[Development] Clarification needed on a crash caused by ignoring ownership of an object

[Andy Shaw]

Thanks Marc, that covers what I needed to know to make a decision ☺

Andy

Development på vegne av Marc Mutz <development-bounces+andy.shaw=qt.io at qt-project.org på vegne av marc.mutz at kdab.com> skrev følgende den 04.05.2017, 10.08:

    On Thursday 04 May 2017 09:58:45 Andy Shaw wrote:
    > Hi,
    > 
    > I am trying to determine what we should be doing inside Qt in a certain
    > case. When a QUndoCommand is deleted after it has been given another
    > QUndoCommand as a parent it will cause a crash since the parent has not
    > been informed that the child was deleted. The documentation for
    > QUndoCommand is explicit on the fact that when it has a parent the
    > ownership of the QUndoCommand is transferred to its parent. As such I
    > would not expect it to be possible to delete the child directly anymore as
    > a result as we no longer own it.
    > 
    > So the question is, since it is still a crash, should we still be
    > accounting for this in the Qt code so as to prevent the crash or is it a
    > case of it is documented to transfer ownership and as such we don’t want
    > to protect against it being deleted out of Qt’s hands?
    
    Seeing as about half of the ubsan reports are about shutdown sequences of 
    object hierarchies which allow parent-owned children to be deleted by the 
    user, I'm tempted to say: add an assert (set a flag when the parent deletes a 
    child, check that the flag is set in the child's dtor).
    
    OTOH, QObject parent-child relationships, and QTreeWidgetItems set a strong 
    precedent.
    
    I'd decide based on how much complexity this adds to the implementation, but 
    if you prepare a patch, the pressure to merge it is going to be a great one. 
    The work is already done, after all. So, please decide yourself. You have my 
    blessing to close as WONTFIX, or else enable the new use-case if it's not too 
    complex (a very subjective criterion, I agree).
    
    Thanks,
    Marc
    
    -- 
    Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
    KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
    Tel: +49-30-521325470
    KDAB - The Qt, C++ and OpenGL Experts
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