MPI.jl is broken on Julia master

[eschnett]

This recent change to Julia master

commit 9427f339c90d9c726deeaf05517c2ecabbf02bd6
Author: Sam Schweigel <sam.schweigel@juliahub.com>
Date:   Fri Sep 12 08:54:20 2025 -0700

    jl_dlfind: do not find symbols in library dependencies (#58815)

broke MPI.jl:

julia +nightly
julia> ]activate --temp
julia> ]add MPI
julia> using MPI
ERROR: InitError: could not load symbol "MPI_UNWEIGHTED":
dlsym(0xb80342a1, MPI_UNWEIGHTED): symbol not found
Stacktrace:
  [1] init_consts()
    @ MPI.API ~/.julia/packages/MPI/hNJm0/src/api/api.jl:82
  [2] __init__()
    @ MPI ~/.julia/packages/MPI/hNJm0/src/MPI.jl:168```

The reason seems to be that we are looking for the MPI constants (e.g. MPI_UNWEIGHTED) in the global namespace, but we now need to look explicitly in the respective MPI library.

[giordano]

% nm ~/.julia/artifacts/6778371a814e4a0259e40af26c12e4ab419af75a/lib/libmpi.12.dylib | grep MPI_UNWEIGHTED
                 U _MPI_UNWEIGHTED
% nm ~/.julia/artifacts/6778371a814e4a0259e40af26c12e4ab419af75a/lib/libpmpi.12.dylib | grep MPI_UNWEIGHTED
00000000002e7bd8 S _MPI_UNWEIGHTED

Handling this in a generic way is going to be extremely not fun, as we can't know for sure where the symbol is coming from precisely.

[eschnett]

I agree. There are some jlls (OpenMPI, but also others) which provide several libraries, all linked into one main library. Sorting this out will be inconvenient.

Maybe there can be a helper function which searches through a list of libraries – e.g. all libraries provided by a jll – and picks the right one? This would run at precompile time.

[giordano]

The behaviour change is being reverted in JuliaLang/julia#59560