[racket-dev] Should `register-finalizer` unwrap impersonators?

I am imagining that the type compilation of type Finalizer and such things would be parameterized over programmer code which would yield a 'trusted' 'thing' in this case except that this would open the door for other such things. 




On Aug 17, 2014, at 3:39 PM, Sam Tobin-Hochstadt wrote:

> How would that change things here? The issue is about
> finalizer-for-what, and that chaperones/impersonators affect object
> identity.
> 
> Sam
> 
> On Sun, Aug 17, 2014 at 3:37 PM, Matthias Felleisen
> <matthias at ccs.neu.edu> wrote:
>> 
>> Could we benefit from an abstract/opaque Finalizer type here? I know we don't have those yet but it may address the general problem. -- Matthias
>> 
>> 
>> 
>> 
>> On Aug 16, 2014, at 8:55 AM, Neil Toronto wrote:
>> 
>>> Short version: the contract system doesn't allow `register-finalizer` to be used in Typed Racket.
>>> 
>>> Long version: consider the following Typed Racket program, in which instances of `os-resource-wrapper` represent an operating system resource `os-resource`, which itself is just a counter. It attempts to register a finalizer for allocated wrappers, which decrements the counter.
>>> 
>>> 
>>> #lang typed/racket
>>> 
>>> (require/typed
>>> ffi/unsafe
>>> [register-finalizer  (All (A) (-> A (-> A Any) Void))])
>>> 
>>> (: os-resource Integer)
>>> (define os-resource 0)
>>> 
>>> (struct os-resource-wrapper ())
>>> 
>>> (: alloc-os-resource (-> os-resource-wrapper))
>>> (define (alloc-os-resource)
>>> (set! os-resource (add1 os-resource))
>>> (define w (os-resource-wrapper))
>>> (register-finalizer w (λ (w) (set! os-resource (sub1 os-resource))))
>>> w)
>>> 
>>> (define w (alloc-os-resource))
>>> (printf "os-resource = ~v~n" os-resource)
>>> (collect-garbage)
>>> (sleep 1)  ; give finalizers a chance to run
>>> (printf "os-resource = ~v~n" os-resource)
>>> 
>>> 
>>> I get this output:
>>> 
>>> os-resource = 1
>>> os-resource = 0
>>> 
>>> The finalizer is being run while the program still has a pointer to the wrapper object. I think it's because the wrapper object is being impersonated when it's sent across the contract barrier, and the *impersonator* is getting the finalizer. (Or it's a chaperone, or an impostor, or a charlatan, or whatever. Let's go with impersonator.)
>>> 
>>> In my specific case, the OS resources are OpenGL objects; e.g. vertex object arrays. The call to `register-finalizer` *must* be in Typed Racket code because the wrapper contains an (Instance GL-Context<%>), which can't have a contract put on it, so it can't pass from untyped to typed code.
>>> 
>>> Is there any reason for `register-finalizer` to behave this way? Does it ever make sense to register a finalizer on an impersonator?
>>> 
>>> Neil ⊥
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>> 
>> 
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