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

Levente Uzonyi leves at elte.hu
Thu Mar 4 00:39:35 UTC 2010


On Thu, 4 Mar 2010, Nicolas Cellier wrote:

> 2010/3/4 Levente Uzonyi <leves at elte.hu>:
>> On Thu, 4 Mar 2010, Nicolas Cellier 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...
>>
>> I'd give the responsibility to the developer to write termination blocks
>> which are safe to evaluate. So I assume that all such blocks behave well. If
>> something goes wrong you can kill the process with a more aggressive
>> method (as Andreas suggested), but you lose the guarantee that the current
>> termination block will be evaluated.
>>
>>
>> Levente
>>
>
> I would tend to focus first on the senders of terminate since
> terminating is unsafe...

The comment suggest to me that it's safe to use it, because it will 
evaluate the termination blocks.


Levente

> How many of them would deserve to be transformed in a terminateRequest ?
>
> Nicolas
>
>>>
>>> Nicolas
>>>
>>>>
>>>> Levente
>>>>
>>>>>
>>>>> Nicolas
>>>>>
>>>>>>>
>>>>>>> Cheers,
>>>>>>>  - Andreas
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>
>>
>>
>>
>
>


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