ENH: additional control and access methods for PstreamBuffers

- PstreamBuffers nProcs() and allProcs() methods to recover the rank
  information consistent with the communicator used for construction

- allowClearRecv() methods for more control over buffer reuse
  For example,

      pBufs.allowClearRecv(false);

      forAll(particles, particlei)
      {
          pBufs.clear();

          fill...

          read via IPstream(..., pBufs);
       }

  This preserves the receive buffers memory allocation between calls.

- finishedNeighbourSends() method as compact wrapper for
  finishedSends() when send/recv ranks are identically
  (eg, neighbours)

- hasSendData()/hasRecvData() methods for PstreamBuffers.

  Can be useful for some situations to skip reading entirely.
  For example,

      pBufs.finishedNeighbourSends(neighProcs);

      if (!returnReduce(pBufs.hasRecvData(), orOp<bool>()))
      {
          // Nothing to do
          continue;
      }
      ...

  On an individual basis:

      for (const int proci : pBufs.allProcs())
      {
          if (pBufs.hasRecvData(proci))
          {
             ...
          }
      }

  Also conceivable to do the following instead (nonBlocking only):

      if (!returnReduce(pBufs.hasSendData(), orOp<bool>()))
      {
          // Nothing to do
          pBufs.clear();
          continue;
      }

      pBufs.finishedNeighbourSends(neighProcs);
      ...

applications/test/processorTopology/Make/files

@@ -0,0 +1,3 @@
+Test-processorTopology.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-processorTopology

applications/test/processorTopology/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */

applications/test/processorTopology/Test-processorTopology.C

@@ -0,0 +1,113 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2022 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Description
+    Test/output processor topology
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "Time.H"
+#include "polyMesh.H"
+#include "globalMeshData.H"
+#include "OFstream.H"
+
+using namespace Foam;
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// Main program:
+
+int main(int argc, char *argv[])
+{
+    argList::noFunctionObjects();
+    argList::addNote
+    (
+        "Create a metis graph file representation for an OpenFOAM mesh"
+    );
+
+    #include "setRootCase.H"
+
+    if (!Pstream::parRun())
+    {
+        FatalErrorInFunction
+            << "Only meaningful in parallel"
+            << exit(FatalError);
+    }
+
+    #include "createTime.H"
+    #include "createPolyMesh.H"
+
+    const globalMeshData& globData = mesh.globalData();
+
+    const labelListList& connectivity = globData;
+
+    if (Pstream::master())
+    {
+        OFstream os("processorTopology.dot");
+
+        os << "// processorTopology" << nl << nl;
+        os.beginBlock("graph");
+
+        forAll(connectivity, proci)
+        {
+            label nconn = 0;
+
+            for (const label neighProci : connectivity[proci])
+            {
+                if (proci < neighProci)
+                {
+                    if (nconn++)
+                    {
+                        os << "  ";
+                    }
+                    else
+                    {
+                        os << indent;
+                    }
+                    os << proci << " -- " << neighProci;
+                }
+            }
+
+            if (nconn)
+            {
+                os  << nl;
+            }
+        }
+
+        os.endBlock();
+
+        Info<< "Wrote processorTopology graph: "
+            << runTime.relativePath(os.name()) << nl;
+
+        Info<< nl
+            << "Use neato, circo or fdp graphviz tools" << nl;
+    }
+
+    Info<< nl << "End\n" << endl;
+
+    return 0;
+}
+
+// ************************************************************************* //

applications/utilities/parallelProcessing/redistributePar/parLagrangianRedistributor.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2015 OpenFOAM Foundation
-    Copyright (C) 2015-2018 OpenCFD Ltd.
+    Copyright (C) 2015-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -185,9 +185,8 @@ Foam::parLagrangianRedistributor::redistributeLagrangianPositions
     }
 
 
-    // Start sending. Sets number of bytes transferred
-    labelList allNTrans(Pstream::nProcs());
-    pBufs.finishedSends(allNTrans);
+    // Start sending
+    pBufs.finishedSends();
 
 
     {
@@ -205,18 +204,15 @@ Foam::parLagrangianRedistributor::redistributeLagrangianPositions
             IDLList<passivePositionParticle>()
         );
 
-
         // Retrieve from receive buffers
-        forAll(allNTrans, procI)
+        for (const int proci : pBufs.allProcs())
         {
-            const label nRec = allNTrans[procI];
+            //Pout<< "Receive from processor" << proci << " : "
+            //    << pBufs.hasRecvData(proci) << endl;
 
-            //Pout<< "From processor " << procI << " receiving bytes " << nRec
-            //    << endl;
-
-            if (nRec)
+            if (pBufs.hasRecvData(proci))
             {
-                UIPstream particleStream(procI, pBufs);
+                UIPstream particleStream(proci, pBufs);
 
                 // Receive particles and locate them
                 IDLList<passivePositionParticle> newParticles