Fix PFC on root level elements

[Niklas997]

Closes #2281

Describe your changes here:

When partition for coarsening is applied and the border element is a root level element, a PFC-Message is constructed and the parent element therein is the actual pointer to the root element in the mesh. Upon destruction of the PFC-Message, the parent element (in this case the actual root element in the mesh) is destroyed. In all other cases (i.e. the border element is not a root element) additional storage for the parent is allocated and it is filled with the corresponding information and this newly constructed element will be deallocated within the destructor. Currently, this leads to a crash in case the partition boundary falls on a root element. In order to account for this, there needs to be an additional element-allocation for the parent in the root level case as well which is then filled by an element_copy of the actual root element, such that it can be safely dealloacted upon destruction of the mesasge.

In order to re-create the issue, one can just construct a uniform forest with refinement level 0 from a cmesh and call partiton for caoarsening on it.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.69%. Comparing base (92212f7) to head (591bc9e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/t8_forest/t8_forest_pfc_message.hxx	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2280      +/-   ##
==========================================
- Coverage   78.69%   78.69%   -0.01%     
==========================================
  Files         115      115              
  Lines       19206    19207       +1     
==========================================
  Hits        15115    15115              
- Misses       4091     4092       +1     

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[spenke91]

Thanks a lot for catching this! :-) I just have one minor suggestion, hope you don't mind...

[spenke91]

Would you mind moving this part before the if statement (and deleting line 226) to avoid code duplication?

[Niklas997]

Yes, we can do this. Do you think we need to add an additional test or include this as a test case in the PFC test? Since this root level case is not yet captured in the code coverage, the codecov/patch test in this PR fails or is the quick fix enough for now?

[spenke91]

I'm sorry, but it would be much cleaner to test it. The easiest (though not the most runtime-efficient) option might be to add the level as a test parameter in test/t8_forest/t8_gtest_partition_for_coarsening.cxx , e.g., like this:

INSTANTIATE_TEST_SUITE_P (t8_gtest_partition_for_coarsening, t8_test_partition_for_coarsening_test,
                          testing::Combine (AllSchemeCollections, AllCmeshsParam, testing::Values(0,2)));

And then of course adjust the std::tuple and read the level from std::get<2> (GetParam ()) instead of hard-coding it to 2...

This way, we would test all cmeshes with level 0 and level 2 (because as you said, running with level 0 causes exatcly this issue). If this gets too expensive, we might also change it later e.g. to using level 1 instead of two for TEST_LEVEL_BASIC. But the important thing for me right now would be to cover this issue you found 👍

Would you mind to / know how to adjust the test case? 🤔

[Niklas997]

Okay, thank you for the hints to do so. Yes, I will try to adapt the test and add it to this PR.