mpi::reduce hangs on intel MPI

Start with the reproducer in the issue and inspect src/point_to_point.cpp around MPI_Mprobe, together with include/boost/mpi/config.hpp around BOOST_MPI_USE_IMPROBE. Run the sample with Intel MPI 2019 and multiple task counts, then compare behavior with the flag disabled and with OpenMPI. Done means the sample completes reliably with the flag enabled on Intel MPI without regressing the other reported configuration.

The following program hangs on calling boost::mpi::reduce when run on an Intel MPI environment.

#include <algorithm>
#include <iostream>
#include <vector>
#include <boost/mpi/collectives.hpp>
#include <boost/mpi/operations.hpp>
#include <boost/serialization/vector.hpp>

struct sum_vec_vec {
  std::vector<double> operator()(const std::vector<double>& a, const std::vector<double>&b) const
  {
    std::vector<double> res(a.size());
    std::transform(a.begin(), a.end(), b.begin(), res.begin(), [](double x, double y) { return x + y; });
    return res;
  }
};

namespace boost {
  namespace mpi {
    template <>
    struct is_commutative<sum_vec_vec, std::vector<double>> : mpl::true_ { };
  }
}

int main()
{
  namespace mpi = boost::mpi;

  mpi::environment env;
  mpi::communicator world;

  std::size_t size = 1000;
  std::size_t L = 32;

  std::vector<std::vector<double>> correlations;

  // Fill up with some data
  for (std::size_t i = 0; i < size; ++i)
    {
      int l = 0;

      std::vector<double> corr(L);
      for (auto&x : corr)
        x = l++;
      correlations.emplace_back(std::move(corr));
    }

  std::vector<std::vector<double>> av_correlations(correlations.size());
  std::cout << "Ready for mpi::reduce" << std::endl;
  boost::mpi::reduce(world, &correlations.front(), correlations.size(), &av_correlations.front(), sum_vec_vec{}, 0);

  return 0;
}

The specific of the MPI environment are:

MPI_Get_library_version: Intel(R) MPI Library 2019 Update 6 for Linux* OS
MPI_VERSION: 3
I_MPI_NUMVERSION: 20190006300

Boost version is 1.74.0, and it defines:

BOOST_MPI_VERSION: 3
BOOST_MPI_USE_IMPROBE: 1

The program above very often hangs when run for example with >6 tasks, and always hangs when, say, Ntasks=192.

The reason apparently lies in the use of the MPI_Mprobe routines in point_to_point.cpp.
On recompiling the library with the flag BOOST_MPI_USE_IMPROBE disabled in config.hpp, the program ends without issues. Also, the program runs without problems, on openmpi and enabled BOOST_MPI_USE_IMPROBE.

All in all, I suspect that the bug mentioned here is still there.