Allow using P4est for preferences not set correctly

[JoshuaLampert]

With this PR using P4est doesn't crash anymore if a system-provided MPI installation together with the P4est_jll p4est version is used, but gives the user a warning to set the preferences for p4est accordingly. This way, it is also possible to first set the MPIPreferences, restart the REPL and then set the preferences for P4est.jl.

Additionally, I have added three functions: path_p4est_library() and path_sc_library(), which returns the path to the libraries or P4est_jll if the default p4est installation is used. This way one can conveniently check if P4est.jl uses the correct libraries.

Lastly, I added the function preferences_set_correctly(), which returns false if P4est.jl is not usable due to missing preference for P4est.jl. This function can be used in Trixi.jl to determine if init_p4est() can be called or not. By adding if preferences_set_correctly() ... end together with my corresponding PR in T8code.jl (DLR-AMR/T8code.jl#40), I was able to run Trixi.jl with system-provided MPI installation on a TreeMesh-example without setting the preferences for P4est.jl and T8code.jl. Thus, with these two PRs and the corresponding PR in Trixi.jl (that I will prepare once this PR and the corresponding T8code.jl PR are merged) it is only necessary to set the preferences for P4est.jl and T8code.jl if you really need them.

Closes #91.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (e2a5e00) 16.18% compared to head (17d4a75) 16.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
+ Coverage   16.18%   16.25%   +0.08%     
==========================================
  Files           3        3              
  Lines        1533     1538       +5     
==========================================
+ Hits          248      250       +2     
- Misses       1285     1288       +3     

Flag	Coverage Δ
unittests	16.25% <50.00%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/LibP4est.jl	14.41% <ø> (ø)
src/P4est.jl	48.39% <50.00%> (-1.61%)	⬇️

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[sloede]

I am wondering: Right now, the preferences check happens at the module level, i.e., during precompilation, if I am not mistaken. Wouldn't it be better if this warning were shown (also) at loading time, i.e., from __init__?

Also, this will lead to no variable libp4est/libsc being defined in the module - is this intentional?

[JoshuaLampert]

Wouldn't it be better if this warning were shown (also) at loading time, i.e., from init?

Yes, definitely. I added that in 17d4a75.

Also, this will lead to no variable libp4est/libsc being defined in the module - is this intentional?

Yes, this is intentional. If MPIPreferences are set, but no P4est.jl preferences, we can neither set libp4est/libsc to the path from P4est_jll (this was done before and fails unless your system MPI is compatible with the P4est_jll MPI) nor to a local one (we have no preference, so we don't know of any local one). Thus, libp4est/libsc are undefined and P4est.jl is not usable, but you can still load it and set the preference (and therefore libp4est/libsc) to make it usable.

[ranocha]

Thanks a lot!

[ranocha]

Thanks! @sloede Do you want to have a final look as well?

[sloede]

Thanks!