From: Peter Zotov <whitequark@...>
Date: 2012-04-14T09:49:48+09:00
Subject: Re: GC on two C extension objects

Mike Owens писал 13.04.2012 21:02:
> I am running into a problem with to C extension objects and I'm not 
> sure
> what I'm missing. I put together a simple example illustrating the 
> point
> here:
>
> https://github.com/linterra/ruby-c-ext-gc
>
> Basically one C extension class (child) references another (parent). 
> To
> keep parent from being GC'd before the child referencing it, I assign 
> a
> reference in the child using rb_ivar_set(). Additionally, I also add 
> a mark
> function in the child to mark the parent.
>
> Still, when GC runs, the parent still gets cleaned up first and the 
> child
> GC segfaults because it references the GC'd parent. I can see that 
> the
> child's mark function is being called.
>
> <snip>
>
> Mike

This is the intended behavior. Objects are not sweeped in an order 
related to
their reference contents; instead, the mark phase just searches for 
islands
of unused objects and sweep phase destroys these islands. Why does it 
work
this way? Easy. First, there is no sensible sweep order in case of 
circular
reference. Second, GC should be as fast as possible, and ensuring a 
particular
order of sweeping would incur significant computational cost.

You should keep all your data in a single "pivot" object and only 
return thin
references to that data in accessors. Think of a DOM document and a 
node list.

-- 
   WBR, Peter Zotov.