Small fixes to better enable running with XIOS 3

[EdHone]

PR Summary

Sci/Tech Reviewer: @mo-marqh
Code Reviewer: @mo-lottieturner

• Run LFRic-XIOS technical tests with XIOS 3 #215

This PR fixes a few issues in our code that are fine with XIOS2 but fail with XIOS3, namely:

• incorrect formatting of the buffer_size_factor config parameter
• fields in read mode files which do not have read_access=.true.

This PR fixes them and the fixes are backwards compatible with XIOS 2.
With the additional changes from #204, the lfric-xios technical tests are able to be run with both XIOS2 and XIOS3.

Code Quality Checklist

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Testing

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trac.log

Test Suite Results - lfric_core - lfric_core-215-xios3-fixes/run2

Suite Information

Item	Value
Suite Name	lfric_core-215-xios3-fixes/run2
Suite User	edward.hone
Workflow Start	2026-01-20T11:49:52
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	EdHone/lfric_core@215-xios3-fixes	False
SimSys_Scripts	MetOffice/SimSys_Scripts@2025.12.1	True

Task Information

✅ succeeded tasks - 372

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[mo-marqh]

i have sci-tech reviewed these changes.
the buffer_size¬factor is a trivial mistake fix that was ignored in XIOS2 but fails in XIOS3. So, this was always wrong. Now updated to correct for both versions.

The setting of read_access to true is essential for XIOS3 and neutral for XIOS2, the implementation is clean, with the extension of a simple xios_set_attr call to include the logical from the lfric-xios object.

[mo-marqh]

@mo-lottieturner this change passes sci/tech and is now ready for code review. (it's a stand-alone lfric_core change)

[mo-marqh]

hi @EdHone

it would be helpful if the setting of read_access was also enabled for lfric_xios created checkpoint definitions when actually restarting

e.g.

--- a/components/lfric-xios/source/lfric_xios_metafile_mod.F90
+++ b/components/lfric-xios/source/lfric_xios_metafile_mod.F90
@@ -269,6 +269,11 @@ subroutine add_field(metafile, dict_field_id, mode, operation, id_as_name, legac
           end if
         end if
       end if
+      if (mode == RESTARTING) then
+        ! Is restarting, then set read_access to True.
+        call xios_set_field_attr(field_id, read_access=.true.)
+      end if
+

[mo-marqh]

reverting back to sci/tech review based on recent reanalysis

[EdHone]

Branch update with the change to enable read_access for checkpointing - tested successfully against apps and core test suite successful and in PR description above.

Passing back to you @mo-marqh and then on to @mo-lottieturner