Regression in 2.18: Index to size-1 array fails

[AndreasAlbertQC]

Zarr version

v2.18.0

Numcodecs version

v0.12.1

Python Version

3.11

Operating System

Reproduced on mac and linux

Installation

micromamba / conda-forge

Description

Indexing to unit-length arrays fails since zarr 2.18. Indexing other array lengths still works. See reproducer script and traceback below. In zarr 2.17, this still worked. I don't understand the problem exactly, but based on the 2.18 changelog, I would guess that this is somehow related to #1800

Steps to reproduce

import zarr
import numcodecs

assert zarr.__version__ == "2.18.0"
assert numcodecs.__version__ == "0.12.1"

x = zarr.array([1, 2])
y = zarr.array([1])

# OK
print(x[:])
print(y[:])

# OK
print(x[0])

# Not OK
print(y[0])

Additional output

Traceback (most recent call last):
  File "/Users/x/Library/Application Support/JetBrains/PyCharm2023.3/scratches/scratch_42.py", line 14, in <module>
    print(x[0])
          ~^^^
  File "/Users/x/micromamba/envs/envname/lib/python3.11/site-packages/zarr/core.py", line 800, in __getitem__
    result = self.get_basic_selection(pure_selection, fields=fields)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/x/micromamba/envs/envname/lib/python3.11/site-packages/zarr/core.py", line 926, in get_basic_selection
    return self._get_basic_selection_nd(selection=selection, out=out, fields=fields)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/x/micromamba/envs/envname/lib/python3.11/site-packages/zarr/core.py", line 968, in _get_basic_selection_nd
    return self._get_selection(indexer=indexer, out=out, fields=fields)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/x/micromamba/envs/envname/lib/python3.11/site-packages/zarr/core.py", line 1343, in _get_selection
    self._chunk_getitems(
  File "/Users/x/micromamba/envs/envname/lib/python3.11/site-packages/zarr/core.py", line 2181, in _chunk_getitems
    self._process_chunk(
  File "/Users/x/micromamba/envs/envname/lib/python3.11/site-packages/zarr/core.py", line 2049, in _process_chunk
    self._compressor.decode(cdata, dest)
  File "numcodecs/blosc.pyx", line 564, in numcodecs.blosc.Blosc.decode
  File "numcodecs/blosc.pyx", line 365, in numcodecs.blosc.decompress
  File "numcodecs/compat_ext.pyx", line 16, in numcodecs.compat_ext.Buffer.__cinit__
ValueError: buffer source array is read-only

[jhamman]

Thanks for reporting. This is likely the same as issue as #1874.

[AndreasAlbertQC]

Thanks for the quick turnaround @jhamman , I appreciate it!