From: ara.t.howard@...
Date: 2006-07-07T00:56:09+09:00
Subject: Re: [BUG] thread/sync.rb memory corruption

On Thu, 6 Jul 2006, Robert Klemme wrote:

> Yes, but they are still queued up against the mutex.  I completely agree
> with Tom's analysis.  Having an Observer trigger the notification again is
> definitively a bad idea - at least if events are not queued up.

ah - but they are indeed queued - this is precisely what sync.rb is supposed
to do.  you can prove this for yourself by adding

   p 'Thread.list.size' => Thread.list.size

in the notify method.  if you do so you'll see something like

   on @ 1152201072.40667
   {"Thread.list.size"=>1}
   off @ 1152201072.41042
   {"Thread.list.size"=>2}
   on @ 1152201072.41508
   {"Thread.list.size"=>3}
   off @ 1152201072.4219
   {"Thread.list.size"=>3}
   on @ 1152201072.43386
   {"Thread.list.size"=>2}
   off @ 1152201072.44186
   {"Thread.list.size"=>3}
   on @ 1152201072.46079
   {"Thread.list.size"=>2}


the number of threads running will never exceed three.  the bug does not seem
to be directly caused by sync.rb in any case.  here is a more minimal script
which will produce the error:

     require 'sync'
     class A
       def initialize
         extend Sync_m
         @observers = []
       end
       def meth
         synchronize(:EX){
           @observers.each do |o|
             if ENV['CORRUPT']
               o.notify nil
             else
               o.notify
             end
           end
         }
       end
       def add_observer o
         synchronize(:EX){
           @observers << o
         }
       end
     end

     class B
       def initialize a
         @a = a
         @a.add_observer self
       end
       def notify *a
         Thread.new{ @a.meth }
       end
     end

     a = A.new
     b = B.new a
     a.meth
     STDIN.gets


if you run it normally

   ruby a.rb

it will run fine.  if you run with

   CORRUPT=true ruby a.rb

you will trash memory.  if you have ef and do

   CORRUPT=true ef ruby a.rb

electric fence will core dump almost immediately.

note that the only difference between crashing and not is calling notify with
and argument - when that occurs the number of threads will remain steady, but
the GC will stop collecting finished threads even though no reference to them
is held.

regards.

-a
-- 
suffering increases your inner strength.  also, the wishing for suffering
makes the suffering disappear.
- h.h. the 14th dali lama