ENH: return old file handler on change

- intended for the following type of use:

      auto oldHandler = fileHandler(fileOperation::NewUncollated());

      ... do something that only works with uncollated

      // Restore previous (if any)
      if (oldHandler)
      {
          fileHandler(std::move(oldHandler));
      }

ENH: make fileOperation distributed(bool) mutable

- use is "static-like" and akin to Pstream::parRun(bool),
  thus allow toggling of the switch without a const_cast

applications/utilities/parallelProcessing/redistributePar/redistributePar.C

@@ -2303,7 +2303,7 @@ int main(int argc, char *argv[])
     //                than it writes to
     // - reconstruct - reads parallel, write on master only and to parent
     //                 directory
-    const_cast<fileOperation&>(fileHandler()).distributed(true);
+    fileHandler().distributed(true);
 
 
     #include "foamDlOpenLibs.H"

applications/utilities/postProcessing/noise/noise.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2015 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -116,7 +116,7 @@ int main(int argc, char *argv[])
     #include "setRootCase.H"
 
     // As much as possible avoid synchronised operation
-    const_cast<fileOperation&>(fileHandler()).distributed(true);
+    fileHandler().distributed(true);
 
     #include "createTime.H"
 

src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.C

@@ -729,6 +729,14 @@ Foam::fileOperation::New
 
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
+bool Foam::fileOperation::distributed(bool on) const noexcept
+{
+    bool old(distributed_);
+    distributed_ = on;
+    return old;
+}
+
+
 Foam::fileName Foam::fileOperation::objectPath
 (
     const IOobject& io,
@@ -1461,6 +1469,17 @@ Foam::label Foam::fileOperation::detectProcessorPath(const fileName& fName)
 }
 
 
+// * * * * * * * * * * * * * Static Member Functions * * * * * * * * * * * * //
+
+Foam::autoPtr<Foam::fileOperation> Foam::fileOperation::NewUncollated()
+{
+    return autoPtr<fileOperation>
+    (
+        new fileOperations::uncollatedFileOperation(false)
+    );
+}
+
+
 // * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
 
 const Foam::fileOperation& Foam::fileHandler()
@@ -1481,7 +1500,8 @@ const Foam::fileOperation& Foam::fileHandler()
 }
 
 
-void Foam::fileHandler(autoPtr<fileOperation>&& newHandler)
+Foam::autoPtr<Foam::fileOperation>
+Foam::fileHandler(autoPtr<fileOperation>&& newHandler)
 {
     if
     (
@@ -1490,10 +1510,14 @@ void Foam::fileHandler(autoPtr<fileOperation>&& newHandler)
      && newHandler->type() == fileOperation::fileHandlerPtr_->type()
     )
     {
-        return;
+        return nullptr;  // No change
     }
 
+    autoPtr<fileOperation> old(std::move(fileOperation::fileHandlerPtr_));
+
     fileOperation::fileHandlerPtr_ = std::move(newHandler);
+
+    return old;
 }
 
 

src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.H

@@ -112,7 +112,7 @@ protected:
         const label comm_;
 
         //- Distributed roots (parallel run)
-        bool distributed_;
+        mutable bool distributed_;
 
         //- Detected processors directories
         mutable HashTable<dirIndexList> procsDirs_;
@@ -181,6 +181,9 @@ public:
     //- Static fileOperation
     static autoPtr<fileOperation> fileHandlerPtr_;
 
+    //- Static construct the commonly used uncollatedFileOperation
+    static autoPtr<fileOperation> NewUncollated();
+
 
     // Constructors
 
@@ -239,13 +242,9 @@ public:
             return distributed_;
         }
 
-        //- Set distributed roots on/off, return old value
-        bool distributed(bool on) noexcept
-        {
-            bool old(distributed_);
-            distributed_ = on;
-            return old;
-        }
+        //- Set distributed roots on/off (mutable)
+        //  \return old value
+        bool distributed(bool on) const noexcept;
 
 
         // OSSpecific equivalents
@@ -595,14 +594,16 @@ public:
 };
 
 
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-// Global declarations: defined in fileOperation.C
+// * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
+
+// Note: defined in fileOperation.C
 
 //- Get current file handler
 const fileOperation& fileHandler();
 
-//- Replace, reset file handler
-void fileHandler(autoPtr<fileOperation>&& newHandler);
+//- Replace, reset file handler.
+//  \return old handler on change, null otherwise
+autoPtr<fileOperation> fileHandler(autoPtr<fileOperation>&& newHandler);
 
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //