OpenMPI apparent bug in MPI_File_seek() using MPI_SEEK_END

fileseekbug.cpp

#include <mpi.h>

#include <string>
#include <vector>

#define DATA_SIZE (0x100000 + 20)

int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);
    int commRank;
    MPI_Comm_rank(MPI_COMM_WORLD, &commRank);
    int commSize;
    MPI_Comm_size(MPI_COMM_WORLD, &commSize);

    MPI_File fileHandle;
    MPI_File_open(MPI_COMM_WORLD, "fileseekbug.out", MPI_MODE_CREATE | MPI_MODE_WRONLY, MPI_INFO_NULL, &fileHandle);

    MPI_Offset mpiOffset = (commRank - 1) * DATA_SIZE;
    if (commRank == 0)
        mpiOffset = (commSize - 1) * DATA_SIZE;

    std::vector<char> data(DATA_SIZE, 0);
    data[DATA_SIZE - 1] = 0x01;
    MPI_File_seek(fileHandle, mpiOffset, MPI_SEEK_SET);
    MPI_File_write(fileHandle, reinterpret_cast<void const*>(&data.front()), static_cast<int>(data.size()), MPI_CHAR, MPI_STATUS_IGNORE);

    if (commRank == 0)
    {
#if defined THIS_WORKS
        MPI_File_get_size(fileHandle, &mpiOffset);
        MPI_File_seek(fileHandle, mpiOffset, MPI_SEEK_SET);
#else // This doesn't work:
        mpiOffset = 0;
        MPI_File_seek(fileHandle, mpiOffset, MPI_SEEK_END);
#endif
        std::string message("This should be at the end of the file");
        MPI_File_write(fileHandle, reinterpret_cast<void const*>(message.c_str()), static_cast<int>(message.size() + 1), MPI_CHAR, MPI_STATUS_IGNORE);
    }

    MPI_File_close(&fileHandle);
    MPI_Finalize();
    return 0;
}

Run the sample code with 4 MPI ranks to demonstrate the bug. The output file should consist of all zeros except for a 1 at the end of each rank's data block, and the end of the file should have the message "This should be at the end of the file" The observed behavior is that the message is written to file position 0x400000, which is not the end of the file. This behavior was observed on Ubuntu 20.04 LTS with GCC 9.3.0, with both OpenMPI 3.1.5 and 4.0.5.