Yeah, that's what the rb_ensure(…) nonsense is that I posted below the problem code. I guess I forgot to change the topic after I peeked at Gosu's source. :D

For the curious, this is its signature in Ruby 1.9.2:

VALUE rb_ensure(VALUE(*)(ANYARGS),VALUE,VALUE(*)(ANYARGS),VALUE);

For the masochistic, here's its source code:

VALUE
rb_ensure(VALUE (*b_proc)(ANYARGS), VALUE data1, VALUE (*e_proc)(ANYARGS), VALUE data2)
{
    int state;
    volatile VALUE result = Qnil;

    PUSH_TAG();
    if ((state = EXEC_TAG()) == 0) {
  result = (*b_proc) (data1);
    }
    POP_TAG();
    /* TODO: fix me */
    /* retval = prot_tag ? prot_tag->retval : Qnil; */     /* save retval */
    (*e_proc) (data2);
    if (state)
  JUMP_TAG(state);
    return result;
}

That "fix me" comment is a little unnerving. Anyway, from this we can at least figure out which parameters do what. If Ruby's ensure block were somehow implemented using Ruby's ensure block, it might look something like this:

def rb_ensure b_proc, data1, e_proc, data2
  result = nil
  begin
    result = b_proc.call data1
  ensure
    e_proc.call data2
  end
  return result
end

The bigger question is actually pretty easy to answer, but difficult for me to describe. Better to show you with an example from my own code.

# Recursively tests whether +point+ falls within one of the container's
# subviews. Returns the first subview that tested positively, +self+ if
# no subviews contain the point, or +nil+ if the container does not
# contain the point.
def point_query point
  if frame.include? point
    # query each subview and return the first non-nil result
    point -= frame.offset
    @subviews.each do |subview|
      found_view = subview.point_query point
      return found_view if found_view
    end
   
    # fall back on self if no subview contains the point
    self
  else
    nil
  end
end

If blocks didn't behave this way, then that return statement would basically be a no op. The block would return, the enumerator would move on to the next item in @subviews and the method would eventually return self. If I wanted to return the result of the subview's point_query, I'd have to write something significantly nastier using a catch/throw or something. I'm sure if it worked that way I'd figure something out.

Let me know if you're not convinced this is the right way to do it. :)