[Vm-dev] [ENH] A better finalization support (VM & language side)

Igor Stasenko siguctua at gmail.com
Wed Mar 10 03:23:14 UTC 2010

On 10 March 2010 05:16, Igor Stasenko <siguctua at gmail.com> wrote:
> On 10 March 2010 04:34, Levente Uzonyi <leves at elte.hu> wrote:
>> On Tue, 9 Mar 2010, Igor Stasenko wrote:
>>> 2010/3/9 Levente Uzonyi <leves at elte.hu>:
>>>> On Tue, 9 Mar 2010, Igor Stasenko wrote:
>>>>> Well, if someone cares, then he actually can make own registry class,
>>>>> which allows copying. But why we should care, by leaving a potential
>>>>> security hole open?
>>>>> I don't think that this is normal to rely on good manners of users and
>>>>> expect them to not attempt to do something wrong. In contrast, a
>>>>> protocols and interfaces should discourage user from abuse. At least,
>>>>> an author could state where it is safe to use and where is not.
>>>>> A copy protocol for weak registry is far from being safe.
>>>>> In that situation is better to generate an error, indicating that such
>>>>> use is not foreseen by author, rather than trying to implement
>>>>> something which 'possibly could work' :)
>>>> I still don't see how is it unsafe.
>>> A simple copy is fine. A copy, which then registered in weak
>>> dependents creating a problem.
>>> I even think that weak registry should not behave as collection at all,
>>> and having only #add: method, with no ability to remove items.
>>> The only use of #remove: i see is in Socket and StandardFileStream,
>>> which implement #unregister: in own class side.
>>> Now , if you look at my implementation, you could see that there is a
>>> way to completely avoid the need in removing items from a
>>> valueDictionary which is pairs of  <weakref> -> <executor>.
>>> A solution:
>>> - add a 'finalizer' ivar to Socket/StandardFileStream
>>> - by registering a socket in registry, retrieve an instance of
>>> WeakFinalizerItem as a result of registration and store it into
>>> 'finalizer' ivar.
>>> - on #destroy, simply nil-out all of the finalizer's ivars, so there
>>> is no chances that once socket become garbage, it will trigger an
>>> executor's #finalize method, which were registered previously in
>>> registry.
>>> - forget about removing the finalizer manually from registry, because
>>> an instance of WeakFinalizerItem which is held by 'valueDictionary' in
>>> registry will eventually be reclaimed, once dictionary will discover
>>> that corresponding key is nil.
>>> What you think?
>> I took a deeper look and found that WeakFinalizationRegistry doesn't support
>> multiple finalizers per object. It does what WeakRegistry did before: throw
>> away the previous finalizer and replace it with a new one.
>> I like the current features of WeakRegistry (removing, adding, multiple
>> finalizers per object) and I think it's easy to modify it to use your vm
>> support.
> Sure, support for multiple finalizers per object could be added. But
> as to me, this looks like an over-engineering.
> Btw, the current implementation of WeakRegistry is buggy, because of
> use non-identity based dictionary.
> Try explore contents of it, after evaluating:
> 10 timesRepeat: [ registry add: (Array with:10) ].
> it adds only a single key/value pair into valueDictionary,
> while i'd expect to have 10 entries, because i adding 10 distinct array objects.
> The flaw in logic is easy to illustrate:
> array1 := Array with: 10.
> array2 := Array with: 10.
> registry add: array1; array2.

oops, the line above should be:

registry add: array1; add: array2.

> array2 at: 1 put: 12.
> registry add: array2.
> array1 := nil "remove strong reference to array1, while keep it for array2"
> The point is, that if two disctinct objects answering true on #= ,
> when we adding them to registry
> it doesn't means that they will keep to be 'equal' after we added
> them, because these objects can mutate.
> So, if one of them eventually die, other(s) may still be in use, which
> will lead to receiving a death notice to wrong recipient.
>> I don't really like your suggestion for files and sockets, but it's doable.
>> I was thinking about a simpler registry which would use an OrderedCollection
>> instead of a WeakKeyDictionary. It'd need a new instance variable in
>> WeakFinalizerItem (though it can be another class/subclass too) which would
>> store the index of the finalizer in this OrderedCollection. It wouldn't have
>> #do:, #keys or #remove:ifAbsent: (though all of them could be implemented)
>> and #add: wouldn't replace existing finalizers, but just add them to the
>> registry.
>> This would have a bit better performance, simpler implementation and less
>> features. But if you don't need #remove:, i'm sure it'd fit your needs.
>> (It's a bit tricky though. Your socket/file ideas wouldn't work out of the
>> box, because all of the WeakFinalizerItems would have to be removed from the
>> OrderedCollecion somehow to avoid leaking memory. One solution would be to
>> add it to the list on "remove" (when replacing the ivars with nil), another
>> would be to remove it from the OrderedCollection by index, but the items
>> don't know their collection so it would need another ivar).
>> All in all, I'd keep WeakRegistry with the current features and optionally
>> add a new simpler and faster registry if needed.
> Yes, i'm also thought about different ways how to organize things in
> less memory & CPU hungry manner.
> The main role of weak registry is to receive a notification, when
> particular object become garbage.
> Obviously, for this, we need to store a reference to notification
> handler (also known as executor) somewhere to
> be able to handle it. That's why current implementation using WeakKeyDictionary.
> Once such notification is handled, we usually no longer need to keep
> this object as well (thus we have to remove/unlink it from registry).
> My implementation is not perfect, it was mainly a quick and dirty hack
> (you pointed on not using semaphores already).
> But my intent was mainly to show how to use new VM feature, i added.
> So, once it will be approved (i hope) and adopted, we could implement
> things properly.
>> Levente
> --
> Best regards,
> Igor Stasenko AKA sig.

Best regards,
Igor Stasenko AKA sig.

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