[Pharo-project] [squeak-dev] Re: WeakRegistry>>remove: - when you'll be in trouble

Levente Uzonyi leves at elte.hu
Sun Oct 10 19:35:52 EDT 2010


On Mon, 11 Oct 2010, Igor Stasenko wrote:

> On 10 October 2010 23:43, Levente Uzonyi <leves at elte.hu> wrote:
> On Sun, 10 Oct 2010, Igor Stasenko wrote:
>
>> A current implementation of this method
>>
>> remove: oldObject ifAbsent: exceptionBlock
>>        "Remove oldObject as one of the receiver's elements."
>>
>>        oldObject ifNil: [ ^nil ].
>>        ^(self protected: [ valueDictionary removeKey: oldObject ifAbsent:
>> nil ])
>>                ifNil: [ exceptionBlock value ]
>>
>> simply removes a previously registered object from registry and voila.
>>
>> Now lets get back to our discussion about multiple finalizers per
>> object and using them in weak subscriptions etc.
>>
>> Suppose i am added a socket to weak registry,
>> and suppose i am added a weak subscription to it.
>>
>> Now, if i do 'socket close' , it tells weak registry to remove it from
>> list.
>> And what we'll have:
>> - socket handle is closed
>> - socket is wiped from weak registry
>> - but weak subscription still listed somewhere in a list of subscriptions
>>
>>
>> My suggestion is, that upon #remove:,
>> a weak registry should notify all executors that object of interest
>> are no longer takes part in finalization scheme,
>> so they should not count on receiving #finalize eventually.
>>
>> In other words:
>>
>> remove: oldObject ifAbsent: exceptionBlock
>>        "Remove oldObject as one of the receiver's elements."
>>
>>        oldObject ifNil: [ ^nil ].
>>        ^(self protected: [ | executor |
>>            executor := valueDictionary removeKey: oldObject ifAbsent: nil.
>>            executor discardFinalization.
>>        ])
>>        ifNil: [ exceptionBlock value ]
>
> It's only an issue with the new WeakRegistry implementation, previous
> implementations don't have such problem. I think changing the method as you
> suggested, implementing WeakFinalizerItem >> #discardFinalization as
> "executor := nil" and changing WeakFinalizerItem >> #finalizaValues to
> ignore the executor if it's nil will fix the problem. Am I right?
>
> I don't get how "multiple finalizers per object" is related to this problem
> at all.
>

No, you miss the point.
When you removing object from weak registry, it is important , time to
time to tell all of its executors,
that they will no longer receive #finalize, because object is no
longer a member of weak registry.

If you simply set executor := nil, it does nothing, an executor itself
did not notified that he won't be needed
in any possible future.
So, if your finalization action is to remove some object(s) from the
list , you'll get your list dirty after object will die,
because #finalize will be never sent to your executor.

Here the simple test case:

| coll obj someWrapper |
coll := OrderedCollection new.
obj := Object new.
someWrapper := WeakArray with: obj.
coll add: someWrapper.

obj toFinalizeSend: #remove: to: coll with: someWrapper.
obj finalizationRegistry remove: obj.
obj := nil.
Smalltalk garbageCollect.
self assert: coll isEmpty


the point is, that once you doing a #remove: ,
your finalization scheme is broken.


I don't get you. When you remove an object from a WeakRegistry, you 
tell the system that you want the object removed and all related executors 
deleted. So the executors should never receive #finalize, they should be 
thrown away.
Even if there's action to be done with the executors (which is pointless 
IMHO since those will be thrown away, but who knows), doing them is the 
responsibility of the sender of #remove:.


Levente

>
> Levente
>

-- 
Best regards,
Igor Stasenko AKA sig.

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