From: Leslie Viljoen <leslieviljoen@...>
Date: 2007-04-19T05:15:47+09:00
Subject: Re: Why does this code deadlock?

On 4/18/07, Bill Kelly <billk@cts.com> wrote:
>
> From: "Yukihiro Matsumoto" <matz@ruby-lang.org>
> >
> > In message "Re: Why does this code deadlock?"
> >    on Wed, 18 Apr 2007 16:21:48 +0900, Nobuyoshi Nakada <nobu@ruby-lang.org> writes:
> >
> > |At Wed, 18 Apr 2007 08:34:03 +0900,
> > |Bill Kelly wrote in [ruby-talk:248300]:
> > |> require 'net/ftp'
> > |> Thread.abort_on_exception = true
> > |> ftp = Net::FTP.new('ftp.idsoftware.com')
> > |  th = Thread.new {$SAFE=4; ftp.login('anonymous', 'abc@def.com')}
> > |  Thread.critical = false
> > |  p th.value
> >
> > Who set Thread.critical?  It should be fixed, I think.
>
> It seems to be something called by ftp.login:  (I added some
> sleeps to coordinate the two threads.)
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> require 'fastthread'
> require 'net/ftp'
> Thread.abort_on_exception = true
> puts "1. crit=#{Thread.critical}"
> ftp = Net::FTP.new('ftp.idsoftware.com')
> puts "2. crit=#{Thread.critical}"
> th = Thread.new {sleep(2); $SAFE=4; sleep(2); ftp.login('anonymous', 'abc@def.com')}
> puts "3. crit=#{Thread.critical}"
> sleep(3)
> puts "4. crit=#{Thread.critical}"
> sleep(2)
> puts "5. crit=#{Thread.critical}"
> # Thread.critical = false
> p th.value
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> $ ruby -v ftp_safe_test.rb
> ruby 1.8.6 (2007-03-16 patchlevel 3) [i686-linux]
> 1. crit=false
> 2. crit=false
> 3. crit=false
> 4. crit=false
> 5. crit=true
> ftp_safe_test.rb:14:in `value': Thread(0x525ad708): deadlock (fatal)
>         from ftp_safe_test.rb:14
>
>
> If I uncomment the `Thread.critical = false', it prints:
>
> $ ruby -v ftp_safe_test.rb
> ruby 1.8.6 (2007-03-16 patchlevel 3) [i686-linux]
> 1. crit=false
> 2. crit=false
> 3. crit=false
> 4. crit=false
> 5. crit=true
> /opt/lib/ruby/1.8/monitor.rb:284:in `mon_acquire': Insecure: can't modify instance variable (SecurityError)
>         from ftp_safe_test.rb:14:in `value'
>         from ftp_safe_test.rb:14
>
>
> This is in MonitorMixin::ConditionVariable:
>
>   def mon_acquire(queue)
>     while @mon_owner && @mon_owner != Thread.current
>       queue.push(Thread.current)
>       Thread.stop
>       Thread.critical = true
>     end
>     @mon_owner = Thread.current
>   end

ftp.login calls synchronize in thread.rb, which calls lock in
thread.rb, which sets critical:

def lock
   while (Thread.critical = true; @locked)
     @waiting.push Thread.current
     Thread.stop
   end
   @locked = true
   Thread.critical = false
   self
end

Maybe I'm barking up the wrong tree.