### What changes were proposed in this pull request?
Currently, we do not have a suitable definition of the `user` concept in Spark. We only have a `sparkUser` app widely but do not support identify or retrieve the user information from a session in STS or a runtime query execution.
`current_user()` is very popular and supported by plenty of other modern or old school databases, and also ANSI compliant.
This PR add `current_user()` as a SQL function. And, they are the same. In this PR, we add these functions w/o ambiguity.
1. For a normal single-threaded Spark application, clearly the `sparkUser` is always equivalent to `current_user()` .
2. For a multi-threaded Spark application, e.g. Spark thrift server, we use a `ThreadLocal` variable to store the client-side user(after authenticated) before running the query and retrieve it in the parser.
### Why are the changes needed?
`current_user()` is very popular and supported by plenty of other modern or old school databases, and also ANSI compliant.
### Does this PR introduce _any_ user-facing change?
yes, added `current_user()` as a SQL function
### How was this patch tested?
new tests in thrift server and sql/catalyst
Closes#32718 from yaooqinn/SPARK-21957.
Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
345d35ed1a