From: Dossy <dossy@...>
Date: 2002-05-25T03:38:28+09:00
Subject: Re: Threading oddness

On 2002.05.25, Sean Russell <ser@germane-software.com> wrote:
> 
> Exceptions thrown in the body of a Thread aren't handled the same way they 
> are elsewhere in Ruby.  If an Exception occurs in a Ruby Thread and you 
> don't handle (rescue) it, it is silently ignored -- IE, you'll never see 
> the exception output.  Therefore, it is /very/ important to wrap your 
> thread code (inside the block) with begin...rescue...end and report your 
> own exceptions.

Perhaps Thread needs a little event handler for thread death?  So,
you could do something like:

  t = Thread.new { ... do something ... }
  t.on_exception { ... handle the exception here ... }

The only problem is the race condition: the time between when the
thread is started and when the #on_exception message gets sent
to the thread.

I think if you do a Thread#join on the thread that died due to an
unrescued exception, you can rescue the exception there.  Like this:

  t = Thread.new { raise }
  # thread dies due to unrescued RuntimeError
  begin
    t.join
  rescue => e
    p e         # => RuntimeError
  end

Maybe the work-around is to create a system where new threads
get created then added to an Array, and a special single thread
gets created and polls for Thread#alive? to return false then
do a "begin ... Thread#join ... rescue ... end" on the dead
thread to catch thread death exceptions.

It's an idea I've been toying with ...

-- Dossy

-- 
Dossy Shiobara                       mail: dossy@panoptic.com 
Panoptic Computer Network             web: http://www.panoptic.com/ 
  "He realized the fastest way to change is to laugh at your own
    folly -- then you can let go and quickly move on." (p. 70)