broken_monitor.rb

require 'monitor'

Thread.report_on_exception = false
mon = Monitor.new
loop do
  exit = false
  locked = false

  th = Thread.new do
    begin
      mon.synchronize { locked = true }
    rescue
      raise "monitor was not unlocked" if mon.mon_locked?
    end
  end

  1 until locked
  th.raise

  th.join
  puts :ok
end

fix.diff

diff --git a/lib/ruby/stdlib/monitor.rb b/lib/ruby/stdlib/monitor.rb
index 64b1f85e7c..1597ee9c40 100644
--- a/lib/ruby/stdlib/monitor.rb
+++ b/lib/ruby/stdlib/monitor.rb
@@ -230,11 +230,15 @@ module MonitorMixin
   def mon_synchronize
     # Prevent interrupt on handling interrupts; for example timeout errors
     # it may break locking state.
-    Thread.handle_interrupt(EXCEPTION_NEVER){ mon_enter }
-    begin
-      yield
-    ensure
-      Thread.handle_interrupt(EXCEPTION_NEVER){ mon_exit }
+    Thread.handle_interrupt(EXCEPTION_NEVER) do
+      mon_enter
+      begin
+        Thread.handle_interrupt(EXCEPTION_IMMEDIATE) do
+          yield
+        end
+      ensure
+        mon_exit
+      end
     end
   end
   alias synchronize mon_synchronize