[Kde-perl] Memory fault when inserting QTableItem into QTable

[Ashley Winters]

--- Richard Dale <Richard_Dale at tipitina.demon.co.uk> wrote:
[snip]
> PerlQt overrides the takeItem() virtual method as part of its garbage
> 
> collection tracking, and so when the perl version of takeItem() is
> called the 
> recently deleted instance is found to not have a corresponding perl
> instance 
> and another is created. Inside the perl takeItem() method the call 
> 'setAllocated( $_[0], 1 );' means that PerlQt owns this newly created
> 
> instance owns the (already deleted) QTableItem and is responsible for
> 
> deleting it.
> 
> sub Qt::Table::takeItem
> {
>     package Qt::_internal;
>     delete ${ this()->{"hidden children"} } { sv_to_ptr($_[0]) };
>     delete $_[0]->{"has been hidden"};
>     setAllocated( $_[0], 1 );
>     no strict 'refs';
>     $Qt::AutoLoad::AUTOLOAD = 'Qt::Table::takeItem';
>     my $autoload = " Qt::Table::_UTOLOAD";
>     dontRecurse();
>     $autoload->( $_[0] );
> }

Hmm...

> 
> On garbage collection of the '$_[0]' passed to takeItem(), PerlQt
> attempts to 
> delete an already deleted instance and there is a crash. Now I
> understand 
> what's going on I can start to think of a fix..

The takeItem() family of functions need to know who their caller is.
When being called from C++, they should bypass all garbage-collection
voodoo. It's either being called from C++ or from Perl, and whichever
one is calling it beeds to take responsibility for deleting the object
thereafter.

When called from Perl, there is already a perfectly valid $item
(otherwise, how did it get passed to takeItem()?), and we are obligated
to call setAllocated(1) on the object, as well as remove any
pre-existing 'hidden children' object remaining in the parent object.
That way, Perl's garbage collection will properly destroy the object
(just like the programmer was asking in the first place by calling
takeItem()).

When called from C++, the item being taken must be setAllocated(0), to
allow the calling function to move or delete the item as it wishes.
It's explicitly taking responsibility for the destruction of the object
AWAY from Perl by calling takeItem(), and we need to honor it.

So, the patch is a simple one-line fix:

-     setAllocated( $_[0], 1 );
+     setAllocated( $_[0], was_called_from_perl() );

Of course, someone has to write was_called_from_perl(), which needs to
pass the following test:

package test1;
use Qt::isa 'Qt::Table';

# This does NOT qualify as being called from Perl if it is called from
# C++
sub takeItem { SUPER->takeItem(@_) }

package test2;
use Qt::isa 'Qt::Table';

# How do you propose to pass this one when called from C++?
sub takeItem { this->Qt::Table::takeItem(@_) }

Okay, test2 counts as being called from Perl even when
test2::takeItem() is called from C++. Use SUPER for proper behavior.

Of course, all the takeItem() functions need the above one-line patch.
:)

Ashley Winters


		
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