mmap failures in KPixmapCache

[Andreas Pakulat]

On 19.11.09 14:49:06, David Nolden wrote:
> Am Donnerstag 19 November 2009 13:57:18 schrieb Thiago Macieira:
> > Em Quarta-feira 18 Novembro 2009, às 23:31:25, você escreveu:
> > > I see it here on "Linux parkotron 2.6.31-ARCH #1 SMP PREEMPT Tue Nov
> > > 10 19:01:40 CET 2009 x86_64 Intel(R) Core(TM) i5 CPU 750 @ 2.67GHz
> > > GenuineIntel GNU/Linux"
> > >
> > > Unfortunately, I know nothing about, mmapping or QFile::map. It
> > > doesn't makes sense for me to report it as I can't provide the
> > > relevant details. David, are you able to report it?
> > 
> > We found the issue. It's a KDE bug.
> > 
> > KPixmapCache was trying to extend the file by mapping a section of the file
> >  past the end, then writing to it. That was never guaranteed to work. In
> >  fact, it didn't work outside Linux.
> > 
> > In Qt 4.6, when testing mmap on other platforms, we realised that this
> >  didn't work on Mac, so we introduced a check to ensure that you don't rely
> >  on non- cross-platform behaviour. That's what KDE was tripping on.
> 
> This is definitely not the case for KDevelop. On my system, alone this simple 
> test application fails:
> 
> "
> #include <QFile>
> 
> int main(int argc, char** argv)
> {
>   {
>     QFile f("maptest.data");
>     f.open(QIODevice::WriteOnly);
>     f.write("aaaaaaaaaaaaaaaaaaaa", 20);
>   }
>   {
>     QFile f("maptest.data");
>     f.open(QIODevice::ReadOnly);
>     Q_ASSERT(f.isOpen());
>     Q_ASSERT(f.map(0, f.size()));
>   }
> }
> "
> 
> It happens with both kernel versions 2.6.31 and 2.6.27. I'm using a 64-bit 
> kernel and userspace.
> 
> Is this failure reproducible somewhere else? Unfortunately, I don't have the 
> qt sources installed, else I would dig into them and check for the actual 
> problem.

I can't reproduce any problem here either (Qt4.6 from kde-qt, last commit
is 69720650b2283). I'll probably do the full rebuild cycle tomorrow
night, will retry after updating Qt.

Andreas

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