Do not override DATA_NUMBER_OF_GHOST_LEVELS() unnecesarily.

If an algorithm produces more ghost levels than asked for, it
should be able to set DATA_NUMBER_OF_GHOST_LEVELS() and let the
pipeline know. This was not possible because the executive was
overwriting the value.

Common/ExecutionModel/vtkStreamingDemandDrivenPipeline.cxx

@@ -1060,7 +1060,22 @@ vtkStreamingDemandDrivenPipeline
         {
         dataInfo->Set(vtkDataObject::DATA_PIECE_NUMBER(), piece);
         dataInfo->Set(vtkDataObject::DATA_NUMBER_OF_PIECES(), numPieces);
-        dataInfo->Set(vtkDataObject::DATA_NUMBER_OF_GHOST_LEVELS(), ghostLevel);
+        // If the source or filter produced a different number of ghost
+        // levels, honor it.
+        int dataGhostLevel = 0;
+        if (dataInfo->Has(vtkDataObject::DATA_NUMBER_OF_GHOST_LEVELS()))
+          {
+          dataGhostLevel =
+            dataInfo->Get(vtkDataObject::DATA_NUMBER_OF_GHOST_LEVELS());
+          }
+        // If the ghost level generated by the algorithm is larger than
+        // requested, we keep it. Otherwise, we store the requested one.
+        // We do this because there is no point in the algorithm re-executing
+        // if the downstream asks for the same level even though the
+        // algorithm cannot produce it.
+        dataInfo->Set(
+          vtkDataObject::DATA_NUMBER_OF_GHOST_LEVELS(),
+          ghostLevel > dataGhostLevel ? ghostLevel : dataGhostLevel);
         }
 
       // In this block, we make sure that DATA_TIME_STEP() is set if: