mask in waitQSem

[Simon Marlow]

On 25/11/2014 14:03, Yuras Shumovich wrote:
> On Tue, 2014-11-25 at 13:53 +0000, Simon Marlow wrote:
>> On 24/11/2014 11:12, Yuras Shumovich wrote:
>>> On Mon, 2014-11-24 at 10:09 +0000, Simon Marlow wrote:
>>>> On 14/11/2014 21:23, Yuras Shumovich wrote:
>>>>>
>>>>> I was reviewing some exception handling code in base library, and I
>>>>> found this line:
>>>>> https://phabricator.haskell.org/diffusion/GHC/browse/master/libraries/base/Control/Concurrent/QSem.hs;165072b334ebb2ccbef38a963ac4d126f1e08c96$74
>>>>>
>>>>> Here mask is used, but I looks completely useless for me. waitQSem
>>>>> itself should be called with async exceptions masked, otherwise there is
>>>>> no way to prevent resource leak.
>>>>>
>>>>> Do anyone know why mask is used here?
>>>>
>>>> I wrote that code.  It looks like mask_ is important, otherwise an async
>>>> exception might leave the MVar empty, no?  We can't assume that waitQSem
>>>> is always called inside a mask_, so it does its own mask_.
>>>
>>> Hmm, yes, you are right.
>>>
>>> Note, that in case of failure (and without external `mask`) it is not
>>> possible to determine, whether a unit of resource was reserved or not.
>>> It makes the particular instance of `QSem` unusable anymore, and we have
>>> to create other one. Probably there are situations where it is OK
>>> though.
>>
>> I don't think that's true: if waitQSem fails, then the semaphore has not
>> been acquired.  This guarantee is important for "bracket waitQSem
>> signalSQem" to work.  I just peered at the code again and I believe this
>> is true - have you spotted a case where it might be false?  If so that's
>> a bug.
>
> The note was about the case when waitQSem is *not* called inside mask
> (as you said we can't assume that). In that case it can be interrupted
> by async exception either before entering mask_ or after leaving it. Is
> it true? That makes waitQSem not very useful outside bracket.

Well, that's the case for *any* side-effecting operation outside of 
mask: when an exception is raised, you have no idea whether the effect 
took place or not.  That's why we have mask.

It might still be useful outside mask, if the QSem is being used locally 
and it's ok to discard it along with the computation.

Cheers,
Simon


> When used inside bracket, it seems to be safe.
>
>>
>> Cheers,
>> Simon
>>
>>
>>>
>>> Thanks,
>>> Yuras
>>>
>>>>
>>>> Cheers,
>>>> Simon
>>>>
>>>>
>>>>> I wonder whether an author of the code tried to do something different,
>>>>> so there actually can be a bug hidden here. Probably
>>>>> uninterruptibleMask_ should be used here? (I don't think so though.)
>>>>>
>>>>> Thanks,
>>>>> Yuras
>>>>>
>>>>>
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>>>>>
>>>
>>>
>
>