fix problem when using PETSC_COMM_SELF with Msg::GetCommSize()>1

Solver/linearSystemPETSc.hpp

@@ -117,6 +117,9 @@ void linearSystemPETSc<scalar>::allocate(int nbRows)
   //since PETSc 3.3 GetOwnershipRange and MatGetSize cannot be called before MatXXXSetPreallocation
   _localSize = nbRows;
   #ifdef HAVE_MPI
+  PetscMPIInt commSize;
+  MPI_Comm_size(_comm,&commSize);
+  if (commSize>1){
     _localRowStart = 0;
     if (Msg::GetCommRank() != 0) {
       MPI_Status status;
@@ -127,6 +130,12 @@ void linearSystemPETSc<scalar>::allocate(int nbRows)
       MPI_Send((void*)&_localRowEnd, 1, MPI_INT, Msg::GetCommRank() + 1, 1, MPI_COMM_WORLD);
     }
     MPI_Allreduce((void*)&_localSize, (void*)&_globalSize, 1, MPI_INT, MPI_SUM, MPI_COMM_WORLD);
+  }
+  else{
+    _localRowStart = 0;
+    _localRowEnd = nbRows;
+    _globalSize = _localSize;
+  }
   #else
   _localRowStart = 0;
   _localRowEnd = nbRows;