Monitor#exit: check monitor ownership.

Monitor#exit should be called by only onwer Thread. However, there
is not check for it.

ext/monitor/monitor.c

@@ -86,10 +86,25 @@ monitor_enter(VALUE monitor)
 }
 
 static VALUE
-monitor_exit(VALUE monitor)
+monitor_check_owner(VALUE monitor)
 {
     struct rb_monitor *mc = monitor_ptr(monitor);
+    if (!mc_owner_p(mc)) {
+        rb_raise(rb_eThreadError, "current thread not owner");
+    }
+    return Qnil;
+}
+
+static VALUE
+monitor_exit(VALUE monitor)
+{
+    monitor_check_owner(monitor);
+
+    struct rb_monitor *mc = monitor_ptr(monitor);
+
+    if (mc->count <= 0) rb_bug("monitor_exit: count:%d\n", (int)mc->count);
     mc->count--;
+
     if (mc->count == 0) {
         RB_OBJ_WRITE(monitor, &mc->owner, Qnil);
         rb_mutex_unlock(mc->mutex);
@@ -111,16 +126,6 @@ monitor_owned_p(VALUE monitor)
     return (rb_mutex_locked_p(mc->mutex) && mc_owner_p(mc)) ? Qtrue : Qfalse;
 }
 
-static VALUE
-monitor_check_owner(VALUE monitor)
-{
-    struct rb_monitor *mc = monitor_ptr(monitor);
-    if (!mc_owner_p(mc)) {
-        rb_raise(rb_eThreadError, "current thread not owner");
-    }
-    return Qnil;
-}
-
 static VALUE
 monitor_exit_for_cond(VALUE monitor)
 {

test/monitor/test_monitor.rb

@@ -35,6 +35,29 @@ class TestMonitor < Test::Unit::TestCase
     assert_equal((1..10).to_a, ary)
   end
 
+  def test_exit
+    m = Monitor.new
+    m.enter
+    assert_equal true, m.mon_owned?
+    m.exit
+    assert_equal false, m.mon_owned?
+
+    assert_raise ThreadError do
+      m.exit
+    end
+
+    assert_equal false, m.mon_owned?
+
+    m.enter
+    Thread.new{
+      assert_raise(ThreadError) do
+        m.exit
+      end
+    }.join
+    assert_equal true, m.mon_owned?
+    m.exit
+  end
+
   def test_enter_second_after_killed_thread
     th = Thread.start {
       @monitor.enter