vtkXMLMultiBlockDataReader: fix when reading nested multipiece datasets

When a vtkMultiBlockDataSet contains a vtkMultiPieceDataSet,
vtkXMLPMultiBlockDataWriter will split each piece into another
vtkMultiPieceDataSet. As a result, the written XML data file will
contain nested "Piece" elements. Since vtkMultiPieceDataSets are not
hierarchical, this dataset cannot be recreated as described.

To fix, have the reader replace the top-level vtkMultiPieceDataSet
with a vtkMultiBlockDataSet instead when nested Piece elements are
found.

Modified test to exercise writing out a vtkMultiPieceDataSet block and
read it back in.

IO/ParallelXML/Testing/Python/testParallelMultiBlockWriter.py

@@ -43,6 +43,13 @@ else:
 #   |   |-- [0,1,..nranks](0,1,...nranks)
 #   |   |-- [0,1,..nranks](0,1,...nranks)
 #   |   ... (up to total number of ranks)
+#   |
+#   |-- <MP>    # level where a multipiece dataset is present on all ranks
+#   |  |
+#   |  |-- [0,1,..nranks](0,1,...nranks)
+#   |  |-- [0,1,..nranks](0,1,...nranks)
+#   |  |-- [0,1,..nranks](0,1,...nranks)
+#   |  ... (up to total number of ranks)
 
 
 def createDataSet(empty):
@@ -62,12 +69,20 @@ def createData(non_null_ranks, non_empty_ranks, num_ranks):
         mb.SetBlock(i, createDataSet(i not in non_empty_ranks))
     return mb
 
+def createMP(num_pieces):
+    mp = vtk.vtkMultiPieceDataSet()
+    mp.SetNumberOfPieces(num_pieces)
+    for i in range(num_pieces):
+        mp.SetPiece(i, createDataSet(False))
+    return mp
+
 def createMB(piece, num_pieces):
     output = vtk.vtkMultiBlockDataSet()
-    output.SetNumberOfBlocks(3)
+    output.SetNumberOfBlocks(4)
     output.SetBlock(0, createData([piece], [piece], num_pieces))
     output.SetBlock(1, createData(range(num_pieces), [piece], num_pieces))
     output.SetBlock(2, createData(range(num_pieces), range(num_pieces), num_pieces))
+    output.SetBlock(3, createMP(num_pieces))
     return output
 
 writer = vtk.vtkXMLPMultiBlockDataWriter()
@@ -104,6 +119,6 @@ if rank == 0:
     # set of files, let's just look at the files we wrote out.
     files = os.listdir(prefix)
 
-    expected_file_count = nranks + nranks + (nranks*nranks)
+    expected_file_count = nranks + nranks + 2*(nranks*nranks)
     print ("Expecting %d files for the leaf nodes" % expected_file_count)
     assert (len(files) == expected_file_count)

IO/XML/vtkXMLMultiBlockDataReader.cxx

@@ -172,7 +172,20 @@ void vtkXMLMultiBlockDataReader::ReadComposite(vtkXMLDataElement* element,
     // Child is a multipiece dataset. Create it.
     else if (mblock != nullptr && strcmp(tagName, "Piece") == 0)
     {
-      vtkMultiPieceDataSet* childDS = vtkMultiPieceDataSet::New();
+      // Look ahead to see if there is a nested Piece structure, which can happen when
+      // the dataset pieces in a vtkMultiPieceDataSet are themselves split into
+      // vtkMultiPieceDataSets when saved in parallel.
+      vtkCompositeDataSet* childDS;
+      if (childXML->FindNestedElementWithName("Piece"))
+      {
+        // Create a multiblock to handle a multipiece child
+        childDS = vtkMultiBlockDataSet::New();
+      }
+      else
+      {
+        // Child is not multipiece, so it is safe to create a vtkMultiPieceDataSet
+        childDS = vtkMultiPieceDataSet::New();
+      }
       this->ReadComposite(childXML, childDS, filePath, dataSetIndex);
       const char* name = childXML->GetAttribute("name");
       mblock->SetBlock(index, childDS);
@@ -284,7 +297,21 @@ int vtkXMLMultiBlockDataReader::FillMetaData(vtkCompositeDataSet* metadata,
     // Child is a multipiece dataset. Create it.
     else if (mblock != nullptr && strcmp(tagName, "Piece") == 0)
     {
-      vtkMultiPieceDataSet* childDS = vtkMultiPieceDataSet::New();
+      // Look ahead to see if there is a nested Piece structure, which can happen when
+      // the dataset pieces in a vtkMultiPieceDataSet are themselves split into
+      // vtkMultiPieceDataSets when saved in parallel.
+      vtkCompositeDataSet* childDS;
+      if (childXML->FindNestedElementWithName("Piece"))
+      {
+        // Create a multiblock to handle a multipiece child
+        childDS = vtkMultiBlockDataSet::New();
+      }
+      else
+      {
+        // Child is not multipiece, so it is safe to create a vtkMultiPieceDataSet
+        childDS = vtkMultiPieceDataSet::New();
+      }
+
       this->FillMetaData(childDS, childXML, filePath, dataSetIndex);
       mblock->SetBlock(index, childDS);
       childDS->Delete();