[Debichem-devel] Bug#1075979: [Help] molds: FTBFS with mpich as default MPI provider: /usr/bin/ld: cannot find -lmpi_cxx: No such file or directory

[Drew Parsons]

On 2024-11-08 20:26, Bill Allombert wrote:
> Le Thu, Nov 07, 2024 at 08:12:22PM +0100, Andreas Tille a écrit :
>> Control: tags -1 help
>> Thanks
>> 
>> Hi,
>> 
>> today molds (from Debichem team) came up as candidate for the Bug of 
>> the
>> Day[1].  I considered bug #1075979 easy to fix by simply adding
>> libopenmpi-dev to Build-Depends since
>> 
>> $ apt-file search libmpi_cxx.so | grep -- -dev
>> libopenmpi-dev: /usr/lib/x86_64-linux-gnu/libmpi_cxx.so
>> libopenmpi-dev: /usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so
>> 
>> but this is obviously not the case for the 5.x series of 
>> libopenmpi-dev
>> which does not contain this library any more.  Any hints for
>> replacements or other ways to make the linker happy are welcome since
>> the problem remains.
> 
> Andreas,
> 
> debian/rules carries the line
> export LIBSBASE=-lstdc++ -lm -lpthread -lgomp -L/usr/lib/lapack/ 
> -llapacke -lmpi -lmpi_cxx
> I am pretty sure this is wrong, mpicxx should take care of -lstdc++ 
> -lmpi -lmpi_cxx
> 
> I checked and the package build fine without -lstdc++ -lmpi -lmpi_cxx

We're in the middle of transitioning to OpenMPI 5.  OpenMPI 5 removed 
libmpi_cxx, which is why this bug happened.

Bill is certainly right, -lstdc++ -lmpi -lmpi_cxx should not be 
hardcoded in build rules.
and just using mpicxx is the simplest method.

But Build-Depends: libopenmpi-dev is also wrong, unless the program is 
badly written and not conformant with the MPI standard.  openmpi 5 does 
not build on 32-bit arches, so we use mpich.

Build-Depends: mpi-default-dev is the correct dependency
(alongside removing the hardcoded -lmpi -lmpi_cxx and using CXX=mpicxx)