[PyQt] sip: Accessing members results in incorrect/missing data

[Phil Thompson]

On 16/01/2020 20:52, MatthijsBurgh wrote:
> I have done some more investigating.
> 
> As mentioned 
> https://github.com/orocos/orocos_kinematics_dynamics/issues/41,
> problems where occuring in with SIP 4.13.2.
> 
> Which is explainable as you fixed this issue in the commit:
> https://www.riverbankcomputing.com/hg/sip/rev/0dd3cb4eff0e. This makes 
> sure
> the member variable contains a reference to the containing class. This
> prevents the class from being garbage collected while there are still
> references to the member variable.
> 
> You apply a fix to this in commit
> https://www.riverbankcomputing.com/hg/sip/rev/fbb9cdbad791.
> 
> The commit, https://www.riverbankcomputing.com/hg/sip/rev/11a92ebd4840,
> which I also mentioned in my first message, is the important one.
> 
> In this commit you introduce a kind of caching for member variables. 
> The
> python wrapper of the members of a class are stored in a dict in the 
> python
> wrapper of the class. To add the member to the cache, you use
> "sipKeepReference(sipPySelf, %d, sipPy)". So the class contains a 
> reference
> to the member variable.
> 
> While previously "sipKeepReference(sipPy, -1, sipPySelf)", was used to 
> store
> the reference to the class in the dict of the member variable. Focus 
> here on
> the switch of "sipPy" and "sipPySelf". So by the last commit, 
> 11a92ebd4840,
> you added a new feature, but you broke another one.
> 
> I suggest to restore the feature which stores the reference to the 
> class in
> the member variable dict.
> 
> I did some testing in 4.19.20. I didn't experience a significant 
> performance
> increase with the caching with my Frame object and its member p, a 
> vector
> with 3 variables. (I tweaked the code,
> https://www.riverbankcomputing.com/hg/sip/file/7c54145e55b6/sipgen/gencode.c#l4904,
> to always get the caching.)
> Also I added manually the "sipKeepReference(sipPy, -1, sipPySelf)" to 
> the
> generated cpp for the class reference. When both features were used at 
> the
> same time, the "release" function of the class was not called. (I added 
> a
> "printf" to it, which didn't show in that situation, but did if just of 
> the
> two was used.)

Thanks for the analysis, that was very helpful.

This changeset is also a problem...

https://www.riverbankcomputing.com/hg/sip/rev/6ec87337d5e2

Can you try the latest version of the 4.19-maint branch, ie. including 
this changeset...

https://www.riverbankcomputing.com/hg/sip/rev/137b9be794a1

...and let me know how you get on.

Phil