MPI_START / MPI_STARTALL semantics clarification

[tonyskjellum]

Problem

Although MPI_START and MPI_STARTALL take persistent requests by reference, the implementation has never been intended to change the handles (the bits in the request itself; the underlying internals are not at issue). Stating this fact is important, because the user has no expectation that handles change values. In some sense, MPI_START should have taken an IN argument for the request, but the API wasn't defined that way in MPI-1.0.

Proposal

Persistent requests don't change their value when used in START/STARTALL operations.

Changes to the Text

Text will be added to indicate that MPI_START and MPI_START_ALL receive persistent requests by reference, but may not
change the requests (const pointer, const array is implied).

Impact on Implementations

No known implementations do this unwanted behavior.

Impact on Users

No expected impact. The lack of this clarification could cause major impacts if a future implementation
chose to use this loophole.

References and Pull Requests

TBD.

[wesbland]

I’m going to propose moving this to MPI 5.0. There’s more discussion to be had here, but it sounds like there’s nothing in progress. If someone objects and thinks we’ll be ready to read this soon, leave a comment and we can discuss bringing it back into MPI 4.1.