[!733] Update generic mpi op

Merge branch 'update-generic-mpi-op' into 'master'

See merge request [!733]

  [!733]: Nonecore/dune-common/merge_requests/733

dune/common/parallel/mpicommunication.hh

@@ -43,7 +43,11 @@ namespace Dune
     {
       if (!op)
       {
-        op = std::shared_ptr<MPI_Op>(new MPI_Op);
+        op = std::make_unique<MPI_Op>();
+        // The following line leaks an MPI operation object, because the corresponding
+        //`MPI_Op_free` is never called.  It is never called because there is no easy
+        // way to call it at the right moment: right before the call to MPI_Finalize.
+        // See https://gitlab.dune-project.org/core/dune-istl/issues/80
         MPI_Op_create((void (*)(void*, void*, int*, MPI_Datatype*))&operation,true,op.get());
       }
       return *op;
@@ -61,12 +65,12 @@ namespace Dune
     }
     Generic_MPI_Op () {}
     Generic_MPI_Op (const Generic_MPI_Op& ) {}
-    static std::shared_ptr<MPI_Op> op;
+    static std::unique_ptr<MPI_Op> op;
   };
 
 
   template<typename Type, typename BinaryFunction, typename Enable>
-  std::shared_ptr<MPI_Op> Generic_MPI_Op<Type,BinaryFunction, Enable>::op = std::shared_ptr<MPI_Op>(static_cast<MPI_Op*>(0));
+  std::unique_ptr<MPI_Op> Generic_MPI_Op<Type,BinaryFunction, Enable>::op;
 
 #define ComposeMPIOp(func,op)                                           \
   template<class T, class S>                                            \