Re: [ruby-core:99185] [Ruby master Bug#17023] How to prevent String memory to be relocated in ruby-ffi

[Aaron Patterson <aaron.patterson@gmail.com>]

> On Jul 15, 2020, at 4:35 PM, Eric Wong <normalperson@yhbt.net> wrote:
> 
> tenderlove@ruby-lang.org wrote:
>> Right, that makes sense.  I really need to document this (and
>> I apologize for not doing so already), but
>> `rb_gc_register_address` will pin your objects.  When you know
>> you're done with the reference, you can release it with
>> `rb_gc_unregister_address`.  Of course if you don't call the
>> unregister function, the reference will stay alive forever.
> 
> Btw, does rb_gc_register_mark_object pin?  A quick glance at
> gc.c tells me it doesn't, and I'll need to revert commit
> 2a6cb76d5010cb763ef5a2c305728465d15eb7c9 in unicorn:
> https://yhbt.net/unicorn-public/20181226050857.6413-1-e@80x24.org/

Yes, it does pin.  I’m not super proud of this code, but here is where objects passed to rb_gc_register_mark_object get pinned:

  https://github.com/ruby/ruby/blob/c2a6295ec04a191c689d22254ac1ad5d665e27ad/vm.c#L2307-L2320

I don’t know why the mark object array is an array of arrays (I assume so as not to waste space in the array buffer?).  Maybe this could be a more friendly data structure.

I created a pinned list in compile.c so that objects allocated and used at compile time don’t move (they become free to move once iseq assembly is finished). It seems that might be a more generally useful thing, but so far I’ve only seen two places that need this feature.