A new case where download_point with size=1 on null data fails

[ceesem]

When doing a supervoxel lookup from a point outside of the volume bounds of a graphene segmentation, we are getting the an error using cv.download_point(pt, size=1). In this case, the region is within the bounding box, but there is no data at the location. I'm attaching a screenshot of the bad location with the z-axis along the horizontal, so you can see how the segmentation comes and goes at the same x,y location.

The same point correctly returns an array of zeros with size=2 or higher, and points outside of the dataset bounding box work correctly now.

The error is:

File Read Error: 0 bytes, Bbox([166333, 115585, 3569],[166589, 115841, 3601], dtype=int64), 9.7_9.7_45/166333-166589_115585-115841_3569-3601, errors: index out of range

Traceback (most recent call last):
  File "/app/materializationengine/workflows/ingest_new_annotations.py", line 495, in get_sv_id
    cv.download_point(pt=pos_array, size=1, coord_resolution=coord_resolution)
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/frontends/graphene.py", line 122, in download_point
    return self.download(bbox, mip, parallel=parallel, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/frontends/graphene.py", line 327, in download
    img = super(CloudVolumeGraphene, self).download(bbox, mip=mip, parallel=parallel, renumber=renumber)
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/frontends/precomputed.py", line 630, in download
    bbox.astype(np.int64), mip, parallel=parallel, renumber=bool(renumber)
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/datasource/precomputed/image/__init__.py", line 183, in download
    background_color=int(self.background_color),
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/datasource/precomputed/image/rx.py", line 148, in download
    secrets, renumber, background_color,
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/datasource/precomputed/image/rx.py", line 254, in download_single_voxel_unsharded
    partial(decode_single_voxel, requested_bbox.minpt - chunk_bbx.minpt)
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/datasource/precomputed/image/rx.py", line 429, in download_chunk
    background_color=background_color)
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/datasource/precomputed/image/rx.py", line 533, in decode_single_voxel
    mip, background_color,
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/datasource/precomputed/image/rx.py", line 559, in _decode_helper
    background_color=background_color
  File "/usr/local/lib/python3.7/site-packages/cloudvolume/chunks.py", line 296, in read_voxel
    out[0,0,0,0] = arr[tuple(xyz)]
  File "src/compressed_segmentation.pyx", line 399, in compressed_segmentation.CompressedSegmentationArray.__getitem__
  File "src/compressed_segmentation.pyx", line 358, in compressed_segmentation.CompressedSegmentationArray.get
IndexError: index out of range