Clarify how PMIX_PREFIX should be handled by the server side in PMIx_Spawn context

[krempel-pt]

We stumbled over this point while implementing our PMIx host server for the ParaStation MPI process management.

Is in unclear how and by whom PMIX_PREFIX if specified should be applied to the command. Discussing this with @rhc54 in openpmix/openpmix#3321 lead to implementing it in the server library, but then it must not be passed upwards to the host server, since it would not be able to detect whether it is already applied or not. So the suggestion is to clearly define that in the standard:

Suggested Clarification

If the implementation handles the prefix, then it must remove the PMIX_PREFIX attribute before passing the keys up to the server. If the server sees a PMIX_PREFIX key, then it must handle it.

References

17.3.11

[dsolt]

Our working group is working on fixes to improve the process management chapter. (there is a PR open currently). As of now we do not have any textual changes around this, but I wanted to follow this as we can include changes in that PR if it helps move things along quicker.

[abouteiller]

24Q2: we like the proposed text, advice to implementor.