interaction between vtk and pyhull causes segfault

[Stephen Sinclair]

I don't know whether to report this to vtk6 or to qhull, but I'll report it to vtk6 since there is no python-pyhull package.

Install the packages via

~~~
sudo apt-get install python-vtk6
sudo pip install pyhull
~~~

The following program then segfaults for me on 16.10 during creation of the ConvexHull object. Works fine on 16.04.

If the line "import vtk" is commented out, the program proceeds without error. So something in vtk 6.3 is causing a problem with pyhull, very strange.

~~~
import numpy
import vtk
from pyhull.convex_hull import ConvexHull

coords = numpy.array([[ 0.05753396, 0.0594523 , 0.04074307],
                      [ 0.02653826, -0.02544384, 0.08457577],
                      [-0.03997982, 0.01372854, 0.08196262],
                      [-0.03134984, -0.02472897, 0.08312879],
                      [ 0.08184259, 0.01582656, -0.03944668],
                      [ 0.05668982, -0.03320603, -0.06471749],
                      [-0.06028319, 0.06223122, -0.03159022],
                      [-0.00628884, -0.00626133, -0.09179312]])

hull = ConvexHull(coords)
print hull
~~~

A backtrace reveals errors during initialization.

~~~
#0 0x00007fff7ccf4a98 in qh_initstatistics () at src/libqhull/stat.c:462
#1 0x00007fff7ccfbaf2 in qh_initqhull_start (
    infile=infile@entry=0x55555624da60, outfile=outfile@entry=0x555555ed3080,
    errfile=errfile@entry=0x7ffff7bb5520 <_IO_2_1_stderr_>)
    at src/libqhull/global.c:1825
#2 0x00007fff7ccfbb3c in qh_init_A (infile=0x55555624da60,
    outfile=0x555555ed3080, errfile=0x7ffff7bb5520 <_IO_2_1_stderr_>, argc=3,
    argv=0x7fffffffdcc0) at src/libqhull/global.c:487
#3 0x00007fff7ccf180a in py_qconvex (self=<optimized out>,
    args=<optimized out>) at src/_pyhull.c:106
#4 0x000055555564e37a in PyEval_EvalFrameEx ()
~~~

It *appears* to be a problem with initialization qhull, but I report it to vtk6 because the problem happens with vtk6.3 but *not* with vtk6.2 (on Xenial). I am not sure how to proceed to debug further.