backend: Add function to check whether file is in queue dir

Another function will need to check whether its parameters
are job queue files.

Reviewed-by: iustinp

lib/backend.py

@@ -1655,14 +1655,25 @@ def RenameFileStorageDir(old_file_storage_dir, new_file_storage_dir):
   return result
 
 
-def JobQueueUpdate(file_name, content):
-  """Updates a file in the queue directory.
+def _IsJobQueueFile(file_name):
+  """Checks whether the given filename is in the queue directory.
 
   """
   queue_dir = os.path.normpath(constants.QUEUE_DIR)
-  if os.path.commonprefix([queue_dir, file_name]) != queue_dir:
+  result = (os.path.commonprefix([queue_dir, file_name]) == queue_dir)
+
+  if not result:
     logging.error("'%s' is not a file in the queue directory",
                   file_name)
+
+  return result
+
+
+def JobQueueUpdate(file_name, content):
+  """Updates a file in the queue directory.
+
+  """
+  if not _IsJobQueueFile(file_name):
     return False
 
   # Write and replace the file atomically