Add tests for NULL objects in memoryviews (GH-4871)

Follow up on https://github.com/cython/cython/pull/4859 by adding tests for memoryviews too.

Additional refactoring to avoid invalid decref calls on test failures. Instead, the item is safely cleared directly before the access.

tests/buffers/bufaccess.pyx

@@ -10,7 +10,7 @@
 from __future__ import unicode_literals
 
 from cpython.object cimport PyObject
-from cpython.ref cimport Py_INCREF, Py_DECREF
+from cpython.ref cimport Py_INCREF, Py_DECREF, Py_CLEAR
 cimport cython
 
 __test__ = {}
@@ -1013,17 +1013,14 @@ def check_object_nulled_1d(MockBuffer[object, ndim=1] buf, int idx, obj):
     >>> rc1 = get_refcount(a)
     >>> A = ObjectMockBuffer(None, [a, a])
     >>> check_object_nulled_1d(A, 0, a)
-    >>> decref(a)  # new reference "added" to A
     >>> check_object_nulled_1d(A, 1, a)
-    >>> decref(a)
     >>> A = ObjectMockBuffer(None, [a, a, a, a], strides=(2,))
     >>> check_object_nulled_1d(A, 0, a)  # only 0 due to stride
-    >>> decref(a)
     >>> get_refcount(a) == rc1
     True
     """
-    cdef void **data = <void **>buf.buffer
-    data[idx] = NULL
+    cdef PyObject **data = <PyObject **>buf.buffer
+    Py_CLEAR(data[idx])
     res = buf[idx]  # takes None
     buf[idx] = obj
     return res
@@ -1037,14 +1034,12 @@ def check_object_nulled_2d(MockBuffer[object, ndim=2] buf, int idx1, int idx2, o
     >>> rc1 = get_refcount(a)
     >>> A = ObjectMockBuffer(None, [a, a, a, a], shape=(2, 2))
     >>> check_object_nulled_2d(A, 0, 0, a)
-    >>> decref(a)  # new reference "added" to A
     >>> check_object_nulled_2d(A, 1, 1, a)
-    >>> decref(a)
     >>> get_refcount(a) == rc1
     True
     """
-    cdef void **data = <void **>buf.buffer
-    data[idx1 + 2*idx2] = NULL
+    cdef PyObject **data = <PyObject **>buf.buffer
+    Py_CLEAR(data[idx1 + 2*idx2])
     res = buf[idx1, idx2]  # takes None
     buf[idx1, idx2] = obj
     return res

tests/memoryview/memslice.pyx

@@ -5,7 +5,7 @@
 from __future__ import unicode_literals
 
 from cpython.object cimport PyObject
-from cpython.ref cimport Py_INCREF, Py_DECREF
+from cpython.ref cimport Py_INCREF, Py_DECREF, Py_CLEAR
 
 cimport cython
 from cython cimport view
@@ -1130,6 +1130,49 @@ def assign_temporary_to_object(object[:] buf):
     """
     buf[1] = {3-2: 2+(2*4)-2}
 
+@testcase
+def check_object_nulled_1d(object[:] buf, int idx, obj):
+    """
+    See comments on printbuf_object above.
+
+    >>> a = object()
+    >>> rc1 = get_refcount(a)
+    >>> A = ObjectMockBuffer(None, [a, a])
+    >>> check_object_nulled_1d(A, 0, a)
+    >>> check_object_nulled_1d(A, 1, a)
+    >>> A = ObjectMockBuffer(None, [a, a, a, a], strides=(2,))
+    >>> check_object_nulled_1d(A, 0, a)  # only 0 due to stride
+    >>> get_refcount(a) == rc1
+    True
+    """
+    cdef ObjectMockBuffer omb = buf.base
+    cdef PyObject **data = <PyObject**>(omb.buffer)
+    Py_CLEAR(data[idx])
+    res = buf[idx]  # takes None
+    buf[idx] = obj
+    return res
+
+@testcase
+def check_object_nulled_2d(object[:, ::1] buf, int idx1, int idx2, obj):
+    """
+    See comments on printbuf_object above.
+
+    >>> a = object()
+    >>> rc1 = get_refcount(a)
+    >>> A = ObjectMockBuffer(None, [a, a, a, a], shape=(2, 2))
+    >>> check_object_nulled_2d(A, 0, 0, a)
+    >>> check_object_nulled_2d(A, 1, 1, a)
+    >>> get_refcount(a) == rc1
+    True
+    """
+    cdef ObjectMockBuffer omb = buf.base
+    cdef PyObject **data = <PyObject**>(omb.buffer)
+    Py_CLEAR(data[idx1 + 2*idx2])
+    res = buf[idx1, idx2]  # takes None
+    buf[idx1, idx2] = obj
+    return res
+
+
 #
 # Test __cythonbufferdefaults__
 #