Prepartitioning with threading race condition fix

[Matthew Walker (mattatmet)]

PR Summary

Sci/Tech Reviewer: mo-marqh
Code Reviewer: Benjamin Went (@MetBenjaminWent)

This PR is to address a race condition in the threading employed by the partitioning section of the cubedsphere_mesh_generator code. This was originally found using offline partitioning in the apps PR #688 (linked to this one) where a seg fault was occurring for really high partition numbers and a custom mesh decomposition task. I believe that the linked list pointer in the global_mesh_map_collection code was being overwritten by competing threads and that by separating the declaration and nullification of the pointer, the save attribute is no longer being applied, making the loop variable local to each thread calling the object.

• linked Offline partitioning of mesh in weekly tests lfric_apps#688

Code Quality Checklist

• I have performed a self-review of my own code
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Testing

• I have tested this change locally, using the LFRic Core rose-stem suite
• If required (e.g. API changes) I have also run the LFRic Apps test suite using this branch
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trac.log

Test Suite Results - lfric_core - partitioned_null_pointer_dev_tests/run1

Suite Information

Item	Value
Suite Name	[partitioned_null_pointer_dev_tests/run1](https://cylchub/services/cylc-review/cycles/matthew.walker/? suite=partitioned_null_pointer_dev_tests%2Frun1)
Suite User	matthew.walker
Workflow Start	2026-08-18T09:47:17
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	mattatmet/lfric_core@offline_partition	False
SimSys_Scripts	MetOffice/SimSys_Scripts@cab3315	True

Task Information

✅ succeeded tasks - 431

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Documentation

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Sci/Tech Review

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(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

• All dependencies have been resolved
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• Security considerations have been addressed
• Performance impact is acceptable

[mo-marqh]

A change is necessary to fix the observed edge case bug.
This is a neat and well contained alternative that meets requirements

[mo-marqh]

ready for code review Benjamin Went (@MetBenjaminWent)

[Benjamin Went (MetBenjaminWent)]

Should this instead be, (for consistency)? : loop => null()

[Benjamin Went (MetBenjaminWent)]

Question for CO too Ricky Wong (@mo-rickywong)

[Matthew Walker (mattatmet)]

Thanks Benjamin Went (@MetBenjaminWent), agreed, have pushed this change now.

[Ricky Wong (mo-rickywong)]

No issue with this, It's actually recommended practice which we try to remove null() assignments on the declaration statement. This is obviously legacy code that hasn't been revisited.

[Benjamin Went (MetBenjaminWent)]

Thanks for the changes.

Developer testing all passes, partitioning runs as expected still with lfric apps as well, and with the latest offline partitioning updates.

CR Approved