[Vm-dev] Re: [Pharo-dev] FogBugz (Case [Issue]10840) Collection - WeakSet>>#size utterly broken

[Eliot Miranda]

On Fri, Jun 7, 2013 at 2:34 AM, Camille Teruel <camille.teruel at gmail.com>wrote:

>  On 7 juin 2013, at 11:19, Pharo Issue Tracker wrote:
>
> A FogBugz case was edited by Stephane Ducasse.
>
> Case ID:      10840
> Title:        WeakSet>>#size utterly broken
> Status:       Resolved (Fix Review Needed)
> Category:     Bug
> Project:      Collection
> Area:         Misc
> Priority:     3 - Must Fix
> Milestone:    Pharo3.0: 30/03/2014
> Assigned To:  Camille Teruel
>
> URL:          https://pharo.fogbugz.com/f/cases/10840
>
> Last message:
> Please can you send a mail to the mailing-list so that we all discuss this
> problems.
>
>
> Hello everyone,
>
> WeakSet>>#size is broken as demonstrated by the following snippet:
>
> set := WeakSet new.
> set add: Object new.
> Smalltalk garbageCollect.
> set size. "answers 1 instead of 0"
>
> That is because a WeakSet has no mean to know when its items get garbage
> collected, and thus the tally cannot be updated.
> So we need to override #size in Weak, I propose:
> size
> | counter |
> counter := 0.
> self do: [ :e | counter := counter + 1 ].
> ^ counter
> But that it rather inefficient for such a simple query.
> Any proposition?
>

One way is to add WeakSets to FInalizationDependents and implement
finalizeValues to update the size.  But that's making work.  A lazy
solution might be better.

You *could*, I suppose, use the VM's count of the number of incremental and
full GCs (alas there's no single count).  I think this is too ugly and one
would need something neater, but it would work like this:

size
      | newGCCount |
      newGCCount := self getGCCount.
      newGCCount == existingGCCount ifTrue:
          [^tally].
      tally := self computeActualSIze.
      existingGCCount := newGCCount

where
    getGCCount
        ^(Smalltalk vmParameterAt: 7) + (Smalltalk vmParameterAt: 9)

There are obviously thread-safety issues here, but there are issues with
the existing code and your suggestion too ;)

The most useful single count here would be a count that was incremented
once in each GC that finalized one or more references.

Note also that for this to be reliable instances' record of the count would
have to be invalidated on snapshot or start-up, which could entail an
expensive allInstancesDo:.

Perhaps the class WeakSet should maintain its instances in (of course) a
WeakSet and then add only itself to FinalizationDependents and enumerate
the set on finalization?



I note that in the VisualWorks VM, objects that have references finalized
are added to a queue so every instance that loses a reference gets sent
finalize, instead of only dependents as in Squeak.  This has its won
problems (the VM must cope with a potentially large list) but I think it
provides more convenient control.


>
> The bug entry is: https://pharo.fogbugz.com/f/cases/10840
>
>
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-- 
best,
Eliot
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