From: Alex Young <alex@...>
Date: 2012-02-27T01:48:34+09:00
Subject: [ruby-core:42955] Re: [ruby-trunk - Feature #5958] ThreadGroup#join

On 26/02/2012 08:32, Masaki Matsushita wrote:
>
> Issue #5958 has been updated by Masaki Matsushita.
>
>
> Koichi Sasada wrote:
>> What should happen when at least one thread causes an exception?
>
> Under the patch, joining thread also causes the exception if the thread is being joined.
> Otherwise, nothing will happen on joining thread.
> It behaves the same as thgrp.list.each(&:join).
>
> I think ThreadGroup#join may be more useful if it catches exceptions from all its threads and handles them like the following.
>
> thgrp.join do |th| # each thread which causes an exception
>    # handle an exception
> end

It'd be much less surprising to go through the normal rescue mechanism 
if possible.  How about something like this:

   module ThreadGroupExceptions
     attr_accessor :next_in_group

     def each
       current = self
       while current
         yield current
         current = current.next_in_group
       end
     end
   end

   class ThreadGroup
     def join
       group_exc = nil
       list.each do |thread|
         begin
           thread.join
         rescue => exc
           exc.extend ThreadGroupExceptions
           group_exc, exc.next_in_group = exc, group_exc
         end
       end
       raise grp_exc if grp_exc
     end
   end

It's slightly messy in that if you're not expecting there to be a 
ThreadGroup involved then you won't be aware of the mixin, but if you 
are, then everything's available as a linked list (which an Enumerable 
could be layered on, if needed), and you can just `rescue 
ThreadGroupExceptions` in that case.

-- 
Alex