[BUG] Fatal recycling of SystemStackErrors

Moin!

In Ruby 1.8.0 old SystemStackErrors seem to be reused when
there should have been raised a new one instead:

irb(main):001:0> y = proc { y.call }; y.call rescue nil
=> nil
irb(main):002:0> eval 'z = proc { z.call }; z.call'
SystemStackError: (irb):1:in `irb_binding': stack level too deep

This can cause serious problems when you're running multiple Threads 
with different levels of safeness, because of fatal exceptions. This 
code snippet:

   2.times {
     begin
       Thread.new {
         $SAFE = 4 # error isn't caused on lower $SAFE levels
         r = proc { r.call }; r.call # create a SystemStackError
       }.value
     rescue Exception => error; end
     p error
   }

produces this output for me:

   nil
   #<fatal: exception reentered>

I'm using ruby 1.8.0 (2003-08-04) [i386-mswin32] and the errors also 
occur with linux releases of ruby 1.8.0.

I haven't been able to run the above test code on an 1.6.8 release yet, 
so the bug might have been introduced only recently.

That fatal exception is a show stopper for me because my application is 
instantly killed by it and because weirdly I'm not able to rescue it in 
the context where it happens.

Regards,
Florian Gross