Throw an exception on invalid BufferView instantiation

Also adjust tests to skip mmap under Python 2.

capnp/lib/capnp.pyx

@@ -15,6 +15,8 @@ from libc.stdlib cimport malloc, free
 from libc.string cimport memcpy
 from cython.operator cimport dereference as deref
 from cpython.exc cimport PyErr_Clear
+from cpython cimport Py_buffer
+from cpython.buffer cimport PyBUF_SIMPLE
 
 from types import ModuleType as _ModuleType
 import os as _os
@@ -3692,7 +3694,9 @@ cdef class _BufferView:
     cdef char * buf
 
     def __init__(self, other):
-        PyObject_GetBuffer(other, &self.view, 0)
+        cdef int ret = PyObject_GetBuffer(other, &self.view, PyBUF_SIMPLE)
+        if ret < 0:
+          raise ValueError("Invalid buffer passed to BufferView")
         self.buf = <char*>self.view.buf
 
     def __dealloc__(self):
@@ -3717,7 +3721,7 @@ cdef class _FlatArrayMessageReader(_MessageReader):
         cdef char * ptr
         if type(buf) == _mmap.mmap:
             view = _BufferView(buf)
-            ptr = <char*>view.view.buf
+            ptr = view.buf
             self._object_to_pin = view
         else:
             ptr = buf

test/test_serialization.py

@@ -6,6 +6,7 @@ import test_regression
 import tempfile
 import pickle
 import mmap
+import sys
 
 this_dir = os.path.dirname(__file__)
 
@@ -41,6 +42,7 @@ def test_roundtrip_bytes(all_types):
     msg = all_types.TestAllTypes.from_bytes(message_bytes)
     test_regression.check_all_types(msg)
 
+@pytest.mark.skipif(sys.version_info.major < 3, reason="mmap doesn't implement the buffer interface under python 2.")
 def test_roundtrip_bytes_mmap(all_types):
     msg = all_types.TestAllTypes.new_message()
     test_regression.init_all_types(msg)