ENH: additional protection against zero-sized graph offset lists

- for metis-like graphs there is no guarantee that a zero-sized graph
  has an offsets list with size 1 or size 0, so always use

     numCells = max(0, xadj.size()-1)

  this was already done in most places, but missed in the
  decomposeGeneral method

STYLE: use sumOp<label>() instead of plusOp<label>()

applications/utilities/preProcessing/setExprFields/setExprFields.C

@@ -208,8 +208,8 @@ bool setField
 
     doCorrectBoundaryConditions(ctrl.correctBCs, output);
 
-    const label numTotal = returnReduce(output.size(), plusOp<label>());
-    reduce(numValuesChanged, plusOp<label>());
+    const label numTotal = returnReduce(output.size(), sumOp<label>());
+    reduce(numValuesChanged, sumOp<label>());
 
     if (numValuesChanged == numTotal)
     {

src/functionObjects/field/expressions/fvExpressionField.C

@@ -265,8 +265,8 @@ bool Foam::functionObjects::fvExpressionField::setField
 
     if (action_ == actionType::opModify && log)
     {
-        const label numTotal = returnReduce(output.size(), plusOp<label>());
-        reduce(numValuesChanged, plusOp<label>());
+        const label numTotal = returnReduce(output.size(), sumOp<label>());
+        reduce(numValuesChanged, sumOp<label>());
 
         Info<< this->name() << ": set ";
         if (numValuesChanged == numTotal)

src/parallel/decompose/decompose/decompositionInformation.C

@@ -49,8 +49,10 @@ void Foam::decompositionInformation::populate
         subdist.resize(nDomain, Zero);
     }
 
-    const label nCells = max(0, xadj.size()-1);
-    for (label celli = 0; celli < nCells; ++celli)
+    // Protect against zero-sized offset list
+    const label numCells = max(0, (xadj.size()-1));
+
+    for (label celli = 0; celli < numCells; ++celli)
     {
         const label ownProc = decomp[celli];
 

src/parallel/decompose/decompositionMethods/decompositionMethod/decompositionMethod.C

@@ -29,7 +29,6 @@ License
 #include "decompositionMethod.H"
 #include "globalIndex.H"
 #include "syncTools.H"
-#include "Tuple2.H"
 #include "faceSet.H"
 #include "regionSplit.H"
 #include "localPointRegion.H"
@@ -481,7 +480,7 @@ void Foam::decompositionMethod::calcCellCells
     // Create global cell numbers
     // ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-    globalIndex globalAgglom
+    const globalIndex globalAgglom
     (
         nLocalCoarse,
         Pstream::msgType(),
@@ -680,7 +679,7 @@ void Foam::decompositionMethod::calcCellCells
     // Create global cell numbers
     // ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-    globalIndex globalAgglom
+    const globalIndex globalAgglom
     (
         nLocalCoarse,
         Pstream::msgType(),

src/parallel/decompose/decompositionMethods/metisLikeDecomp/metisLikeDecomp.C

@@ -56,8 +56,11 @@ Foam::label Foam::metisLikeDecomp::decomposeGeneral
             << " Decomposing all of graph on master processor." << endl;
     }
 
+    // Protect against zero-sized offset list
+    const label numCells = max(0, (xadj.size()-1));
+
     const globalIndex globalAdjncy(adjncy.size());
-    const globalIndex globalCells(xadj.size()-1);
+    const globalIndex globalCells(numCells);
 
     List<label> allAdjncy(globalAdjncy.gather(adjncy));
 
@@ -107,13 +110,13 @@ Foam::label Foam::metisLikeDecomp::decomposeGeneral
     {
         // Send my part of the graph (local numbering)
 
-        if (xadj.size() <= 1)
+        if (!numCells)
         {
             // Nothing to do
         }
         else
         {
-            SubList<label> procSlot(xadj, xadj.size()-1);
+            SubList<label> procSlot(xadj, numCells);
 
             OPstream::write
             (

src/parallel/decompose/decompositionMethods/multiLevelDecomp/multiLevelDecomp.C

@@ -253,11 +253,11 @@ void Foam::multiLevelDecomp::subsetGlobalCellCells
     labelList& cutConnections
 ) const
 {
+    const globalIndex globalCells(cellCells.size());
+
     // Determine new index for cells by inverting subset
     labelList oldToNew(invert(cellCells.size(), set));
 
-    globalIndex globalCells(cellCells.size());
-
     // Subset locally the elements for which I have data
     subCellCells = UIndirectList<labelList>(cellCells, set);
 
@@ -275,7 +275,7 @@ void Foam::multiLevelDecomp::subsetGlobalCellCells
     // subCellCells : indexes into oldToNew and allDist
 
     // Globally compact numbering for cells in set.
-    globalIndex globalSubCells(set.size());
+    const globalIndex globalSubCells(set.size());
 
     // Now subCellCells contains indices into oldToNew which are the
     // new locations of the neighbouring cells.
@@ -417,7 +417,7 @@ void Foam::multiLevelDecomp::decompose
                 nOutsideConnections
             );
 
-            label nPoints = returnReduce(domainPoints.size(), plusOp<label>());
+            label nPoints = returnReduce(domainPoints.size(), sumOp<label>());
             Pstream::listCombineGather(nOutsideConnections, plusEqOp<label>());
             Pstream::listCombineScatter(nOutsideConnections);
             label nPatches = 0;
@@ -525,7 +525,7 @@ void Foam::multiLevelDecomp::decompose
                     }
                 }
 
-                reduce(nPoints, plusOp<label>());
+                reduce(nPoints, sumOp<label>());
                 Pstream::listCombineGather
                 (
                     nOutsideConnections,