Re: why does this code leak?

On Jan 9, 2008 5:18 PM, ara howard <[e-mail]> wrote:
>
> On Jan 9, 2008, at 2:52 PM, Rick DeNatale wrote:
>
> > Looking at eval.c, it looks like lambda actually copies information
> > from the invocation stack, not just from the current frame. In the
> > leaky finalizer case we have the following on the stack when
> > leaky_finalizer is called, with the binding represented in hash
> > notation.
> >
> > leaky_finalizer   :self => Class
> > finalizer               :self => Class
> > new                      :self=> Class, :object => the new Array
> > instance
> > code which called Array.new
> >
> > The leak free finalizer lambda was created once at a time when no
> > instance to be finalized was on the stack.
>
> yeah i think that may be true - but it doesn't make sense.  the
>
>    def leaky_finalizer
>      lambda{}
>    end
>
> is the current paradigm for preventing lambdas from enclosing a
> reference to an object.  what you are saying is that this call
> encloses a local variable from another (the calling in the this case)
> function.

Well I'm positing that based on a rather superficial read of eval.c.

> how would that not be a bug?  why enclose a variable that cannot
> possible be reached in the code ran?

Well I suspect that it's because no real analysis is done of what's
inside the block when a proc is created, so the assumption is that the
entire binding is needed.  The Smalltalk compilers I recall would
produce different types of block objects depending on whether or not
the block contained references to variables outside the block, and/or
contained a return.

Now why it goes back down the stack, if it indeed does, I'm not sure.
Perhaps it has something to do with the lambda vs. Proc.new
differences. I think that Proc.new and lambda/proc both use the
proc_alloc function where this seems to  be happening.

In any event this is probably more a topic for ruby-core so I'm
cross-posting this reply there.

> this seems, to me just like this
> code
>
>    void * leak (){ return(&malloc(42)) }
>
> i just cannot image why lambda would crawl up the stack outside the
> current function.  if that is true then they are useless and *any*
> invocation means every object in memory at the time of creation can
> never be freed while that lambda exists.  doesn't that seem
> excessive?  also i use tons of lambdas in code that does not leak so
> this just seems impossible.
>
> nevertheless you may be right!

Or not <G>
-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/