lib/monitor.rb: prevent to initialize MonitorMixin twice

and allow to initialize again when obj.dup.

Suggested by Benoit Daloze.  [ruby-core:88504] [Feature #15000]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66215 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/monitor.rb

@@ -255,9 +255,13 @@ 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) && @mon_mutex_owner_object_id == object_id
+      raise ThreadError, "already initialized"
+    end
+    @mon_mutex = Thread::Mutex.new
+    @mon_mutex_owner_object_id = object_id
     @mon_owner = nil
     @mon_count = 0
-    @mon_mutex = Thread::Mutex.new
   end
 
   def mon_check_owner