fix: shape the value buffer for coordinate selections in sharded writes

[dylanpulver]

Summary

Nightly Slow Hypothesis CI on main filed #4280 on 2026-08-22 and hit it again on 2026-08-25 (run 32792180919): ValueError: shape mismatch: value array of shape (3,1) could not be broadcast to indexing result of shape (3,). An orthogonal set on a sharded array with two array-indexed dimensions reproduces it:

a = zarr.create_array(MemoryStore(), shape=(4, 4), chunks=(2, 4), dtype="int32",
                      serializer=ShardingCodec(chunk_shape=(2, 2), codecs=(BytesCodec(),)))
a.oindex[np.array([3, 1, 2]), np.array([0, 2])] = np.arange(6).reshape(3, 2)

OrthogonalIndexer hands such a chunk selection down as an np.ix_ pair (indexing.py:989). get_indexer reads it back as a coordinate selection, whose projections address shard_array flat while the caller shaped it like sel_shape. _decode_partial_single reshapes out to sel_shape on the way out (sharding.py:1085); this does the same on the way in. Reads were unaffected.

For reviewers

Guarded on shard_array.shape == sel_shape, so a flat value passes through. Checked against a numpy np.ix_ oracle over 1568 combinations of chunk grid, sharding nesting, per-dimension selector: 96 failures on main, 0 after, every one a write with two array-indexed dimensions.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Changes documented as a new file in changes/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.12%. Comparing base (d44f9f9) to head (77ce2c2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4284   +/-   ##
=======================================
  Coverage   94.12%   94.12%           
=======================================
  Files          92       92           
  Lines       12831    12835    +4     
=======================================
+ Hits        12077    12081    +4     
  Misses        754      754           

Files with missing lines	Coverage Δ
src/zarr/codecs/sharding.py	96.19% <100.00%> (+0.02%)	⬆️

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[d-v-b]

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Code review

Found 1 issue:

1. The fix is applied to _encode_partial_single (async, BatchedCodecPipeline) but not to its sync twin _encode_partial_sync (used by FusedCodecPipeline), which derives its indexer the same way and still fails with the same ValueError for this PR's own regression scenario when codec_pipeline.path is set to FusedCodecPipeline. The decode side applies the sel_shape reshape in both twins (_decode_partial_single and _decode_partial_sync), so the sync encode path needs the matching reshape too. Parametrizing the new test over both pipelines (as test_sharding_vlen_inner_codec_roundtrip does) would cover it.

Missing-fix location:

zarr-python/src/zarr/codecs/sharding.py

Lines 798 to 806 in 01aac1f

	indexer = list(
	get_indexer(
	selection,
	shape=shard_shape,
	chunk_grid=ChunkGrid.from_sizes(shard_shape, self.chunk_shape),
	)
	)

The reshape added on the async side:

zarr-python/src/zarr/codecs/sharding.py

Lines 1361 to 1374 in 01aac1f

	shard_indexer = get_indexer(
	selection,
	shape=shard_shape,
	chunk_grid=ChunkGrid.from_sizes(shard_shape, chunk_shape),
	)
	# A coordinate indexer flattens the selection, so its projections address
	# `shard_array` as 1-D while the caller shaped it like `sel_shape`. This
	# mirrors the reshape `_decode_partial_single` applies on the way out.
	sel_shape = getattr(shard_indexer, "sel_shape", None)
	if sel_shape is not None and shard_array.shape == sel_shape:
	shard_array = shard_array.reshape(shard_indexer.shape)
	indexer = list(shard_indexer)

Sync decode twin already carrying the mirrored reshape:

zarr-python/src/zarr/codecs/sharding.py

Lines 1306 to 1309 in 01aac1f

	if hasattr(indexer, "sel_shape"):
	return out.reshape(indexer.sel_shape)
	return out

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