hydra/mpiexec: detect missing executables

[hzhou]

Pull Request Description

If the user invokes mpiexec without executables, currently it will segfault. Check for this case and provider proper error messages.

Fixes #6563 - mpirun

Related: #6520 - mpirun -n 2
EDIT: also #5598 - mpirun -n
[skip warnings]

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[hzhou]

test:mpich/pmi ✔️ clear with session_re_init failures addressed in #6564

[hzhou]

After this PR:

$ mpirun -n 2
[mpiexec@tiger] main (mpiexec/mpiexec.c:59): Missing executable. Try -h for usages.

[raffenet]

See also #5598. Is it still necessary? I guess the issue is that its not just the case where there are no argument at all, but specifically no executable to run.

[hzhou]

See also #5598. Is it still necessary? I guess the issue is that its not just the case where there are no argument at all, but specifically no executable to run.

I am afraid so. The 3 fixes are all for different situations albeit looks similar:

• Missing argument altogether, thus command line parsing was skipped
• Missing option parameter (-np []) . We allow arbitrary option syntax thus we have to fix theses on a per-option basis
• Missing executables. Executables are filled in after option parsing, thus need check post parsing.