Logic errors in GeoTIFF validation in rio_cogeo/cogeo.py

[mplough-kobold]

Lines 456-461 of rio_cogeo/cogeo.py read:

            block_offset = int(src.get_tag_item("BLOCK_OFFSET_0_0", "TIFF", bidx=1))
            if not block_offset:
                errors.append("Missing BLOCK_OFFSET_0_0")

            data_offset = int(block_offset) if block_offset else None
            data_offsets = [data_offset]

If BLOCK_OFFSET_0_0 is missing / None, the cast to int will raise an exception and the error string will not be appended.

Later, on line 472:

            if data_offsets[-1] < ifd_offsets[-1]:

The value of data_offsets[-1] may be None. If it is in fact None, a TypeError will be raised.

I don't currently understand the TIFF file format well enough to understand how to handle the missing offset correctly so I don't want to open a PR with a potentially wrong fix, but I'd be glad to get a discussion started on what to do here.

Thanks!

[vincentsarago]

@mplough-kobold would you be able to share a tiff where BLOCK_OFFSET_0_0 is missing ?

FYI looking at https://github.com/OSGeo/gdal/blob/master/gdal/swig/python/samples/validate_cloud_optimized_geotiff.py#L72-L73 it seems that you are indeed right 😉

[mplough-kobold]

Certainly!

The code at https://github.com/OSGeo/gdal/blob/master/autotest/gcore/cog.py#L761 will generate a TIFF file with a missing block offset. (it's currently HEAD; for those looking at this in the future, see the specific commit -- https://github.com/OSGeo/gdal/blob/a9e3b302963f1b560502fea8760807b5506fdfb4/autotest/gcore/cog.py#L761)

I sliced and diced it a bit and ran this snippet in the osgeo/gdal Docker image; the resulting TIFF file is attached here -
cog.tif.zip.

from osgeo import gdal

filename = 'cog.tif'
src_ds = gdal.GetDriverByName('MEM').Create('', 512, 512)
src_ds.GetRasterBand(1).Fill(255)
src_ds.WriteRaster(0, 0, 256, 256, '\x00' * 256 * 256)
src_ds.WriteRaster(256, 256, 128, 128, '\x00' * 128 * 128)
src_ds.BuildOverviews('NEAREST', [2])
gdal.GetDriverByName('COG').CreateCopy(filename, src_ds, options=['BLOCKSIZE=128', 'SPARSE_OK=YES', 'COMPRESS=LZW'])

[mplough-kobold]

🎉 🎉 Thanks @vincentsarago for taking care of this!

[vincentsarago]

@mplough-kobold thanks for reporting it :-)