sandbox timers & block scopes

Two puzzles I am trying to solve:

1. Ruby uses setitimer to switch threads.  I would like to use setitimer as well
to abort a sandbox which runs longer than its timeout.  Can anyone recommend a
signal which can be used for this?  Or could I get access to the thread
switching routine so I can check sandbox timeouts between threads?  Or, perhaps
there is another strategy.

2. Block scopes are causing me some trouble since they have mixed scope.

     3.times do |i|
       Sandbox.safe.eval("local_variables")  #=> ['i']
     end

Method scopes, on the other hand, work fine.

     def box_eval(str)
       i = 1
       Sandbox.safe.eval(str)
     end
     box_eval("local_variables")  #=> []

When eval is called, I swap out ruby_scope for a new, blank struct SCOPE.
Do I need to replace ruby_block and ruby_frame as well?

_why