Remove change in lib/monitor.rb

* It conflicts with using mon_initialize in initialize_copy.
* Fixes #1428.

CHANGELOG.md

@@ -20,6 +20,7 @@ Bug fixes:
 * Fixed problem with `Kernel.public_send` not tracking its caller properly (#1425).
 * `rb_thread_call_without_gvl()` no longer holds the C-extensions lock.
 * Fixed `caller_locations` when called inside `method_added`.
+* Fixed `mon_initialize` when called inside `initialize_copy` (#1428).
 
 Performance:
 

lib/mri/monitor.rb

@@ -239,12 +239,9 @@ def initialize(*args)
   # Initializes the MonitorMixin after being included in a class or when an
   # object has been extended with the MonitorMixin
   def mon_initialize
-    # In TruffleRuby avoid resetting the Mutex if mon_initialize is called twice, such as in Rails tests
-    if RUBY_ENGINE != 'truffleruby' || !@mon_mutex
-      @mon_owner = nil
-      @mon_count = 0
-      @mon_mutex = Thread::Mutex.new
-    end
+    @mon_owner = nil
+    @mon_count = 0
+    @mon_mutex = Thread::Mutex.new
   end
 
   def mon_check_owner