[squeak-dev] Re: [Pharo-project] #ensure: issues

Levente Uzonyi leves at elte.hu
Thu Mar 4 00:45:39 UTC 2010

On Thu, 4 Mar 2010, Igor Stasenko wrote:

> On 4 March 2010 01:56, Nicolas Cellier
> <nicolas.cellier.aka.nice at gmail.com> wrote:
>> 2010/3/4 Levente Uzonyi <leves at elte.hu>:
>>> On Thu, 4 Mar 2010, Nicolas Cellier wrote:
>>>> 2010/3/3 Levente Uzonyi <leves at elte.hu>:
>>>>> On Wed, 3 Mar 2010, Andreas Raab wrote:
>>>>>> On 3/3/2010 2:07 PM, Levente Uzonyi wrote:
>>>>>>> On Wed, 3 Mar 2010, Igor Stasenko wrote:
>>>>>>>> i don't get it. Just before that, you said: ' I'd expect it to be
>>>>>>>> evaluated no matter what happens.' ?
>>>>>>>> But now you saying that it may not be executed in some conditions
>>>>>>>> (when user pressing abandon button, causing process to be terminated).
>>>>>>> It's simple: don't terminate process X from another process if process
>>>>>>> X
>>>>>>> is executing a termiation block (aka #ensure: block). Or if you
>>>>>>> terminate it, make sure that the execution of the block will continue
>>>>>>> somehow (I don't care how).
>>>>>> You're missing Igors point which is that in his example the halt /
>>>>>> Transcript *was* in the ensure block and as a result you're
>>>>>> contradicting
>>>>>> yourself here. Let's go back to Igor's example:
>>>>>> [self boom ] ensure: [ self halt. Transcript show: 'boom']
>>>>>> The halt is inside the ensure block. If you terminate the process from
>>>>>> the
>>>>>> debugger, it would be logical from your statement that the Transcript
>>>>>> message would be executed - after all it's " executing a termiation
>>>>>> block
>>>>>> (aka #ensure: block)" and so it can't be terminated by your reasoning.
>>>>>> However, when Igor was pointing this out you replied with "I didn't say
>>>>>> that. I said evaluate it the same way as normal code." which is
>>>>>> inconsistent
>>>>>> with the other statement.
>>>>> That shows my lack of knowledge about how the debugger works.
>>>>>>> I think every user of #ensure: expects that the termination blocks are
>>>>>>> executed even if the process which is executing the receiver of
>>>>>>> #ensure:
>>>>>>> is terminated. And it actually happens in all but this case.
>>>>>> The question of terminating processes is always tricky. I don't think
>>>>>> that
>>>>>> your proposal would actually work in practice - it could easily result
>>>>>> in
>>>>>> processes that cannot be terminated due to a simple bug in an ensure
>>>>>> block.
>>>>>> Personally, I'd rather say that the more useful behavior would be
>>>>>> something
>>>>>> along the lines of saying that process termination either skips the
>>>>>> current
>>>>>> ensure block (assuming there's a bug and it should get the heck out of
>>>>>> it
>>>>>> but try to evaluate the remaining ones) or that there need to be two
>>>>>> terminations - one that is 'soft' and won't allow ensure blocks to be
>>>>>> skipped and one that is 'hard' (kill -9 hard) and just ignores all the
>>>>>> ensure blocks.
>>>>> I'm only saying that normal usage (aka #terminate) shouldn't do
>>>>> unexpected
>>>>> things like this.
>>>>> If you read the comment of Process >> #terminate, you may assume that
>>>>> #ensure: and #ifCurtailed: blocks will be excuted even if you use
>>>>> #terminate, but that's not true.
>>>>> "Stop the process that the receiver represents forever.  Unwind to
>>>>> execute
>>>>> pending ensure:/ifCurtailed: blocks before terminating."
>>>>> Levente
>>>> The only way I see to solve your problem would be to execute the
>>>> unwind block in another process...
>>>> Quite technical and costly !
>>> It's our problem. Just look at the senders of #ensure: and imagine what will
>>> happen if the termination block is not evaluated.
>>> I think there's another way (though it might be my lack of knowledge again).
>>> After suspending the process which is about to be terminated we can check if
>>> it's executing a termination block. It it's not, we are safe to continue the
>>> termination, otherwise we can do something else which ensures that the
>>> termination block is evaluated.
>> Maybe...
>> Unfortunately, you did not tell how you will distinguish well behaved
>> unwind-blocks from Igor's example...
> Yes, then what prevents me from writing:
> [ [ ] ensure: [ self doCrazyThings ] ] fork.

What prevents you from writing: Object superclass: Object. ?
Nothing, but you don't do that, do you?

> and now given assumption that any code which placed inside ensure
> block should always run to the end, without chances being terminated,
> will have ill side effects.

You can terminate it (maybe not the usual way).

> The #ensure: means, that interpreter will have a chance to enter that
> block eventually, but it should not mean that it will keep running the
> code there until normal or non-local return from that block.

Then it doesn't ensure anything at all, so it should be called 


> Othewise, you losing a way to terminate unwanted, ill-behaved process.
>> Nicolas
>>> Levente
>>>> Nicolas
>>>>>> Cheers,
>>>>>>  - Andreas
> -- 
> Best regards,
> Igor Stasenko AKA sig.

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