Avoid panel_decomposition_mod causing a divide-by-zero

[tom-j-h]

PR Summary

Sci/Tech Reviewer: @thomasmelvin
Code Reviewer: @mike-hobson

This issue was found in development of the JADA benchmark.

Other changes made meant that jedi_lfric_tests was crashing due to a divide by zero here:

lfric_core/infrastructure/source/mesh/panel_decomposition_mod.f90

Line 252 in a4caea1

mp_num_cells_x = num_cells_x / mapping_factor

mp is initialised here:

lfric_core/infrastructure/source/mesh/panel_decomposition_mod.f90

Lines 879 to 885 in a4caea1

	! If no meshes were found, return 1. This is relevant for JEDI where mesh
	! initialisation is run twice.
	if ( shortest_panel_width < huge(0_i_def) ) then
	mp = this_panel_width / shortest_panel_width
	else
	mp = 1
	end if

The solution is to instead set mp = max(1, this_panel_width / shortest_panel_width).

Please note, I did not find this issue or develop the fix - I just need it for the linked PR below, and there wasn't a PR yet.

• linked Align jedi_lfric_tests linear model/adjoint testing to adjoint_tests and linear_model lfric_apps#156

• closes Avoid panel_decomposition_mod causing a divide-by-zero #226

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I can't find my trac.log! It might be because I ran with --no-run-name? Here's the Cylc Review: https://cylchub/services/cylc-review/taskjobs/tom.hill/?suite=fix_jelf_divide_by_zero-develop.

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[thomasmelvin]

I think it's worth modifying this comment to capture the changes in the code, i'd suggest "If no meshes were found, or this_panel_width < shortest_panel_width, then return 1. This is relevant for JEDI where mesh..."

[tom-j-h]

Good point, thank you! Change made.

[thomasmelvin]

One minor suggestion for adding to the existing comment, otherwise I'm happy with the changes

[mike-hobson]

This is a simple fix and I have no issues with it (except for the unnecessary comment).

[tom-j-h]

@mike-hobson change made. Thanks for the quick review!

[mike-hobson]

Great, thanks. I'm happy for this to PASS code review, now.