MPI variants of ADIOS2 libraries always require MPI::MPI_C in CMake

[lahwaacz]

Describe the bug
MPI variants of ADIOS2 libraries always require the MPI::MPI_C target in CMake, even when MPI::MPI_CXX would be more appropriate.

Recall that FindMPI says:

The module exposes the components C, CXX, MPICXX and Fortran. Each of these controls the various MPI languages to search for. The difference between CXX and MPICXX is that CXX refers to the MPI C API being usable from C++, whereas MPICXX refers to the MPI-2 C++ API that was removed again in MPI-3.

Hence, when building a C++ project and linking to e.g. adios2::cxx11_mpi, the MPI::MPI_CXX target is the best option and MPI::MPI_C might even be unavailable (see the reproducer below).

To Reproduce
Create the following CMakeLists.txt and an empty foo.cpp file:

cmake_minimum_required(VERSION 3.28.3)
project(foo LANGUAGES CXX)
add_executable(foo foo.cpp)

find_package(MPI COMPONENTS CXX REQUIRED)
find_package(ADIOS2 REQUIRED)
target_link_libraries(foo PUBLIC MPI::MPI_CXX adios2::cxx11_mpi)

Try to configure the project:

$ cmake -B build -S .
-- Found MPI_CXX: /usr/lib/libmpi.so (found version "3.1")
-- Found MPI: TRUE (found version "3.1") found components: CXX
-- Found MPI: TRUE (found version "3.1")
-- Found ADIOS2: /usr/lib/cmake/adios2/adios2-config.cmake (found version "2.9.2") found components: CXX MPI
-- Configuring done (0.6s)
CMake Error at /usr/lib/cmake/adios2/adios2-cxx11-targets.cmake:78 (set_target_properties):
  The link interface of target "adios2::cxx11_mpi" contains:

    MPI::MPI_C

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  /usr/lib/cmake/adios2/adios2-config-common.cmake:199 (include)
  /usr/lib/cmake/adios2/adios2-config.cmake:29 (include)
  CMakeLists.txt:6 (find_package)


-- Generating done (0.0s)
CMake Generate step failed.  Build files cannot be regenerated correctly.

Note that changing find_package(MPI COMPONENTS CXX REQUIRED) to find_package(MPI COMPONENTS C REQUIRED) and linking to MPI::MPI_C is not enough alone, because the C language is not enabled in this project and it would lead to this error:

$ cmake -B build -S .
CMake Error at /usr/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find MPI (missing: MPI_C_FOUND C)

      Reason given by package: MPI component 'C' was requested, but language C is not enabled.

Call Stack (most recent call first):
  /usr/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake/Modules/FindMPI.cmake:1837 (find_package_handle_standard_args)
  CMakeLists.txt:5 (find_package)


-- Configuring incomplete, errors occurred!

Expected behavior
The adios2::cxx11_mpi target (and others?) should work just fine with MPI::MPI_CXX and MPI::MPI_C should not be required.

Desktop (please complete the following information):

• OS/Platform: Arch Linux
• adios2 2.9.2
• cmake 3.28.3
• openmpi 5.0.2

[vicentebolea]

We use the MPI C library, for that reason we need to link against the mpi c library.
You can avoid the error of mpi_c not found by adding C as a language in your project.

Closing this, please re-open if you still have issues

[lahwaacz]

@vicentebolea Where do you use the MPI::MPI_C target? Note that MPI::MPI_C is not required for linking to libmpi.so, the MPI::MPI_CXX target, which "refers to the MPI C API being usable from C++" according to FindMPI, does the same thing. The deprecated/removed C++ interface for MPI is MPI::MPI_MPICXX.

Requiring the C language in users projects even if they use only C++ code is rather unfriendly, please reopen.

[vicentebolea]

I see, the reason why where were using MPI_C rather than MPI_CXX for the CXX code is that at some point our minimum cmake was 3.10 or lower version meaning that we had to use MPI_C everywhere.

Another problem that I see here is that we do have a C API. This means that we still need to use MPI_C for these cmake targets. These cmake targets belong to a single component so they could be excluded. I'll re-open this while we explore options for this

[lahwaacz]

@vicentebolea Thanks for a quick fix! ❤️