osobhandClaude Opus 5.5 7c1968a34a fix(format): resolve shared fill value messages instead of zero-filling ...

dataset_fill_value treated a shared Fill Value message as "no fill
value", so unwritten storage of a dataset whose fill value lives in the
file's shared-message (SOHM) heap read as zeros rather than its fill
value. libhdf5 shares fill values whenever the file has a SOHM index for
them.

- fill_value::dataset_fill_value_in follows the reference (another object
  header, or the SOHM heap); read_full_with_fill and the facade's
  selection read use it.
- dataset_fill_value, which has no file to follow a reference into, now
  returns UnresolvedSharedMessage for a shared message instead of None.
- shared_message::load_sohm_table / message_data_with_sohm load the SOHM
  table from the superblock extension on demand.
- parse_sohm_table skipped each index's leading version byte, reading
  every field one byte off; SOHM references could never resolve.

Fixture shared_fill_value.h5 (HDF5 2.0, gen_shared_fill.py): sohm_b read
[0,1,2,3,0,0,0,0] and now reads [0,1,2,3,-7,-7,-7,-7], as h5py does.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

2026-09-25 21:18:12 -05:00

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