"--stdin" doesn't work correctly for non-zero

[Klaaktu]

Thank you for taking the time to submit an issue!

Background information

What version of Open MPI are you using?

v5.0.2 (Fedora 40), v5.0.5 (Arch)

Describe how Open MPI was installed

From distribution: Fedora, Arch.

If you are building/installing from a git clone, please copy-n-paste the output from git submodule status.

Please describe the system on which you are running

• Operating system/version: Fedora 40, Arch (in QEMU)
• Computer hardware: x86_64
• Network type: Wi-Fi

---

Details of the problem

When --stdin is changed to any process other than 0, the process reading from stdin will be stuck waiting even though an input is provided.
Here is a minimal example:

#include <mpi.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
    MPI_Init(&argc, &argv);
    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    if (rank == 1)
    {
        printf("rank %d\n", rank);
        getchar();
    }
    MPI_Finalize();
    return 0;
}

$ mpicc -Wall -pedantic -o test test.c
$ mpirun -n 2 --stdin 1 ./test
rank 1
a
b

^C

Debugging shows that the process is stuck on getchar().

p.s. Does not happen on 4.1.6 (Ubuntu).