BUG: vtk write of uniform field in parallel (fixes #2349)

- used low-level MPI gather, but the wrapping routine contains an
  additional safety check for is_contiguous which is not defined for
  various std::pair<..> combination.

  So std::pair<label,vector> (which is actually contiguous, but not
  declared as is_contiguous) would falsely trip the check.

  Avoid by simply gathering unbundled values instead.

applications/test/contiguous/Test-contiguous.C

@@ -104,6 +104,8 @@ int main(int argc, char *argv[])
     printInfo<FixedList<word, 2>>();
     printInfo<Pair<word>>();
 
+    printInfo<std::pair<int, vector>>();
+
     printInfo<FixedList<FixedList<int, 2>, 2>>();
     printInfo<segment>();
 

applications/test/gatherValues1/Test-gatherValues1.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2021 OpenCFD Ltd.
+    Copyright (C) 2021-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -180,6 +180,36 @@ int main(int argc, char *argv[])
         }
     }
 
+    // This will likely fail - not declared as is_contiguous
+    // Cannot even catch since it triggers an abort()
+
+    #if 0
+    {
+        std::pair<label,vector> sendData(Pstream::myProcNo(), vector::one);
+
+        const bool oldThrowingError = FatalError.throwing(true);
+
+        try
+        {
+            List<std::pair<label,vector>> countValues
+            (
+                UPstream::listGatherValues<std::pair<label, vector>>
+                (
+                    sendData
+                )
+            );
+
+            Pout<< "listGather: " << flatOutput(countValues) << nl;
+        }
+        catch (const Foam::error& err)
+        {
+            Info<< err.message().c_str() << nl;
+        }
+
+        FatalError.throwing(oldThrowingError);
+    }
+    #endif
+
     Info<< "\nEnd\n" << endl;
 
     return 0;

src/fileFormats/vtk/output/foamVtkOutputTemplates.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2016-2021 OpenCFD Ltd.
+    Copyright (C) 2016-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -140,20 +140,15 @@ void Foam::vtk::writeValueParallel
     }
 
     // Gather [count, value] tuples, including from master
-    const List<std::pair<label, Type>> countValues
+    const List<label> counts(UPstream::listGatherValues(count));
-    (
+    const List<Type> values(UPstream::listGatherValues(val));
-        UPstream::listGatherValues<std::pair<label, Type>>
-        (
-            std::pair<label, Type>(count, val)
-        )
-    );
 
     if (Pstream::master())
     {
-        for (const auto& countVal : countValues)
+        forAll(counts, i)
         {
             // Write [count, value] tuple
-            vtk::write(fmt, countVal.first, countVal.second);
+            vtk::write(fmt, counts[i], values[i]);
         }
     }
 }