[Pharo-project] Another finalization concern: error handling

Schwab,Wilhelm K bschwab at anest.ufl.edu
Mon Oct 11 10:10:13 EDT 2010


A similar discussion arose around Dolphin's event (#trigger*) mechanism.  My recollection is that it was not fully addressed due to performance concerns.  Forking and error handlers both have their costs.  I'm not saying we should necessarily follow (we probably should not), though with careful design, an interrupted chain of events might survive to be handled on a subsequent attempt.

I am far more worried about having multiple executors per object (when did p=malloc();free(p);free(p);free(p) become good style?) than I am about getting the finalizer process itself completely robust at this point.


From: pharo-project-bounces at lists.gforge.inria.fr [pharo-project-bounces at lists.gforge.inria.fr] On Behalf Of Levente Uzonyi [leves at elte.hu]
Sent: Monday, October 11, 2010 9:51 AM
To: Pharo Development
Cc: The general-purpose Squeak developers list
Subject: Re: [Pharo-project] Another finalization concern: error handling

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.

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

> 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?


> 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

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