Support both forward and backwards ABI?

[bwbarrett]

Problem

There is an assumption in this issue that the MPI forum will continue to add things to the MPI standard which require additional functions.

Do we want to provide forward compliance only (ie, an app compiled to the MPI 5.0 ABI works with the MPI 6.0 ABI, but an app compiled against the MPI 6.0 ABI does not work with the MPI 5.0 ABI, even if it only uses functions that didn't change between the two), or do we want to support forward and backward compliance (where an app compiled against the MPI 6.0 ABI but not using any functions that changed from 5.0 would still work)?

If we want both directions, we need to consider using symbol versioning over library versioning, which is going to require additional thought (and likely platform dependencies, as the mechanisms differ from platform to platform). We need to discuss this early, because switching versioning systems is an ABI break itself. I'm not an expert here, but hopefully we can find some experts. First, though, I'd like us to agree on what properties we want for the user experience.