[sheikhayaan]

Fixes #4531.

Following the suggestion by @rabernat, this PR exposes blosc.use_threads through the zarr.config system.

Added blosc.use_threads (default False) to zarr.core.config.

Updated zarr.codecs.blosc to read this config value.

Overrode Config.set and Config.refresh in zarr.core.config to dynamically sync the numcodecs.blosc.use_threads global state whenever the config is updated.

Added a test in tests/test_config.py to verify both the default state and dynamic updates.

Note: I couldn't run the full pytest suite locally due to a Windows Application Control policy blocking the hypothesis native DLL, but I verified the config logic manually with a script (default False -> set True -> set False) and expect CI to run the full suite.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.71%. Comparing base (ce10c0b) to head (0abb755).
⚠️ Report is 137 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4534      +/-   ##
==========================================
+ Coverage   94.12%   94.71%   +0.59%     
==========================================
  Files          92       94       +2     
  Lines       12830    13673     +843     
==========================================
+ Hits        12076    12951     +875     
+ Misses        754      722      -32     

Files with missing lines	Coverage Δ
src/zarr/codecs/blosc.py	97.63% <100.00%> (+0.01%)	⬆️
src/zarr/core/config.py	100.00% <100.00%> (ø)

... and 50 files with indirect coverage changes

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[d-v-b]

this will behave counter-intuitively if we change the default value in numcodecs to True

[sheikhayaan]

Thanks for the review @d-v-b. You're right about the default behavior. Right now, Zarr hardcodes numcodecs.blosc.use_threads = False. By exposing it in zarr.config with a default of False, we are explicitly preserving Zarr's current behavior.

If numcodecs changes its default in the future, we would just need to update the Zarr config default to match it, or users can override it via zarr.config.set({"blosc.use_threads": True}).

Since you mentioned the comprehensive thread budget design in the issue thread, should I add a note in the docs here warning that this is a temporary escape hatch until the budget system is implemented?