ganeti-watcher: Don't write file if data didn't change

This is the safest way to detect changes and the amount of data
is small, so keeping a copy around is cheap enough.

Reviewed-by: iustinp

daemons/ganeti-watcher

@@ -136,12 +136,19 @@ class WatcherState(object):
     if "node" not in self._data:
       self._data["node"] = {}
 
+    self._orig_data = self._data.copy()
+
   def Save(self):
     """Save state to file, then unlock and close it.
 
     """
     assert self.statefile
 
+    if self._orig_data == self._data:
+      logging.debug("Data didn't change, just touching status file")
+      os.utime(constants.WATCHER_STATEFILE, None)
+      return
+
     # We need to make sure the file is locked before renaming it, otherwise
     # starting ganeti-watcher again at the same time will create a conflict.
     fd = utils.WriteFile(constants.WATCHER_STATEFILE,