BUG: parallel blocking with faFieldDecomposer, faMeshReconstructor (fixes #2237)

- the patch remapping in faFieldDecomposer calls weights
  internalField() which can trigger parallel communication on the
  complete mesh for some processors only (ie, blocks).

  Force a priori creation of weights instead.

- ensure that the complete mesh (reconstruction helper)
  is serial when adding patches.

src/parallel/decompose/faDecompose/faFieldDecomposer.C

@@ -293,7 +293,7 @@ void Foam::faFieldDecomposer::reset
                 new processorEdgePatchFieldDecomposer
                 (
                     procMesh_.boundary()[patchi].size(),
-                    static_cast<const labelUList&>(localPatchSlice)
+                    localPatchSlice
                 )
             );
         }
@@ -309,6 +309,15 @@ void Foam::faFieldDecomposer::reset(const faMesh& completeMesh)
     processorAreaPatchFieldDecomposerPtrs_.resize(nMappers);
     processorEdgePatchFieldDecomposerPtrs_.resize(nMappers);
 
+    // Create weightings now - needed for proper parallel synchronization
+    (void)completeMesh.weights();
+
+    // faPatches don't have their own start() - so these are invariant
+    const labelList completePatchStarts
+    (
+        completeMesh.boundary().patchStarts()
+    );
+
     forAll(boundaryAddressing_, patchi)
     {
         const label oldPatchi = boundaryAddressing_[patchi];
@@ -324,7 +333,7 @@ void Foam::faFieldDecomposer::reset(const faMesh& completeMesh)
                 (
                     completeMesh.boundary()[oldPatchi].size(),
                     localPatchSlice,
-                    completeMesh.boundary()[oldPatchi].start()
+                    completePatchStarts[oldPatchi]
                 )
             );
         }
@@ -346,7 +355,7 @@ void Foam::faFieldDecomposer::reset(const faMesh& completeMesh)
                 new processorEdgePatchFieldDecomposer
                 (
                     procMesh_.boundary()[patchi].size(),
-                    static_cast<const labelUList&>(localPatchSlice)
+                    localPatchSlice
                 )
             );
         }

src/parallel/decompose/faDecompose/faFieldDecomposerFields.C

@@ -219,9 +219,9 @@ void Foam::faFieldDecomposer::decomposeFields
     const PtrList<GeoField>& fields
 ) const
 {
-    forAll(fields, fieldI)
+    forAll(fields, fieldi)
     {
-        decomposeField(fields[fieldI])().write();
+        decomposeField(fields[fieldi])().write();
     }
 }
 

src/parallel/reconstruct/faReconstruct/faMeshReconstructor.C

@@ -517,7 +517,13 @@ void Foam::faMeshReconstructor::createMesh()
         );
     }
 
+    // Serial mesh - no parallel communication
+
+    const bool oldParRun = Pstream::parRun(false);
+
     completeMesh.addFaPatches(completePatches);
+
+    Pstream::parRun(oldParRun);  // Restore parallel state
 }