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List:       pykde
Subject:    Re: [PyQt] Conversion to wrong SubClass
From:       Elvis Stansvik <elvstone () gmail ! com>
Date:       2019-01-25 20:10:16
Message-ID: CAHms=ea7nyhfgxzapzZvwX80xf-eGhGQ-i0x0_RFQLQE++p6Wg () mail ! gmail ! com
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Den tis 22 jan. 2019 kl 18:35 skrev Matthias Kuhn <matthias@opengis.ch>:
> 
> On 1/22/19 10:04 AM, Phil Thompson wrote:
> > On 21 Jan 2019, at 11:13 pm, Matthias Kuhn <matthias@opengis.ch> wrote:
> > > Hi Phil
> > > 
> > > On 1/18/19 2:13 PM, Phil Thompson wrote:
> > > > On 18 Jan 2019, at 12:48 pm, Matthias Kuhn <matthias@opengis.ch> wrote:
> > > > > Or does this mean that if the refcount for this Python object dropped to
> > > > > 0 the cache will be invalidated anyway (and the real issue here is, that
> > > > > the python object is still alive somewhere)?
> > > > It should be invalidated when the ref count reaches 0.
> > > It seems to be doing that. But when an additional /Transfer/ annotation
> > > comes into play things start to go worse.
> > > 
> > > The following code snippet reproduces the issue:
> > > 
> > > https://issues.qgis.org/issues/20661#note-2
> > > 
> > > 
> > > The interesting bits are the (almost) last two lines of the loop
> > > 
> > > * pt_g.clone() is a /Factory/ method
> > > 
> > > * The QgsGeometry constructor has a /Transfer/ annotation on the
> > > parameter, if this one is not called, everything goes well.
> > > 
> > > 
> > > Here are the first couple of lines of the output:
> > > 
> > > ------------------------------------------
> > > 
> > > new pt  0x5d98240
> > > new pt  0x5f257c0
> > > new pt  0x4283da0
> > > new pt  0x5d98240
> > > WHAT??????!!!!!  0x5f257c0
> > > new pt  0x629fa60
> > > new pt  0x5d3c470
> > > WHAT??????!!!!!  0x5d98240
> > > new pt  0x2e2ac90
> > > new pt  0x4283da0
> > > new pt  0x5dcef50
> > > new pt  0x5f770d0
> > > new pt  0x629fa60
> > > WHAT??????!!!!!  0x5d98240
> > > 
> > > ------------------------------------------
> > > 
> > > 
> > > Any idea what the additional Transfer of the object triggers?
> > See the docs.
> > 
> > I can't really comment as you haven't explained the class hierachy or the use of \
> > annotations in the ctors.
> 
> To make it easier, I have extracted all the relevant information about
> class hierarchy etc. here:
> 
> https://gist.github.com/m-kuhn/368c017f05183df707ce054c2f431830
> 
> 
> The complete and complex version can be found here:
> 
> https://github.com/qgis/QGIS/tree/master/python/core/auto_generated/geometry
> 
> > 
> > Does the cloned object have a parent (in which case it's not a factory)?
> 
> There are no QObjects involved here.
> 
> 
> My interpretation of what's going on:
> 
> * There's a `QgsGeometry` which owns a point `QgsPoint(QgsAbstractGeometry)`
> 
> * this point is cloned (ownership of the clone goes to Python at this
> point) (/Factory/)
> 
> * the clone is then transferred to C++ by calling the `QgsGeomtery(
> QgsAbstractGeometry* /Transfer/ )` constructor
> 
> * the python wrapper of the clone is garbage collected (that's where I
> think cached type information should be cleared)
> 
> * then the QgsGeometry is garbage collected and the clone deleted along
> (in C++, that's what Python can know nothing about, but by that time any
> caching of information regarding the clone should have been invalidated
> before if I interpret your previous statements correctly)

"There are two hard things in computer science: cache invalidation,
naming things, and off-by-one errors".

Sorry, could not resist :) Following this mystery with interest, even
if I don't have the knowledge to contribute :/

Elvis

> 
> 
> Is there something I am missing here?
> 
> Matthias
> 
> 
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