Palapeli crashes (was Palapeli and kdelibs4)

[Christian Ehrlicher]

Am 01.09.2017 um 15:10 schrieb Ian Wadham:
> I had a look at some of these crashes back in KDE 4 days, but it is indeed hard to
> reproduce any of them - except for bug 336075, the one where you restart the puzzle
> while dragging a piece.
>
> FWIW, my theory is that the crashes are (all?) caused by deleting a piece-object
> while it is being operated on by an interactor.  How so?  Obviously all pieces get
> deleted and re-created when you restart the puzzle, so that is one way.  Another,
> from what I remember (bearing in mind that I am getting old :-)) is when two pieces
> are joined or merged (i.e. the user takes one step in solving the puzzle).  At that time a
> new composite piece-object is created and the component piece-objects are deleted,
> or so I understand.
>
> If you move the pieces to be merged close together, but not quite touching, then let
> go of the mouse button, Palapeli drags them together automatically and merges them.
> This is the normal user-action and there is then no problem: the move-interactor has
> terminated before the merge.
>
> However, if the move-interactor is still busy when the merge occurs, there is code that
> is supposed to take care of this by signalling the replacement piece to the interactor,
> using the replaced pieces as senders.  I am not convinced that the relevant code
> works in all cases and I think this is where the crashes may lie.  Maybe some sort of
> race condition internally in Palapeli or maybe some user-action that somehow re-
> selects one of the merging or merged pieces at an awkward time.
>
> The code I have been looking at is in palapeli/src/engine, files mergegroup.cpp,
> interactors.cpp and piece.cpp, in particular the methods
>        void Palapeli::MergeGroup::createMergedPiece()
>        void Palapeli::Piece::announceReplaced(Palapeli::Piece* replacement)
>        void Palapeli::MovePieceInteractor::pieceReplacedBy(Palapeli::Piece* replacement)
>
> After all that is said, the crashes in Palapeli do not have a huge impact on the user,
> because Palapeli saves its state to disk about every two seconds.
>
> However the crashes are not a good look, especially if the user has just moved the last
> piece into place and completed the puzzle… :-)
I've had a look at the same places and tried to reproduce it by e.g. 
delaying the animation or trigger it with some small source code 
modifications but really had no luck.
Maybe it would be an idea to use QPointer<Piece> in the interactors so 
we can at least check for != nullptr...
Maybe I've some time during the next week... :)


Christian