Introduced preliminary support for asynchronous file read requests,

ReaderWriter::ReadResult now has a FILE_REQUEST enum.
  ReaderWriter::Options now has a s/getAsynchronousFileReadHint() parameter methods.

  libcurl based plugin now detects enabing of the AsynchronousFileReadHint, but
  as yet does not handle async requests - handling everything syncronously.
  
  DatabasePager now by default will enable AsynchronousFileReadHint for http
  based file requests

include/osgDB/ReaderWriter

@@ -90,17 +90,21 @@ class OSGDB_EXPORT ReaderWriter : public osg::Object
 
                 Options():
                     osg::Object(true),
-                    _objectCacheHint(CACHE_ARCHIVES) {}
+                    _objectCacheHint(CACHE_ARCHIVES),
+                    _asynchronousFileReadHint(false) {}
+                    
                 Options(const std::string& str):
                     osg::Object(true),
                     _str(str), 
-                    _objectCacheHint(CACHE_ARCHIVES) {}
+                    _objectCacheHint(CACHE_ARCHIVES),
+                    _asynchronousFileReadHint(false) {}
                 
                 Options(const Options& options,const osg::CopyOp& copyop=osg::CopyOp::SHALLOW_COPY):
                     osg::Object(options,copyop),
                     _str(options._str),
                     _databasePaths(options._databasePaths),
-                    _objectCacheHint(options._objectCacheHint) {}
+                    _objectCacheHint(options._objectCacheHint),
+                    _asynchronousFileReadHint(options._asynchronousFileReadHint) {}
 
                 META_Object(osgDB,Options);
 
@@ -119,12 +123,29 @@ class OSGDB_EXPORT ReaderWriter : public osg::Object
                 /** Get the const database path which is used a hint of where to look when loading models.*/
                 const FilePathList& getDatabasePathList() const { return _databasePaths; }
 
+
                 /** Set whether the Registry::ObjectCache should be used by default.*/
                 void setObjectCacheHint(CacheHintOptions useObjectCache) { _objectCacheHint = useObjectCache; }
 
                 /** Get whether the Registry::ObjectCache should be used by default.*/
                 CacheHintOptions getObjectCacheHint() const { return _objectCacheHint; }
 
+
+                /** Set Asynchrnous file read hint. 
+                  * This hint is used by plugins like the libcurl http reader plugin to inform them that
+                  * they should make an internal file read requests to their background threads to load files,
+                  * with the plugin returning immediately with a ReadResult::FILE_REQUESTED status.  It is 
+                  * assumed that calls will continue to be made to the plugin until the background threads
+                  * have read or failed to read the request file, at which point the return status which change
+                  * to FILE_LOADED and the objects will be returned.
+                  * Note, this facility is particular useful when using DatabasePager in conjunction with
+                  * internet based databases where file load latency is relatively high.*/
+                void setAsynchronousFileReadHint(bool flag) { _asynchronousFileReadHint = flag; }
+
+                /** Get Asynchrnous file read hint. */
+                bool getAsynchronousFileReadHint() const { return _asynchronousFileReadHint; }
+
+
                 /** Sets a plugindata value PluginData with a string */
                 void setPluginData(const std::string& s, void* v) const { _pluginData[s] = v; }
 
@@ -148,6 +169,7 @@ class OSGDB_EXPORT ReaderWriter : public osg::Object
                 std::string         _str;
                 FilePathList        _databasePaths;
                 CacheHintOptions    _objectCacheHint;
+                bool                _asynchronousFileReadHint;
 
                 typedef std::map<std::string,void*> PluginDataMap;
                 mutable PluginDataMap _pluginData;
@@ -160,11 +182,12 @@ class OSGDB_EXPORT ReaderWriter : public osg::Object
 
                 enum ReadStatus
                 {
-                    FILE_NOT_HANDLED, //!< file is not appropriate for this file reader, due to some incompatibility, but *not* a read error
-                    FILE_NOT_FOUND, //!< file could not be found or could not be read
-                    FILE_LOADED, //!< file successfully found, loaded, and converted into osg
-                    FILE_LOADED_FROM_CACHE, //!< file found in cache and returned
-                    ERROR_IN_READING_FILE //!< file found, loaded, but an error was encountered during processing
+                    FILE_NOT_HANDLED, //!< File is not appropriate for this file reader, due to some incompatibility, but *not* a read error.
+                    FILE_NOT_FOUND, //!< File could not be found or could not be read.
+                    FILE_LOADED, //!< File successfully found, loaded, and converted into osg.
+                    FILE_LOADED_FROM_CACHE, //!< File found in cache and returned.
+                    ERROR_IN_READING_FILE, //!< File found, loaded, but an error was encountered during processing.
+                    FILE_REQUESTED, //!< Asyncronous file read has been requested, but returning immediatiely, keep polling plugin till file read has been completed.
                 };
 
                 ReadResult(ReadStatus status=FILE_NOT_HANDLED):_status(status) {}