MPI behavior when building MPI_Group/MPI_Comm with ranks=[0,0]?

Question

I'm trying to understand what would be the behavior of an MPI implementation (mine is OpenMPI 4.1.0) dictated by the MPI standard of this code :

#include <iostream>

#include <mpi.h>

int main (int argc, char ** argv) {

    MPI_Init(&argc, &argv);
    
    int idProc, nbProc;
    MPI_Comm_rank(MPI_COMM_WORLD,&idProc);
    MPI_Comm_size(MPI_COMM_WORLD,&nbProc);

    if (idProc == 0 || idProc == 1){
        MPI_Group worldGroup;
        MPI_Comm_group(MPI_COMM_WORLD, &worldGroup);

        int* ranks = new int[2];
        ranks[0] = 0;
        ranks[1] = 0;

        MPI_Group intrfGroup;
        MPI_Group_incl(worldGroup, 2, ranks, &intrfGroup);

        MPI_Comm mpiComm;
        std::cout << "MPI_Comm_create_group before ! By proc " << idProc << std::endl;
        MPI_Comm_create_group(MPI_COMM_WORLD, intrfGroup, 0, &mpiComm);
        std::cout << "MPI_Comm_create_group after ! By proc " << idProc << std::endl;

        int result;
        MPI_Comm_compare(mpiComm, MPI_COMM_WORLD, &result);

        if (result == MPI_IDENT) {
            std::cout << "mpiComm is MPI_COMM_WORLD." << std::endl;
        } else {
            std::cout << "mpiComm is not MPI_COMM_WORLD." << std::endl;
        }       
    }


    MPI_Finalize();
  }

It returns :

[ACH@is227051 bin]$ mpiexec -n 4 Allreduce 
MPI_Comm_create_group before ! By proc 1
MPI_Comm_create_group after ! By proc 1
MPI_Comm_create_group before ! By proc 0
[is227051:03157] *** An error occurred in MPI_Comm_compare
[is227051:03157] *** reported by process [3464888321,1]
[is227051:03157] *** on communicator MPI_COMM_WORLD
[is227051:03157] *** MPI_ERR_COMM: invalid communicator
[is227051:03157] *** MPI_ERRORS_ARE_FATAL (processes in this communicator will now abort,
[is227051:03157] ***    and potentially your MPI job)

I tried if (idProc == 0) to let only proc 0, which is the only proc in the rank vector enter the if, but if I do that it just hang at :

[ACH@is227051 bin]$ mpiexec -n 4 Allreduce 
MPI_Comm_create_group before ! By proc 0

What I'm trying to understand here, is the behavior of MPI when you creates group/communicator from an array of int rank that list the ranks of the proc that you want to create a group/communicator but have doublons, ultimately just the same rank twice. I could not find anything in the MPI standard document describing this.

Answer 1

Ok, actually the answer was right here in the documentation of OpenMPI :

Each of the n elements of ranks must be a valid rank in group and all elements must be distinct, or else the program is erroneous.

Moreover, it is worth noticing that it also says :

This implementation does not currently check to ensure that there are no duplicates in the list of ranks.

So the behavior I've got with my code is completely normal and now explained.

I hope this helps other people in the future.