[JAVA-289]: convenience methods to obtain max bson size object

Author: agirbal

src/main/com/mongodb/Mongo.java

@@ -514,6 +514,26 @@ void _applyMongoOptions() {
         setWriteConcern( _options.getWriteConcern() );
     }
 
+    /**
+     * Gets the maximum size for a BSON object supported by the current master server.
+     * Note that this method may make a request to the server to obtain the size.
+     * @throws MongoException
+     * @return the size supported, or 0 if it cannot be determined.
+     */
+    public int getMaxBsonObjectSize() {
+        int size = 0;
+        if (_connector.getReplicaSetStatus() != null)
+            size = _connector.getReplicaSetStatus().getMaxBsonObjectSize();
+        if (size == 0) {
+            // need to fetch size
+            DB db = getDB("admin");
+            CommandResult res = db.command("isMaster");
+            if (res.containsField("maxBsonObjectSize"))
+                size = ((Integer)res.get( "maxBsonObjectSize" )).intValue();
+        }
+        return size;
+    }
+
     final ServerAddress _addr;
     final List<ServerAddress> _addrs;
     final MongoOptions _options;

src/main/com/mongodb/ReplicaSetStatus.java

@@ -169,6 +169,10 @@ synchronized void update(Set<Node> seenNodes){
                             seenNodes.add(node);
                     }
                 }
+
+                if (_isMaster && res.containsField("maxBsonObjectSize"))
+                    _maxBsonObjectSize = ((Integer)res.get( "maxBsonObjectSize" )).intValue();
+
             }
             catch ( MongoException e ){
                 Throwable root = e;
@@ -388,11 +392,15 @@ void close(){
         _closed = true;
     }
 
+    public int getMaxBsonObjectSize() {
+        return _maxBsonObjectSize;
+    }
 
     final List<Node> _all;
     Updater _updater;
     Mongo _mongo;
     String _setName = null; // null until init
+    int _maxBsonObjectSize = 0;
     Logger _logger = _rootLogger; // will get changed to use set name once its found
 
     String _lastPrimarySignal;