Every decode stage was capped at the chunk's decoded size. That holds
only when every filter ahead of the codec preserves size; Fletcher32
does not (it appends a 4-byte checksum), so a pipeline with Fletcher32
before deflate (NetCDF-4's fletcher32 -> shuffle -> deflate ordering,
h5repack's "all filters") failed with "deflate: output exceeds size
limit" on every chunk.
decompress_chunk_masked now computes each stage's bound by running the
chunk size forward through the filters that precede it in write order
(and that the chunk's mask did not skip): shuffle keeps the size,
Fletcher32 adds 4, any codec adds at most n/8 + 64. The cap is still a
small constant factor of the chunk, so a decompression bomb is rejected
as before (tested).
Shuffle also had to learn libhdf5's handling of a length that is not a
whole number of elements (chunk + checksum): shuffle the whole elements
and leave the trailing bytes in place, in both directions. It used to
refuse such data.
Regression: h5py_fletcher32_before_deflate_reads (fletcher->shuffle->
gzip, fletcher->gzip, shuffle->fletcher->gzip, and a 2-D i32 grid),
fletcher32_ahead_of_deflate_stays_bounded and
shuffle_leaves_a_partial_trailing_element_in_place.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
