Add Trixi.jl downstream test

[benegee]

I would like to add a downstream test targeting Trixi.jl. This would allow to test new versions (in CI) before actually releasing them.

[benegee]

The downstream tests worked: https://github.com/DLR-AMR/T8code.jl/actions/runs/36401884708
After that I changed the workflow to workflow_dispatch only.

[benegee]

I also changed the precompile step introduced in the last PR. I now use try / finalize to make sure that in the end at least T8code.jl loads successfully.

[JoshuaLampert]

Does the workflow_dispatch allow running the downstream tests on a PR or only after a PR was merged? It would be good if we could trigger the downstream tests to test a PR. We could, e.g., trigger it on a certain comment posted on a PR.

[benegee]

My idea was

• use Actions (top)
• select the new workflow on the left (similar to current "Run tests")
• "Run workflow"
• specify the branch of the PR to be checked (and optionally the Trixi.jl branch)

[JoshuaLampert]

Ah, sounds good. I didn't know you can set the branch there, but for Spell Check for instance you can switch the branch. Then it probably also works for the downstream tests once this PR is merged.

[JoshuaLampert]

LGTM! I lost a bit track: Are the errors we see in CI with the system MPI (e.g. for SYSTEM_MPI nightly job in both the "Precompile (nightly)" and the "Run Julia tests" steps) expected?

[benegee]

LGTM! I lost a bit track: Are the errors we see in CI with the system MPI (e.g. for SYSTEM_MPI nightly job in both the "Precompile (nightly)" and the "Run Julia tests" steps) expected?

Yes, unfortunately.

The dedicated precompile step is required for Julia 1.14 because the behavior regarding failed precompilation was changed. We have to use Pkg.precompile(; skip_dependents=false) to precompile T8code, even if its direct dependency t8code_jll failed. Once it is precompiled, we can actually use it. We make sure using T8code actually works.

In the tests, precompilation is triggered again because Julia realizes not all packages in the project are available in the cache. That is why we see the errors.

The dedicated precompilation step is not required for Julia <1.14 but I decided to leave it in just to trigger the failure and to make sure using T8code actually works.