Problematic find_package calls in axom-config.cmake

[ldowen]

The export target logic in axom-config.cmake calls things like find_package(MPI REQUIRED) if conduit and MPI are both enabled. This seems excessive and poses an issue for any project that might only want to find the MPI package for certain languages. It also seems unnecessary in the event that MPI has already been found, which is likely if the project is using MPI locally.

axom/src/cmake/axom-config.cmake.in

Line 160 in 21504d4

find_package(MPI REQUIRED)

[rhornung67]

This line can be removed from export logic (and tested to make sure it still works), but does not solve issue in BLT. Maybe @white238 know what this means....

See also llnl/blt#755

[kennyweiss]

Related issue to investigate/fix when working on this:

PR #1956 added the following to support non-MPI Axom configs (-DENABLE_MPI=OFF) w/ MPI-based MFEM

if(TARGET mfem AND MFEM_USE_MPI)
    blt_patch_target(NAME mfem DEPENDS_ON mpi)
endif()

But, BLT only creates an mpi target (in BLTSetupMPI.cmake) when BLT_ENABLE_MPI is set.

Since blt_patch_target appends DEPENDS_ON entries to INTERFACE_LINK_LIBRARIES without checking they are targets,
this silently becomes -lmpi on the link line rather than a configure error, and also doesn't append an MPI include directory.

Once we have language-dependent targets (per this issue), we can change this check to something like:

if(TARGET mfem AND MFEM_USE_MPI)
    if(NOT TARGET MPI::MPI_C)
        find_package(MPI QUIET COMPONENTS C)
    endif()
    if(TARGET MPI::MPI_C)
        blt_patch_target(NAME mfem DEPENDS_ON MPI::MPI_C)
    else()
        message(WARNING "MFEM was built with MPI but no MPI C target was found; linking against MFEM may fail")
    endif()
endif()