fix: resolve chunks=-1 to chunk size 1 on zero-length axes - #315

…opers#4277)

* feat: name packages that provide a codec zarr cannot find

Closes zarr-developers#4271.

When zarr fails to resolve a codec it now says which Python packages are known to
provide it, instead of raising a bare KeyError holding only the codec name:

    An implementation for codec 'wavpack' is not available. Register one explicitly
    using the codec registry (see <docs>), or install a Python package that
    registers a codec implementation with numcodecs. Known packages supporting this
    codec: wavpack-numcodecs.

Two hand-maintained tables in zarr/registry.py hold the mapping, one per Zarr
format, because the two formats resolve codecs through different registries and the
same name can mean different things in each: `imagecodecs_*` names are declared by
`virtual-tiff` under the `zarr.codecs` entry point group and by
`imagecodecs-numcodecs` under `numcodecs.codecs`, and `crc32c` is a codec zarr
implements itself in format 3 while in format 2 it needs `numcodecs[crc32c]`. Each
table has an exact-match and a prefix-match half, since packages that provide many
codecs namespace them behind a shared prefix. Entries cover third-party packages and
the codecs numcodecs gates behind its own optional dependencies -- `zfpy`, `pcodec`,
`crc32c` and `msgpack2` -- which are the most common missing-codec case in practice.

Backwards compatibility: `get_codec_class` now raises `zarr.errors.UnknownCodecError`
instead of `KeyError`, both for a codec with no registered implementation and for a
codec whose configured implementation is not registered. `get_numcodec` raises it
instead of the ValueError numcodecs raises for an unregistered format 2 codec id.
All are subclasses of `ValueError`. Carrying the message on a `KeyError` was not an
option: `KeyError.__str__` reprs its argument, so a multi-sentence message comes back
quoted and escaped. `UnknownCodecError` is now exported from `zarr.errors`, since
users are being told to catch it.

`get_numcodec` supports numcodecs down to the declared 0.14 floor: `numcodecs.errors`
only exists from 0.15.1, so the unregistered-codec check prefers that exception type
where it is importable and falls back to matching the message otherwise.

Signed-off-by: arcusbuilds <srijankeshri007@gmail.com>

* fix: address review feedback

1. parse_codecs converts KeyError from from_dict again. The removed try/except
   wrapped the whole expression, not just the registry lookup, so a codec whose
   from_dict indexes a malformed configuration leaked a bare KeyError out of
   metadata parsing. On the zarr.open fallback path that KeyError was swallowed
   and reported as an unrelated group error: with mode="a" it surfaced as
   `TypeError: open_group() got an unexpected keyword argument 'shape'`.

   The catch is narrow, around from_dict only, since get_codec_class now raises
   for the lookup half. It raises MetadataValidationError naming the codec and
   the missing key rather than restoring the old message, which reported the
   missing configuration key as though it were the codec name
   ("Unknown codec: 'required_option'").

2. The config-pin branch raises BadConfigError, matching get_pipeline_class,
   get_buffer_class and get_ndbuffer_class, which all use it for this exact
   situation. This also stops migrate_v3._find_numcodecs_zarr3 misreporting a
   config typo as a missing numcodecs codec.

3. Three tests assumed the advertised packages were absent. Both registries are
   entry-point driven, so they failed in any environment with zarr-n5 or
   wavpack-numcodecs installed, which are the packages the messages recommend.
   Two fixtures now remove the specific entry for the duration of the test.
   Verified by installing both packages and re-running.

4. test_mapping_does_not_shadow_builtin_codecs selected on "registry is
   non-empty", conflating loaded-in-this-process with implemented-by-zarr. It
   now selects on the implementing class's module, so a lazy-loaded third-party
   codec cannot fail it.

5. get_numcodec's Raises section notes that numcodecs' own error propagates
   unchanged when data carries no string "id".

6. Dropped the `pragma: no cover` on the numcodecs < 0.15.1 fallback. The
   min_deps env pins numcodecs==0.14.* and runs run-coverage, so that branch is
   measured.

Also hoisted the repeated in-function imports in tests/test_registry.py to the
module level.

Signed-off-by: arcusbuilds <srijankeshri007@gmail.com>

* fix: address the second review round

Four defects, all found by review after the previous round was reported clean.

get_numcodec no longer wraps the numcodecs call in an exception handler. Catching
cannot distinguish "this id is unregistered" from "a registered codec rejected its
configuration" or "a wrapper codec failed to resolve an inner codec", and it was
relabelling both of the latter with the outer id plus a package hint that was
wrong. Reproduced: a wrapper registered as `wavpack` whose from_config resolved a
missing inner codec reported "An implementation for codec 'wavpack' is not
available ... install wavpack-numcodecs", when wavpack was installed and the
missing codec was something else entirely.

It now performs the lookups numcodecs performs, before delegating. That also fixes
reading `id` off a non-mapping input, which raised AttributeError where a
ValueError used to propagate. And it removes _is_missing_numcodec_error, and with
it the numcodecs <0.15.1 compatibility branch, since there is no longer an
exception to classify. Note this narrows the zarr error to Mapping inputs; a
duck-typed mapping now gets numcodecs' error instead, as it did before this PR.

The imagecodecs_ prefix in the Zarr format 3 table pointed at virtual-tiff, which
declares 15 of the 81 imagecodecs_* names under zarr.codecs; imagecodecs-numcodecs
declares all 81, but under numcodecs.codecs. Users of the other 66 names were told
to install a package that does not provide them. The format 3 side now lists the
15 exact names, so the rest get no hint rather than a wrong one. The format 2 side
keeps the prefix, where it is correct.

test_parse_codecs_converts_keyerror_from_from_dict leaked test_picky into the
global codec registry, in the file that also reads that global.

Tests: mutation testing showed six mutations surviving. Added the missing coverage
for a registered codec rejecting its configuration, wrapper codecs resolving an
inner codec by either route, non-mapping input, the imagecodecs_ over-match, and
the _resolve_codec entry point. Message assertions now pin the whole string and the
URL constants rather than substrings, which had allowed both documentation URLs to
be replaced with wrong ones and half the message body to be deleted with every test
still passing.

Also corrects the docs path to src/zarr/registry.py, and rewrites the changelog to
lead with the exception-type changes and to document the parse_codecs change it had
omitted.

Signed-off-by: arcusbuilds <srijankeshri007@gmail.com>

---------

Signed-off-by: arcusbuilds <srijankeshri007@gmail.com>
Co-authored-by: Davis Bennett <davis.v.bennett@gmail.com>

…al arrays (zarr-developers#4305)

* Fix infinite loop in shard size guessing for 0-dimensional arrays

_guess_num_chunks_per_axis_shard looped forever for rank-0 arrays
because neither loop condition depends on the counter when
chunk_shape is empty. Return 1 immediately in that case, which makes
the budget path produce shards=() just like the no-budget path.

Closes zarr-developers#4304

* Add changelog fragment

Assisted-by: ClaudeCode:claude-fable-5-1