added a log rolling function to file appender

lib/log4js.js

@@ -342,22 +342,69 @@ module.exports = function (fileSystem, standardOutput, configPaths) {
     }
 
     /**
-     * File Appender writing the logs to a text file.
+     * File Appender writing the logs to a text file. Supports rolling of logs by size.
      * 
      * @param file file log messages will be written to
      * @param layout a function that takes a logevent and returns a string (defaults to basicLayout).
+     * @param logSize - the maximum size (in bytes) for a log file, if not provided then logs won't be rotated.
+     * @param numBackups - the number of log files to keep after logSize has been reached (default 5)
+     * @param filePollInterval - the time in seconds between file size checks (default 30s)
      */
-    function fileAppender (file, layout) {
+    function fileAppender (file, layout, logSize, numBackups, filePollInterval) {
 	layout = layout || basicLayout;	
 	//syncs are generally bad, but we need 
 	//the file to be open before we start doing any writing.
-	var logFile = fs.openSync(file, 'a', 0644);    
+	var logFile = fs.openSync(file, 'a', 0644);
+
+        if (logSize > 0) {
+            setupLogRolling(logFile, file, logSize, numBackups || 5, (filePollInterval * 1000) || 30000);
+        }
 	
 	return function(loggingEvent) {
 	    fs.write(logFile, layout(loggingEvent)+'\n', null, "utf8");
 	};
     }
 
+    function setupLogRolling (logFile, filename, logSize, numBackups, filePollInterval) {
+        fs.watchFile(filename, 
+                     { 
+                         persistent: false, 
+                         interval: filePollInterval 
+                     },
+                     function (curr, prev) {
+                         if (curr.size >= logSize) {
+                             rollThatLog(logFile, filename, numBackups);
+                         }
+                     }
+                    );
+    }
+
+    function rollThatLog (logFile, filename, numBackups) {
+        //first close the current one.
+        fs.closeSync(logFile);
+        //roll the backups (rename file.n-1 to file.n, where n <= numBackups)
+        for (var i=numBackups; i > 0; i--) {
+            if (i > 1) {
+                if (fileExists(filename + '.' + (i-1))) {
+                    fs.renameSync(filename+'.'+(i-1), filename+'.'+i);
+                }
+            } else {
+                fs.renameSync(filename, filename+'.1');
+            }
+        }
+        //open it up again
+        logFile = fs.openSync(filename, 'a', 0644);
+    }
+
+    function fileExists (filename) {
+        try {
+            fs.statSync(filename);
+            return true;
+        } catch (e) {
+            return false;
+        }
+    }
+
     function logLevelFilter (levelString, appender) {
 	var level = Level.toLevel(levelString);
 	return function(logEvent) {