I think this is a good idea, and for the reasons you outline in your Rationale. This definitely bites people from time to time at Big Blue as well, and a gentle warning will certainly help.
Those who care about the openib BTL:
What do you think about warning when fork() is used with the openib BTL? See https://svn.open-mpi.org/trac/ompi/ticket/1244 for details.
Rationale: Several Cisco customers have been bitten by not realizing that they had calls to system() in their MPI codes when switching away from older mVAPI-based stacks to OFED (the older Cisco/Topspin mVAPI stack was a bit more tolerable of fork()). Newer kernels and OFED versions can handle fork() better, but I've still had spurious reports of MPI codes failing when system() was used (never had much chance to followup to see what was actually happening, though -- it *should* have worked...?).
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