[bugfix] Only communicate on non-empty interfaces in VariableSizeCommunicator

Before this commit communcation would be initiated even on empty
interfaces. While this does not lead to overwriting data or producing
segmentation faults, it does lead to accessing the first component of
empty vectors to get a pointer to pass to MPI functions. This produces
error output with valgrind and other memory debuggers. Therefore with this
commit we will first check whether the interface contains values and do
nothing if it doesn't. We also augmented the test to catch the case wher only
some processes have an empty interface.

dune/common/parallel/test/variablesizecommunicatortest.cc

@@ -125,6 +125,12 @@ int main(int argc, char** argv)
     }
     else
     {
+        // We also want to check the case where the interface is empty on some
+        // processes. Therefore we artificially lower the numer of processes
+        // if it is larger than two. Thus the last rank will not send anything
+        // and we check for deadlocks.
+        if(procs>2)
+            --procs;
         int N=100000;
         int num_per_proc=N/procs;
         int start, end;
@@ -142,7 +148,7 @@ int main(int argc, char** argv)
             assert(N==end);
         typedef Dune::VariableSizeCommunicator<>::InterfaceMap Interface;
         Interface inf;
-        if(rank)
+        if(rank && rank<procs) // rank==procs might hold and produce a deadlock otherwise!
         {
             Dune::InterfaceInformation send, recv;
             send.reserve(2);
@@ -165,6 +171,9 @@ int main(int argc, char** argv)
             recv.add(end);
             inf[rank+1]=std::make_pair(send, recv);
         }
+        if(rank==procs)
+            std::cout<<" rank "<<rank<<" has empty interface "<<inf.size()<<std::endl;
+
         Dune::VariableSizeCommunicator<> comm(MPI_COMM_WORLD, inf, 6);
         MyDataHandle handle(rank);
         comm.forward(handle);

dune/common/parallel/variablesizecommunicator.hh

@@ -1145,6 +1145,11 @@ template<class Allocator>
 template<bool FORWARD, class DataHandle>
 void VariableSizeCommunicator<Allocator>::communicate(DataHandle& handle)
 {
+  if( interface_->size() == 0)
+    // Simply return as otherwise we will index an empty container
+    // either for MPI_Wait_all or MPI_Test_some.
+    return;
+
   if(handle.fixedsize())
     communicateFixedSize<FORWARD>(handle);
   else