[Octave-bug-tracker] [bug #45540] Segfault when moving the mouse over Qt figures

[Dan Sebald]

Follow-up Comment #8, bug #45540 (project octave):

The backtrace: you can see how many things are involved with the opengl call. 
Took me a long time to build all the tools needed for the driver before I
could debug some things in swrast.

Anyway, the comments I made about


warning: opengl_selector::select: selection buffer overflow


here https://savannah.gnu.org/bugs/?45604#comment7 pertain to the technique
described in comment #7.  Basically, with a very dense plot, the Select mode
can overflow.  If as you describe, only the axes hover is important, the
warning message could be removed because an overflowing Select mode isn't a
concern--you just want to know if there is activity there, which an overflow
inherently indicates.

There might be some technique whereby a bogus object the size of the axes is
put on the screen and all the other objects do not cause select events (but if
ever the day comes where the dream scenario of hand-decorating plots is
reality then individual object selection is of interest).  But, in principle
opengl is indicating selection which is what is wanted, albeit verbosely.  My
suspicion is that this is a threading issue.  From what I've seen of this Qt
plotting, it hasn't had the semaphore protection necessary from keeping
threads from conflicting with one another.  I'm guessing opengl events are in
one thread, Octave core in another, GUI maybe another.

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