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

From: Sam Tobin-Hochstadt (samth at cs.indiana.edu)
Date: Sun Aug 17 15:39:30 EDT 2014

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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