ARROW-8105: [Python] Fix segfault when shrunken masked array is passe…

…d to pyarrow.array

Needed to validate that the mask of the masked array wasn't equal to the nomask
constant which indicates that the masked array was shrunk

Closes #6612 from nugend/master

Lead-authored-by: Dan Nugent <nugend@gmail.com>
Co-authored-by: Daniel Nugent <nugend@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

python/pyarrow/array.pxi

@@ -208,7 +208,8 @@ def array(object obj, type=None, mask=None, size=None, from_pandas=None,
                 raise ValueError("Cannot pass a numpy masked array and "
                                  "specify a mask at the same time")
             else:
-                mask = values.mask
+                # don't use shrunken masks
+                mask = None if values.mask is np.ma.nomask else values.mask
                 values = values.data
 
         if hasattr(values, '__arrow_array__'):

python/pyarrow/tests/test_array.py

@@ -1656,6 +1656,13 @@ def test_array_from_masked():
         pa.array(ma, mask=np.array([True, False, False, False]))
 
 
+def test_array_from_shrunken_masked():
+    ma = np.ma.array([0], dtype='int64')
+    result = pa.array(ma)
+    expected = pa.array([0], type='int64')
+    assert expected.equals(result)
+
+
 def test_array_from_invalid_dim_raises():
     msg = "only handle 1-dimensional arrays"
     arr2d = np.array([[1, 2, 3], [4, 5, 6]])