lib/monitor.rb: prevent to initialize MonitorMixin twice

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

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65822 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/monitor.rb

@@ -251,9 +251,12 @@ 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

@@ -0,0 +1,37 @@
+# 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