ENH: support change of Pstream buffer format via flag modifier

- allows changing the format of the sending OPstream at an arbitrary
  point in the transmission. The information is passed through the
  buffer and the receiving IPstream changes its format accordingly.
  This allows a temporary toggling of ASCII/BINARY mid-stream.

src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C

@@ -29,6 +29,24 @@ License
 #include "token.H"
 #include <cctype>
 
+// * * * * * * * * * * * * * * * Local Functions * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+// Adjust stream format based on the flagMask
+inline static void processFlags(Istream& is, int flagMask)
+{
+    if ((flagMask & token::ASCII))
+    {
+        is.format(IOstream::ASCII);
+    }
+    else if ((flagMask & token::BINARY))
+    {
+        is.format(IOstream::BINARY);
+    }
+}
+}
+
 
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
@@ -121,19 +139,48 @@ Foam::UIPstream::~UIPstream()
 Foam::Istream& Foam::UIPstream::read(token& t)
 {
     // Return the put back token if it exists
+    // - with additional handling for special stream flags
     if (Istream::getBack(t))
     {
-        return *this;
+        if (t.isFlag())
+        {
+            processFlags(*this, t.flagToken());
+        }
+        else
+        {
+            return *this;
+        }
     }
 
-    char c;
+    // Read character, return on error
+    // - with additional handling for special stream flags
 
-    // Return on error
-    if (!read(c))
+    char c;
+    do
     {
-        t.setBad();
-        return *this;
+        if (!read(c))
+        {
+            t.setBad();   // Error
+            return *this;
+        }
+
+        if (c == token::FLAG)
+        {
+            char flagVal;
+
+            if (read(flagVal))
+            {
+                processFlags(*this, flagVal);
+            }
+            else
+            {
+                t.setBad();   // Error
+                return *this;
+            }
+        }
     }
+    while (c == token::FLAG);
+
 
     // Set the line number of this token to the current stream line number
     t.lineNumber() = lineNumber();

src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C

@@ -193,7 +193,8 @@ bool Foam::UOPstream::write(const token& tok)
     {
         case token::tokenType::FLAG :
         {
-            // silently consume the flag
+            writeToBuffer(char(token::tokenType::FLAG));
+            writeToBuffer(char(tok.flagToken()));
 
             return true;
         }