[Pharo-project] [squeak-dev] Re: Another finalization concern: error handling

Igor Stasenko siguctua at gmail.com
Mon Oct 11 10:30:44 EDT 2010

On 11 October 2010 16:51, Levente Uzonyi <leves at elte.hu> wrote:
> On Mon, 11 Oct 2010, Igor Stasenko wrote:
>> Hello,
>> here a situation, with which we can deal in more safer manner:
>> Suppose you have a weak registry, populated by different objects and
>> their executors.
>> Now, when some of them died, a weak registry performs finalization.
>> The potential danger is , that if there's an error triggered by some
>> executor(s),
>> then rest of executors will have no chance to run and will be
>> forgotten, causing memory leakage.
>> What you think, should we handle this more graciously?
>> (Consider a following meta-code)
>> WeakRegistry>>finalizeValues
>> | executors |
>>  executors := self gatherExecutorsToRun.
>>  executors do: [:ex |
>>    [ ex finalize ] fork.
>>  ].
> The "gatherExecutorsToRun" part was implemented in the previous version of
> WeakRegistry (history is not available from the image...). #finalize wasn't
> sent from the protected block. The current version lacks it, so it will
> deadlock if a finalizer will try to access the same WeakRegistry. Other kind
> of deadlocks are also possible. For example when a finalizer can access a
> semaphore which is also used by another process that uses the same semaphore
> and also uses the WeakRegistry, but locks them in a different order
> (JNIPort).
> This only happens if the VM doesn't support the new finalization scheme
> which is the case for most current VMs.
> There is another issue with removal, and this affects the new finalization
> scheme too, because the finalizer of valuesDictionary is #finalizeValues.
> This means that finalization can happen in a process other than the
> finalization process. This can lead to random errors.
err no. valuesDictionary are held privately by weak registry. and any
access to it possible only
through instance of weak registry, which always using protect: [].

> So I think parts of the old WeakRegistry implementation should be restored,
> like:
> - WeakRegistry should collect executors
> - executors should be evaluated outside the protected block
agreed on this one.

But weak registry lost this feature before my intervention:

This is a code from Squeak 4.1. image:
	"Some of our elements may have gone away. Look for those and activate
the associated executors."
	self protected: [ valueDictionary finalizeValues ]

ul 2/22/2010 14:23 · finalization · 2 implementors · in no change set ·

So, i thought, it not a big sacrifice, besides it runs faster.
And to my thinking, the code which needs to manipulate weak registry
during finalization smells badly.

>> in this way, if any executor's #finalize causing error, it won't
>> interfere with other executors, and they will accomplish their task
>> normally.
>> Of course, i'm not saying that we should use #fork for this, because
>> it is costly. Similar could be done w/o forking.
>> I just wanted to show a simplest code with which we could achieve a
>> more gracious error handling.
> Wouldn't it be better to use an exception handler?

Well, its a bit complicated, since what i want is to:

 - report an error
 - but continue run #finalize for rest executors


 - report an error (stop finalization process), then
 - continue running for next executor , if user press 'abandon'
 - or continue with running fixed code

If i'm not mistaken, currently, if you put halt inside a finalization
process (like in some #finalize method) and then abandon it,
you won't have finalization process in system anymore, because it will
be terminated.

> Levente
>> P.S. of course, in a first place it would be good to make sure that we
>> writing executors, which can't cause an error during finalization.
>> But bad things happen, and we should make sure that rest of system
>> won't be put on its knees because of some stupid bug in a single
>> #finalize.
>> --
>> Best regards,
>> Igor Stasenko AKA sig.
>> _______________________________________________
>> Pharo-project mailing list
>> Pharo-project at lists.gforge.inria.fr
>> http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/pharo-project

Best regards,
Igor Stasenko AKA sig.

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