Automatic legacylaunchers (mpiexec, mpirun) in OpenMPI 5.0.x ?

For a while spack replaces the mpiexec & mpirun commands with a warning "use srun because of performance issues.

https://github.com/spack/spack/pull/10340

One of the comments points out that since OpenMPI 5.0.x moved to PMIx, these commands need to be restore instead of srun. (I'm a little confused at that point, since wouldn't the mpirun/mpiexec still need to interface with Slurm's allocation?).

OpenMPI says that the performance issue is "no longer true" - https://docs.open-mpi.org/en/v5.0.x/launching-apps/slurm.html#launching-with-slurm and that mpirun is the preferred way ("mpirun’s Slurm support should always be available").

Slurm's documentation suggests the opposite, but perhaps they haven't updated with respect to OpenMPI 5.0.x: https://slurm.schedmd.com/mpi_guide.html#open_mpi

I'm out of my depth as to who is "right" here, but perhaps when building scheduler=slurm with OpenMPI 5 or greater, +legacylaunchers is always set?

Link to the current logic:

https://github.com/spack/spack/blob/4bbf4a5e79483f9b2cd7e4f3b778b33da1f6f76a/var/spack/repos/builtin/packages/openmpi/package.py#L1260

Start with var/spack/repos/builtin/packages/openmpi/package.py around line 1260 and compare the linked OpenMPI and Slurm documentation about mpirun, mpiexec, srun, and +legacylaunchers. Done means resolving the version- and scheduler-specific behavior and updating the package logic accordingly.