Add new PROTECTED property attribute

simgear/props/props.cxx

@@ -827,7 +827,7 @@ SGPropertyNode::trace_read () const
  * Last used attribute
  * Update as needed when enum Attribute is changed
  */
-const int SGPropertyNode::LAST_USED_ATTRIBUTE = PRESERVE;
+const int SGPropertyNode::LAST_USED_ATTRIBUTE = PROTECTED;
 
 /**
  * Default constructor: always creates a root node.

simgear/props/props.hxx

@@ -814,7 +814,8 @@ public:
     TRACE_READ = 16,
     TRACE_WRITE = 32,
     USERARCHIVE = 64,
-    PRESERVE = 128
+    PRESERVE = 128,
+    PROTECTED   = 1 << 8,
     // beware: if you add another attribute here,
     // also update value of "LAST_USED_ATTRIBUTE".
   };
@@ -1854,7 +1855,7 @@ private:
   mutable std::string _buffer;
   simgear::props::Type _type;
   bool _tied;
-  int _attr;
+  int _attr = NO_ATTR;
 
   // The right kind of pointer...
   union {

simgear/props/props_io.cxx

@@ -251,6 +251,9 @@ PropsVisitor::startElement (const char * name, const XMLAttributes &atts)
         setFlag(mode, SGPropertyNode::USERARCHIVE, val, location);
       else if( att_name == "preserve" )
         setFlag(mode, SGPropertyNode::PRESERVE, val, location);
+      // note we intentionally don't handle PROTECTED here, it's
+      // designed to be only set from compiled code, not loaded
+      // dynamically.
 
       // Check for an alias.
       else if( att_name == "alias" )