorMixin)
-    cond = a.new_cond
-    queue1 = Queue.new
-    th = Thread.start do
-      queue1.deq
-      a.synchronize do
-        a.replace("bar")
-        cond.signal
-      end
-    end
-    th2 = Thread.start do
-      a.synchronize do
-        queue1.enq(nil)
-        assert_equal("foo", a)
-        result1 = cond.wait
-        assert_equal(true, result1)
-        assert_equal("bar", a)
-      end
-    end
-    assert_join_threads([th, th2])
-  end
-
-  def test_initialize_twice
-    a = Object.new
-    a.extend(MonitorMixin)
-    assert_raise(ThreadError) do
-      a.send(:mon_initialize)
-    end
-  end
-end