`MPI.REQUEST_NULL` only pretends to be a constant

[vchuravy]

using MPI
MPI.Init()

reqs = Vector{MPI.Request}(undef, 2)
fill!(reqs, MPI.REQUEST_NULL) 

# first foot-gun `reqs[1]` and `reqs[2]` are now aliases

MPI.Irecv!(Ref(1.0), MPI.COMM_WORLD, reqs[1])

julia> reqs
2-element Vector{MPI.Request}:
 incomplete request
 incomplete request

But worse...

julia> MPI.REQUEST_NULL
MPI.Request: incomplete request

[eschnett]

We may have made a weird design decision in MPI.jl.

In the MPI standard, a request is an opaque value object. Functions such as irecv have the request as output argument. A straightforward translation to Julia would return the request from irecv, or – a lower level API – would accept an uninitialized Ref{} argument that is filled in.

Instead, we have mutable Request objects. This explains the "first foot-gun" above; things are working as designed for fill!, fill! doesn't copy the objects.

Instead of introducing a new type, it might work smoother if we stopped wrapping MPI_REQUEST_NULL. Instead, one would call the constructor of the Request object, which would return a new request object. (This could be a state different from MPI_REQUEST_NULL, or it could actually be MPI_REQUEST_NULL.) With mutable types, one needs to expose object creation to the caller. Using a constant such as MPI.REQUEST_NULL` doesn't work.

I don't think that a new type NullRequest would work. Such an object, when passed to Irecv!, would need to convert itself (???) to an actual request.

The best solution (which would change the API) would be to use value objects (or something with value semantics) instead. Value objects are never modified (by the caller), and it thus doesn't matter that fill! doesn't copy.

That is, our problem is that Irecv! takes a request as input. Instead, it should return a request, or take a reference to a request as input. It does not matter whether we implement a request as Cint, or as a mutable struct so that we can hold on to buffers. Conceptually, the request is created by Irecv!, and it needs to create it and give it to us.

Otherwise – in our current API – requests are created by us. We cannot use a constant such as MPI.REQUEST_NULL since we want objects that can be modified. We need a constructor that creates a new null object.