From: Motohiro KOSAKI <redmine@...>
Date: 2011-01-17T14:46:06+09:00
Subject: [ruby-dev:43050] [Ruby 1.9-Bug#4284][Open] Timeout.timeout may cause application exit unintetionally, again

Bug #4284: Timeout.timeout may cause application exit unintetionally, again
http://redmine.ruby-lang.org/issues/show/4284

起票者: Motohiro KOSAKI
ステータス: Open, 優先度: Normal
担当者: Yukihiro Matsumoto, カテゴリ: lib, Target version: 1.9.3
ruby -v: ruby 1.9.3dev (2010-12-22 trunk 30291) [x86_64-linux]

This issue was discovered during [Bug#4266] discussion.
Current timeout is racy.

Now, timeout module has following code.
-------------------------------------------------------------------------------
  def timeout()
    begin
      x = Thread.current
      y = Thread.start {
        begin
          sleep sec
        rescue => e
          x.raise e
        else
          x.raise exception, "execution expired" if x.alive?
        end
      }
      return yield(sec)
    rescue exception => e
      raise Error, e.message, e.backtrace
    ensure
      if y and y.alive?
        y.kill
        y.join # make sure y is dead.
      end
    end
  end
---------------------------------------------------------------

Unfortunatelly, 

   y = Thread.start {}

is not an atomic operation. Then, A following race can occur.


  CPU0(thread x)                     CPU1(thread y)         remark
---------------------------------------------------------------------------
  enter begin block
  [thread construct]                                        but no assign y yet
                                     sleep sec
                                     wakeup from sleep
                                     x.raise
  if y                                                      return false. (see above)


Therefore, CPU0 don't call y.join and leak y's thread resource. C# have solved
this two-step-construction vs asynchrounous exception race by RAII.

But unfortunately, Ruby don't have such language feature. So, We can't write
async-exception-safe code. One of solution is to move timeout module from ruby code
into c code as JRuby does. But I don't think timeout is only asynchrounos exception user.
we also have Interrupt class (for Ctrl-C) and I think we need to allow to write async 
exception safe code by ruby.

Or, Am I missing something?


----------------------------------------
http://redmine.ruby-lang.org