A type-1 (raw data chunk) B-tree key holds the chunk size, the filter
mask and one offset per dimension, and those offsets are always 8 bytes:
they are dataset coordinates, not file addresses. The reader used the
superblock's size-of-offsets for them, so in a file with 4-byte offsets
every key was misparsed. Unfiltered chunked datasets read as zeros (with
stray bytes where a misread address landed on data) and filtered ones
failed with "deflate: truncated stream".
Only the sibling and child addresses follow size-of-offsets now. The
unit-test B-tree builder wrote keys the same wrong way, which is why its
tests passed; it now matches the format.
Regression: h5py_four_byte_offsets_chunked_reads (h5py, set_sizes(4, 4)
and (4, 8); 1-D and 2-D, unfiltered and gzip) and the unit test
collect_chunks_with_four_byte_addresses.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
