mpi_aggregator: don't pass a local variable to MPI_Isend

The data passed to MPI_Isend needs to remain available until after
MPI_Wait, so use a class member variable instead.

source/adios2/toolkit/aggregator/mpi/MPIAggregator.cpp

@@ -57,16 +57,12 @@ MPIAggregator::IExchangeAbsolutePosition(BufferSTL &bufferSTL, const int step)
 
     if (m_Rank == step)
     {
-        const size_t position = (m_Rank == 0)
-                                    ? m_SizeSend
-                                    : m_SizeSend + bufferSTL.m_AbsolutePosition;
-
-        // While the MPI_Isend function should take a const void* as it's first
-        // argument, some MPICH implementations provide a broken signature
-        // which takes a non-const first argument.  The explicit const_cast
-        // here works around this.
+        m_AbsolutePositionSend =
+            (m_Rank == 0) ? m_SizeSend
+                          : m_SizeSend + bufferSTL.m_AbsolutePosition;
+
         helper::CheckMPIReturn(
-            MPI_Isend(const_cast<size_t *>(&position), 1, ADIOS2_MPI_SIZE_T,
+            MPI_Isend(&m_AbsolutePositionSend, 1, ADIOS2_MPI_SIZE_T,
                       destination, 0, m_Comm, &requests[0]),
             ", aggregation Isend absolute position at iteration " +
                 std::to_string(step) + "\n");

source/adios2/toolkit/aggregator/mpi/MPIAggregator.h

@@ -52,6 +52,7 @@ class MPIAggregator
     int m_ConsumerRank = -1;
 
     size_t m_SizeSend = 0;
+    size_t m_AbsolutePositionSend;
 
     MPIAggregator();