[C++][Python] PyArrow crash when filter/take empty Extension array

[asfimport]

PyArrow is crashing when applying filter or take on already empty extension arrays.

The bug can be reproduced with the documentation example:

import pyarrow as pa

class Point3DArray(pa.ExtensionArray):
    def to_numpy_array(self):
        return self.storage.flatten().to_numpy().reshape((-1, 3))


class Point3DType(pa.PyExtensionType):
    def __init__(self):
        pa.PyExtensionType.__init__(self, pa.list_(pa.float32(), 3))

    def __reduce__(self):
        return Point3DType, ()

    def __arrow_ext_class__(self):
        return Point3DArray

storage = pa.array([[1, 2, 3], [4, 5, 6]], pa.list_(pa.float32(), 3))
arr = pa.ExtensionArray.from_storage(Point3DType(), storage)
arr = arr.filter(pa.array([False, False]))

# Crashing here...
arr.filter(pa.array([], pa.bool_()))
# Crashing as well...
arr.take(pa.array([], pa.int32()))

The underlying issue seems to be that the function nulls is not implemented for extension types in the C++ codebase: 

arrow/cpp/src/arrow/array/util.cc

Line 472 in 6db88a9

Status Visit(const DataType& type) {

Environment: Python 3.7, Ubuntu 20.04
Reporter: Paul Balanca / @balancap
Assignee: Joris Van den Bossche / @jorisvandenbossche

PRs and other links:

• GitHub Pull Request #11227

_{Note: This issue was originally created as ARROW-13474. Please see the migration documentation for further details.}

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
@balancap Thanks for the clear bug report! I can confirm the issue and your analysis looks correct. You are looking into a fix for this?

[asfimport]

Paul Balanca / @balancap:
Thanks @jorisvandenbossche  I don't want to over-promise. Next two weeks are a bit busy for me, hopefully after I will get a bit more time. There are a couple of methods not supported currently by PyArrow on extension types (e.g. nulls, concat_arrays, combine_chunks) that I would be interested in implementing for our internal tooling at FiveAI.

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
I opened a PR to fix this (so we can get it into the next release).

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 11227
#11227