[Cython] memoryview slice transpose

[Syam Gadde]

I wanted to re-offer for discussion a fix for a bug I reported a while 
back.  The code that breaks is this:

import numpy
cimport numpy

def testfunc():
    a = numpy.arange(12).reshape([3,4])
    cdef numpy.int_t[:,:] a_view = a
    cdef numpy.int_t[:,:] b = a_view[:,:].T

One problem is that when a memoryview slice is decremented, the .memview 
and .data fields are not set to NULL, and any error that causes a jump 
to cleanup code will attempt to decref it again.

The second problem (I think) is that assignment of a transpose of 
memoryview slices to a variable is not resulting in an incref on the 
underlying slice, even though the variable is not a temporary and 
continues to persist.

Here is a patch that solves the problem for me.

https://github.com/SyamGadde/cython/commit/5739d8b908f18c4fc9103ef04e39964d61af3495

The part I am least sure about is removing the "if" qualifications on 
the incref and decref:

   if self.obj.is_name  or self.obj.is_attribute and self.obj.is_memslice_transpose:

that was obviously there for a reason, but it causes problems in the above case.  If there is a less extreme solution I'd be happy to take it.

Thanks,

-syam