mask in waitQSem

[Yuras Shumovich]

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.

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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