Demonstration of measurement duplication after repeated collective merge

merge.cpp

#include <iostream>
#include <string>

#include <alps/accumulators.hpp>
#include <alps/utilities/mpi.hpp>
#include <alps/hdf5/archive.hpp>

int main (int argc, char *argv[]) {
    alps::mpi::environment env(argc, argv, false);
    alps::mpi::communicator c;

    alps::accumulators::accumulator_set measurements;
    measurements << alps::accumulators::MeanAccumulator<double>("dummy");

    // measure some data
    for (size_t i = c.rank() * 10; i < (c.rank() + 1) * 10; ++i)
        measurements["dummy"] << i;

    // write checkpoint before collecting
    {
        std::stringstream ss;
        ss << "before_rank_" << c.rank() << ".h5";
        alps::hdf5::archive ar(ss.str(), "w");
        ar["measurements"] << measurements;
        ar.close();
    }

    // collect results
    if (c.rank() == 0) {
        measurements["dummy"].collective_merge(c, 0);
        alps::accumulators::result_set results;
        results.insert("dummy", measurements["dummy"].result());

        alps::hdf5::archive ar("results.h5", "w");
        ar["/"] << results;
        ar.close();
    } else {
        measurements["dummy"].collective_merge(c, 0);

        // resetting slaves manually solves the issue
        // measurements["dummy"].reset();
    }

    // measure some more
    for (size_t i = c.rank() * 10; i < (c.rank() + 1) * 10; ++i)
        measurements["dummy"] << i;

    // final checkpoint
    {
        std::stringstream ss;
        ss << "after_rank_" << c.rank() << ".h5";
        alps::hdf5::archive ar(ss.str(), "w");
        ar["measurements"] << measurements;
        ar.close();
    }

    // collect results a second time
    if (c.rank() == 0) {
        measurements["dummy"].collective_merge(c, 0);
        alps::accumulators::result_set results;
        results.insert("dummy", measurements["dummy"].result());

        alps::hdf5::archive ar("results2.h5", "w");
        ar["/"] << results;
        ar.close();
    } else {
        measurements["dummy"].collective_merge(c, 0);
        // measurements["dummy"].reset();
    }
    return 0;
}

results2.h5 will show 110 measurements of dummy and yield the wrong mean of 20.863. Uncommenting line 41 in contrast yield the correct result of 19.5 at 80 samples.