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

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 ⊥
> _________________________
> Racket Developers list:
> http://lists.racket-lang.org/dev