add api that enables checking if a user has certain permissions on a file

For some of the authorization modes in Hive, the servers in Hive check if a given user has permissions on a certain file or directory. For example, the storage based authorization mode allows hive table metadata to be modified only when the user has access to the corresponding table directory on hdfs. There are likely to be such use cases outside of Hive as well.

HDFS does not provide an api for such checks. As a result, the logic to check if a user has permissions on a directory gets replicated in Hive. This results in duplicate logic and there introduces possibilities for inconsistencies in the interpretation of the permission model. This becomes a bigger problem with the complexity of ACL logic.

HDFS should provide an api that provides functionality that is similar to access function in unistd.h - http://linux.die.net/man/2/access .